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SMITHSONIAN CONTRIBUTIONS TO PALEOBIOLOGY • NUMBER 49

Megaspores and a Palynomorph
from the Lower Potomac Group
in Virginia

Francis M. Hueber

SMITHSONIAN INSTITUTION PRESS

City of Washington

1982

ABSTRACT

Hueber, Francis M. Megaspores and a Palynomorph from the Lower Potomac Group in Virginia. Smithsonian Contributions to Paleobiology, number 49, 69 pages, 1 figure, 24 plates, 1982.—A plant microfossil assemblage comprising seven species of megaspores; Verrutriletes carbunculus (Dijkstra) Potonié, Echitriletes cf. E. lanatus (Dijkstra) Potonié, Erlansonisporites erlansonii (Miner) Potonié, Thylakosporites retiarius (Hughes) Potonié, Arcellites disciformis (Miner) Ellis and Tschudy, Arcellites cf. A. pyriformis (Dijkstra) Potter, and Paxillitriletes species Hall and Nicolson; two species of the microspore Crybelosporites Dettmann, C. striatus (Cookson and Dettmann) Dettmann adherent to specimens of Arcellites disciformis, and Crybelosporites species adherent to specimens of Echitriletes cf. E. lanatus; and the palynomorph Dictyothylakos pesslerae Horst; is recorded from the Patuxent Formation, Potomac Group, Lower Cretaceous (Barremian-Aptian) in Virginia, USA. A preliminary analysis of the enclosing matrix for microspores and pollen has related the collection site closely to lowermost Zone I of the Potomac Group as described by Hickey and Doyle (1977). The megaspore assemblage supported by acceptance of the oldest possible date derived from the microspore and pollen analysis suggests correlation with the Barremian-Aptian horizons in the English Wealden, Lower Cretaceous, and specifically with the “Arcellites Flora” of Hughes. Megafossils comprising two seed cones belonging to the Pinaceae, Pityostrobus hueberi Robison and Miller and Pityostrobus virginiana Robison and Miller have been reported from the site. A fruit or cupule of Caytonia has been found along with numerous seeds, fern fragments, coniferous woods, and cycadopsid cuticles. This array of megafossils is not described or illustrated herein. A backswamp area of sedimentation and type of habitat is suggested on the basis of the lithofacies and generalized composition of the flora. The writer fully agrees with Tschudy (1976) as to the importance of searching for megaspores in continental Mesozoic rocks to aid in correlating and subdividing the deposits more effectively.

OFFICIAL PUBLICATION DATE is handstamped in a limited number of initial copies and is recorded in the Institution’s annual report, Smithsonian Year. SERIES COVER DESIGN: The trilobite Phacops rana Green.

Library of Congress Cataloging in Publication Data

Hueber, Francis M.

Megaspores and a Palynomorph from the Lower Potomac Group in Virginia

Smithsonian contributions to paleobiology; no. 49

Bibliography: p.

1. Spores (Botany), Fossil. 2. Palynology. 3. Paleobotany—Cretaceous. 4. Paleobotany—Virginia. I. Title. II. Series.

QE701.S56 no. 49 [QE996] 560s [561′.131 81-607852 AACR2

Contents

Introduction

Locality

Materials and Methods

Acknowledgments

Systematics

Verrutriletes Potonié, 1956

Verrutriletes carbunculus (Dijkstra) Potonié

Echitriletes Potonié, 1956

Echitriletes cf. E. lanatus (Dijkstra) Potonié

Erlansonisporites Potonié, 1956

Erlansonisporites erlansonii (Miner) Potonié

Thylakosporites Potonié, 1956

Thylakosporites retiarius (Hughes) Potonié

Crybelosporites Dettmann, 1963

Crybelosporites striatus (Cookson and Dettmann) Dettmann

Crybelosporites species

Arcellites Miner, 1935

Arcellites disciformis (Miner) Ellis and Tschudy

Arcellites cf. A. pyriformis (Dijkstra) Potter

Paxillitriletes Hall and Nicolson, 1973

Paxillitriletes species

Dictyothylakos Horst, 1954

Dictyothylakos pesslerae Horst, 1954

Conclusions

Literature Cited

Plates

Requirements for Smithsonian Series Publication

Megaspores and a Palynomorph
from the Lower Potomac Group
in Virginia

Francis M. Hueber

___________

Francis M. Hueber, Department of Paleobiology, National Museum of National History, Smithsonian Institution, Washington, D. C. 20560.

Introduction

The present study grew out of finding specimens of the megaspore species Arcellites disciformis (Miner) Ellis and Tschudy 1964 during a routine examination of a collection of fossil plants I made in 1973. My original interest in the collection centered on the woods and gymnosperm cones; however, the beauty of the megaspore diverted my attention and led me to research the literature for any records of its occurrence in the Cretaceous Potomac Group. The search was a very brief one. I found the summary paper for the genus by Ellis and Tschudy (1964), in which there is citation of the occurrence of the species in the Patuxent Formation based on a written communication from W. G. Chaloner. The occurrence represented stratigraphically the earliest for the genus, but no subsequent photographic documentation was published.

I had obtained only three specimens from my original preparation of the matrix. I mounted the best one for viewing by means of the scanning electron microscope. The results were excellent; however, the additional details of anatomy and morphology needed to fully substantiate the identification of the species required more specimens. Preparation of three more samples of the matrix and careful search yielded 84 well-preserved specimens and 15 badly distorted ones. The even more significant results of those preparations was the accumulation of other species of megaspores in adequate numbers to form the basis for this expanded and hopefully more useful report.

Tschudy in 1976 and in later conversation gave added inspiration regarding the significance of this small assemblage of megaspores. In addition, an analysis of a sample of the matrix for microspore and pollen content was made by J. A. Doyle, whose findings served as the key to the stratigraphic correlations discussed later herein.

Tschudy (1976) was correct when he wrote:

I am convinced that once megaspores have been routinely searched for in continental Mesozoic rocks they, and the smaller palynomorphs accompanying them, will provide for the stratigraphic subdivision of non-marine rocks as effectively as ammonites and baculites now serve to subdivide marine rock sequences.

LOCALITY.—Stratum 2 to feet (60–76 cm) thick and undetermined areal extent, comprising lignitized logs and other plant debris enclosed in a medium gray clay, with irregularly distributed pockets of silt and medium to coarse sand grains. Exposed in 1973 during construction of new sections of the road bed of Henry Shirley Memorial Highway, Alexandria City, Virginia, on the slope between Seminary Road and Richenbacher Avenue at Lat. 38°49′46″ N, Long 77°07′07″ W, 7.5′ Alexandria Quadrangle.

The locality is given USNM Locality No. 14252.

Age is Lower Cretaceous (Barremian-Aptian), Potomac Group, Patuxent Formation, Lower Zone I of Hickey and Doyle (1977).

MATERIALS AND METHODS.—The collection comprises lumps of clay and sandy clay matrix containing lignitized woods, cones, leaf fragments and a large array of seeds. Preparation of a small sample was made, immediately after the collection was obtained, in order to evaluate the quality of the fossil material before the matrix became dry. Most of the larger plant fragments disintegrate after drying, even if the matrix is allowed to dry slowly within its protective paper wrappings. The first specimens of the spore genus Arcellites were observed and notes made indicating the importance of documenting photographically the presence of the spores in the Potomac Group. The collection was stored until time for additional preparations was available.

The matrix when fresh and moist was a medium gray color; however, on drying a noticeable change to dusky yellow took place. I suspect that the change was due to oxidation of finely divided pyrite that contributes significantly to the original gray color. An efflorescence of melanterite is present on most of the samples, which are now quite dry, and its presence as an oxidation by-product lends credence to the probable explanation of the color change of the matrix.

The sand and silt grains, distributed in irregular pockets throughout the clay, primarily are angular, transparent to translucent quartz. A small fraction of the silt grains are euhederal zircon, along with mineral fragments tentatively identified as ilmenite and garnet. No attempt was made to carry out a complete qualitative and quantitative analysis of the matrix.

The matrix will gradually disintegrate when soaked in water for at least one week. Unfortunately, however, the plant fossils remain coated with clay particles and sand grains. Complete maceration of the matrix with subsequent free and clean release of the plant material was best accomplished with concentrated (48%) hydrofluoric acid.

Transparent, polystyrene, lidded containers, the type used for food storage, were used to hold the acid and matrix samples during maceration. A platform to support the matrix was constructed from plastic containers of the type used in marketing fruits such as strawberries. Polyester screen of standard 0.5 mm mesh was placed on top of the supporting platform and then the specimen of matrix. The container was filled with concentrated (48%) hydrofluoric acid in sufficient quantity to cover the matrix sample. The chemical reaction was exothermic and it was necessary to keep the containers in a cold water bath during the early part of the maceration. The macerations and first washings were done in a chemical fume hood.

After 18 to 24 hours the maceration was complete. The larger fragments of plant remains were left lying on the screen while the smaller particles settled through to the bottom of the container. The screen was slowly lifted from the container by holding opposing edges with large, long forceps. Particular care was taken to avoid disturbing the fossil plant fragments more than necessary. A one gallon polyethylene cylinder, filled with tap water and fitted with a plastic supportive platform positioned about one inch below the water surface, was close at hand. The screen and plant fragments were carefully lowered into the water and placed on the platform. After one hour the water in the container was siphoned off and discarded. The container was immediately refilled with fresh tap water. This washing was repeated four times. Sediment at the bottom of the container was not removed during the siphoning process but saved to be combined later with the sediment that had settled through the screen during maceration. Additional washing of the larger plant fragments was accomplished by inverting the supportive screen into a 0.5 mm mesh plastic sieve that was partially submerged in a constant-flow water bath set up in the laboratory sink. The plant fragments generally settled in a single area in the sieve but by sweeping the piece of supportive screen forcefully through the water and near the specimens they were eventually well separated from one another. Washing continued for five hours. The washed material was then distributed among large (14 cm) petri plates for sorting and picking of the specimens.

The sediment in the maceration containers was treated differently from the coarse fraction. First the supernatant liquid was carefully decanted into another plastic container for subsequent disposal. The container holding the sediment was then filled with tap water, the filling being done forcefully in order to lift the sediment and disperse it. The sediment was allowed to settle completely, until the wash water was clear. The water was decanted and forcefully replaced again. This process was repeated ten times. The washed material was distributed among several small, shallow, white glass containers for sorting and picking the specimens.

Sorting was done at × 20 magnification, using a binocular dissecting microscope. Picking was facilitated by using a dropper pipette or a small scoop-shaped piece of 0.25 mm mesh copper screen. Significant large fragments of plant material were sorted into storage vials filled with distilled water and the megaspores were placed in small petri plates. No further chemical treatment was given to the sorted materials except to include granules of thymol in order to prevent growth of bacteria and fungi. This measure is important because the fossil remains are attacked by bacteria within 24 hours and only a short while later by aquatic fungi.

It should be noted that at least 40% of the Arcellites were found floating along the margin of the meniscus in the sieve frame or plastic wash containers. The spores were held in place by surface tension and capillary action of the water. Several specimens of Thylakosporites were also found under the same circumstances, including the tetrahedral tetrads of the species as described herein. I recommend that future workers pay particular attention to this peculiarity, otherwise many megaspores will be lost during the washing and decanting processes.

After isolation, the megaspores were transferred to distilled water and then further cleaned by being transferred rapidly from the distilled water to 100% ethyl alcohol and back to distilled water. The micro-convection currents caused by the immersion of the spore in the water after immersion in the alcohol swept away all of the minute debris that still may have clung to the spore body. This process was repeated four times, ending with transfer of the spore into the alcohol bath. The transfers were facilitated by supporting the spore or several spores at a time on a scoop-shaped, 0.25 mm mesh, copper screen. This technique also works extremely well for cleaning larger plant fragments. The spores were individually removed from the alcohol bath, air dried and gently dropped into a depression slide for storage. They were generically and specifically sorted at that time.

Manipulation of the spores for mounting on cover slips was facilitated by using a single eyelash attached with acetate glue to a round toothpick. The tip of the hair is extremely finely pointed and when moistened with distilled water can be used to pick, move, and position spores at will. Circular coverslips were coated on one surface with a thin film of white glue (Elmer’s brand) that was allowed to dry. The individual spore was picked up with the moistened hair. The moisture absorbed by the spore from the hair in turn served to moisten the surface of the glue when the spore was touched to it. Drying was rapid and the spore remained firmly attached. By this means spores and spore fragments can be oriented for scanning electron microscopy as illustrated in the specimens of Arcellites.

Dissections of Arcellites illustrated herein were accomplished by sharpening the point of an insect pin into the form of a microknife blade. The spore was moistened slightly with distilled water and then cut open in order to reveal the Y-mark. The dissected specimens were air dried and mounted in the same manner as the whole spores.

The coverslips with the spores were mounted on aluminum stubs. The spores were sputter coated first with carbon and then with gold-palladium. They were viewed and photographed using Cambridge Mark—IIA, Cambridge S4-10, and Coates and Welter 106B scanning electron microscopes.

ACKNOWLEDGMENTS.—I am grateful to the work crew of the Shirley Highway project for alerting me to their discovery of the fossil plant horizon that resulted in saving a small portion of it for the national collections, to Mary-Jacque Mann and Susann Braden for their enthusiastic and skillful operation of the scanning electron microscopes and processing of the photographic negatives, to James P. Ferrigno for the photographic prints used to illustrate this paper, to Dr. James A. Doyle for his analysis of the matrix for associated palynomorphs and his comments concerning the stratigraphic correlations, to Garland R. Upchurch, Jr., for a preparation containing a specimen of Arcellites disciformis from Lower Zone I at Dutch Gap Canal, Virginia, to William G. Chaloner for locality data concerning the occurrence of Arcellites reported by Ellis and Tschudy from the Patuxent Formation, to Lawrence B. Isham for drawing the diagram in Figure 1, and to Robert H. Tschudy and Roberta Townsend for comments and critical reading of the manuscript.

Systematics

The descriptions of the seven megaspores and two microspore species presented herein are arranged according to the morphologic system of classification proposed by Potonié (1956). The single specimen of Dictyothylakos pesslerae Horst, 1954 is treated as a species of uncertain affinities because its position within Potonié’s orderly arrangement remains obscure.

Anteturma Sporites Potonié, H., 1893

Turma Triletes (Reinsch, 1881) Dettmann, 1963

Subturma Azonotriletes Luber, 1935

Infraturma Apiculati (Bennie and Kidston, 1886)
Potonié, 1956

Genus Verrutriletes Potonié, 1956

Verrutriletes carbunculus (Dijkstra, 1949)

Genus Echitriletes Potonié, 1956

Echitriletes cf. E. lanatus (Dijkstra, 1951)

Infraturma Muronati Potonié and Kremp, 1954

Genus Erlansonisporites Potonié, 1956

Erlansonisporites erlansonii (Miner, 1932)

Infraturma Perinotriletes Erdtman, 1947

Genus Thylakosporites Potonié, 1956

Thylakosporites retiarius (Hughes, 1955)

Genus Crybelosporites Dettmann, 1963

Crybelosporites striatus (Cookson and Dettmann, 1958)

Crybelosporites species

Subturma Pyrobolosporites Potonie, 1956

Genus Arcellites Miner, 1935

Arcellites disciformis (Miner, 1935)

Arcellites cf. A. pyriformis (Dijkstra, 1951)

Turma Barbates Madler, 1954

Genus Paxillitriletes Hall and Nicolson, 1973

Paxillitriletes species

Uncertain Affinities

Genus Dictyothylakos Horst, 1954

Dictyothylakos pesslerae Horst, 1954

Verrutriletes Potonié, 1956

Verrutriletes carbunculus (Dijkstra) Potonié

PLATES 1–3

Triletes carbunculus Dijkstra, 1949:10, pl. I: fig. 12.

Triletes cf. carbunculus Dijkstra, 1951:10, pl. 2: fig. 11.

Verrutriletes carbunculus (Dijkstra).—Potonié, 1956:28, pl. 3: fig. 26.

DESCRIPTION (from Dijkstra, 1949:22).—

Shape spherical. Diameter 600–1000 µ (the mean being 820 µ, 4 spores measured). Tri-radiate ridge conspicuous, 50 µ broad, 40 µ high, its length being ca. 0.6 of the radius of the spore. Arcuate ridges lacking or scarcely visible. Spore wall, inclusive of the tri-radiate ridges and with exception of the contact faces, covered with hemispherical, red translucent 5–30 µ broad objects. Some specimens have but a few of such objects, which lie single or are joined to small groups; on other specimens they have been united to great complexes. Spore wall 30–36 µ thick, dark brown.

The description given by Dijkstra in 1951 for another specimen, but from the English Wealden, varies little from the original one as presented above.

Potonié’s description (1956:28), when he established the new combination, translates as follows:

The type, ca. 940 µ, has been shown to me. Equator more or less round to slight triangular. The “carbuncles” adhere irregularly distributed on the dull exine like solidified, globular, glassy, transluscent, red, liquid droplets. They look like outflowed resin, which becomes more conspicuous by the fact that they occur in groups, leave asymmetrical spaces and are of very variable sizes. The form may be only provisionally placed in the series here. Similar ones in Hughes’ collection.

REMARKS.—The two complete, although distorted, specimens and the fragment illustrated herein conform to the descriptions given by Dijkstra and Potonié with exceptions of the breadth and height of the tri-radiate ridge and the thickness of the spore coat. The tri-radiate ridge, as clearly evident in Plate 1 (figures 1, 2, and 4), is 28 µm broad and ≈ 21 µm high, just over one-half the dimensions given for the type specimen. The thickness of the spore coats illustrated in vertical section in Plate 2 (figures 1 and 3) and Plate 3 (figure 2) is 15 µm and 32 µm respectively. These exceptions cannot be attributed totally to shrinkage of the spores during dehydration by reason that the dimensions of the spore bodies are well within the range of size given in the description of the type. The dimensions of the tri-radiate ridge, I feel, do not have so much significance as to be critical in species determination. The thinness of the spore coat of the fragment shown in Plate 1 (figure 6), which serves as the source for the photographs in Plate 2 (figures 1–3), is probably attributable to corrosion and degradation of the wall following fragmentation of the spore. Its more porous structure is in obvious contrast to the dense structure of the spore coats of the complete but distorted spores shown in Plate 1 (figures 1–5) and Plate 3 (figures 1 and 2).

The specific epithet, carbunculus, is exceptionally appropriate. Viewed through a binocular microscope at magnifications of 40 to 60 diameters, the spores seem encrusted over most of their surfaces with minute, lustrous, ruby cabochons. The ornamentations, best defined morphologically as gemmae (Plate 1: figure 3), are not present on the contact areas (Plate 1: figures 1 and 2; Plate 2: figure 4; Plate 3: figure 1). They are greatly reduced in size and number or are altogether absent on the distal surface of the spore body (Plate 2: figure 5). Potonié’s allusion to them as resin that had flowed out onto the spore surface is appropriate. The gemmae appear to have been exuded from within the spore as a resinous substance, retaining its glossy surface and viscous appearance after solidifying (Plate 1: figures 3 and particularly 5; Plate 2: figures 1 and 2). Structurally there is a smooth, thin film covering a foam-like inner layer (Plate 3: figures 2 and 4), the whole of which is subtended by a papilla of the outer exoexine (Plate 2: figure 2, papilla at arrow; Plate 3: figure 2). Viewed from the interior of the spore, in this instance the partially degraded spore wall fragment (Plate 2: figure 3), a depression marks the position of a gemma on the outer surface. This characteristic cannot be seen when the endexine is intact (Plate 2: figure 6). Circular holes in the thin film covering the foam-like inner layer of the gemmae is a common feature seen on the spores that have been partially eroded (Plate 2: figure 7).

OCCURRENCES.—Boring Sm. Maurits No. 554, South Limburg, Netherlands; Upper Cretaceous (Aachenian (Senonian)); Dijkstra, 1949.

Epen and Vaals, South Limburg, Netherlands; Upper Cretaceous (Aachenian (Senonian)); Dijkstra, 1949.

Bore, D’Arcy Expl. Co. Kingsclere No. 1, 466: Weald, southern England; Lower Cretaceous (Barremian-Aptian); Dijkstra, 1951.

This report, USNM Locality No. 14252, Patuxent Formation, Potomac Group; Lower Cretaceous (Barremian-Aptian).

SPECIMENS.—GG-7A, USNM 304311; GG-7B, USNM 304312; GG-7C, USNM 304313.

Echitriletes Potonié, 1956

Echitriletes cf. E. lanatus (Dijkstra) Potonié

PLATES 4, 5

Triletes lanatus Dijkstra, 1951:11, pl. 2: fig. 22.

Echitriletes lanatus (Dijkstra).—Potonié 1956:36, pl. 4: fig. 36.

DESCRIPTION (from Dijkstra, 1951:11).—

Spore hemispherical, top area flattened. Diameter 600–800 µ (the mean being 700 µ, 4 specimens measured). Tri-radiate ridges nearly as long as the spore radius, 20–30 µ broad, 50–100 µ high. Arcuate ridge not clearly distinguishable. Spore coat covered with 10 µ large papillae bearing hair-like wooly appendages, 150 µ long, 3–5 µ thick. Spore coat brown, 25 µ thick.

Potonié (1956:36) described the species as follows:

Genotype around 780 µ (from the illustration); trilete, simple megaspores, equator subtriangular to circular, trilete rays not always present, exine ornamented on all sides with capilli to spinae, in the genotype provided with rounded verrucae, which subtend more or less long, hair-like, contorted capilli, otherwise with simple or hook-like spinae (translated by author).

REMARKS.—Morphologically the two specimens illustrated herein conform to the descriptions given for the species, with the exceptions of the density of the capilli on the specimens (Plate 4: figures 1, 2; Plate 5: figures 1, 3) and the low or poorly developed tri-radiate ridges. Accurate measurements of the spore body are precluded by the density of the capilli. Measurements excluding the capilli can be estimated as low as 800 µm or, including the capilli, can be obtained as high as 1066 µm. These dimensions exceed the range given for the type specimen. I am assuming that the measurements of the type specimen did not include the capilli, that is, the dimension was of the spore body alone. If, however, one were to add the given length of the capilli, 150 µm, to the diameter of the spore body, 780 µm, the total of 930 µm would then fall within the measurable size given for the two specimens at hand. The specimens in this report are larger, but not so much so as to be excluded from the species.

The capilli, which range from 65 to 135 µm long, are proximally broadened, in which case it could be interpreted that they are subtended by verrucae (Plate 4: figures 3, 4). Their apices are either pointed or simply to several times divided (Plate 5: figure 4). This latter character is not described for the type-species; however, most of the forked apices, because of their fragility, can be easily broken off during processing and thus are not readily observed (Plate 5: figure 3).

A finely reticulate pattern can be seen as characteristic of the outer exoexine between the bases of the capilli (Plate 4: figure 4; Plate 5: figure 2).

I have only two specimens of this megaspore and although their morphological differences from the type-species are probably significant enough to establish a new species, I prefer to designate them as E. cf. E. lanatus. Acquisition of additional specimens will permit more significant analysis of the similarities and differences than is possible at this time. Commonly, in both specimens, microspores, herein assumed to be related to this species, are found among the matted capilli. They are all of the same morphology and dimensions (Plate 4: figures 5, 6; Plate 5: figures 5, 6). Description of these microspores appears later, where they are assigned to Crybelosporites Dettmann, 1963 as Crybelosporites species.

OCCURRENCES.—Boring B; Bore, D’Arcy Expl. Co. Kingsclere No. 1 at 706 feet and (?) 466 feet, Weald, southern England; Lower Cretaceous (Barremian-Aptian); Dijkstra, 1951.

This report, USNM Locality No. 14252, Patuxent Formation, Potomac Group, Lower Cretaceous (Barremian-Aptian).

SPECIMENS.—HH-1A, USNM 304314; HH-1B, USNM 304315.

Erlansonisporites Potonié, 1956

Erlansonisporites erlansonii (Miner) Potonié

PLATE 6

Selaginellites erlansonii Miner, 1932:500, fig. 1.

Erlansonisporites erlansonii (Miner).—Potonié, 1956:46, pl. 5: fig. 53.

DESCRIPTION (from Miner, 1932:500).—

Body of exine round, 465–1000 µ in diameter the mean being 690 µ; exine reticulate with irregular diaphanous appendages, 17–122 µ in width, giving the spore a total diameter of 500–1,155 µ with the mean at 790 µ; no commisural ridges or clefts visible.

Potonié (1956:47) describes the species as follows:

Genotype apart from the membranes of the muri 899 µ (from the photograph); equator circular, trilete mark not or poorly discernible probably because of the heavy reticulation. The muri of the reticulum merge into filmy lamellae that may be uniformly developed on the entire exine, however, in the photograph of the genotype they become visible chiefly on the periphery of the spore (translated by the author).

REMARKS.—One spore from among five specimens that were obtained from the prepared samples is shown in Plate 6 (figures 1–6). It is 800 µm in diameter and is representative in form and structure of all of the specimens. The size range of the five specimens is 695 to 840 µm in diameter. The location and general size of the trilete mark is visible (Plate 6: figure 1), probably because of increased height of the muri of the reticulum along the leasurae instead of an extension of lips along the laesurate margins. Support for this observation requires microtome preparations and such are not within the scope of this report. The muri range from 14 to 45 µm in height between the bastion prongs, the prongs reaching as much as 78 µm in height.

The outer exoexine is psilate (Plate 6: figure 4). Bastion prongs and the diaphanous muri connecting them are quite obvious in Plate 6 (figures 1–3, 5). Sculpture of the muri is reticulate with minute, ≈ 1.5 µm, clavae rising from the intersections of the muri of the reticulum (Plate 6: figure 6).

A precious opal-like iridescence of the spore coat is a notable characteristic of the species. Structural details of the spore coat relevant to this phenomenon were not obtained; however, I shall presume that they are identical to those described and discussed for Thylakosporites retiarius (Hughes) Potonié, the next species in this report. In that discussion it will be noted that the spore coat structure of this species and T. retiarius is compared to that of the two modern species of Selaginella. This evidence strongly suggests that it would be appropriate to return E. erlansonii to the original taxon Selaginellites, as proposed by Miner in 1932. However, the transfer will not be made herein because neither isotype material from Miner’s collection nor additional specimens from the site described herein have been obtained for direct comparison and confirmation.

OCCURRENCES.—Skansen, east of Disko Island, Greenland; Upper Cretaceous (?Campanian); Miner, 1932.

This report, USNM Locality No. 14252, Patuxent Formation, Potomac Group; Lower Cretaceous (Barremian-Aptian).

SPECIMENS.—GG-10B, USNM 304316; depression slide with four unmounted specimens, USNM 304317.

Thylakosporites Potonié, 1956

Thylakosporites retiarius (Hughes) Potonié

PLATES 7–10

Triletes retiarius Hughes, 1955:213, pl. 11: figs. 3, 4.

Thylakosporites retiarius (Hughes).—Potonié, 1956:49, figs. 55–57 [not 57 in text. Type-specimen not designated. Lectotype designated by Jansonius and Hills, 1976, fig. 3 of Hughes.]

DESCRIPTION (from Hughes, 1955:213).—

Trilete megaspore probably spherical but the whole upper (apical) surface may be preserved infolded [figure citation]. Diameter 520–700 µ (mean 620 µ, 6 specimens). Tri-radiate ridges 200 µ long, 10 µ broad, and 30 µ high; contact faces smooth except in one specimen where the perispore covers the whole spore surface. Distal hemisphere entirely covered [figure citation] with a mesh-like (net) perispore sometimes in different layers of different mesh size. The chief mesh strands are 15–35 µ wide and 10 µ thick [figure citation] but may also be circular in cross-section exactly as described and figured by Horst (1954) and figured by Michael (1936). Spore coat is brown but shows a red luminous effect in places; thickness of exoexine 15 µ intexine very thin 2–3 µ and continuous with tri-radiate lips.

Potonié (1956:49) gives an abbreviated description that translates as follows:

Genotype ca. 650 µ (from the illustration), trilete megaspore, probably spherical, however the entire proximal hemisphere can be strongly folded, Y-radii extending nearly to the equator; exine smooth, however always surrounded on the whole distal side, in one specimen of the genotype also on the proximal side, by a perispore. The latter consists of an often multilayered reticulum [figure citation] similar to that which is described by Horst 1954, page 610, as Dictyothylakos pesslerae. Dictyothylakos is found in the German Wealden coals of southern Mecklenburg and Odenwald.

It was correct not to equate Dictyothylakos and Thylakosporites because there could be other spore genera that have a similarly constructed perispore. (In Cystosporites no perispore is present, but at the same time the exine shows a felt-like and densely matted, many layered reticulum; compare Potonié and Kremp, 1955, Plate 10, figure 79.) Detached fragments of perispores belonging to Thylakosporites are found (translation by the author).

REMARKS.—Thirty-four specimens of this species were obtained from the samples prepared for this report. This number afforded the opportunity to observe a wide range of structural details as well as various degrees of mechanical and chemical degradation of the spore body and its ornamentation.

Three specimens were recovered in which the spores are still united in the form of tetrahedral tetrads; a typical one is illustrated in Plate 7 (figures 1, 2). A very thin, amorphous layer of organic material covers the whole tetrad and nearly completely masks the reticulum of the outer exoexine on the distal portions of the spores. The layer is thickened along the margins of the contact areas and becomes more evident when a single spore is broken away from the tetrad (Plate 7: figures 3, 4). When the thin, amorphous, outer layer is degraded or eroded the reticulum becomes more prominent (compare Plate 7: figures 5 and 6). The reticulum on some specimens is partially lost to degradation or erosion (Plate 8: figures 2–4), or completely missing except for individual points of attachment to the spore body (Plate 9: figures 1, 3).

The diameter of the spore body, excluding the reticulum, ranges from 600 to 785 µm, or, including the reticulum, ranges from 640 to 950 µm. No meaningful measurements of the height of the extension of the reticulum above the spore body can be given because the variation is a function of the amount of compression that the layer has undergone. The specimens at hand tend to exceed the size range given for the type material; however, I do not consider the difference so great as to suggest establishing a new species.

A tri-radiate ridge is prominent on the spores. It varies from 185 to 210 µm long, 12 to 23 µm wide, and 24 to 32 µm high. The variation appears to be due to the degree of compression, the more convex, less compressed specimens have narrower and higher ridges than those that have been distorted by compression.

Three layers from the spore coat (Plate 8: figures 1, 5–8; Plate 9: figures 2, 4–6; Plate 10: figure 3). The reticulate, outer exoexine layer on the spore body varies from 1.5 to 4 µm thick. the outer exoexine layer extends into and forms the coarse reticulum that covers the equatorial and distal portions of the spore body (Plate 8: figure 4; Plate 10: figure 4). The fine reticulate structure of the individual elements of the reticulum is shown in Plate 10 (figures 5, 6). The next layer, inner exoexine, with an opal-like structure, averages 16.75 µm in thickness. The reticulate endexine varies from 1 to 2 µm in thickness; the average being 1.4 µm. The contact areas are free of the ornamental reticulum (Plate 9: figures 1, 3; Plate 10: figure 1).

The opal-like quality of the spore wall is, beyond question, the most striking characteristic of the species. I feel that the characteristic is briefly alluded to in Hughes’ comment: “ … spore coat … shows red luminous effects in some places.” The spore stands out, sparkling with the colors of a fine, precious opal, against the blackness of the surrounding carbonized plant debris. I regret that it is not possible to reproduce a color photograph to illustrate this phenomenon. The structure of the inner exoexine (Plate 8: figures 5, 7; Plate 9: figures 4, 6) is precisely that which has been shown by Sanders and Darragh (1971) for the structure of precious opal. Rows of contiguous spherules, each spherule averaging 0.25 µm in diameter, are uniformly and alternately aligned, giving the appearance of stacked lead shot, or on a grander scale, stacked cannon balls. The alignment can be seen to change direction (Plate 9: figure 4) through an angle approximating 60° This alignment of spherular bodies produces a diffraction grating effect by separating and reflecting incident light into the spectrum colors. The various alignments of the spherules produce the various colors seen as the spore body is rotated. At first it was thought that the spore had been preserved through permineralization of the spore wall by opal. However, only this species and Erlansonisporites erlansonii from among all of the other species found at this site exhibit the precious opal effect and it would be quite unusual for such selective preservation to take place. I made the assumption that the organic layers of the outer exoexine and the endexine had protected what I thought was the siliceous inner exoexine from attack by the hydrofluoric acid solution. This assumption was encouraged by the specimen illustrated in Plate 10 (figures 1–3), which exhibits a peculiar corrosion of the inner exoexine layer of the spore wall.

Electron dispersive x-ray (EDX) analysis of the inner exoexine layer produced the results shown in the spectograph reproduced in Figure 1. Silicon should be the primary element recorded if the inner exoexine layer were composed of opal. Instead, only a minor amount of silicon is indicated. Analyses of five different specimens of the spore yielded essentially the same results as shown here. The analyses clearly indicated that the inner exoexine layer in Thylakosporites retiarius is not composed of opal, even though its microstructure and light refractive qualities parallel those of precious opal. All of the elements indicated in the analyses can be anticipated as components of the structure of the spore coat, with the exception of the copper (Cu), aluminum (Al), and tin (Sn), which are interpreted herein as contaminants derived from the metal screening used in the isolation, cleaning, and drying of the individual spores. Taylor, et al. (1976) present analysis of the spore walls of the liverwort species, Conocephalum conicum, and discuss the significance of the various elements found there. Their bibliography contains several additionally useful references for the intepretation of the results of spore wall analyses. I recommend that the interested reader consult Taylor, et al. Further discussion of the subject is beyond the scope of this report.

FIGURE 1.—Electron dispersive x-ray spectrograph showing analysis of inner exoexine layer in spore coat of Thylakosporites retiarius (Hughes) Potonié (specimen KK-1B, USNM 304318).

An exciting and completely unexpected body of information relevant to the structure of the inner exoexine of T. retiarius was found in a paper by Tryon and Lugardon (1978). The writers discuss the structure and mineral content of the spore coats of the megaspores of two species of Selaginella. They illustrate their paper with SEM and TEM photographs as well as EDX spectrographs. The SEM photographs in their pl. IV (fig. 1) and pl. V (fig. 1) are so similar in appearance to those in my Plate 8 (figure 5) and Plate 9 (figure 4) as to appear to be the same specimens. However, their photographs are of the megaspore wall of Selaginella galeottii Spring, a modern, tropical, American species. Their EDX analyses of the opal-like inner exoexine of the spores indicate, as did my analyses, the presence of only a minor amount of silicon. Their analyses are more detailed than mine and include spectrographic mapping whereby they are able to demonstrate that the silicon is concentrated in the outer limits of the inner exoexine layer. The silicon, presumably as the oxide mineral quartz, is randomly distributed among the interstices of the rows of spherular bodies that form the layer. The opal-like iridescence of the spore coat of the megaspore of S. galeottii is not mentioned by Tryon and Lugardon; however, I have observed the phenomenon in specimens from Veracruz, Mexico filed in the United States National Herbarium.

Pierre Martens (1960a) illustrates, by means of TEM photographs, the same opal-like wall structure in the megaspores of another modern species of Selaginella, S. myosurus (Swartz) Alston. In a second paper on the same species (1960b) he describes the iridescent quality of the spore wall when observed microscopically in reflected light. This is the only reference to the phenomenon that I have been able to find in the literature. Selaginella myosurus is native to the region of Zaire, Africa.

Françoise Stanier (1965, 1967) presents additional details regarding the structure of the megaspore wall of S. myosurus and quotes the observation made by Martens on the iridescent quality of the spore wall. The TEM photographs in her pl. 1: figs. 3, 4 (1967) lend supportive evidence for the interpretation of the structure of the spore wall of the fossil T. retiarius, illustrated herein with SEM photographs.

Additional, refined EDX analyses and preparations, such as sections for transmission electron microscopy of the spore coat of T. retiarius, are required. Certainly, precisely the same study of the spore coat structure of Erlansonisporites erlansonii is also required when one considers the significance of the iridescent quality of the spore coats of the two fossil species. Nevertheless, despite lack of elaborative details, the evidence at hand strongly indicates that the spore species, T. retiarius and E. erlansonii, are fossil evidence for the existence of ancestral forms of some of the modern species of Selaginella during the Lower Cretaceous of eastern North America.

As a final note, I agree completely with Potonié’s opinion (1956) that Dictyothylakos and Thylakosporites are not to be combined. I illustrate a specimen of Dictyothylakos late in this report to show that structurally there is some similarity between the reticulum of the outer exoexine of Thylakosporites and the morphology of Dictyothylakos. I present a conjectural analysis of Dictyothylakos in the discussion of the specimen.

OCCURRENCES.—Hughes’ locality S 7; Cliffs of Compton Bay, Isle of Wight: Wealden Marls, Bed 11; Lower Cretaceous (Barremian-Aptian); Hughes, 1955.

This report, USNM Locality No. 14252, Patuxent Formation, Potomac Group; Lower Cretaceous (Barremian-Aptian).

SPECIMENS.—BB-12A, USNM 304319; GG-8A, USNM 304320; GG-10A, USNM 304321; GG-10C, USNM 304322; GG-10D, USNM 304323; HH-2B, USNM 304324; KK-1B, USNM 304318; KK-2A, USNM 304325; KK-2B, USNM 304326; and 26 specimens stored in a depression slide, USNM 304327 for use in further study.

Crybelosporites Dettmann, 1963

Crybelosporites striatus (Cookson and Dettmann) Dettmann

PLATE 11

Perotrilites striatus Cookson and Dettmann, 1958, 4(1):43, pl. 1: figs. 8–12.

DESCRIPTION (from Dettmann, 1963:80).—

Microspores trilete, spheroidal. Sclerine stratified, cavate proximally, 4–5 µ thick (inclusive of sculptural/structural elements), consisting of a smooth, homogeneous, inner layer (1–1.5 µ thick) and a ruffled, proximally cavate, two-layered sculptine, the outer of which forms a conical gula-like projection over the proximal pole. Sculptine 3 µ thick in optical section, striated proximally, reticulate distally and equatorially; reticulum composed of membranous muri (1.5–3 µ high) which enclose circular to polygonal lumina 1–3 µ in diameter. Laesurae developed on two inner wall layers only; straight, length 3/4 spore radius with weakly thickened lips.

Dimensions: equatorial diameter 28(37)45 µ; polar diameter 34(45)56 µ; diameter of inner layer 20(29)29 µ.

REMARKS.—Scanning electron microscope observations preclude complete comparison of the spores illustrated herein to those observed by means of optical light microscopy. The two-layered, proximally cavate sculptine is quite obvious in Plate 11 (figures 3, 4). The reticulate sculpture is rather faintly discernible on the distal and equatorial portions of the spore body. The size range of the specimens among the folds of the acrolamellae on specimens of Arcellites disciformis in Plate 11 (figures 1, 3) and Plate 14 (figure 3) is 30 to 42 µm in equatorial diameter, which is within the size range given for the type of the species.

Significantly this microspore has been found adhering to the type specimen of the megaspore species Arcellites disciformis and on subsequent specimens illustrated by Ellis and Tschudy (1964). Also, the specimen of A. disciformis sent by Garland Upchurch, Jr., mentioned later herein, has two of these same microspores adhering to the base of the acrolamellae. This repeated association clearly indicates that these megaspores and microspores will eventually be identified with a single sporophyte species. However, the sporophyte is certain to be a delicate aquatic fern of which the remains will be found only as a result of ideal conditions of burial and preservation.

I am not convinced that the natural position of occurrence of these microspores is within the neck formed by the acrolamellae of the megaspore as described by Ellis and Tschudy (1964). Instead, their position appears to have been one of adhesion to the smooth outer surface of the folds of the acrolamellae. Additional discussion of this question is presented later in the remarks on Arcellites disciformis.

OCCURRENCES.—Aptian through Albian in Australia and New Guinea; Dettmann, 1963.

No. 1 Mann well, 4686 feet, Fall River Sandstone; Upper Cretaceous (Albian); Ellis and Tschudy, 1964.

This report, USNM Locality No. 14252, Patuxent Formation, Potomac Group; Lower Cretaceous (Barremian-Aptian).

SPECIMENS.—BB-11C, USNM 304328; GG-6A, USNM 304329.

Crybelosporites species

PLATES 4, 5

DESCRIPTION (incomplete).—Trilete microspores; equatorial outline of spore body circular; structure of the spore coat requiring thin sections for transmission electron microscopy or examination by optical, light microscopy for complete description; however, sculptine of two layers is suggested by folded, cavate, outer layer (Plate 4: figure 5) that covers and masks the sculpture of the inner layer; sculpture of inner layer may be granular as described for C. punctatus Dettmann, 1963 (Plate 4: figure 6); Y-mark small (Plate 5; figure 6), rays approximately one-half of spore radius; diameter of the spore 23 to 25 µm (14 measured).

This microspore was found lodged among the capilli on Echitriletes cf. E. lanatus, described herein, in relatively large numbers and to the apparent exclusion of other species. I suggest that these spores, the megaspore as well as microspores, were produced by the same sporophyte, which, unfortunately, is yet to be identified.

OCCURRENCE.—This report, USNM Locality No. 14252, Patuxent Formation, Potomac Group; Lower Cretaceous (Barremian-Aptian).

SPECIMENS.—HH-1A, USNM 304314 and HH-1B, USNM 304315.

Arcellites Miner, 1935

Arcellites disciformis Miner emend. Ellis and Tschudy

PLATES 12–19

Arcellites disciformis Miner, 1935:600, pl. 20: figs. 61, 64–66.

Arcellites disciformis Miner, emend. Ellis and Tschudy, 1964: 75.

DESCRIPTION (emendation by Ellis and Tschudy, 1964:75).—

Nearly spherical spore body having an equatorial diameter of 185–350 µ (mean 259 µ); neck consisting of six leaf-like folded appendages forming a long flamelike organ; length of neck 261–477 µ (mean 357 µ). Each “leaf” folded lengthwise and inward, characteristically crenulate or fimbriate on its margin. “Leaves” commonly exhibit torsion of about 180°. Spore body bearing a variable number (11 to 26, mean 16) of blunt tubelike appendages, which are usually constricted at their bases. Appendages possess a thick, probably originally spherical, reticulate bladder on their extremities. Spore coat of three layers; exoexine about 11 µ thick, thinning at neck and appendages and divisible into two layers, outer about 8 µ thick and inner about 3 µ thick, sometimes seen detached from the exoexine. The inner layer of the exoexine and the intexine are continuous across the floor of the neck. A tri-radiate scar is present on the floor of the neck. Exoexine penetrated by characteristic pits which are somewhat elliptical in cross section; intexine smooth.

REMARKS.—Nearly all specimens examined for this report are preserved in three-dimensional, undistorted form. The equatorial diameter of the spore bodies varies from 188 to 245 µm, which places the variation below the mean given in the original description. The measurements I have taken are from dry spores. I did not measure specimens in wet mounts wherein I am sure the measurements would be higher due to swelling of the spore walls as a result of absorption of water. Movement can be observed as the spores dry or as they are remoistened during the mounting processes, suggesting that a certain amount of shrinkage does take place during dehydration. The flame-like organ varies in length and width, as can be seen in the equatorial and proximal views of the examples in Plates 11–13 and Plate 19 (figures 1, 3). Again the variation falls below the mean given in the original description, and the reason for that is probably the same as for the spore body variation.

Morphologically and anatomically the specimens are identical to those described and illustrated by Ellis and Tschudy (1964). Three layers form the spore coat and are clearly demonstrated in Plate 17 (figure 2). Pits in the outer exoexine layer, illustrated in surface view in Plate 16 (figures 1–4), penetrate the outer exoexine layer, as can be seen in Plate 17 (figures 2–4). The inner exoexine layer has a reticulate structure while the endexine (intexine) is compact and almost amorphous. The tubular appendages (Plate 18: figure 1) are formed by extension of the outer exoexine layer (Plate 18: figure 3). In some instances formation of the appendages was not complete as indicated by protuberances on the spore surface (Plate 16: figure 4). The inner exoexine extends very slightly into the base of each appendage (Plate 17: figures 3, 4; Plate 18: figures 3, 5). The apices of the appendages are reticulate (Plate 18: figures 1, 2, 4) and I agree that they probably were formerly slightly expanded in bladder-like form (Plate 18: figure 2). The reticulation is similar to that of the margins of the acrolamellae. Minute, pleat-like folds along the margins of the acrolamellae end in finger-like extensions that appear to be entangled but not fused with those of the neighboring margin (Plate 15: figures 24). Pores and fissures are present between the folds and among the elements of the fimbriate margins. They probably gave access to the neck cavity (Plate 15: figure 1) formed by the acrolamellae. Occasionally the acrolamellae are separated from one another (Plate 13: figure 4; Plate 14: figures 1, 2). They, like the appendages, are formed by extension of the outer exoexine (Plate 19: figure 3) as a single layer (Plate 15: figure 2). The Y-mark can be clearly observed only by removal of the flame-like organ or by cutting the spore body in half along its equator. The best results were obtained by cutting the spore body in half (Plate 19: figure 1) and examining the inner surface of the proximal polar area. The Y-mark is clearly visible in the spore coat that forms the floor of the neck cavity (Plate 19: figure 2). The endexine and the inner exoexine are continuous across the floor of the neck cavity (Plate 19: figures 3, 4) but are split by the trilete suture.

Microspores were found adhering to the outer, smooth surfaces of the folds in the acrolamellae (Plate 14: figure 3; Plate 11: figures 1–4). I agree with Ellis and Tschudy (1964: 76) that the flamelike organ, the neck, may have had a gelatinous material associated with it that served to trap microspores. The outer layer of the sculptine of the microspore may also have had a sticking quality that would cause it to adhere to the surface of the megaspore after the gelatinous exudate of the megaspore had disintegrated. Germination of the microspore could have occurred outside of the neck cavity and the sperm could have swam through the fissures and pores along the margins of the acrolamellae, into the cavity and to the female gametophyte at the base of the neck.

I have illustrated these megaspores in the position suggested by Ellis and Tschudy as the life position when the spores were released from the sporophyte. This is the position that they assume when found floating in the wash waters during preparation of the matrix. Note that many can be lost if wash waters are discarded without first being examined for “floaters.”

Ellis and Tschudy (1964) place Arcellites crillensis Schemel, 1950 and Arcellites cf. A. hexapartitus (Dijkstra) Potter, 1963 in synonymy with A. disciformis. I agree with this decision. I must admit that I am uneasy with Arcellites hexapartitus as illustrated by Hughes, 1955 (pl. 10: fig. 4). However, without seeing specimens of the species at first hand I must accept the identification of the specimen.

OCCURRENCES (from Ellis and Tschudy, 1964, table 1).—In descending order of age:

Skansen, Disko Island, Greenland; Upper Cretaceous (Cenomanian); Miner, 1935.

Crill coal, Dakota Group, Iowa, USA; Upper Cretaceous (Albian); Schemel, 1950.

“D” sandstone, 4480 feet, Plains Exploration Company Parker 1; C NW SE Sec. 19, T. 3N., R. 51 W., Washington County, Colorado, USA; Upper Cretaceous (Cenomanian); Ellis and Tschudy, 1964.

“Huntsman” shale, samples 4531 and 4580 feet, Marathon Oil Company Holt 2; SE SE SE Sec. 7, T. 17N., R. 49W., Morrill County, Nebraska, USA; Upper Cretaceous (Albian); Ellis and Tschudy, 1964.

Newcastle Sandstone (type section); NW NW NW Sec. 28, T. 45N., R. 61W., Weston County, Wyoming, USA; Upper Cretaceous (Albian); Ellis and Tschudy, 1964.

“J” sandstone, 4865 feet, Marathon Oil Company Gurschke 1; SW NE SE Sec. 3, T. 14N., R. 50W., Cheyenne County, Nebraska, USA; Upper Cretaceous (Albian); Ellis and Tschudy, 1964.

Glencairn Shale Member of the Purgatoire Formation, Beaver Creek surface section; NW Sec. 10, T. 18S., R. 68W., Fremont County, Colorado, USA; Upper Cretaceous (Albian); Ellis and Tschudy, 1964.

Skull Creek Shale, South Platte Formation, 4698 feet, Marathon Oil Company, Knievel 1; NE SE SE Sec. 2, 15N., R. 49W., Cheyenne County, Nebraska, USA; Upper Cretaceous (Albian); Ellis and Tschudy, 1964.

Fall River Sandstone, 4686 feet, California Company Mann 1; SE NE NW Sec. 27, T. 30N., R. 56W., Sioux County, Nebraska, USA; Upper Cretaceous (Albian); Ellis and Tschudy, 1964.

Patuxent Formation, Potomac Group, Drewry’s Bluff, James River, near Richmond, Virginia; Lower Cretaceous (Barremian-Aptian); W. G. Chaloner, pers. comm. 22 Sep 1980.

Dutch Gap Canal, Virginia; Lower Zone I, Potomac Group, Lower Cretaceous (Barremian-Aptian); specimen sent for examination by Garland Upchurch, Jr.

This report, USNM Locality No. 14252, Patuxent Formation, Potomac Group; Lower Cretaceous (Barremian-Aptian).

SPECIMENS. BB-11A, USNM 304330; BB-11B, USNM 304331; BB-11C USNM 304328; BB-11F, USNM 304332; GG-3A, USNM 304333; GG-4A, USNM 304334; GG-4B, USNM 304335; GG-4C, USNM 304336; GG-5A, USNM 304337; GG-5C, USNM 304338; GG-6A, USNM 304329; GG-6B, USNM 304339; II-2, USNM 304340; depression slide with 70 specimens unmounted, USNM 304341.

Arcellites cf. A. pyriformis (Dijkstra) Potter

PLATES 20, 21

Triletes pyriformis Dijkstra, 1951:14, pl. II: fig. 9

Pyrobolospora pyriformis (Dijkstra).—Hughes, 1955:209, pl. XI: figs. 1, 2.

Arcellites pyriformis (Dijkstra).—Potter, 1963:228.

DESCRIPTION (from Dijkstra, 1951:14).—

Spore pyriform, mostly flattened in lateral direction. Length of spore, including neck-like projection, 375–700 µ (the mean being 591 µ 17 specimens measured), breadth about 300–700 µ. Neck-like projection to 250 µ long, 300 µ broad. Tri-radiate ridges 250–300 µ long, 50 µ broad, about 60 µ high. Arcuate ridge not distinguishable. Between the tri-radiate ridges some plications running in lateral direction, 100–200 µ long, 30 µ broad, 10 µ high. Spore body covered with numerous bluntly ended appendages, 30 µ long, 20 µ broad. Spore coat black.

Hughes’ emendation (1955:209) is as follows:

Spore-body originally nearly spherical but usually equatorial diameter exceeds the axial. Equatorial diameter 290–920 µ (mean 651 µ, 35 specimens or mean 631 µ including Dijkstra’s specimens). The axial measurement with or without the neck is meaningless, as the majority of specimens are asymmetrically laterally flattened; this suggests that the top of the spore below the neck is the most rigid part. Only the maximum neck dimensions are given as the neck is so frequently incomplete; length up to 350 µ, width 350–400 µ at extremity, narrowing to the base. The neck arises from the spore body as three prominent folds and consists basically of three bulging lobes between these folds; the specimen in Plate XI, fig. 2, is a small one with somewhat irregular appendages but it shows at the neck attachement a fold centrally, flanked by two downward bulging lobes, with the two other folds at the margins in this view. In most cases the main part of the neck is opaque and ochreous in colour, and lacking in precise form; broken surfaces reveal the upper part to be composed of a solid mass of sponge-like tissue without any central canal. Certain complete specimens seem to show a six-fold symmetry of leaves of the neck on the top surface, and others in which the neck is reduced to translucency by maceration show the same number. Spore-body covered evenly with blunt simple or compound appendages 30–40 µ broad, and seldom less than 15 µ apart in an average specimen; slight decrease in size of appendages towards the neck.

Spore-coat brown to black, consisting of opague exoexine layer 25–30 µ thick and intexine 5 µ thick and transluscent; exoexine layer is likely to be divided as in P. vectis but good confirmatory sections have not been prepared because of the brittleness of specimens. Tri-radiate mark (laesurae 200 µ long) seen on inner side of apex of intexine and inner part of exoexine in broken specimen. Ideally a neck-chamber is present in the base of the neck but it has not been explored; the swollen lobes mentioned above were seen in some good specimens to be hollow.

REMARKS.— Three specimens were obtained but only one of them is complete (Plate 21: figures 1–3). Its diameter is 647 µm and the range of sizes of the appendages is in agreement with that given in the type-description. However, I cannot be certain of the morphology of the neck. Hughes begins his morphological description saying that “the neck arises from the spore-body as three prominent folds and consists basically of three bulging lobes between these folds.” My specimen shows three folds arising from the spore body but no bulges are present between the folds; the neck remains three-parted (Plate 21: figures 3, 4). I shall withhold judging whether there are only three acrolamellae on my specimen or six that are distorted by preservation. Hughes placed emphasis on the presence of six acrolamellae. I cannot prove which number is correct for my specimen. I shall therefore suggest that it be designated Arcellites cf. A. pyriformis in light of the strong agreement of the other structural details with the type-specimen.

The two fragmentary specimens (Plate 20: figures 1–7, and Plate 21: figures 6–9) exhibit identical morphological characteristics to the intact spore and provide excellent anatomical details. The spore coat comprises three layers (Plate 20: figure 4, and Plate 21: figure 7); the inner exoexine, which is coarsely reticulate, and the endexine, which is very thin (≈ 1 µm). The outer exoexine forms the appendages. All of the layers tend to separate from one another (Plate 20: figures 4–6) and, in the case of the fragment in Plate 21 (figure 6), the endexine is missing from most of the inner wall. When preserved, the endexine structure is more finely reticulate (Plate 20: figure 8) than that of the other layers. Evidence for a neck chamber at the base of the neck is illustrated in Plate 20 (figure 3) and Plate 21 (figure 6). Details of the Y-mark were not obtained from the specimens at hand.

I have illustrated this species with the neck pointing downward, suggesting the position of the spore when it was free floating after release from the sporophyte. Unfortunately, no microspores have been found attached to the neck as has been the case with A. disciformis. My feeling is that the microspores would most likely adhere to the apex of the neck rather than to the sides. The texture of the apex, in the fossil condition, suggests that it was spongy and thus porous and the probable site of any gelatinous exudate that might have entrapped microspores.

OCCURRENCES.—Lower part Bore B, Netherlands; Lower Cretaceous (Wealden); Dijkstra, 1951.

Cliffs in middle Fairlight Clay below Coast Guard Station, halfmile NE of Fairlight Glen Foot, England; Hughes’ Locality 80A; Lower Cretaceous (Berriasian); Batten, 1969.

Wealden Marls of the Isle of Wight, England; Lower Cretaceous (Barremian); Hughes, 1955.

This report, USNM Locality No. 14252, Patuxent Formation, Potomac Group; Lower Cretaceous (Barremian-Aptian).

SPECIMENS.—BB-12, USNM 304342; HH-2A, USNM 304343; HH-2C, USNM 304344.

Paxillitriletes Hall and Nicolson, 1973

Paxillitriletes species

PLATES 22, 23

Paxillitriletes Hall and Nicolson, 1973:319 [type-species P. reticulatus (Mädler) Hall and Nicolson].

DIAGNOSIS (from Madler, 1954: 150).—“Macrospores with pilose sculpture along the tetrad rays or only in their immediate region; equatorial flange narrow, rays of Y-mark extending to or slightly beyond it.”

REMARKS.—Only two specimens were found and this paucity of material precludes assigning a specific name to these beautifully ornamented megaspores. The specimen shown in Plate 22 (figures 1–4) and Plate 23 (figures 1, 2) is 284 µm in diameter, excluding the equatorial flange. The distal surface has a reticulate sculpture. Short appendages, 5.75 to 17.5 µm long, and truncate bases of longer ones, extend from the contact points of the muri. The exine is uniformly pitted (Plate 23: figure 2). The contact areas on the proximal surface also have a reticulate sculpture with appendages extending from the intersections of the muri. The appendages are 4–6 µm long near the equatorial flange and gradually increase in length to form capilli 57–115 µm long at the border of the laesurae. The Y-mark extends to the outer margin of the equatorial flange and is bordered by labra 58–75 µm high that are formed by the fused, broadened bases of blunt-ended appendages. The equatorial flange is formed in the same way; however, the appendages are much shorter, 23–40 µm high.

The second specimen (Plate 23: figures 3, 4) is laterally flattened and is 325 µm in diameter. The formation, size, and distribution of the appendages is the same as on the other specimen. The labra seem to be torn or perhaps fimbriate. The spore is not so well preserved as the other.

These specimens, in their form and distribution of sculptural elements, most resemble P. alatus (Batten, 1969) Hall and Nicolson and P. fairlightensis (Batten, 1969) Hall and Nicolson from among the 12 species listed by Hall and Nicolson (1973). P. alatus is larger in all dimensions than the specimens at hand and has capilli over the whole of the distal surface of the spore. The broken or truncated bases of appendages on the distal surface of the first of the two specimens described herein suggest that there were capilli present but that they were eroded prior to burial or subsequent to maceration and processing. P. fairlightensis is smaller than the spores at hand; however, in sculptural detail they are very similar. I tend to favor identification of the spores with P. alatus but more specimens are required in order to arrive at a convincing analysis and conclusion.

OCCURRENCE.—The genus Paxillitriletes ranges from Lower Jurassic, P. mensurus (Harris, 1935), to Upper Cretaceous, e.g., P. dakotensis (Hall, 1963). A summary of all of the species is given by Hall and Nicolson, 1973.

This report establishes a record of the genus at USNM Locality No. 14252, Patuxent Formation, Potomac Group; Lower Cretaceous (Barremian-Aptian).

SPECIMENS.—GG-3C, USNM 304345; GG-3D, USNM 304346.

Dictyothylakos Horst, 1954

Dictyothylakos pesslerae Horst, 1954

PLATE 24

DESCRIPTION (translated by the author from Horst, 1954:610).—

The structures are uniformly orange colored. In oblique light, dark-field of the ultropak they appear rosin-like and strongly reflective. They are brittle and break easily.

The photographs [plate citation] clearly show the formation of the body from a net-like framework. The individual meshes are in part three to five cornered, in part polygons are formed through irregular development of the framework; their inner edges are often rounded. From the thick, 15–35 µm, primary strands depart secondary strands, about 10 µm in diameter, which serve as reinforcement to the meshes.

All strands are nearly round in cross-section. They are hollow, which has been proved during the course of the research.

The fragment, only briefly mentioned by Michael (1936), was found in the coal petrographic collection of the Staatlichen Geologischen Kommission (glycerine-jelly preparation No. 6 from the Osterwald occurrence). The comparison demonstrated a conformity in both accounts. Similar fossils, to my knowledge to this time, have not been described. A classification of the very well-characterized objects into known fossil species is at present still not possible.

Diagnosis: Oblong hollow body, net-like, orange colored framework, 4.5 mm maximum length and average 1.7 mm width. Meshes of net in part three to five cornered, in part polygonal. All strands are hollow inside. Cross-section round. Diameter of the primary strands 15–35 µm, the secondary strands about 10 µm. Weight of a fully preserved specimen 0.192 mg.

REMARKS.—Although only one specimen of this enigmatic palynomorph was obtained from my preparations, I felt that the significance of its occurrence was adequate reason to include illustrations and description of it. The dorsiventral, discoid form of the specimen (Plate 24: figures 1, 2) causes me to hesitate in associating it taxonomically with Thylakosporites retiarius. However, structurally the margin of the disc, with its looping and anastomosing elements (Plate 24: figures 1, 2, 4, 5) does recall to a certain degree the sculptural detail of the outer exoexine of Thylakosporites. The remainder of the specimen bears no similarity. The surface shown in Plate 24 (figure 1) is interpreted as the outer surface because of the more pronounced relief in the patterns surrounding the pore-like openings (Plate 24: figure 3). The surface comprises strands of structural material laid down in much the way coils of clay would be used to form a decorative ceramic object. The inner surface (Plate 24: figure 2) has less relief, the surfaces between the pores are relatively flat. Particular attention is directed toward the frayed aspect of the margin of the specimen. This characteristic leads me to suggest that this specimen is operculum-like and may have broken away from its parent organ as a result of stresses applied to the structurally weaker filaments along the margin.

Descriptively, for what it is worth, and I give little value to color in fossil materials, the specimen at hand is orange and quite lustrous. In color and lustre there is agreement with the description of the type-specimen. The strands along the margin of the specimen are clearly comparable, morphologically, to the strands forming the framework of the body of the type-specimen. However, the remainder of the specimen illustrated herein is thicker and of heavier construction overall due to the layering of structural elements around the pores or meshes. Also, all of the strands are solid rather than hollow. This latter characteristic is, unfortunately, not clearly illustrated in the original account of the type-species.

Horst (1954) made every effort to determine the composition of the material through chemical analysis, staining reactions, x-ray analysis, and spectrochemical techniques. None gave a positive indication of the composition. He remained convinced that Dictyothylakos was of plant origin and stressed that it probably was the “framework of an alga” and not the remains of a seed coat as suggested by Michael (1936).

Hughes (1955) was strongly of the opinion that some of Dictyothylakos illustrated by Horst was identical in form to the perispore he was describing as part of his new species, Triletes retiarius, now Thylakosporites retiarius. The result was that he placed part of Horst’s material, although he did not state which part, in synonymy with T. retiarius. This action has not been accepted by most palynologists.

I should like to present a conjectural analysis of Dictyothylakos pesslerae that simply requires the extension of insufficient evidence into a discussion. I shall not go so far as to prepare drawings because they can pass through the veil of reality and miraculously, in some later time, be reproduced as facts.

Horst, in his figures 2 and 3, illustrates individual, apparently hollow, oblong bodies, the walls of which are coarsely reticulate. In his figure 2, one end of the body is tapered to a somewhat blunt point while the opposite end is truncated, appearing as though broken. In figure 3, one end of the body is broken away and very irregular in outline. The body is split longitudinally for about one-half of its length and would probably exhibit a tapered form if reconstructed. Unfortunately, the end opposite the very roughly broken one is also incomplete. My reason for describing these two bodies is to have a basis for the suggestion that the specimen described in this report represents a covering or closure (operculum) on the broader end of a morphologically similar if not identical body. A reconstruction would represent the complete form as a lidded, cylindrical container reminiscent of a pyxis. The net-like structure would be represented as strands of material laid down on the inner surface of the container. I shall, from here on, refer to the container as a sporangium. Thin-walled cells forming the outer layer of the sporangium were lost to decay. I shall suggest further that, if Dictyothylakos were part of a sporangium, the sporangium was a megasporangium.

In summary, Dictyothylakos, because of the similarity between its structural components and those of the outer exoexine of Thylakosporites, may represent a part of a megasporangium from which a spore like Thylakosporites originated. This suggestion is not wholly original, as Hughes (1955: 214) did casually infer such an interpretation. I reinforce the interpretation with the specimen described and illustrated herein. I suggested earlier that Thylakosporites is probably the megaspore of an ancient Selaginella. No present-day megasporangium of Selaginella even approaches the conjectured appearance of Dictyothylakos.

OCCURRENCES.—Bore W. III, 870 m, Wealden coal (Lower Cretaceous), south Mecklenburg and Osterwald, Germany; Horst, 1954.

Seven separate but undocumented localities in the Wealden (Lower Cretaceous) of England; Hughes, 1955.

This report, USNM Locality No. 14252, Patuxent Formation, Potomac Group, Lower Cretaceous (Barremian-Aptian).

SPECIMENS.—II-4B, USNM 304347.

Conclusions

Evidence for correlating a part of the Potomac Group with a part of the English Wealden section is a significant contribution of this report. The megaspores described herein, though significant, would not be so effective in correlation were it not for some added precision afforded by the preliminary palynological analysis of the matrix prepared by Dr. J. A. Doyle (pers. comm.). In that analysis he found a large number of ferns, Eucommidites, Vitreisporites (=Caytonia), some saccate conifers, but very few Classopollis, bisaccate pollen related to Decussosporites, a beautiful suite of angiospermous monosulcates such as Retimonocolpites peroreticulatus, Clavatipollenites, Liliacidites species B and others, familar to Zone I of the Potomac Group. Although the listing is only a preliminary one, Doyle has no doubt that the Shirley Highway locality is in Zone I, as described by Hickey and Doyle in 1977. The assemblage, as he sees it, resembles most closely the 770–765′ level in Delaware Well D12, that is, the lower part of Zone I, which would place the age in the Barremian-Aptian level in the section. As a point of reference I refer the reader to Hickey and Doyle’s fig. 3, in which the subdivisions of the Potomac Group and correlations of the fossil localities are shown. The horizon with which the material from the Shirley Highway excavation closely correlates is at the lower right of their figure.

Turning now to the early Cretaceous Wealden of England, Hughes (1958:44) suggests certain megaspore floras as representative of particular stages within the Wealden section. The “Verrutriletes Flora” represents Berriasian; the “Thomsonia Flora” (now, because of nomenclatural change, “Paxillitriletes Flora”) is Valanginian; the “Pyrobolospora Flora” (now, because of nomenclatural change, “Arcellites Flora”) is Barremian-Aptian.

A review of the species indicates the presence of Verrutriletes carbunculus, which was first reported from the Upper Cretaceous (Senonian) by Dijkstra (1949) and then again by him from the Barremian-Aptian (1951). The Barremian-Aptian occurrence was at 466’ in the Kingsclere Bore Hole No. 1 in England that directly associates the species with the “Arcellites Flora” of Hughes. That occurrence and association are also true for Echitriletes, to which I compare some specimens in this report. Thylakosporites retiarius, Arcellites disciformis, A. pyriformis, and Dictyothylakos pesslerae can all be associated with the “Arcellites Flora.” Thus most of the megaspores and the palynomorph discussed herein are typical to the Barremian-Aptian stages in the Wealden section. The megaspores, in association with the preliminary analysis of the microspore and pollen flora by Doyle, suggest correlation between lowermost Zone I (Patuxent Formation) of the Potomac Group and the Barremian-Aptian horizons of the English Wealden.

Of the species recorded herein that are not directly associated with the “Arcellites Flora,” Crybelosporites striatus may be added by reason of its relationship as the microspore belonging with Arcellites disciformis. Also Paxillitriletes can be anticipated as a member of the “Flora” because its range is from Lower Jurassic to Upper Cretaceous and certainly one or more of the 12 species will be shown to be of Barremian-Aptian age. On the other hand, Erlansonisporites erlansonii to this time is known only from the Upper Cretaceous (?Campanian) on Disko Island, Greenland, and this report extends its range considerably downward in the section. I am concerned about my identifications of Echitriletes cf. E. lanatus, Arcellites cf. A. pyriformis, and Paxillitriletes species because I have had too few specimens and have relied on SEM studies alone for the analyses.

Among the megafossil remains from this locality Pityostrobus hueberi Robison and Miller and P. virginiana Robison and Miller (1977), assigned to the Pinaceae, represent the first species of this genus to be reported from the North American Atlantic Coastal Plain. The age in Robison and Miller was given as Albian, which, at the time, was based on correlations projected from outcrops of known age. The pollen and spore analysis, although preliminary, indicates that the site is within the Barremian-Aptian Patuxent Formation of the Potomac Group.

A cupule or fruit belonging to Caytonia, comparable morphologically to C. nathorsti (Thomas) Harris (1964), was found during the sorting of the plant remains for megaspores. Seeds comparable to those found in the cupule were also isolated from the debris.

The megafossil flora is rich in fern, cycadopsid, and conifer remains and accordingly deserves further attention. The collection of the matrix is available in the paleobotanical collections of the National Museum of Natural History, Smithsonian Institution, as a study resource for qualified researchers.

Interpretation of the lithofacies of the site, along with the general composition of the megafossil flora, suggests a backswamp deposit as discussed by Hickey and Doyle (1977). It is unfortunate that there was very little time to collect from the deposit before it was destroyed. The megafossil flora can only be hinted at on the basis of the collection at hand. Potentially some of the sporophytes may have been found that were the sources of the sporomorphs described herein. It would have been particularly exciting if sporophytes of Selaginella, similar to the modern species S. galeottii and S. myosurus, were found and shown to be the sources of Erlansonisporites erlansonii and Thylakosporites retiarius.

Literature Cited

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1960b.   Nouvelles observations sur la structure des parois megasporales de Selaginella myosurus (Swartz) Alston. Compte Rendu de l’Académie des Sciences de Paris, 250:1774–1775.

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1954.     Die Gattung der palaeozoischen Sporae dispersae und ihre Stratigraphie. Geologische Jahrbuch, 69: 111–194, 5 diagrams, 17 plates.

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1977.     Anatomically Preserved Seed Cones of the Pinaceae from the Early Cretaceous of Virginia. American Journal of Botany, 64(6):770–779, 16 figures.

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1971.     The Microstructure of Precious Opal. Mineralogical Record, 2(6):261–268, 13 figures.

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Stainier, F.

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1967.     Morphological Study of the Walls of Mega- and Micro-Spores of Selaginella myosurus (Sw.) Alston. Review of Palaeobotany and Palynology, 3:47–50, 1 plate.

Taylor, J., G. LeVee, P. Hollingsworth, and W. Bigelow

1976.     Sporopollenin and Major Mineral Constituents of the Spore, Elators, and Capsule Wall of Conocephalum conicum. Michigan Botanist, 15:167–172, 7 figures.

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PLATES

PLATE 1

FIGURES 1–6.—Verrutriletes carbunculus (Dijkstra) Potonié: 1, Proximal view, × 100; 2, equatorial view, × 100; 3, area x in figure 1, × 500; 4, area y in figure 1, × 1000; 5, surface of exine between gemmae on ornamented portion of spore, × 1000. (Specimen GG-7A, USNM 304311.)

6, Fragment, source of subsequent views of structure of wall and ornamentations, × 75 (Specimen GG-7C, USNM 304313).

PLATE 2

FIGURES 1–7.—Verrutriletes carbunculus (Dijkstra) Potonié: 1, Vertical section of spore coat with gemma in place, × 2000; 2, outer exoexine surface with gemma and former site of gemma attachment (arrow), × 1000; 3, oblique view of inner surface of spore coat showing depression immediately beneath gemma, × 1000. (Specimen GG-7C, USNM 304313.)

4, Proximal view, × 85; 5, distal view × 85; 6, view at x in figure 4 at 45° tilt, × 500; 7, view of sculpture at y in figure 5, × 350. (Specimen GG-7B, USNM 304312.)

PLATE 3

FIGURES 1–4.—Verrutriletes carbunculus (Dijkstra) Potonié: 1, View of broken margin of one of laesurae and of morphology of gemma, × 200; 2, vertical section of sporoderm (end = endexine, i-ex = inner exoexine, g = gemmae), × 1000; 3, fused gemmae with rupture revealing inner structure, × 3500; 4, corroded gemma showing interior structure, × 3000. (Specimen GG-7B, USNM 304312.)

PLATE 4

FIGURES 1–4.—Echitriletes cf. E. lanatus (Dijkstra) Potonié: 1, Assumed proximal view, × 75; 2, assumed distal view, × 75; 3, view at x in figure 1 showing density and morphology of capilli, × 150; 4, view at y in figure 1 showing bases of capilli and surface of outer exoexine, × 1000. (Specimen HH-1A, USNM 304314.)

FIGURES 5, 6.—Crybelosporites species: 5, Microspores lodged among capilli at z in figure 2 (note folds of outer layer of sculpture (perine) on lower-most spore), × 1000; 6, single microspore showing granulate surface of inner layer of “sculptine” impressed on outer layer of sculptine (perine), × 3000. (On specimen HH-1A, USNM 304314.)

PLATE 5

FIGURES 1–4.—Echitriletes cf. E. lanatus (Dijkstra) Potonié: 1, Assumed proximal view, × 75: 2, outer exoexine surface at x in figure 1, × 1000; 3, assumed distal view, × 75; 4, divided tips of capilli at y in figure 3, × 150. (Specimen HH-1B, USNM 304315.)

FIGURES 5, 6.—Crybelosporites species: Microspores lodged among capilli at z in figure 3, × 1000; 6, distal view of microspore collapsed, and with Y-mark evident as depression; outer layer of sculptine (perine) suggested by folds on left and lower right margins of spore, × 3000. (On specimen HH-1B, USNM 304315.)

PLATE 6

FIGURE 1–6.—Erlansonisporites erlansonii (Miner) Potonié: 1, Proximal view, × 115; 2, equatorial view (clockwise rotation), × 115; 3, distal view, × 115; 4, low murus and texture of outer exoexine at x in figure 1, × 2300; 5, diaphanous muri extending between bastions, apices of bastions rounded, view at y in figure 1, × 550; 6, reticulate surfaces of muri, × 1500. (Specimen GG-10B, USNM 304316.)

PLATE 7

FIGURES 1–6.—Thylakosporites retiarius (Hughes) Potonié: 1, Apical view of tetrahedral tetrad, × 40; 2, basal view of tetrahedral tetrad of spores, × 40; 5, area at x in figure 2 showing reticulum of outer exoexine partially free of layer of organic material that coats whole tetrahedral tetrad, × 125. (Specimen GG-8A, USNM 304320.)

3, Probable equatorial view, broken area in lower left representing margin of contact point in tetrahedral tetrad, × 70; 4, reverse side of specimen, × 70; 6, view of morphology of reticulum at y in figure 3, × 200. (Specimen HH-2B, USNM 304324.)

PLATE 8

FIGURES 1–8.—Thylakosporites retiarius (Hughes) Potonié: 1, Surface of outer exoexine between elements of reticulum, × 5000 (specimen HH-2B, USNM 304324).

2, Distal view, × 100; 3, proximal view with area of commissure broken away, × 100; 4, view at x in figure 3 with specimen tilted toward viewer at about 30° showing points of attachment of elements of reticulum, both intact and broken, × 325; 5, transverse section of spore coat (o-ex = outer exoexine, i-ex = inner exoexine, end = endexine), × 2500; 6, finely reticulate sculpture of the outer exoexine, × 10,000; 7, orderly arrangement of spherular elements forming inner exoexine, × 10,000; 8, reticulate structure of endexine as seen in surface view, × 10,000. (Specimen GG-10C, USNM 304322.)

PLATE 9

FIGURES 1–6. Thylakosporites retiarius (Hughes) Potonié: 1, Proximal view of crushed and eroded specimen, Y-mark exaggerated due to folding, × 150; 2, transverse section of spore coat at x in figure 1, (o-ex = outer exoexine) eroded except at points of attachment of reticulum (i-ex = inner exoexine, end = endexine) loosened and folded, × 600; 3, near-equatorial view showing remnants of points of attachment of reticulum (Plate 8, figure 4), × 140; 4, orderly arrangement of spherular elements forming inner exoexine and in turn giving rise to opal-like quality of spore coat as seen in reflected light, view at y in figure 2, × 5000; 5, remnants of surface of outer exoexine particularly at arrow, × 5000; 6, additional view of spherular elements seen at y in figure 2, × 10,000. (Specimen BB-12A, USNM 304319.)

PLATE 10

FIGURES 1–6.—Thylakosporites retiarius (Hughes) Potonié: 1, Slightly oblique proximal view showing erosion and corrosion of spore coat, × 150; 2, oblique distal view showing areas of corrosion and separation of spore coat elements, × 150; 3, area x in figure 2 (end = endexine, i-ex = inner exoexine, o-ex = outer exoexine), × 500. (Specimen GG-10A, USNM 304321.)

4, Fragment of distal portion of spore with reticulum intact, × 130; 6, transverse section of strand of reticulum viewed at y in figure 4, × 3000. (Specimen GG-10D, USNM 304323.)

5, Reticulate surface structure of one reticulum strand, × 4500. (Specimen GG-10C, USNM 304322.)

PLATE 11

FIGURES 1–4.—Crybelosporites striatus (Cookson and Dettmann) Dettmann: 1, Microspores resting in folds of acrolamellae of megaspore, × 170; 2, folding of outer exoexine (perine) of spores, × 750; 4, single spore at x in figure 2 showing reticulate surface of inner exoexine and outer savate exoexine, × 2000. (Specimen of Arcellites disciformis (Miner) Ellis and Tschudy, GG-6A, USNM 304329 with seven adherent microspores.)

3, Additional examples of this microspore on second specimen of Arcellites disciformis, × 1250. (Specimen of Arcellites disciformis (Miner) Ellis and Tschudy, BB-11C, USNM 304328, with adherent microspores.)

PLATE 12

FIGURE 1–4.—Arcellites disciformis (Miner) Ellis and Tschudy: 1, Equatorial view showing tubelike appendages ornamenting body of spore and plicate, crenulate-margined acrolamellae forming neck and chamber above Y-mark, × 170; 2, proximal view showing torsion of acrolamellae forming neck, × 240. (Specimen BB-11B, USNM 304331.)

3, Equatorial view of extremely well-preserved specimen except for flattening of tube-like appendages, × 170; 4, proximal view shows greater expansion of acrolamellae forming neck, × 240. (Specimen BB-11A, USNM 304330.)

PLATE 13

FIGURES 1–4.—Arcellites disciformis (Miner) Ellis and Tschudy: 1, Equatorial view of specimen mounted by apices of acrolamellae, tubular nature of body appendages clearly demonstrated, × 170; 2, distal view showing distribution of body appendages and punctate texture of outer exoexine, × 170. (Specimen GG-5C, USNM 304338.)

3, Equatorial view showing narrow, elongate neck, × 170; 4, proximal view showing separation of crenulate margins of some of acrolamellae, × 280. (Specimen GG-5A, USNM 304337.)

PLATE 14

FIGURES 1–4.—Arcellites disciformis (Miner) Ellis and Tschudy: 1, Crushed specimen showing separation of individual acrolamellae, × 170; 2, enlarged view to show detail of morphology of margins of acrolamellae, × 350. (Specimen GG-4B, USNM 304335.)

3, Equatorial view of obliquely crushed specimen showing microspores (Crybelosborites striatus) lodged in plications of acrolamellae, × 170; 4, proximal view showing pronounced torsion of acrolamellae, × 280. (Specimen BB-11C, USNM 304328.)

PLATE 15

FIGURE 1–4.—Arcellites disciformis (Miner) Ellis and Tschudy: 1, Proximal view showing chamber or neck cavity formed by acrolamellae, apices of acrolamellae were removed by dissection, × 340; 2, separation of acrolamellae and demonstration of single-layered construction of individual acrolamellae, × 1700. (Specimen II-2, USNM 304340.)

3, Marginal contact of two acrolamellae showing intertanglement of fimbrate margins and pores and interstices between crenae, × 2000 (specimen GG-5A, USNM 304337).

4, Base of acrolamellae at point of origin from body of spore, × 1325 (specimen BB-11A, USNM 304330).

PLATE 16

FIGURES 1–4.—Arcellites disciformis (Miner) Ellis and Tschudy: 1, Pitted structure of outer exoexine of spore body and continuation of structure into tubular appendage, × 1000; 2, pitted surface structure of outer exoexine × 5500. (Specimen BB-11A, USNM 304330.)

3, Irregularity in size of pits possibly due to corrosion as shown by thinned areas that transmit electron beam through specimen, × 4000 (specimen GG-5A, USNM 304337).

4, Pitted surface structure of outer exoexine clearly visible as well as undeveloped body appendages represented by rounded protuberances from body surface, × 1600 (specimen BB-11F, USNM 304332).

PLATE 17

FIGURES 1–4.—Arcellites disciformis (Miner) Ellis and Tschudy: 1, Spore body sectioned transversely to demonstrate spore coat structure, × 240; 2, view at x in figure 1 (o-ex = outer exoexine, i-ex = outer exoexine, end = endexine), × 4000. (Specimen GG-4A, USNM 304334.)

3, Vertical section through base of body appendage and onward through spore wall (note outer exoexine and inner exoexine rising into base of appendage, endexine layer seen at lowest level of section), × 3000 (specimen GG-4C, USNM 304336).

4, Transverse section of base of body appendage showing outer exoexine and limit of extension of inner exoexine into appendage, × 3500 (specimen GG-5C, USNM 304338).

PLATE 18

FIGURES 1–5.—Arcellites disciformis (Miner) Ellis and Tschudy: 1, Body appendages, intact and with broken apex, × 750; 2, appendage with ruptured apex, suggesting partially collapsed bulbous tip, × 1500; 5, view downward into interior of body appendage showing reticulate structure of inner exoexine at bottom and characteristic structure of outer exoexine, × 3200. (Specimen GG-5C, USNM 304338.)

3, Longitudinal section through body appendage showing outer exoexine forming wall of appendage and inner exoexine extending only slightly into base, endexine clearly discernible, forming inner layer of spore coat, × 1000 (specimen GG-6A, USNM 304329).

4, Inwardly contracted apex of body appendage with structure similar to fimbrate margins of acrolamellae, × 7000 (specimen GG-5A, USNM 304337).

PLATE 19

FIGURES 1–4.—Arcellites disciformis (Miner) Ellis and Tschudy: 1, Equatorial view of specimen showing level of section required to reveal position of Y-mark, × 170; 2, view into spore body cavity with Y-mark in position that corresponds to area directly beneath the cavity formed by acrolamellae, × 350. (Specimen GG-3A, USNM 304333.)

3, Equatorial view of specimen badly broken during attempt to prepare transverse sections, chamber formed by acrolamellae imperfectly demonstrated but can be interpreted in this view, × 230; 4, view into spore body cavity showing Y-mark and layering of spore coat, × 480. (Specimen GG-6B, USNM 304339.)

PLATE 20

FIGURES 1–7.—Arcellites cf. A. pyriformis (Dijkstra) Potter: 1, Near equatorial view, folds of neck-like projection broken away at lower left, × 90; 2, view at x in figure 1 showing texture of outer exoexine surface and variation in morphology of the body appendages, × 180; 3, broken side of spore from which some details of spore coat structure were obtained, × 90: 4, detail at y in figure 3, the o-ex (outer exoexine) quite thick and forming body appendages seen in section; a thinner, i-ex (inner exoexine) appearing more loosely structured and end (endexine) seen as extremely thin layer, × 1000; 5, detail at z in figure 3, × 500; 6, detail at arrow in figure showing sculpture of exoexine and separation between outer exoexine and inner exoexine (note textural differences), × 5000; 7, detail of outer exoexine surface texture in figure 5, × 8,500. (Specimen BB-12, USNM 304342.)

PLATE 21

FIGURES 1–9.—Arcellites cf. A. pyriformis (Dijkstra) Potter: 1, Equatorial view, × 85; 2, equatorial view, specimen rotated 90° counter-clockwise, × 85; 3, equatorial view, 180° rotation from position in figure 1, × 85; 4, proximal view, × 85; 5, area at x in figure 1 after rotation of specimen few degrees toward viewer, × 300. (Specimen HH-2A, USNM 304343.)

6, Fragment of spore in near-equatorial view, × 85; 7, section of spore coat (o-ex = outer exoexine, i-ex = inner exoexine), × 4000; 8, structure of inner-most surface of endexine, × 2000; 9, surface texture of outer exoexine, × 950. (Specimen HH-2C, USNM 304344.)

PLATE 22

FIGURES 1–4.—Paxillitriletes species Hall and Nicolson: 1, Proximal view, × 260; 2, equatorial view, 90° rotation, counter-clockwise, on equatorial axis, × 260; 3, distal view, × 260; 4, equatorial view, 90° rotation, counterclockwise on equatorial axis, × 260. (Specimen GG-3D, USNM 304346.)

PLATE 23

FIGURES 1–4.—Paxillitriletes species Hall and Nicolson: 1, View at x in Plate 22 (figure 4) showing long capillae along margins of laesurae, shorter capillae arising at connecting points of muri, part of equatorial flange (lower right) and fused bases of appendages immediately bordering laesurae, × 600; 2, detail at x in figure 1 showing reticulate sculpture of surface of outer exoexine, × 2200. (Specimen GG-3D, USNM 304346.)

3, Equatorial view, × 175; 4, proximal view, × 175. (Specimen GG-3C, USNM 304345.)

PLATE 24

FIGURES 1–5.—Dictyothylakos pesslerae Horst: 1, View interpreted as upper or outer surface of specimen (note “frayed” margin), × 90; 2, slightly oblique view interpreted as lower or inner surface, × 90; 3, detail of surface at x in figure 1, × 375; 4, margin of specimen at y in figure 1, showing broken elements and structural detail, × 200; 5, view at z in figure 2 showing broken structural elements rounded in cross-section and apparently not hollow, × 350. (Specimen II-4B, USNM 304347.)

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