Reflections on the Morphology, Anatomy, Evolution, and Classification of the Class Stenolaemata (Bryozoa)

Abstract

Thin sections of 37 stenolamate species with soft parts within the skeletons unexpectedly reveal 10 variations in polypide anatomy and related methods of tentacle protrusion. Six of the variations modify attachment organs, and four occur in species that lack such organs. Attachment organs are required for the majority of “progressive” degeneration-regeneration polypide cycles that advance new polypides as the skeletons grow, so that functionally constant protrusion distances are maintained. In most species lacking attachment organs, polypides retract and regenerate in the bottoms of their living chambers so polypide cycles are “stationary” and protrustion distances increase as the skeletons grow in length. Internal studies indicate that skeletal microstructure and polypide anatomy in Recent stenolaemates are generally qualitatively uniform within colonies and within species. Unexpectedly, skeletal microstructure and polypide anatomy often do not correlate with each other or with the skeletal structures of the five informal taxa (horneroids, tubuliporines, fasciculates, disporelloids, and heteroporoids) used to group Recent species. These groupings have been established historically using only the relatively few external characters available. Internally, four of the five groupings have more than one kind of attachment organ, and three of the five have polypides both with and without attachment organs. Furthermore, organs that apparently occur in relatively few taxa occur in more than one grouping. Skeletally, more than one microstructure occurs in four of the five groupings. Noncorrelations of internal character states are so numerous that it seems almost any polypide can occur in almost any skeleton. Genetically controlled taxonomic characters of fossil stenolaemates that seem promising in ellucidating relationships are skeletal microstructure, skeletal structures that reflect polypide anatomy and function, such as nutrient exchange systems, and mode of growth. Species in which laminae in the vertical zooidal walls grew inward from zooidal boundaries under outer body cavities dominated throughout the Paleozoic and Triassic and that inward-growing microstructure has continued to the present. Communication pores and frontal walls developed and disappeared in a meager number of species that became extinct in the Paleozoic. Species that grew laminae outwardly from zooidal boundaries, or grew crystallites at right angles to zooidal growth, apparently appeared first in the Jurassic. Three Paleozoic orders are inferred to have survived into the Jurassic, and, with the new Mesozoic clades, to have independently evolved communication pores and frontal walls. The three surviving Paleozoic orders are inferred to have interchanged primitive characteristics with the advanced features of newly evolved Mesozoic clades to produce the detailed noncorrelations described here in Recent species. Post-Triassic stenolaemates, therefore, are inferred to be polyphyletic at the order level. With just 37 Recent species available, it is not clear whether polythetically described taxa ultimately can be distinguished in a manner that adequately reflects phylogenetic patterns in younger stenolaemates. Differing, but functionally similar, parts might have been so pervasive across clades that the resulting noncorrelations have produced a mosaic distribution of combined primitive and advanced character states that makes any sort of natural classification unlikely.