Abstract:
We report fossil plants and traces from the Arroyo de Alamillo Formation of the Yeso Group, a lithologic succession of siltstone, sandstone, and minor dolostone and gypsum of early Permian (early Leonardian/late Artinskian) age. These Yeso strata formed under a semi-arid to arid climate regime on a vast coastal plain, conditions generally unfavorable for the preservation of fossil remains. Two fossil plant assemblages were collected from a single outcrop in Socorro County, New Mexico, ~9 and ~13 m above the base of the Arroyo de Alamillo Formation. The lower of the two assemblages occurs in a mud-draped, tabular siltstone bed, approximately 1 m in thickness; the mud drape accounts for the upper 1 cm of the bed. This bed is similar to much of the underlying Abo Formation and is interpreted to have formed in a playa lake. Plant remains are concentrated in the mud drapes and occur with animal trackways, mudcracks, and “raindrop” imprints. Identified plants include the walchian conifer, Brachyphyllum tenue, and remains tentatively identified as a coniferophyte similar to Dicranophyllum, but lacking key features of that genus. The upper plant assemblage is ~ 4 m above the first. Plant remains are present in a 0.5 m thick, fine-grained sandstone bed. Planar bedded and lacking trough cross bedding, with climbing ripples, we interpret this deposit as a small-scale sheet flood, unchannelized flow, into a standing water pond, possibly an oasis in an otherwise arid, sand-rich landscape. The fossil plants occur in a small area approximately 2 m in width and 30 cm in depth, length limited by the erosional boundaries of the outcrop to about 3 m. With the exception of a few specimens, all plant remains can be assigned to the peltasperm (callipterid) Autunia naumanii. Single specimens are tentatively identified as the peltasperm Arnhardtia scheibei and reproductive organs. These are among the youngest occurrences of plant fossils so far reported in the early Permian of western Pangea.
