New Mexico Geological Society Annual Spring Meeting & Ft. Stanton Cave Conference — Abstracts

Few fossil remains of any kind have been reported from the Arroyo de Alamillo Formation of the Yeso Group, a lithologic succession of siltstone, sandstone and minor dolostone and gypsum of early Permian (Leonardian/Kungurian) age, formed under a semi-arid to arid climate regime on a vast coastal plain, conditions generally unfavorable for the preservation of fossil remains. Here we report a fossil plant assemblage from Socorro County, NM, ~ 14 m above the base of the Arroyo de Alamillo Formation. The plant remains occur in a 0.5 m thick, fine-grained sandstone bed of local areal extent. 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 on an otherwise arid, sand-rich landscape. The fossil flora occurs in a small area approximately 2 m in width and 30 cm in depth, its 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, the filicalean fern Oligocarpia sp., and some small seeds. The specimens are preserved as large fragments, leading us to suspect that had the hard, resistant enclosing rock matrix been more susceptible to splitting along large surfaces, much larger, possibly even entire, fronds might have been recovered. The dominant plant, Autunia naumannii has been widely reported across tropical Pangea, from the easternmost regions (China) to the west (New Mexico) and many areas in between. It is associated with environmental indicators of moisture seasonality, but still with moderate soil moisture. The material found in the Yeso deposit appears to have had small, forked fronds bearing multilobed pinnules and small, rounded intercalary pinnules. It deviates to some degree from typical A. naumannii in having more rounded pinnule lobes than is typical for the species; in this trait it might be considered a subspecies or even a new species. This is the youngest occurrence of plant fossils so far reported in the early Permian of far western Pangea.