New Mexico Geological Society Annual Spring Meeting — Abstracts

Continental strata of the Abo Formation that crop out around the Sandia Mountains have been previously described as poorly represented by tetrapod fossils; and from the underlying, dominantly marine Madera Formation, none have been reported. However, just south of where these strata disappear at the San Francisco fault north of Placitas, a noteworthy vertebrate fossil assemblage has been discovered. This is the first Paleozoic section in New Mexico where rocks containing diverse tetrapod body fossils interdigitate with highly fossiliferous marine units and thus provides a direct and precise cross-correlation of nonmarine and marine biostratigraphy.

The bone-bearing strata belong to the uppermost Madera Formation and the marine strata indicate they are of Middle Virgilian age. Deposition took place on a coastal floodplain, which may account for the occurrence of taxonomically and ecologically distinctive vertebrates previously unknown in New Mexico. The general observation, moreover, that the Abo/Cutler deposits are poorly represented by aquatic forms compared to other classic localities is, in part, redressed at this site.

Vertebrate fossils have been identified at various places along 1 km of outcrop, but the preponderance of material is localized within about 100 m. Recovered elements are mostly fragmentary and, with the notable exception of a single collection, are unassociated. Due to its fragmentary nature, the material thus far obtained is of uncertain value for morphological studies. It is, however, sufficient to characterize a diverse assemblage.

At least three chondrichthyans are represented by isolated teeth, including a holocephalian tooth plate and a large cladodont tooth, possibly referable to the elasmobranch order Symmoriiformes. Diplodont teeth of the freshwater xenacanth shark Orthacanthus are common, though rarely intact.

The dipnoan genus Sagenodus is represented by several tooth plates. While recorded from the Lower Permian of Utah and north-central Texas, this is the first record of a non-aestivating lungfish in the Permo-Pennsylvanian of New Mexico. Its occurrence implies existence of permanent bodies of fresh water and mesic conditions. Heretofore, only the aestivating lungfish Gnathorhiza has be found in New Mexico and is an indicator of seasonal drought.

Temnospondyl amphibians are represented by at least two species. The bizarre dissorophoid Platyhystrix is represented by several of its distinctive neural spines. A larger, typical rhachitome is represented by several characteristic intercentra and part of a neural spine. The latter are consistent with a form such as Eryops, common in the Permo-Pennsylvanian of New Mexico.

Especially interesting is the first record of a large anthracosaur amphibian west of the mid-continent. A collection of associated vertebral elements and a jaw fragment with intact teeth is indicative of the embolomere genus Neopteroplax. Rare material from elsewhere in the state represents a small embolomere sometimes identified as Archeria.

Diadectomorphs are early derivative amniotes of high diversity in the Permo-Pennsylvanian of New Mexico. Portions of a dentary with intact teeth have been recovered that are characteristic of the diadectomorph family Diadectidae, the members of which may represent the earliest herbivorous tetrapods.

At least one large predatory pelycosaur is represented by diverse postcranial elements including vertebrae that are assignable as either ophiacodontid or sphenacodontid. Two edaphosaurs may be present. Certainly, a larger species consistent with Edaphosaurus novomexicanus, an early herbivore, is well represented by large neural spines with characteristic lateral tubercles. Very small neural spines appear consistent with those of the insectivorous and early derivative edaphosaur lanthasaurus from the Late Pennsylvanian of Kansas and Colorado.

Further collecting at the site and more intensive study of the material will refine some of the above identifications and continue to add others. The Placitas site and its assemblage can serve as an important temporal and stratigrapgic bench mark for continued study of this region's unique system-transgressive vertebrate chronofauna.