New Mexico Geological Society Annual Spring Meeting — Abstracts

Since the 1880s, El Cobre Canyon (BCC) has been a collecting area for early tetrapod fossils, most notably the complete skeleton of Limnoscelis that has figured prominently in debate about the origin of reptiles. A section of nonmarine red beds at least 250 m thick assigned to the Cutler Formation is exposed in ECC and has long been assigned either a Late Pennsylvanian or Early Permian age based on its megafossil plants and tetrapod fossils. A detailed lithostratigraphy coupled with new biostratigraphic data allows us to discriminate both Late Pennsylvanian and Early Permian strata in the ECC section.

Part of the biostratigraphic confusion at ECC began with Smith et al. (1961, NMBMMR Bulletin 75), who claimed that a plant locality at the northern end of the canyon is faulted into the section and much older than strata exposed elsewhere in ECC. However, no fault is associated with this locality, it is part of the Cutler Formation section and it is stratigraphically overlapped by the bulk of the ECC vertebrate fossil assemblage. The plant locality yields a megaflora that includes Calamites, Cordaites, SphenophyZlum, Danaeites, Asterophyllites, Macroneuropteris and Alethopteris and is certainly Late Pennsylvanian, an upper Conemaugh-Monangahela equivalent (Missourian?). Preliminary palynological analysis of the locality yields 97% trilete spores (Apiculatisporis sp., Calamospora parva,Convolutispora varicosa, Punctatisporites glaber, Reticulitriletes sp. cf. R. mediareticulatus), 2% taeniate disaccate pollen (Illinites unicus, Protohaploxypinus sp.) and 1% monosaccate pollen (Potonieisporites sp.). These species are generally long ranging and not precise biostratigraphic indicators, but are consistent with a Late Pennsylvanian age. The bulk of the ECC vertebrate fossil assemblage includes taxa such as Desmatodon and Limnoscelis, only found elsewhere in Late Pennsylvanian assemblages.

Previous workers were confounded by the presence of Sphenacodon at ECe, a taxon only known elsewhere in Wolfcampian rocks. However, Sphenacodon fossils from ECC come from a stratigraphic level much higher than the remainder of the vertebrate fossil assemblage. Indeed,
this level is above a marked physical break and lithofacies change in the section, and is in Wolfcampian strata that probably are equivalent to the Early Permian Abo Formation to the south.