Abstract:
A recently discovered tracksite in the Abo Formation in the Quebradas region of Socorro County is distinguished by unusually high ichnodiversity and exceptional preservation. This site, NMMNH (New Mexico Museum of Natural History) locality 12617, is north of Tinajas Arroyo in the Cañon de Espinoso Member of the Abo Formation, about 5 meters below the base of the overlying Yeso Group. The fossil-bearing stratum is a 0.7-1.5 m thick interval of thin-bedded, ripple-laminated, very fine sandstone with extensive mudcracks. At locality 12617, many surfaces with trace fossils have microbially induced sedimentary structures (MISS), and microbial mediation of preservation likely caused some of the exceptional ichnofossil preservation at this site. The invertebrate trace (ribbon trail) Olivellites (= Psammichnites) is reported here from the Abo Formation for the first time. Tetrapod ichnogenera from locality 12617 are Amphisauropus, Batrachichnus, Dromopus, Dimetropus, Limnopus and Varanopus. Plant specimens consist of the conifer Walchia, the peltasperm Supaia, and the callipterid Autunia conferta. Walchia and Supaia are typical of the Abo Formation throughout its extent; most Abo fossil plant sites are dominated by conifer remains of several types, but some fewer by Supaia. Mixed assemblages are uncommon. Autunia conferta, of the small-pinnule type, is common only in the upper Abo Formation (Cañon de Espinoso Member). The ichnofaunal composition at NMMNH locality 12617, which is dominated by anamniote, parareptile/eureptile and synapsid tracks, is very different from that of the Erpetopus biochron just above it. This is consistent with data from North America and Europe that identify a substantial change in the composition of footprint ichnoassemblages at the beginning of the Erpetopus biochron (close to the beginning of the Leonardian) due to the diversification of sauropsid reptiles, likely driven by climate changes (drying) across much of Pangea. Indeed, paleoenvironmental changes across the Abo-Yeso transition—regional marine transgression and a trend towards drier climates—are well reflected in the changes in trace fossil and floral composition during the early Leonardian.
