New Mexico Geological Society Annual Spring Meeting — Abstracts

Vertebrate and invertebrate trackways are locally abundant in Upper Triassic strata of the Tucumcari basin in east-central New Mexico. The oldest trackways are from the Los Esteros Member of the Santa Rosa Formation (latest Carnian) in Santa Fe County and pertain to limulids (horseshoe crabs) and represent the genus Koupichnium. The Bull Canyon Formation (early-middle Norian) contains many invertebrate trackways in Quay County. All trackways represent arthropods with trackway widths less than 2 cm. Such trackways are locally abundant in the basal Bull Canyon near Logan and the middle Bull Canyon at Bull Canyon and Barranca Creek. The Barranca area also includes several vertebrate tracks and trackways. Two isolated tracks represent "swimming traces" and consist of parallel scratch marks. One slab contains several trackways of a small (pes length = 6 cm), undescribed reptile with a tetradactyl manus and pes.

The Redonda Formation (Rhaetian) is rich in invertebrate trace fossils and vertebrate tracks and trackways are common at three. localities in Quay County: (1) Mesa Redonda; (2) Red Peak; and (3) Apache Canyon. These footprints occur in epirelief on the undersides of laterally continuous calcarenites that represent cyclic lacustrine shoreline environments. At Mesa Redonda, these cycles represent the highest strata in the Redonda and contain numerous tracks of a tridactyl theropod. One slab contains a well preserved trackway of Pseudotetrasauropus. Two isolated tracks pertain to Brachychirotherium. Most tracks at Red Peak represent a tridactyl theropod, but Pseudotetrasauropus also occurs here. At Apache Canyon, where the trackbearing cycles are overlain by terrestrial strata, Pseudotetrasauropus tracks dominate and Brachychirotherium and the tridactyl theropod track are less common.

The Tucumcari basin contains the best sequence of superposed ichnofaunas in the Chinle Group of the western United States. Several ichnotaxa are biochronologically significant. For example, Pseudotetrasauropus is restricted to Rhaetic strata (Redonda, Sloan Canyon Formations of New Mexico, Rock Point Formation of utah).

The Tucumcari basin also contains the best sequence of superposed bodyfossil fauna in the Chinle Group. All formations are dominated by specimens of phytosaurs with aetosaurs and metoposaurs constituting the majority of other fossils. The body-fossil and trace-fossil record of the Tucumcari basin exhibit poor correspondance: (1) limulid body fossils are unkown; (2) footprints of phytosaurs are absent; (3) Brachychirotherium may represent an aetosaur but it is common in only one formation; (4) saurischian bones and teeth are rare but their tracks are common in the Redonda Formation; and (5) Pseudotetrasauropus may represent a prosauropod but body fossils of these dinosaurs are absent. Most formations of the Tucumcari basin are of Type 4b (bones more abundant than footprints and of different animals) of Lockley (1989). The Redonda is of Type 3b (bones subequal in numbers with tracks and of different animals).