New Mexico Geological Society Annual Spring Meeting — Abstracts

We report here a diverse microfauna and flora from NMMNH locality 325 in the type section of the Upper Triassic (Apachean: late Norian-Rhaetian) Sloan Canyon Formation in Union County, northeastern New Mexico. The calcareous microflora and microfauna of the Sloan Canyon Formation contain a new species of charophyte (Porochara n. sp.), two species of the ostracode Darwinula, one new (Darwinula accuminata Belousova, 1961; Darwinula n. sp.), and two species of the ostracode Gerdalia, one new (Gerdalia triassica (Belousova), 1961; Gerdalia n. sp.). The fine (100-230 mesh) screens used to recover the calcareous microfossils also facilitated the recovery of many microvertebrates, including selachians, osteicthyans, and tetrapods. Shark fossils from the Sloan Canyon locality are predominantly small teeth of the hybodont Lissodus. The osteichthyan fauna is more diverse and includes numerous redfieldiid fossils as well as less abundant colobodontids, semionotids and palaeoniscids. The tetrapod fauna includes sphenodontians, cynodonts, and fragmentary teeth of larger tetrapods, principally phytosaurian archosaurs. These records are an important addition to the Sloan Canyon vertebrate fauna, which is otherwise dominated by indeterminate phytosaurs and metoposaurid amphibians. Vertebrate trace fossils from the Sloan Canyon Formation include tracks of aetosaurs (Brachychirotherium) and theropod dinosaurs (cf. Grallator) and sauropodomorph dinosaurs (Tetrasauropus).

NMMNH locality 325 occurs in an intraformational conglomerate and limestone we interpret as a lacustrine beach deposit. Fish scales comprise at least 20% of the clasts in the conglomerate. The Sloan Canyon microfauna apparently lived in a permanent water body with a depth of about 6 m or less. This water body was highly mineralized, and the fauna/flora indicates a typical Triassic association of diverse darwinulid ostracodes and rarer charophytes. The associated hybodont shark, Lissodus, may have preyed on the ostracodes.