VERTEBRATE ICHNOFAUNA OF THE UPPER TRIASSIC REDONDA FORMATION (CHINLE GROUP), EAST-CENTRAL NEW MEXICO

The Norian-Rhaetian? Redonda Formation (Chinle Group) formed in a lacustrine basin in northeastern New Mexico. Throughout most of its outcrop belt the Redonda preserves only invertebrate trace fossils. However, in eastern Quay County the lacustrine deposits onlapped on the Frio uplift and there are extensive vertebrate body- and trace-fossils in this area.

The vertebrate ichnofauna has been collected since the 1930’s and significant collections are preserved at the Mesalands Dinosaur Museum, New Mexico Museum of Natural History and Science, the University of Michigan Museum of Paleontology and the Panhandle Plains Historical Museum. Tracks are preserved in tabular calcarenites that represent shoreline deposits, in association with an abundant invertebrate ichnofauna.

The most common tetrapod track-type in the main track faunas is a tetradactyl pedal impression assigned to the prosauropod ichnotaxon Pseudotetrasauropus sp. Next in abundance are quadrupedal tracks of Brachychirotherium sp. which are conventionally considered to be aetosaur prints. The theropod track Grallator sp. is represented by a small number of tracks. The track-bearing beds fine upward and the thinly-bedded upper units yield a distinct ichnofauna of smaller animals including Rhynchosauroides, a large Rhynchosauroides-like track and a small Batrachopus-like track.

Tetrapod tracks in the Chinle Group are widespread from Utah to Mexico, but they are mainly restricted to Apachean-aged strata of the uppermost part of the Group. The Redonda ichnofauna differs from correlative ichnofaunas in the absence of Tetrasauropus and the numerical abundance of Pseudotetrasauropus. The Redonda body-fossil fauna differs from the inferred trackmaker fauna in lacking prosauropod dinosaurs, in possessing abundant phytosaurs and metoposaurid amphibians and in having a paucity of aetosaurs and small tetrapods.