New Mexico Geological Society Annual Spring Meeting — Abstracts

The Redonda Formation (Upper Triassic: Rhaetian) is widely exposed in eastcentral New Mexico and represents lacustrine, and lacustrine-margin deposition. Laterally continous beds of ripple-laminar micrite are common in more easterly exposures in Quay County. These strata represent carbonate mudflats on the lacustrine margin and yield a significant ichnofauna. The vertebrate component of the ichnofauna was first collected in 1934 by E. C. Case. The vertebrate ichnofauna includes Pseudotetrasuropus, Tetrasauropus, Grallator and Rhynchosauroides. In 1995, we found several specimens of a new ichnotaxon in the thinly-bedded upper portion of one of the micrite beds.

The new tracktype is pentadactyl and represents a quadrupedal trackmaker. A typical pedal imprint is 5 cm long and about 5 cm, wide. Digit impressions IV-I exhibit decreasing length or are subequal in length and are incurved. The impression of digit V is always shallow; it diverges from the other digit impressions and is much shorter. A short, narrow heel imprint is deep and has an acute posterior margin.

This ichnotaxon shows morphological similarities to tracks assigned to Rhynchosauroides, such as those from the lower Petrified Forest Formation (Upper Triassic: late Carnian) of Petrified Forest National Park, northeastern Arizona. It differs from most Rhynchosauroides tracks in the depth and shape of the heel impression and in the large size. Most Rhynchosauroides tracks are exhibit pedal lengths less than 5 cm, although some European species compare in size to the Redonda tracks. The ichnogenus Rhynchosauroides is apparently a "wastebasket" taxon that includes all "lizard-like" tracks from the Triassic and is in need of taxonomic revision. The Redonda tracks appear to represent an ichnotaxon similar, but not identical, to Rhynchosauroides.