New Mexico Geological Society Annual Spring Meeting — Abstracts

The Redonda Formation (Chinle Group) of northeastern New Mexico represents a large lacustrine basin of late Norian or Rhaetian age. It yields a diverse ichnofauna and body-fossil fauna. These faunas are of great importance for four reasons: (1) the body-fossil fauna is one of few diverse faunas of this age worldwide; (2) most body-fossil faunas of this age represent terrestrial communities (Germany, Argentina), whereas the Redonda samples both semi-aquatic and terrestrial ecosystems; (3) it isthe only diverse ichnofauna of this age that is associated with a diverse bone record; (4) it offers a unique opportunity to examine differences between animal communities inferred from trace fossils and body fossils because the tracks and bones are not just from the same lithosome, but also from the same lakeshore environment.

The vertebrate ichnofauna of the Redonda Formation consists of, in decreasing order of abundance, Pseudotetrasauropus (prosauropod), Brachychirotherium (crurotarsan -aetosaur?), Rhynchosauroides (sphenodont) and Grallator (theropod). These tracks derive from calcarenites that formed in a beach environmentThe tetrapod fauna of the Redonda Formation comprises, in decreasing order of abundance, phytosaurs (Redondasaurus), metoposaurs (Apachesaurus), aetosaurs (Redondasuchus, Neoaetosauroides), sphenosuchians, Vancleavia-like reptile and small reptiles (cynodont, theropod, sphenodontid, procolophonid). These fossils derive from a variety of lacustrine margin environments including oolitic and non-oolitic beach, delta, river and paleosol.

There is obviously, a marked disparity between the ichnofauna and bone fauna from the same environmental complex. Thus, for example, the ichnofauna suggests that prosauropod dinosaurs are as common as they are in contemporaneous faunas from Europe and South America, yet no bones have been found. The good news is that two distinct animal communities are being sampled; the bad is that neither ichnofaunas nor body-fossil faunas in isolation give an accurate representation of the Redonda lakeshore communities.