TETRAPOD ICHNOLOGY OF THE EARLY JURASSIC NAVAJO SANDSTONE: IMPLICATIONS FOR THE "VERTEBRATE ICHNOFACIES" CONCEPT

The Navajo Sandstone (Colorado Plateau, southwestern USA) is an Early Jurassic age eolian sandstone, comprised predominantly of dune strata. Thin, laterally-restricted (usually a few hundreds of meters) sabkha deposits are often calcareous; many exhibit "crinkly" algal laminations and stromatolitic structures. Tetrapod skeletal remains are rare and often unidentifiable; they include theropod and prosauropod dinosaurs, crocodylomorphs, and tritylodontids. These groups are typically represented by only one or two specimens. In contrast, footprints demonstrate the existence of hundreds of individuals. Ichnotaxa from this unit, and their probable trackmakers, are: Grallator, Anchisauripus, Eubrontes (theropod dinosaurs); Otozoum, Navahopus (prosauropod dinosaurs); Anomoepus (ornithischian dinosaurs); Brasilichnium (synapsids); and Batrachopus (crocodylomorphs). Some small poorly-preserved tracks may represent lepidosauromorphs and/or pterosaurs.

Footprints occur in the dune facies, both on foreset and bounding surfaces, and in the interdune deposits. Brasilichnium, Grallator, Batrachopus, small Anomoepus and lepidosauromorphs/pterosaurs(?) occur on dune foreset surfaces. The remaining horizontally-bedded track-bearing sedimentary facies preserve Anchisauripus, Eubrontes, Otozoum and large Anomoepus. The correlation between facies and ichnotaxa has been attributed to the trackmakers' preference for particular environments, leading to the concept of "vertebrate ichnofacies". In this scenario, synapsids, crocodylomorphs, lepidosauromorphs/pterosaurs(?) and small dinosaurs were restricted in life to dune environments, while the larger dinosaurs were limited to playa environments. Instead, it is suggested here that the facies-taxon relationship reflects preservational bias, due to foot-substrate interactions and kinematic factors, rather than habitat preference. Only the tracks of small animals are likely to be preserved on foresets - the traces of larger animals will be obscured by grainflow and other sedimentological processes. In the playas, small tracks are not preserved. The scale of the algal features is of the same magnitude as the small footprints; this, combined with any subsequent diagenetic alteration, effectively obliterates all but the largest footprints.