EARLY PERMIAN TETRAPOD ICHNOFAUNA FROM THE ROBLEDO MOUNTAINS, NEW MEXICO – REVISORY NOTES AFTER TWO DECADES OF COLLECTION AND RESEARCH

The Early Permian (late Wolfcampian) Robledo Mountains Formation in south-central New Mexico contains one of the most abundant and diverse assemblages of nonmarine Paleozoic trace fossils in the world. As a first-ranked ichnofossil Lagerstätte, the stratum typicum area was protected by law as the Paleozoic Trackways National Monument (PTNM) in March 2009, and this resulted in the intensification of research activities. A recently finished conservative revision of invertebrate trace fossil collections from the PTNM shows 22 assigned taxa, while treating nine other forms in open nomenclature. These are mostly arthropod walking and shallow feeding traces. Though much more work has always been dedicated to the vertebrate tracks of the study area, their taxonomy is still obscure, given that 32 different binominal names are available in the literature for tetrapod footprints from the Robledo Mountains. Based on anatomically-controlled features of the imprint morphology and trackway pattern we identify six recurrent types of tetrapod footprints at the PTNM. These are Batrachichnus, Limnopus, Dimetropus, Dromopus, Hyloidichnus, and a new unnamed ichnotaxon, and these can be referred to anamniote (“amphibian”), synapsid reptile (“pelycosaur”), araeoscelid, and captorhinomorph trackmakers. The assemblage is clearly dominated by footprints of anamniotes, “pelycosaurs”, and araeoscelids, whereas captorhinomorph tracks represent < 1% of the available material. Large samples provide a wide range of extramorphological variation of the recurrent footprint types, and evaluation of this variation makes possible a definitive ichnotaxonomy. After two decades of extensive collection at more than 40 localities of variable sedimentary facies, future discoveries of abundant tracks of other tetrapod groups seem to be rather unlikely. Thus, the PTNM yields a unique impression of the general taxonomic composition of late Early Permian tetrapod faunas in low-latitude nearshore coastal plain areas and an invaluable data base for paleoecological inferences.