THE IMPORTANCE OF VERTEBRATE TRACE FOSSILS TO AN UNDERSTANDING OF THE FOSSIL RECORD: EXAMPLES FROM THE OUTSTANDING ICHNOFAUNAS OF NEW MEXICO

The first vertebrate trace fossils from New Mexico were reported in 1840. New Mexico has an extensive and important vertebrate footprint record, notably in the Early Permian, Late Triassic and Early Cretaceous. Arguably the Early Permian record is the most extensive and significant known. New Mexico has a very diverse and relatively well studied record of vertebrate bromalites, notably coprolites, including more than a dozen holotypes of named bromalite ichnotaxa. The most abundant coprolite ichnofaunas occur in the Late Pennsylvanian, Early Permian, Late Triassic, Late Cretaceous and Paleogene. Vertebrate burrows are relatively uncommon in New Mexico, as elsewhere, and the majority of described specimens are lungfish burrows from Permian and Triassic redbeds. The only fossilized skin impressions from New Mexico are of Late Cretaceous hadrosaurian dinosaurs. Vertebrate ichnofossils are an important adjunct to body fossils in understanding the vertebrate body-fossil record of New Mexico in several areas: (1) ichnotaxonomy - description of the Early Permian ichnofaunas of the Robledo Mountains resulted in a rationalization of the number of Permian ichnotaxa from over 100 to less than 20 and a validation of the concept of extramorphological variation; (2) ichnofacies - Early Permian tracksites in New Mexico represent an ecological transect from piedmont facies in the north to tidal flat deposits in the south and study of this distribution has led to a new understanding of ichnofacies and ichnocoenoses; (3) distribution - the only information about the composition of the dinosaur fauna of the Early Cretaceous in New Mexico derives from the multiple tracksites; (4) biochronology - tracks are an important element of the biochronology in the Late Triassic (e.g., Eosauropus and Evazoum in the Apachean Redonda and Sloan Canyon formations) and Early Permian (e.g., Amphisauropus and Ichniotherium in the Coyotean Abo Formation); and (5) behavior - New Mexico provides the most important evidence for gregarious behavior in ornithopods from several localities in the Early Cretaceous Pajarito Shale and Mesa Rica Formation and one of the few examples of cannibalism in dinosaurs in the Late Triassic theropod Coelophysis.