New Mexico Geological Society Annual Spring Meeting — Abstracts

Although the paleontology of the Upper Cenozoic Santa Fe Group in New Mexico has been collected and studied since the 1870s, little is known of its ichnofossils. A newly discovered vertebrate tracksite in the Pliocene Santa Ana Mesa Member of the Ceja Formation at Rio Rancho, Sandoval County, is the most extensive ichnoassemblage known from the Santa Fe Group. This assemblage is in a large (at least 500 m²), bedding plane surface of a 0.1-0.2-m-thick bed of wavy bedded sandstone in which wave crests are 4-6 cm high and separated by ~ 40 cm; crest shapes indicate that paleoflow was to N30^oE. The surface is also marked by some east-west oriented swales that were water filled during track-making. We cleared ~ 14 m² of this surface to reveal a low diversity invertebrate ichnoassemblage, salamander trackways and over 170 bird footprints that represent at least 18 trackways. Cochlichnus, the sinusoidal, bedding-plane parallel grazing trace of a small arthropod, dominates the invertebrate ichnofossils, which also include a few larger, horizontal and tubular grazing traces assignable to Palaeophycus and Scoyenia. The salamander tracks form at least one trackway of small (15-20 mm long), tridactyl or tetradactyl footprints of a quadruped with long, scratch-like digits. These closely resemble the “Gracilichnium” extramorphological variant of the amphibian footprint ichnogenus Batrachichnus. The bird footprints are mostly tridactyl, but tetradactyl where well preserved. They show three long and pointed, anteriorly directed digits II-IV, and well preserved tracks also have a short, and spur-like digit I impression directed backwards. In better preserved footprints, a sole imprint connects the bases of the forward-directed digits. These bird footprints are assigned to the ichnogenus Gruipeda, most similar to Gruipeda calcarifera Sarjeant & Langston. These are the footprints of anisodactyl shorebirds. On the uneven trackway surface, bird footprints in the swales are more deeply sunk into the sediment than those outside of the swales. Most unusual is a salamander trackway that emerges from a swale, turns to the northeast and walks to disappear among bird footprints, possibly because one of the birds preyed upon the salamander. The sediments and ichnoassemblage at the Rio Rancho tracksite indicate a large, lower energy sandflat, either along a lake or channel margin associated with the large, fluvial, ancestral Rio Grande system. Similar habitats, though less active depositionally, are present along some reaches of the Rio Grande today.