New Mexico Geological Society Annual Spring Meeting — Abstracts

Paleontologists have long taken advantage of the harvester ant’s habit of armoring their nest mounds (ant-hills) with small pebbles and other objects gathered from the surrounding landscape. Ants sometimes incorporate fossils consisting of isolated fossilized teeth of small mammals and other vertebrates that have weathered from nearby exposures of bedrock. However, ant-hill vertebrate assemblages have not been previously reported from the Nacimiento Formation, which is renowned for its early Paleocene mammal faunas. During the 2009 and 2010, we collected approximately 100 kg of sediment from ant-hills developed near exposures of the Nacimiento Formation on the East Flank of Torreon Wash (New Mexico Museum of Natural History [NMMNH] localities 7844, 7845, and 8279). These localities are from the Pantolambda bathmodon – Mixodectes pungens zone, a middle Torrejonian (To2) horizon that is underrepresented by microvertebrate fossils. Ant-hill samples were wet-screened and picked under a binocular microscope. Over 56 identifiable mammal specimens, all consisting of isolated teeth or tooth fragments, representing more than 14 taxa were recovered. Most specimens are of small, well-known mammals including the mioclaenid “condylarth” Promioclaenus acolytus and the mixodectid euarchontan Mixodectes malaris. However, several specimens are of rare and poorly known taxa (e.g., the stemprimate Anasazia) or possibly represent new taxa.

We compared the ant-hill assemblages to those from a microvertebrate site (NMMNH locality L-7583) from a nearby channel-hosted microfossil bonebed from approximately the same stratigraphic horizon in order to assess potential collecting biases. Ant-hill teeth occupied a narrow size range of about 2-5 mm in maximum diameter, which is near the smallest size of specimens typically collected through surface collecting techniques, but significantly larger than the smallest specimens collected from typical microfossil bonebeds (< 1 mm), resulting in significant differences in composition. This preliminary work indicates that ant-hills can provide significant fossils of small, underrepresented Paleocene taxa of the Nacimiento Formation.