New Mexico Geological Society Annual Spring Meeting — Abstracts

The middle Miocene (Late Barstovian) Cerro Conejo Member, Zia Formation on the Ceja del Rio Puerco yielded a small vertebrate fauna based on body fossils that includes the rhino Peraceras, the camels, Michenia sp., Protolabis sp., cf. Procamelus sp., the antilocaprid cf. Ramoceras sp., the mustellid Martes sp., the small canid Leptocyon vafer, an equid, the castorid rodent Eucastor sp., and the gomphotheriid proboscidean Gomphotherium productum. Several localities (e.g., L-2930, 3446,4635) have also yielded a diverse ichnofauna that includes footprints made by small and medium sized camels, a possible rhino, a horse, a large feloid, a large canid, a proboscidean, and a bird. Trackways are typically preserved as positives in an extensive, well-indurated, coarse-grained, cross-bedded sandstone.

The most numerous tracks consist of symmetrical didactyl prints with wedge-shaped hoof impressions. These can be assigned to the ichnogenus Gambapes and are attributed to small (track length = ~4 cm; locality L-2930) medium-sized (length = ~10 cm; localities L-2930 and 4635) camels.Several trackways of the larger prints at locality L-2930 show the manus prints are larger than the pes. One trackway at L-2930 demonstrates that a camel "slipped" on the substrate. The larger Gambapes are consistent with a Protolabis-size camel and the smaller are consistent with a Michenia-size camel. A single track at L-2930 is large (length = ~15 cm), tridactyl, and symmetrical with a longer middle digit. It probably represents a rhino. Peraceras is the inferred trackmaker. At least one impression at locality L-4635 is monodactyl (length = ~6 cm), chevron-or crescentic-shaped with a rounded anterior and concave posterior margin. This is similar to the ichnogenus Hippipeda and is attributed to an equid. Two trackways at L-2930 consist of digitigrade footprints with four digits on both the manus and pes. The digital pads are subequal in size and lack claw impressions. The pes prints partly overlap the larger prints of the manus. These tracks are referred to the ichnogenus Bestiopeda. It can probably be attributed to a large feloid. There are no body fossils of feloids represented in these strata, but the most likely trackmaker is Pseudaelurus. A single track at locality L-4635 is digitigrade with four digits, subequal digital pads and distinct claw impressions (length = ~8 cm) and is referred to ichnogenus Chelipus. It probably represents a borophagine dog. A single large, deep, circular, depression (about 30 cm in diameter) at locality L-4635 is referred to Proboscipeda and probably represents a proboscidean. Several shallow circular marks at localities L-2930 and 3446 may also be referable to this ichnogenus. Gomphotherium productum is the inferred trackmaker. Numerous tracks and trackways at locality L-2930 are tridactyl with a longer middle digit (length = ~2 cm). These are attributed to a medium-sized wading bird. No body fossils of birds have been recovered from the Benevidez Ranch local fauna.

The ichnofauna of the Benevidez Ranch local fauna is similar to the fauna represented by body fossils. One striking difference, however, is the large proportion of large carnivorans represented by tracks but not body fossils and the disproportionate number of small mammals represented by body fossils but not tracks. This is likely the result of taphonomic and collecting biases.