New Mexico Geological Society Annual Spring Meeting — Abstracts

A new fossil specimen from New Mexico Museum of Natural History locality 5585 in the southwestern Jemez Mountains represents the second record of fossil bird tracks from New Mexico. The strata that yielded the specimen were previously mapped as Zia Formation, but recent work by Shari Kelley indicates that the strata are more likely equivalent to the Abiquiu Formation. The Oligocene to Miocene sedimentary rocks in this area can be divided into lower volcaniclastic member and an upper fluvial sandstone member. The tracks are from low in the fluvial unit. The tracks are from low in the fluvial unit. The fluvial unit has an ⁴⁰Ar/³⁹Ar age of 20.61±0.07 Ma on an ash bed about 6.5 km NE of NMMNH locality 5585. This is a lower Miocene (close to the boundary of the Arikareean and Hemingfordian landmammal “ages”) age.

The new specimen consists of a nearly rectangular slab with dimensions of approximately 26 x 12 x 2.5 cm. There at least 16 bird tracks on one surface which all appear to represent one ichnotaxon. The impression of digit II is 2.25-2.5 cm long and is straight or incurved. The digit impressions of II and IV are straight and about 1.5 and 2 cm in length respectively. A distinct hallux impression is located immediately posterior to digit impression II and is 04- 0.6 cm long. There is no preservation of webbing. In at least two instances there are double imprints of digit impressions suggesting that the bird stayed in one location for a period and shifted its position. There are no clear trackways or indication of a preferred orientation to the tracks. These factors suggest terrestrial feeding activity.

The Zia Sandstone has produced an Arikareean body fossil fauna in New Mexico, but it lacks birds. Tetrapod tracks are known from the Eocene, Oligocene, Miocene and Pliocene strata of New Mexico. With the exception of Eocene tracks from the Baca Formation of west-central New Mexico, and Pliocene tracks from the Ogalalla Formation of east-central New Mexico, all Cenozoic tetrapod tracks from New Mexico are in the Rio Grande rift or associated basins (e. g., Tularosa basin). With one exception these tracks all pertain to mammals (camels, artiodactyls, proboscideans, cats) and most of them are of large, terrestrial ungulates. The only other bird tracks in New Mexico are from the Middle Miocene (Barstovian) Cerro Conejo Member of the Zia Formation in Sandoval County.