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Analysis of horse (Equus) metapodials from the late Pleistocene of the lower Nueces valley, south Texas.

Article Excerpt
Abstract.--Ninety-eight relatively complete metapodials (29 metacarpals and 69 metatarsals) of Equus were recovered from late Pleistocene terrace and valley fill deposits along the Nueces River in western Nueces and San Patricio counties, Texas. Sixteen measurements were taken on each metapodial. Three species of Equus were determined to be present using discriminant functions and bivariate and multivariate plots of the data. Equus cf. conversidens, the most abundant species, is a small- to average-sized horse with normal length metapodials. It is similar to members of the E. alaskae group. The second species, represented by 24 metapodials, is assigned to E. cf. scotti. These are larger horses with robust limbs that resemble members of the E. scotti and E. laurentius groups. The third, represented by six specimens, is a stilt-legged horse of the E. francisci group.

The Wright Material Inc., sand and gravel pits along the Nueces River in western Nueces and San Patricia counties, Texas have produced a diverse assemblage of late Pleistocene fossils. Twenty-six species of mammals have been identified from here (Baskin 2000). Equids are among the most common fossils recovered. Living Equus includes horses, asses and zebras. Identification of fossil Equus to species from isolated teeth and bones is difficult at best (Winans 1989; Dalquest & Schultz 1992). Additionally, although most of the approximately 60 North American species that have been named are junior synonyms or invalid, the taxonomy of Equus itself is far from agreed on. Dalquest & Schultz (1992) identified seven or eight species of Equus from the Pleistocene (Irvingtonian and Rancholabrean) of northwestern Texas alone. Azzaroli (1998) recognized up to ten Pleistocene North American species. Winans (1989) recognized five species groups of North American Equus, of which no more than four groups were extant at a give n time. Winans (1989) considered the possibility that each group represented a single species and that therefore only four species of Equus were present in North America during the Pleistocene.

The purpose of this paper is to determine how many species of Equus were present in the late Pleistocene Nueces River Valley deposits of south Texas and identify them. Voucher specimens are deposited with the holdings of the Texas Memorial Museum (TMM) of the University of Texas, Austin.

GEOLOGIC SETTING

Four alluvial terrace units and three younger valley fill units are recognized from late Pleistocene and Holocene sediments in the lower Nueces River Valley, Nueces and San Patricio counties, west of Corpus Christi, Texas, between Odem and Mathis, where the Nueces River is entrenched in the late Pleistocene Beaumont Formation (Corish & Baskin 1995). The valley fill units are included in the Cayamon Creek Alloformation. Most of the metapodials described in this paper come from channel fill and point bar sands and gravels of the Cayamon Creek allomember 1 at the Wright Materials, Inc. quarries (TMM localities 43059 and 43064), approximately 4 km north of Bluntzer, Nueces County, Texas. A log buried in this unit has been carbon dated at 13,230 + 110 YBP (Baskin 1991). Seven metatarsals were recovered across the river in the Angelita Terrace, San Patricio County (TMM locality 18594). These late Quaternary terraces and valley fill deposits have produced a mixed assemblage of early Pliocene and Pleistocene fossil v ertebrates. The Pliocene horses are reworked from older updip deposits, presumably of the upper Goliad Formation (Baskin 1991).

Whether the Pleistocene vertebrates are all contemporaneous with the latest Pleistocene alluvium or are to...