The first postcranial oreodont (mammalia, Artiodactyla, Merycoidodontidae) skeleton from Mexico: Description and paleobiological significance

Abstract

In this study, the first postcranial skeleton of an oreodont (Merycoidodontidae, Artiodactyla) found in Mexico is described and discussed. This family was very successful, quasi endemic of North America, and lived from the Late Duchesnean North American Land Mammal Age, acronymed NALMA (Late Eocene) to the Late Hemphillian NALMA (early Late Miocene), roughly between 38 Ma to 5.2 Ma. Oreodonts were diverse (~27 genera, 69 species), and fairly abundant. However their record in Mexico is modest: two genera in northeastern Chihuahua (Rancho Gaitán local fauna, Late Duchesnean), one genus in Aguascalientes (Zoyatal local fauna, early Hemingfordian), one genus in northwestern Oaxaca (Yolomécatl local fauna, latest Uintan-earliest Duchesnean) and one genus in central Oaxaca (Suchilquitongo local fauna, latest Arikareean-Early Hemingfordian). Therefore, the finding in Tehuitzingo, southern Puebla of an articulated postcranial skeleton referred to the Merycoidodontidae, described below, is quite important and significant.The fossil material was recovered from the Tehuitzingo Formation, which largely fills the namesake basin; such geologic unit is a thick lacustrine-fluvial, fine-grained, indurated sequence that includes strata of silicified limestone and siltstone/sandstone interbedded by tuff/ignimbrite sheets, one of them yielded a K-A date of 29.9 ± 0.8 Ma (Early Oligocene), confirmed by the 29.96 Ma age of the overlying Puerto El Gato Formation. The fossil locality lies near Atopoltitlán, and yielded a medium-sized mammalian, articulated, postcranial skeleton. The recovery, preparation and study of the material were performed using standard paleontological methods and techniques. The fossil exhibits among others, the following features: Antero-posteriorly wide ribs, astragalus with double trochlea, ecto- and mesocuneiform fused, phalanx II smaller than the others, and hoofed ungual phalanges. Such characters set it in the artiodactyl Family Merycododontidae, making it the first of its kind in Mexico. The size of the postcranial skeleton corresponds to that of the Whitneyan-Late Arikareean Eporeodon; however, the lack of skull and jaws prevents a positive assignment to this or other merycoidodont genus.Finally, the Tehuitzingo species was tropical, probably cursorial, (?herding), and lived in an open country ecologic setting, with ready access to water supply. Unusual taphonomic conditions led to its preservation as an articulated, headless carcass.