The first Paleogene mammal record of Middle America: Simojovelhyus pocitosense (Helohyidae, Artiodactyla)

ABSTRACT

Simojovelhyus pocitosense, gen. et sp. nov., is based on a mandibular fragment with m1-3 collected from an unnamed Late Oligocene formation, in an amber mine near Simojovel, in northwestern Chiapas. The source strata consist of black, very fossiliferous, shaley phyllarenitic sandstone and mudstone that were deposited in an estuarine to swampy lagoonal environment, dated in Chrons C10–C9 (~29.4–26.8 Ma), or early Late Oligocene. Simojovelhyus pocitosense possesses small, rectangular, bunodont molars that increase in size posteriorly. This combination of characters sets it apart from the Oligocene bunodont artiodactyls (entelodontids, tayassuids and leptochoerids), and bring it within the Helohyidae. It differs from recognized genera in having a tiny paraconid, and a unique combination of cristids and cuspulids as follows. Cristids: paracristid, anteromesocristid (neomorphic), and anterolingualcristid (neomorphic) on the trigonid anterior slope, cristid obliqua (short to remnant), and a robust postcristid in the talonid. Cuspulids (small, accessory): mesoconid, hypoconulid (large and blunt in m3), postmesoconulid (neomorphic), and pseudohypoconulid (neomorphic). This record extends the geochronologic range and geographic distribution of the Helohyidae from the Late Eocene of North America and Asia to the early Late Oligocene (Early Arikareean) of Middle America. Simojovelhyus pocitosense is also the southernmost record of Paleogene mammals, not only of Mexico, but of all North America. This record significantly adds to the meager, but important, record of Paleogene vertebrates in the tropics.