A NEW TERRESTRIAL VERTEBRATE PLIO-PLEISTOCENE ASSEMBLAGE FROM CENTRAL WASHINGTON

We present a preliminary description of the megafauna from a previously undescribed Plio-Pleistocene assemblage in Central Washington. The biostratigraphic range of the fauna suggests the site is late Blancan to Irvingtonian in age, filling an important gap between the abundant Pliocene and Rancholabrean material in the eastern portion of the state. The fossil-bearing horizon lies above a reworked rhyolitic tephra layer potentially connected to the earliest ca. 3 Ma explosive eruptive phase of the Goat Rocks volcanic complex located 80 km to the southwest.

Artiodactyls are the most common taxa represented at the site (63% of fossil material). Antilocapra cf. americana is the most common artiodactyl (n=5), represented primarily by distal hind limb material. Odocoileus sp. (n=4) is represented by a combination of antlers and distal hind limb material. Other artiodactyls are represented by single bones including the lunate of a large indeterminate cervid, the jaw of a juvenile Palaeolama sp., and a lower tusk of Tayassuidae.

Equus simplicidens is well represented in the site, comprising 31% of the fossils (n = 6). All material is metatarsal or phalanges. A single fragmentary scapula of Mammut sp. is also present. Material for multiple lagomorphs and rodents has also been uncovered but has yet to be identified.

All fossil material is weathering out of two sedimentological units exposed by a roadcut. The majority of the material comes from a pedogenically altered silt layer we interpret as an overbank deposit on a floodplain. Three articulated toes of Equus simplicidens were found in a gravel deposit with low-angled cross-bedding to the south. The two units interfinger with one another and it is likely they represent different depositional environments during the same time period.

The dominance of phalanges and other distal limb elements suggests some form of taphonomic bias was present. All bones are free from extensive wear or sun damage. We found no indication of bite or gnaw marks on the material. Most elements are complete or nearly so. Each skeletal element for each species is unique. It is therefore possible that a single individual animal was present from each species in the locality. With the exception of Palaeolama, all represented individuals appear to be from adult animals.