Two New Oligocene Desmostylians and a Discussion of Tethytherian Systematics

Abstract

A new genus, comprising two new species of desmostylians, is described from marine Oligocene deposits of the Pacific Northwest. <i>Behemotops proteus</i>, new genus, new species, is based on an immature mandibular ramus and apparently associated skeletal fragments from the middle or (more likely) upper Oligocene lower part of the Pysht Formation of Clallam County, Washington. A related new species, <i>Behemotops emlongi</i>, is founded on a mandibular ramus of an old individual and a mandibular fragment with canine tusk from the uppermost Oligocene (early Arikareean equivalent) Yaquina Formation of Lincoln County, Oregon. The two new species are the most primitive known desmostylians and compare favorably with the primitive Eocene proboscideans <i>Anthracobune</i> and <i>Moeritherium</i>, and to the still more primitive tethythere <i>Minchenella</i> from the Paleocene of China. For many years the Desmostylia were widely regarded as members of the mammalian order Sirenia before being accepted as a taxon coordinate with the Sirenia and Proboscidea (Reinhart, 1953). On the basis of cladistic analysis we go a step further and regard the Desmostylia as more closely related to Proboscidea than to Sirenia because the Desmostylia and Proboscidea are interpreted herein to share a more recent common ancestor than either order does with the Sirenia. This analysis also suggests that the common ancestor of the Proboscidea and Desmostylia (but not the Sirenia) had suppressed P5 and the original last molar. These characters may be convergent with some other mammals. The Superorder Tokotheria McKenna, 1975, was originally thought to be characterized by loss of both P5 and M3. However, because early sirenians do not show these losses, they may have occurred independently in the common ancestor of proboscideans and desmostylians and in various other tokotheres. The late Paleocene genus <i>Minchenella</i> Zhang, 1980, from China, is a suitable candidate to be the common ancestor of both the Desmostylia and the Proboscidea. It possesses a small entoconid II on M3. The Eocene genus <i>Lammidhania</i> Gingerich, 1977, from Pakistan, and the late Paleocene and/or early Eocene Chinese and Mongolian phenacolophids had not acquired an entoconid II on M3 but are otherwise similar to <i>Minchenella</i> and the anthracobunids. The Asiatic occurrence of phenacolophids, <i>Lammidhania, Minchenella</i>, and anthracobunids suggests an Asian origin for the Proboscidea and is in accord with the exclusively Pacific distribution of the Desmostylia. We believe that desmostylians were amphibious herbivores that fed on marine algae and angiosperms, and that at least the earlier taxa depended to a large extent on plants exposed in the intertidal zone.