Tapochoerus, a Uintan Dichobunid Artiodactyl from the Sespe Formation of California

Abstract

Hyopsodus egressus Stock, 1934b, from the Uintan (late Eocene) Tapo Ranch faunule, Sespe formation, Southern California, is not a hyopsodont condylarth. The species is here made the type of Tapochoerus, new genus, a dichobunid artiodactyl. A hypocone and non-hypertrophied metaconule on the upper molars and the lack of a metaconid on P₃, combined with presence of a metaconid on P₄, a paraconid on M₁ and M₂, and the nature of the lower molar hypoconulids, indicate affinity with the most primitive members of the dichobunid subfamily Homacodontinae. The recently proposed dichobunid subfamily Antiacodontinae Gazin, 1958, is rejected and its members returned to the Homacodontinae. Among the fossil mammal remains collected by the California Institute of Technology at Locality 180 on the Tapo Ranch in Simi Valley, Southern California, are half a dozen jaws and more than a dozen teeth of a Uintan (late Eocene) mammal which Chester Stock ( 1934b ) named as a new species of the condylarth genus Hyopsodus, H. egressus. In his paper Stock did not make comparisons with remains of any dichobunid artiodactyl. Recent clarifications of dichobunid interrelationships by C. L. Gazin (1952, 1955, 1958) have made it increasingly apparent to the author that Hyopsodus egressus Stock, 1934b, is not a hyopsodont condylarth but rather is a dichobunid artiodactyl related to Microsus, Hexacodus, Antiacodon, and Auxontodon. "H." egressus possesses a combination of characters which indicates that it represents a Uintan member of a primitive homacodont dichobunid lineage that may have been isolated in California west of Mohavia throughout most of the Eocene. By Uintan time the lineage had become generically distinct from its closest dichobunid allies. I wish to thank Drs. D. E. Savage and C. L. Gazin and Mr. W. A. Clemens for reading the manuscript and for criticism. Dr. Theodore Downs and Mr. William Otto were also of considerable assistance. The illustrations were drawn by Howard Hamman (Plate 37, fig. a, c) and Owen J. Poe (Plate 37, fig. b.).