Evolution of Paleocene-Eocene Mesonychidae (Mammalia: Mesonychia).

Abstract

Mesonychia is a group of archaic hoofed mammals present in the Paleocene and Eocene of Asia, Europe, and North America. They are generally considered to be cursorial, Hyaena-like, scavenging carrion-feeders. Mesonychids are important as a group because of their distinctive morphological and functional specializations, because of their broad geographic distribution, and because they may be related to the origin of whales. The purpose of this work is to review mesonychian dental, cranial, and postcranial morphology in order to better understand mesonychians as living animals, and to facilitate broader comparison with other contemporary groups of mammals and comparison with early stages of whale evolution. The great temporal range of mesonychians invites consideration of evolutionary changes they exhibit. Mesonychids are placed in their own order, Mesonychia. Mesonychia include two families, Mesonychidae and Andrewsarchidae. Reference of Hapalodectidae (Szalay and Gould, 1966; Ting and Li, 1987), with one genus Hapalodectes, to the order Mesonychia is uncertain. Two Asian genera, Dissacusium and Yantanglestes, and one dubious species, ?Hukoutherium shimemensis, are separately placed in the Mesonychia. Mesonychids were medium- to large-sized mammals. Body weights of 22 mesonychid species are estimated to range from 7 kg to 193 kg, with a median of 36 kg. Study of Pachyaena gracilis and Pachyaena ossifraga suggests that mesonychids were not sexually dimorphic. Comparison of mesonychid skull morphology with that of Hapalodectes supports removal of Hapalodectinae from Mesonychidae. Postcranial study of five mesonychid genera, Ankalagon, Dissacus, Sinonyx, Pachyaena, and Mesonyx, show that mesonychids achieved greater cursorial capability over time through proportional and structural changes in the limbs. These proportional and structural changes increased the functional lengths of limbs, minimized rotational and mediolateral movements of limb joints, and increasingly limited limb movement to a parasagittal plane.