Mesonychidae

Mesonychidae Temporal range: Early Paleocene — Late Eocene, Suspected Late Cretaceous origin
Harpagolestes immanis skull
Model reconstruction of Mesonyx
Scientific classification
Kingdom:	Animalia
Phylum:	Chordata
Class:	Mammalia
Infraclass:	Placentalia
Order:	†Mesonychia
Family:	†Mesonychidae Cope 1880
Type genus
†Mesonyx Cope, 1872
Genera
List †Ankalagon? †Dissacus †Guilestes †Harpagolestes †Hessolestes †Honanodon †Hukoutherium †Hyaenodictis †Jiangxia †Lohoodon †Mesonyx †Metahapalodectes †Mongolestes †Mongolonyx †Pachyaena †Sinonyx †Synoplotherium †Yantanglestes

Mesonychidae (meaning "middle claws") is an extinct family of small to large-sized omnivorous-carnivorous mammals. They were found in North America and Eurasia during the Early Paleocene to the Late Eocene, and were the earliest group of large carnivorous mammals. Once considered a sister-taxon to artiodactyls, recent evidence now suggests no close relation to any living mammal. Mesonychid taxonomy has long been disputed and they have captured popular imagination as "wolves on hooves", animals that combine features of both ungulates and carnivores.

Taxonomy

Mesonychidae was named by Cope (1880), with its type genus, Mesonyx, being assigned to the order Creodonta by Cope (1880). However, they were reassigned to Carnivora by Peterson (1919). Currently, experts consider mesonychids to be part of mesonychians.

Classification

Because skulls and teeth of mesonychids have similar features to early whales, and the family was long thought to be the ancestors of cetaceans. However, fossil discoveries have overturned this idea; the consensus is that whales are highly derived artiodactyls. Some researchers now consider the family a sister group either to whales or to artiodactyls, close relatives rather than direct ancestors. Other studies define Mesonychia as basal to all ungulates, occupying a position between Perissodactyla and Ferae. In this case, the resemblances to early whales would be due to convergent evolution among ungulate-like herbivores that developed adaptations related to hunting or eating meat.

Description

The mesonychids were an unusual group of condylarths with a specialized dentition featuring tri-cuspid upper molars and high-crowned lower molars with shearing surfaces. They had large heads with relatively long necks. Over time, the family evolved foot and leg adaptations for faster running, and jaw adaptations for greater bite force. Like the Paleocene family Arctocyonidae, mesonychids were once viewed as primitive carnivorans, and the diet of most genera probably included meat or fish. Various genera and species coexisted in some locations, as hunters and omnivores or scavengers. In contrast to arctocyonids, the mesonychids had only four digits furnished with hooves supported by narrow fissured end phalanges.

Evolutionary history

Restoration of Mesonyx

Mesonychids probably originated in Asia, where the most primitive mesonychid, Yantanglestes, is known from the early Paleocene. They were also most diverse in Asia where they occur in all major Paleocene faunas. Since other carnivores such as the creodonts and Carnivora were either rare or absent in these animal communities, mesonychids most likely dominated the large predator niche in the Paleocene of Asia. Throughout the Paleocene and Eocene, several genera, including Dissacus, Pachyaena and Mesonyx would radiate out from their ancestral home in Asia and disperse into North America via the Bering land bridge. Mesonychids fared very poorly at the close of the Eocene epoch, with only one genus, Mongolestes, surviving into the Early Oligocene epoch, going extinct 33 million years ago. However, recent analysis found the supposed Early Oligocene biochronogical units have been revised to the Late Eocene age, suggesting the family went extinct just before the end of the Eocene.

References

Cited sources

• Agusti, Jordi; Anton, Mauricio (2002). Mammoths, Sabertooths, and Hominids: 65 Million Years of Mammalian Evolution in Europe. Columbia University Press. ISBN 0-231-11640-3.
• Cope, E. D. (1880). "On the genera of the Creodonta". Proceedings of the American Philosophical Society. 19: 76–82. OCLC 45016526. Retrieved 11 August 2013.
• Jin, Xun (2005). "Mesonychids from Lushi Basin, Henan Province, China" (PDF). Vertebrata PalAsiatica (in Chinese and English). 43 (2): 151–164. Retrieved 11 August 2013.