Mesonychid

This article is about the prehistoric ungulate. For another, more detailed, article about Mesonychidae, see Mesonychidae.

Mesonychids Temporal range: Early Paleocene–Early Oligocene PreЄ Є O S D C P T J K Pg N
Harpagolestes immanis skull
Scientific classification
Kingdom:	Animalia
Phylum:	Chordata
Class:	Mammalia
Infraclass:	Eutheria
Superorder:	Laurasiatheria
Order:	Mesonychia
Families
Hapalodectidae Mesonychidae Triisodontidae

Mesonychia ("Middle Claws") are an extinct order of medium to large-sized carnivorous mammals that were closely related to artiodactyls (even-toed ungulates) and to cetaceans (dolphins and whales). They first appeared in the Early Paleocene, went into a sharp decline at the end of the Eocene, and died out entirely when the last genus, Mongolestes, became extinct in the Early Oligocene. Mesonychids probably originated in Asia, where the most primitive mesonychid, Yangtanglestes, 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 condylarths were either rare or absent in these animal communities, mesonychids most likely dominated the large predator niche in the Paleocene of Asia. One genus, Dissacus, successfully spread to Europe and North America by the early Paleocene. Dissacus was a jackal-sized carnivore that has been found all over the northern hemisphere,^[1] but its daughter genus, Ankalagon, from the early to middle Paleocene of New Mexico, was far larger, growing to the size of a bear.^[2] Species of the later genus, Pachyaena, entered North America by the earliest Eocene, where they evolved into huge species surpassing even Ankalagon in size. Mesonychids in North America were by far the largest predatory mammals during the early Paleocene to middle Eocene.

Characteristics

The mesonychids bore a strong, albeit superficial, resemblance to wolves. Early mesonychids had five digits on their feet, which probably rested flat on the ground during walking (plantigrade locomotion), but later mesonychids had four digits that ended in tiny hoofs on all of their toes and were increasingly well adapted to running. Like running members of the even-toed ungulates, mesonychids (Pachyaena, for example) walked on its digits (digitigrade locomotion).^[1] These "wolves on hooves" were probably one of the more important predator groups (although they may have been scavengers) in the late Paleocene and Eocene ecosystems of Europe (which was an archipelago at the time), Asia (which was an island continent), and North America. Mesonychid dentition consisted of molars modified to generate vertical shear, thin blade-like lower molars, and carnassial notches, but no true carnassials. The molars were laterally compressed and often blunt and were probably used for shearing meat or crushing bones. Many species are suspected of being fish-eaters, and the largest species are considered to have been scavengers.

Phylogeny and evolutionary relationships

Cladogram showing the position of Mesonychia

Mesonychians were once long considered to be creodonts but have now been removed from that order and placed in three families (Mesonychidae, Hapalodectidae, Triisodontidae), either within their own order, Mesonychia (or in older text, "Acreodi"), or within the order Condylarthra as part of the cohort or superorder Laurasiatheria. Nearly all mesonychids are, on average, larger than most of the Paleocene and Eocene creodonts and miacoid carnivorans. The order is sometimes referred to by its older name "Acreodi." Technically speaking, the term "mesonychid" refers specifically to the members of the family Mesonychidae only, such as the species of the genus Mesonyx. However, as the order is also renamed for Mesonyx, the term "mesonychid" is now used to refer to members of the entire order Mesonychia and the species of other families within it, such as the triisodontid mesonychid, Andrewsarchus, which is, coincidentally, probably the most publicly known member of the entire order. Andrewsarchus is suspected of being one of the largest known terrestrial mammalian predators, due to its metre-long skull, which is substantially larger than that of the largest living terrestrial mammalian predator, the Kodiak bear.

Relationship with whales

Mesonychids possess unusual triangular molar teeth that are similar to those of Cetacea (whales and dolphins), especially those of the archaeocetids, as well as having similar skull anatomies and other morphologic traits. For this reason, scientists had long believed that mesonychids were the direct ancestor of Cetacea, but the discovery of well preserved hind limbs of archaic cetaceans as well as more recent phylogenetic analyses^[3][4][5] now indicates that cetaceans are more closely related to hippopotamids and other artiodactyls than they are to mesonychids, and this result is consistent with many molecular studies.^[6] Most paleontologists now doubt the idea that whales are descended from mesonychids, and instead suggest that whales are either descended from or share a common ancestor with the anthracotheres, the semi-aquatic ancestors of hippos.^[7] However, the close grouping of whales with hippopotami in cladistic analyses only surfaces upon deletion of Andrewsarchus, which has often been included within the mesonychids.^[8][9] One possible conclusion is that Andrewsarchus is not a mesonychid, but rather closely allied with hippopotamids. The current uncertainty may in part reflect the fragmentary nature of the remains of some crucial fossil taxa, such as Andrewsarchus.^[8]

References

1. ^ ^{a b} Jehle, Martin (2006). "Carnivores, creodonts and carnivorous ungulates: Mammals become predators". Paleocene mammals of the world (online). http://www.paleocene-mammals.de/predators.htm. 
2. ^ O'Learya, Maureen A., Spencer G. Lucas, and Thomas E. Williamson (2000). "A New Specimen of Ankalagon (Mammalia, Mesonychia) and Evidence of Sexual Dimorphism in Mesonychians". Journal of Vertebrate Paleontology 20 (2): 387–393. doi:10.1671/0272-4634(2000)020[0387:ANSOAM]2.0.CO;2. 
3. ^ Geisler, J.H. and Uhen, M.D. (2003). "Morphological support for a close relationship between hippos and whales". Journal of Vertebrate Paleontology 23: 991–996. doi:10.1671/32. 
4. ^ Geisler, J.H. and Uhen, M.D. (2005). "Phylogenetic relationships of extinct cetartiodactyls: results of simultaneous analyses of molecular, morphological, and stratigraphic data". Journal of Mammalian Evolution 12 (1 and 2): 145–160. doi:10.1007/s10914-005-4963-8. 
5. ^ Boisserie, J.−R., Lihoreau, F., and Brunet, M. (2005). "The position of Hippopotamidae within Cetartiodactyla". Proceedings of the National Academy of Sciences 102 (5): 1537–1541. doi:10.1073/pnas.0409518102. PMC 547867. PMID 15677331. http://www.pubmedcentral.nih.gov/articlerender.fcgi?tool=pmcentrez&artid=547867. 
6. ^ Gatesy, J., Hayashi, C., Cronin, A., and Arctander, P. (1996). "Evidence from milk casein genes that cetaceans are close relatives of hippopotamid artiodactyls". Molecular Biology and Evolution 13 (7): 954–963. PMID 8752004. 
7. ^ Geisler, J.H., Theodor, J.M., 2009. Hippopotamus and whale phylogeny. Nature 458:E1-E4
8. ^ ^{a b} Thewissen, J.G.M., Cooper, L.N., Clementz, M.T., Bajpai, S., Tiwari, B.N., 2009. Thewissen et al. reply. Nature 458: E4-E5
9. ^ O'Leary, M.A.; Gatesy, J. (2008). "Impact of increased character sampling on the phylogeny of Certiodactyla (Mammalia): combined analysis including fossils". Cladidistics 24: 397–442. doi:10.1111/j.1096-0031.2007.00187.x. 

External links

• Paleocene Mammals of the World: Carnivores, Creodonts and Carnivorous Ungulates
• Mesonychids in Cryptozoology