Mesonychia

Mesonychia Fossil range: Early Paleocene–Early Oligocene PreЄ Є O S D C P T J K Pg N
Andrewsarchus mongoliensis
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). A few experts unite Mesonychia with the whales to form the clade "Cete." They first appeared in the Early Paleocene and 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. Mesonychians 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, mesonychians 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, but its daughter genus, Ankalagon, from the early to middle Paleocene of New Mexico was far larger, growing to the size of a bear. Species of the later genus, Pachyaena, entered North America by the earliest Eocene, where they evolved into huge species surpassing even Ankalagon in size. Mesonychians in North America were by far the largest predatory mammals during the early Paleocene to middle Eocene.

Characteristics

The mesonychians bore a strong, albeit superficial resemblance to wolves. Early mesonychians had five digits on their feet, which probably rested flat on the ground during walking (plantigrade locomotion), but later mesonychians had four digits and ended in tiny hoofs on all of their toes and were increasingly well adapted to running. Like running members of the even-toed ungulates, mesonychians (Pachyaena, for example) walked on its digits (digitigrade locomotion). 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

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. Nearly all mesonychians 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 well-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

Mesonychians 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 mesonychians were the direct ancestor of Cetacea, but the discovery of well preserved hind limbs of archaic cetaceans as well as more recent phylogenetic analyses now indicates that cetaceans are more closely related to hippopotamids and other artiodactyls than they are to mesonychians, and this result is consistent with many molecular studies. Most paleontologists now doubt the idea that whales are descended from mesonychians, and instead suggest that whales are either descended from or share a common ancestor with the anthracotheres, the semi-aquatic ancestors of hippos. Other paleontologists remain adamant that mesonychians are the ancestors of whales, and regard whales, hippos and anthracotheres as their living descendants. Currently, it is believed that the mesonychians are descended from the Condylarths (the first hoofed animals) and are part of the cohort or superorder Laurasiatheria.

While other ancestries of the cetaceans have also been proposed (e.g. from artiodactyls), it has been argued that the transition from mesonychians to cetaceans is easy to follow from the fossil evidence. Mesonychians were often shore dwelling animals that hunted both on land and in the shallows, and so it was not hard to imagine a shore dwelling creature becoming more specialized and eventually returning to the ocean. Intermediate cetacean forms such as Ambulocetus and Pakicetus especially closely resemble Mesonychians with their fully functioning legs and similar tooth morphology. Pakicetus has a similar body design, but a head more closely resembling archaic cetaceans. Ambulocetus is similar in design to Pakicetus, with the addition of flippered feet, and most likely moved better in the water than on land like a modern otter or seal. Beyond Ambulocetus, it is easy to trace the ancestry directly to modern cetaceans: Sinonyx (land-dwelling) -> Pakicetus (swims occasionally) -> Ambulocetus natans (swims predominantly) -> Rodhocetus (paddling reduced hind legs) -> Basilosaurus (vestigial hind limbs) -> Dorudon.

Cryptozoology

Mesonychians have been extinct since the early Oligocene, but there are occasional, unconfirmed sightings of cat or dog-like predators with hooves. One such example is the Beast of Gévaudan, where some witnesses described it as a huge wolf having hooves rather than paws.:

See also

• Evolution of cetaceans
• Cetartiodactyla
• Archaeoceti
• Sinonyx
• Mesonyx
• Andrewsarchus mongoliensis

External links

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