Biology:Mesonychia

Mesonychia ("middle claws") is an extinct order of small to large-sized omnivorous to carnivorous hoofed mammals related to ungulates. Originally, it was hypothesized that mesonychians were a group of ungulates, however recent analysis now found them to be outside of the group entirely. Mesonychians first appeared in the Early Paleocene in East Asia with Dissacus rotundus, Yantanglestes, Hukotherium, and Dissaccussium.

Dissacus would later disperse into North America during the Early Paleocene, before dispersing into Northwestern Europe by the end of it. Dissacus was a jackal-sized predator, but taxon of a closely related or identical genus, Ankalagon, from the early to middle Paleocene of New Mexico, were far larger, growing to the size of a bear. A later genus, Pachyaena, entered North America by the earliest Eocene, where it evolved into species that were at least as large. Mesonychians were largest predatory mammals in North America and Europe during the Late Paleocene to Middle Eocene. In Asia, the record of their history suggests they grew gradually larger and more predatory over time, then shifted to scavenging and bone-crushing lifestyle.

During the Middle Eocene, mesonychians saw a decline in diversity across their distribution. Originally, it was suggested mesonychids, such as Mongolestes, survived into the Early Oligocene. This has proven to be questionable as the Ulan Gochu Formation of Mongolia has been revised to the late Middle to earliest Late Eocene. The latest occurrence of the order was within the Ergilin Dzo Formation.

Taxonomy

Cladogram showing the position of the Mesonychia

Mesonychians were long considered to be creodonts, but have now been removed from that order and placed in three families (Mesonychidae, Hapalodectidae, and Triisodontidae), either within their own order, Mesonychia, or within the order Condylarthra as part of the cohort or superorder Laurasiatheria. The order is sometimes referred to by its older name Acreodi.

Sarah et al. (2015) recovered mesonychians as basal ungulates most closely related to the "arctocyonids" Mimotricentes, Deuterogonodon and Chriacus. With "Triisodontidae" possibly being paraphyletic. However, majority of phylogenetic analyses have recovered mesonychians outside of ungulates entirely, with triisodontids being more closely related to ungulates than to mesonychians.

Some genera may need revision to clarify the actual number of species or remove ambiguity about genera (such as Dissacus and Ankalagon).

Relationship with whales

See also: Biology:Evolution of cetaceans

Mesonychians possess unusual triangular molar teeth that are similar to those of Cetacea (whales and dolphins), especially those of the archaeocetes, 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 indicate cetaceans are more closely related to hippopotamids and other artiodactyls than they are to mesonychians, and this result is consistent with many molecular studies. The similarity in dentition and skull may be the result of primitive ungulate structures in related groups independently evolving to meet similar needs as predators; some researchers have suggested that the absence of a first toe and a reduced metatarsal are basal features (synapomorphies) indicating that mesonychians, perissodactyls, and artiodactyls are sister groups.

Most paleontologists now doubt that whales are descended from mesonychians. Some experts have hypothesized mesonychians were basal ungulates, and that cetaceans are descended from advanced ungulates (Artiodactyla), either deriving from, or sharing a common ancestor with, anthracotheres (the semiaquatic ancestors of hippos). However, mesonychians being recovered within ungulates in cladistic analyses only surfaces following the deletion of Andrewsarchus, which has been recovered as a mesonychid within the cladogram. One possible conclusion is that Andrewsarchus has been incorrectly classified. The current uncertainty may, in part, reflect the fragmentary nature of the remains of some crucial fossil taxa, such as Andrewsarchus. Current analysis recovered Andrewsarchus as a basal Cetancodontamorpha being closely related to entelodonts, cetaceans, and hippopotamuses. Spaulding et al. (2009) only recovered mesonychians being closely related to cetaceans, following the deletion of Carnivora, "Creodonta", Lipotyphia, and Raoellidae.

Cladogram recovered by Spaulding et al. (2009), which recovered the order outside of ungulates entirely:

†Hapalodectes       †Sinonyx     †Mesonyx     †Harpagolestes uintensis     †Dissacus zanabazari       †Dissacus praenuntius       †Harpagolestes orientalis     †Pachyaena         Mesonychia       †Protungulatum       Perissodactyla   Artiodactyla   Tylopoda       Suina       Ruminantia       †Entelodontidae         †Anthracotheriidae     Hippopotamidae         †Raoellidae     Cetacea               Ungulates

Characteristics

Mesonychians have often been reconstructed as resembling wolves albeit superficially, but they would have appeared very different in life. With a short lower spine stiffened by revolute joints, they would have run with stiff backs like modern ungulates rather than bounding or loping with flexible spines like modern carnivorans. While later mesonychians evolved a suite of limb adaptations for running similar to those in both wolves and deer, their legs remained comparatively thick. They would have resembled no group of living animals. Early mesonychians probably walked on the flats of their feet (plantigrade), while later ones walked on their toes (digitigrade). These later mesonychians had hooves, one on each toe, with four toes on each foot. The foot was compressed for efficient running with the axis between the third and fourth toes (paraxonic); it would have looked something like a hoofed paw.

Mesonyx model reconstruction at the Natural History Museum, London

Mesonychians varied in size; some species were as small as a fox, although some species approached the size of moderately sized bears. Some members of the group are known only from skulls and jaws, or have fragmentary postcranial remains. But where skeletons are known, they indicate that mesonychians had large heads with strong jaw muscles, relatively long necks, and robust bodies with robust limbs that could run effectively but not rotate the hand or reach out to the side. An unrelated early group of mammalian predators, the creodonts, also had unusually large heads and limbs that traded flexibility for efficiency in running; large head size may be connected to inability to use the feet and claws to help catch and process food, as many modern carnivorans do. Some mesonychians are reconstructed as predatory (comparable to canids), others as scavengers or carnivore-scavengers with bone-crushing adaptations to their teeth (comparable to the large hyenas), and some as omnivorous (comparable to pigs, humans, or black bears). They may not have included hypercarnivores (comparable to felids); their teeth were not as effective at cutting meat as later groups of large mammalian predators. In some localities, multiple species or genera coexisted in different ecological niches. There is evidence to suggest that some genera were sexually dimorphic.

These "wolves on hooves" were probably one of the more important predator groups 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. Mesonychian dentition consisted of molars modified to generate vertical shear, thin blade-like lower molars, and carnassial notches, but no true carnassials. The largest species are considered to have been scavengers.

References

Template:Mesonychia Wikidata ☰ Q132961 entry