Mesonychia

Mesonychids, as well as mesonychians as a whole, first appeared in fossil record in the Early Paleocene of East Asia with Dissacus rotundus, Yantanglestes, Hukoutherium, 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. Mesonychids 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 later found 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. The extinction of the order was thought to have been the result of changing environments, not competition with replacement clades such as hyaenodonts and carnivoramorphans.

Taxonomy

Classification

Cope (1975) first classified mesonychids as carnivorans due to the trochelar face of the astragalus is completely grooved. However, in a later paper, Cope reclassified mesonychids as creodonts. In their 1966 paper, Van Valen removed mesonychids from creodonts and placed them within the order Condylarthra. Szalay and Gould (1966) would support this classification within their paper. In 1969, Van Valen suggested mesonychids were a separate order of mammals closely related to Artiodactyla, known as Mesonychia. The order is sometimes referred to by its older name Acreodi.

Ting and Li (1987) and Zhou et al. (1995) suggested Hapalodectidae as a separate family of mesonychians, a classification supported by phylogenetic modeling. Some genera may need revision to clarify the actual number of species or remove ambiguity about genera (such as Dissacus and Ankalagon). In 2023, Solé and colleagues reclassified nearly all European species of Dissacus as the newly revived Hyaenodictis. Additionally, the North American species, "Dissacus" willwoodensis, was also reclassified as Hyaenodictis. Within the same study, they noted the close relation between Ankalagon and Dissacus navajovius, which suggests the former may be a large species of Dissacus.

Relationship with Triisodontidae

When Scott (1894) described Triisodontidae, he noted the family was possibly closely related to mesonychids. Triisodontines were considered to have been a subfamily of mesonychids by Matthew (1907), although they were later moved to Arctocyonidae. However, McKenna and Bell (1997) would reclassify them as a family of mesonychians. Szalay (1969) suggested triisodontines were ancestral to mesonychids due to the difference in their dentition. Phylogenetic analysis by Tabuce et al. (2011) recovered mesonychians as a monophyletic group within Acreodi, with triisodontids being a paraphyletic stem to mesonychians.

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

Relationship with whales

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:

Evolution

Mesonychians were believed to have evolved during the Late Cretaceous period, at least 66.7 Ma, with both families were thought to have originate in the Late Cretaceous based on phylogenetic analysis. The earliest known mesonychians fossils were dated to 63 Ma. In East Asia, mesonychids were known from East Asia during the Early Paleocene and were represented by Dissacus rotundus, Yantanglestes, Hukoutherium, and Dissaccussium. Within North America, Dissacus appeared during the late Torrejonian and was contemporary with the "triisodontid" Triisodon, with Ankalagon appearing during the latest Torrejonian. The presence of mesonychids in Asia and in North America suggests mesonychids initial radiation began in the Early Paleocene. Hapalodectids first appeared in the fossil record during the Middle Paleocene in Asia. Before the end of the Paleocene, mesonychids would disperse into Europe with the appearance of Dissacus europaeus. Hapalodectids would disperse into North America during the Paleocene-Eocene Thermal Maximum, which saw the appearance of Hapalodectes anthracinus.

Extinction

The disappearance of Dissacus was contemporary with the appearance of Pachyaena and Palaeonictis, although competition is unlikely to have been the reason for the extinction of Dissacus europaeus. European Pachyaena would go extinct 55 Ma, being replaced by European Hyaenodictis, following the Intra-European Faunal Turnover. Hapalodectids would go extinct near the end of Early Eocene, roughly 49 Ma. European mesonychids would go extinct 48 Ma, which was contemporary with the Ypresian-Lutetian Mammal Turnover event. The cause of the turnover event likely coincided with the cooling period following the Early Eocene Climatic Optimum. Following the extinction of European mesonychians and oxyaenids, hyaenodonts saw a diversification in species and body sizes. Hapalodectids would go extinct during the Middle Eocene roughly 44 Ma.