Mesonychia Temporal range: Early Paleocene — Late Eocene, 63–34 Ma PreꞒ Ꞓ O S D C P T J K Pg N Suspected Late Cretaceous origins[1][2]
Dissacus zanabazari skull (Mesonychidae)
Hapalodectes hetangensis skull (Hapalodectidae)
Scientific classification
Kingdom:	Animalia
Phylum:	Chordata
Class:	Mammalia
Infraclass:	Placentalia
Clade:	Scrotifera
Grandorder:	Ferungulata
Clade:	Pan-Euungulata
Order:	†Mesonychia Van Valen, 1969
Families
†Hapalodectidae †Mesonychidae
Synonyms
Acreodi (Eberle and McKenna, 2002)[3]

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,^[4][5] however recent analysis now found them to be outside of the group entirely. Instead were a basal order of hoofed mammals within Pan-Euungulata.^[6][7][1] Mesonychians first appeared in fossil record in the Early Paleocene of East Asia with Dissacus rotundus, Yantanglestes, Hukotherium, and Dissaccussium.^[8][1] However, it believed that mesonychians evolved during the Late Cretaceous, at least 66.7 Ma.^[1][5]

Dissacus would later disperse into North America during the Early Paleocene,^[9] before dispersing into Northwestern Europe by the end of it.^[10] Dissacus was a jackal-sized predator,^[11][10] 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.^[12] A later genus, Pachyaena, entered North America by the earliest Eocene, where it evolved into species that were at least as large.^[13] Mesonychians were largest predatory mammals in North America and Europe during the Late Paleocene to Middle Eocene.^[14][10] 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.^[15]

During the Middle Eocene, mesonychians saw a decline in diversity across their distribution.^{[16][10][17][1]} Originally, it was suggested mesonychids, such as Mongolestes, survived into the Early Oligocene.^[18][19] This has proven to be questionable as the Ulan Gochu Formation of Mongolia has been revised to the late Middle to earliest Late Eocene.^[20] The latest occurrence of the order was within the Ergilin Dzo Formation.^[21]

Taxonomy

Classification

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.^[22]

Sarah et al. (2015) recovered mesonychians as basal ungulates most closely related to the "arctocyonids" Mimotricentes, Deuterogonodon and Chriacus. With "Triisodontidae" possibly being paraphyletic.^[23] However, majority of phylogenetic analyses have recovered mesonychians outside of ungulates entirely, with "triisodontids" being more closely related to ungulates than to mesonychians.^[6][7][1]

Some genera may need revision to clarify the actual number of species or remove ambiguity about genera (such as Dissacus and Ankalagon).^[24][1][10] In 2023, Solé and colleagues reclassified nearly European species of Dissacus within 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.^[1]

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.^[24] 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.^[31]

Mesonychians varied in size; some species were as small as a fox, although some species approached the size of moderately sized bears.^[31] Some members of the group are known only from skulls and jaws, or have fragmentary postcranial remains.^[31][1] 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.^[32] Although some experts consider Mesonyx to be a hypercarnivore due to the metaconids of the lower molars are completely reduced.^[18]

There is evidence to suggest that some genera were sexually dimorphic.^[33] 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.^[34] Mesonychian dentition consisted of molars modified to generate vertical shear, thin blade-like lower molars, and carnassial notches, but no true carnassials.^[32] The largest species are considered to have been scavengers.^[15]

References