Pachyaena (literally, "thick hyena") was a paraphyletic or monophyletic genus of heavily built, relatively short-legged mesonychids. Mesonychids were part of the now extinct order of hoofed mammals known as Mesonychia, a group mammalian predators that evolved before modern ungulates or carnivorans. Despite this, mesonychians are found to have combined characteristics of both carnivorans and ungulates. The genus first appeared in East Asia during the Gashatan ALMA (Asian Land Mammal Age) of the Late Paleocene. Pachyaena would disperse into North America via the Bering land bridge. From North America, it would disperse into Northwestern Europe via the North Atlantic land bridge. However, Pachyaena presence in Europe was notably short, lasting only 500,000 years. They would go extinct following a faunal turnover event and were later replaced by Hyaenodictis. North American Pachyaena, would go extinct around 52.8 Ma. The last population of Pachyaena persisted in the Lushi Formation, which dates to the Irdinmanhan faunal stage.
Taxonomy
The oldest species within the genus was P. nemegetica is known from Late Paleocene strata of Mongolia. However, the genus' greatest diversity is seen in North America, with P. ossifraga (P. intermedia is considered synonymous with this species), P. gigantea, and P. gracilis all being known from Early Eocene strata of Wyoming, with P. ossifraga being known from the earliest Wasatchian. A 2007 paper by Geisler and colleagues suggested that Pachyaena may have been paraphyletic genus, with P. ossifraga being closer to Synoplotherium, Harpagolestes and Mesonyx than to P. gigantea. Similarly, Solé et al. (2018) also recovered Pachyaena as a paraphyletic. However, their analysis also found P. ossifraga being closely related to Dissacus serior. Both species were recovered within the Mesonyx clade, a derived clade of mesonychids that were known almost exclusively in the Eocene.
However, in a more recent study, Solé et al. recovered P. gigantea as a sister taxon to P. ossifraga recovering the genus as monophyletic.
Mesonychians recovered by Spaulding et al. (2009),which recovers the order outside of ungulates.
Description
Size Pachyaena was one of the largest known mammals from the Early Eocene. P. gracilis was roughly the size of a wolf, with a mean body mass of , making it smaller than a few Wasatchian mammals. The larger species within the genus were about the size of moderately sized bears. Zhou et al. estimated that P. ossifraga could’ve weighed anywhere between , with mean body mass of . They found little size variation between individuals suggesting that P. ossifraga had no sexual dimorphism based on size. However, O'Leary and Rose found even larger sizes for P. ossifraga. With the average body mass of , although individuals could’ve also reached . The largest species was P. gigantea, with a mean mass of , from the range of , making it the larger than all other Wasatchian mammal with the exceptions of panodonts and taeniodonts.J. Sargis, Eric (2009) Mammalian Evolutionary Morphology: A Tribute to Frederick S. Szalay p.130
Postcranial anatomy The scapula of Pachyaena has been recovered, however even the most complete scapula was fragmented and incomplete. The glenoid fossa was symmetrically pyriform within the genus. Within the larger species, it was three-fourths as wide as it is long. In P. gracilis, it was found to have been somewhat narrow compared to the larger species. Compared to Canis, the suprascapular notch was relatively shallow.
The humerus exhibited many characteristics of cursoriality. Despite being relatively robust, the humeral shaft and articular surfaces are modified to reduce most muscular prominences, which limited movement to the sagittal plane. The greater tuberosity is prominent and projects above of the head, although less so in P. gracilis than in larger species, which is a characteristic seen in ungulates and non-ungulate cursorial carnivorous mammals.
Paleobiology
Locomotion Analysis of three wolf to bear-sized species from the early Eocene of Wyoming (Willwood Formation) indicates they all had many adaptations for running, including paraxonic compressed feet with a vestigial first digit, lower sections of the limbs elongated compared with the upper sections, and limb joints with movement mostly restricted to a sagittal plane (back-and-forth movement). All these characteristics are common to both ungulates that run to escape predators and carnivores that run to pursue prey, though they probably evolved independently in mesonychids. Some adaptations are more typical of the grade of cursorial carnivores; others are more specialized, as in ungulates. Pachyaena was likely built for endurance rather than speed; the overall body shape of the genus would have resembled a modern tapir. However, compared to cursorial carnivorans such as spotted hyenas, Pachyaena was relatively short-limbed suggesting it couldn’t have attained the speeds seen in hyenas. Pachyaena was found to have plantigrade or secondarily subdigitgrade locomotion.
Predatory behavior The dental microwear patterns of Pachyaena show that it processed large, irregular objects. They resemble those of the extant genus Crocuta. This suggests that like Crocuta, Pachyaena had an osteophagous diet. Zhou et al. (1992) suggested Pachyaena probably wasn’t an hypercarnivore due to the lack of carnassials for eating fresh meat and it’s not particularly fast dilocomotory locomotion. On the other hand, Szalay & Gould (1966) analyzed the tooth morphology of mesonychids and recovered P. gracilis as an hypercarnivore while P. gigantea was recovered a carnivorous omnivore. However, later studies have recovered all species of Pachyaena as carnivorous omnivores.
Whether or not Pachyaena hunted its prey has also been discussed by experts, Szalay & Gould (1966) suggested Pachyaena still hunted prey as their teeth still had adaptations for piercing and slicing through flesh. Zhou et al. (1992) noted Pachyaena lacked true carnassials and claws, arguing that Pachyaena wasn't able to hunt live prey or consume fresh carrion. They found it unlikely that fish, turtles, mollusks, or other animals to have been prey due to its cursorial locomotion. Its large size also suggests its cursorial build wasn't for protection against larger predators, as its large size alone would've ensured safety. Instead, they suggested it, as well as other large mesonychids, to have been specialized on eating carrion and rarely, if ever, ate fresh meat.
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