Pachyaena

Pachyaena Temporal range: Late Paleocene-Early Eocene (Thanetian to Ypresian), 58.7–55  Ma PreꞒ Ꞓ O S D C P T J K Pg N ↓
Pachyaena gigantea jaws
Scientific classification
Kingdom:	Animalia
Phylum:	Chordata
Class:	Mammalia
Order:	† Mesonychia
Family:	† Mesonychidae
Genus:	† Pachyaena Cope, 1874
Type species
†Pachyaena ossifraga Cope, 1874
Other species
P. gigantea P. intermedia P. nemegetica P. gracilis

Pachyaena (literally, "thick hyena") was a genus of heavily built, relatively short-legged mesonychids. Mesonychids were part of the now extinct order 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 likely originated from Asia and dispersed to Europe, and from there to North America across a land bridge in what is now the North Atlantic ocean. ^[1] Pachyaena would later be replaced by Dissacus in Europe. ^[2]

Taxonomy

Cladogram showing the position of the Mesonychia

The genus' greatest diversity is seen in North America, with P. ossifraga known from Latest Paleocene strata of Wyoming, and P. gigantea, P. intermedia, and P. gracilis known from Early Eocene strata of Wyoming. P. nemegetica is known from Late Paleocene strata of Mongolia. ^[3] However, a 2007 paper suggests that Pachyaena may be paraphyletic, with P. ossifraga being closer to Synoplotherium , Harpagolestes and Mesonyx than to P. gigantea. ^{[4] [5]}

Mesonychians recovered by Spaulding et al. (2009),which recovers the order outside of ungulates. ^[6]

	† 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

Description

Restoration of P. gigantea

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 36 kg (79 lb), making it smaller than a few Wasatchian mammals. The larger species within the genus were about the size of moderately sized bears. ^[3] Zhou et al. estimated that P. ossifraga could’ve weighed anywhere between 60–70 kg (130–150 lb), with mean body mass of 62 kg (137 lb). They found little size variation between individuals suggesting that P. ossifraga had no sexual dimorphism based on size. ^[7] However, O'Leary and Rose found even larger sizes for P. ossifraga. With the average body mass of 78 kg (172 lb), although individuals could’ve also reached 93 kg (205 lb). ^[3] The largest species was P. gigantea, with a mean mass of 191 kg (421 lb), from the range of 129 to 396 kg. ^{[3] [8]}

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. ^[3]

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. ^[3]

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. ^[3] 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. ^[7] Pachyaena was found to have plantigrade or secondarily subdigitgrade locomotion. ^{[3] [9]}

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. ^[10] Despite this, Pachyaena probably wasn’t a derived carnivore due to the lack of carnassials for eating fresh meat and it’s not particularly fast dilocomotory locomotion. ^[7] Experts instead of being a carnivore, Pachyaena was instead an omnivorous predator, with a lean towards carnivory. ^{[11] [9]}

References

1. Eberle, Jaelyn J; McKenna, Malcolm C (June 2002). "Early Eocene Leptictida, Pantolesta, Creodonta, Carnivora, and Mesonychidae (Mammalia) from the Eureka Sound Group, Ellesmere Island, Nunavut". Canadian Journal of Earth Sciences. 39 (6): 899–910. Bibcode:2002CaJES..39..899E. doi:10.1139/e02-001. ISSN   0008-4077.
2. Solé, Floréal; Godinot, Marc; Laurent, Yves; Galoyer, Alain; Smith, Thierry (2018). "The European Mesonychid Mammals: Phylogeny, Ecology, Biogeography, and Biochronology" . Journal of Mammalian Evolution. 25: 339–379. doi:10.1007/s10914-016-9371-8.
3. O'Leary, Maureen A.; Rose, Kenneth D. (1995). "Postcranial Skeleton of the Early Eocene Mesonychid Pachyaena (Mammalia: Mesonychia)". Journal of Vertebrate Paleontology. 15 (2): 401–430. Bibcode:1995JVPal..15..401O. doi:10.1080/02724634.1995.10011238. ISSN   0272-4634. JSTOR   4523639.
4. "Mesonyx and the other mesonychid mesonychians (mesonychians part IV) | ScienceBlogs". scienceblogs.com. Retrieved 2019-05-16.
5. Geisler, Jonathan H.; McKenna, Malcolm C. (2007). "A new species of mesonychian mammal from the lower Eocene of Mongolia and its phylogenetic relationships" (PDF). Acta Palaeontologica Polonica. 52 (1): 189–212.
6. Spaulding, Michelle; O'Leary, Maureen A.; Gatesy, John (2009-09-23). "Relationships of Cetacea (Artiodactyla) Among Mammals: Increased Taxon Sampling Alters Interpretations of Key Fossils and Character Evolution". PLOS ONE. 4 (9) e7062. Bibcode:2009PLoSO...4.7062S. doi: 10.1371/journal.pone.0007062 . ISSN   1932-6203. PMC   2740860 . PMID   19774069.
7. Zhou, X.; Sanders, W. J.; Gingerich, P. D. (1992). "Functional and Behavioral Implications of Vertebral Structure in Pachyaena ossifraga (Mammalia, Mesonychia)" (PDF). Contributions from the Museum of Paleontology the University of Michigan. 28: 289–319.
8. J. Sargis, Eric (2009) Mammalian Evolutionary Morphology: A Tribute to Frederick S. Szalay p.130
9. Solé, Floréal; Fournier, Morgane; Ladevèze, Sandrine; et al. (2023). "New postcranial elements of mesonychid mammals from the Ypresian of France: New hypotheses for the radiation and evolution of the mesonychids in Europe" . Journal of Mammalian Evolution. 30 (2): 371–401. doi:10.1007/s10914-023-09651-x.
10. O'Leary, Maureen A.; Teaford, Mark F. (1992). "Dental microwear and diet of mesonychids". Journal of Vertebrate Paleontology . 12 (3).
11. Szalay, Frederick; Gould, S. J. (1966). "Asiatic Mesonychidae (Mammalia, Condylarthra)". Bulletin of the American Museum of Natural History. 132 (2): 127–174.