| Chriacus Temporal range:
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|---|---|
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| Life reconstruction | |
Scientific classification 
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| Kingdom: | Animalia |
| Phylum: | Chordata |
| Class: | Mammalia |
| Order: | †Arctocyonia |
| Family: | †Arctocyonidae |
| Genus: | †Chriacus Cope, 1883 |
| Type species | |
| †Lipodectes pelvidens Cope, 1881
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| Species | |
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Chriacus is an extinct genus of placental mammals that lived in what is now North America from the Paleocene and early Eocene epochs. In life, members of the genus may have looked something like a kinkajou or binturong, though they were not closely related to any living mammal. Like many early Cenozoic mammals, its relationships are uncertain, with possible affinities to ungulates or Ferae (carnivorans and their relatives).
Paleobiology
[edit]Chriacus were probably omnivores, eating fruit, eggs, insects and small mammals.[1]
Climbing adaptations
[edit]Chriacus has better-preserved fossils than most other Paleocene mammals.[2][3][4][5] The most complete skeleton belongs to an uncertain species (Chriacus sp.) from the Willwood Formation of Wyoming.[3] In life, it would have measured around 1 metre (3.3 ft) long, including a long, robust tail, which may or may not have been prehensile. Other features include a light build, weighing approximately 5–7 kilograms (11–15 lb), and many adaptations typical of animals that live in trees. These include the ability to walk on the soles of their five-toed feet, as well as claws which are long, curved, and compressed. The powerfully-built limbs have flexible joints, especially the ankles, an adaptation that allows an animal to turn its hind feet behind it, like modern tree squirrels, in order to climb downward.[3]
Senses
[edit]Analysis of casts of the brain and inner ear from C. pelvidens and C. baldwini suggest these animals depended more on their sense of smell than sight, may have been able to hear about as well as a modern aardvark, and were slow-moving to moderately agile. The encephalization quotient (EQ) had a range of 0.12–0.41 (1.0 is set as an average brain size in modern mammals of a similar body size) and the neocortex was less developed than later mammals. By the standards of modern mammals, Chriacus would have been neither especially quick nor intelligent, but their brains were comparable to many mammals of their time. Derived features of the inner ear were shared with fossils that are assigned to Euungulata (artiodactyls+perissodactyls), suggesting the genus may be close to the origin of ungulates,[6] though it is too different in form to be a direct ancestor.[3]
Classification
[edit]At least nine species are currently recognized in the genus,[6] though it is unknown whether all of these species share exclusive ancestry with each other. Variation within the genus (at its widest concept) is unusually high. Some Chriacus fossils point towards a slow, climbing lifestyle. Others are oddly similar to the fleet-footed early artiodactyl Diacodexis.[5]
Like most early placental mammals, the classification of Chriacus in relation to other groups is disputed. Halliday et al. (2015)[7] consider it a member of the family Oxyclaenidae, a sister group to palaeoryctids and creodonts, while Tabuce et al. (2011)[8] classify it as an arctocyonid, most closely related to Loxolophus, then Arctocyon, and allied to the Mesonychia.[9]
This difference mirrors the history of the mammals classified as arctocyonids. They were first considered creodonts (imagined to be the ancestors of modern carnivorans), and then "condylarths" (imagined to be the ancestors of hoofed mammals).[10][5] Modern studies suggest the confusion is due to the fact that ungulates, carnivorans, and creodonts are related groups, and flesh-eating lineages and adaptations evolved within each of them. Chriacus lies somewhere within the range of their early relatives.
References
[edit]- ^ Palmer, D., ed. (1999). The Marshall Illustrated Encyclopedia of Dinosaurs and Prehistoric Animals. London: Marshall Editions. p. 234. ISBN 1-84028-152-9.
- ^ Matthew, W.D. (1897). "A revision of the Puerco fauna". Bulletin of the American Museum of Natural History. 9: 259–323.
- ^ a b c d Rose, K. D. (1987-04-17). "Climbing adaptations in the early eocene mammal Chriacus and the origin of artiodactyla". Science. 236 (4799): 314–316. Bibcode:1987Sci...236..314R. doi:10.1126/science.3426662. ISSN 0036-8075. JSTOR 1698647. PMID 3426662.
- ^ Williamson, T.E.; Lucas, S.G. (1993). "Paleocene vertebrate paleontology of the San Juan Basin, New Mexico". New Mexico Museum of Natural History and Science Bulletin. 2: 105–135.
- ^ a b c Rose, K D (1996-02-20). "On the origin of the order Artiodactyla". Proceedings of the National Academy of Sciences. 93 (4): 1705–1709. doi:10.1073/pnas.93.4.1705. ISSN 0027-8424. PMC 40006. PMID 11607634.
- ^ a b Bertrand, Ornella C., Sarah L. Shelley John R. Wible Thomas E. Williamson Luke T. Holbrook Stephen G.B. Chester Ian B. Butler, and Stephen L. Brusatte (2019). "Virtual endocranial and inner ear endocasts of the Paleocene 'condylarth' Chriacus: new insight into the neurosensory system and evolution of early placental mammals". Journal of Anatomy. 236 (1): 21–49. doi:10.1111/joa.13084. PMC 6904649. PMID 31667836. S2CID 204969130.
{{cite journal}}: CS1 maint: multiple names: authors list (link) - ^ Halliday, Thomas J.D.; Upchurch, Paul; Goswami, Anjali (2015). "Resolving the relationships of Paleocene placental mammals". Biological Reviews. 92 (1): 521–550. doi:10.1111/brv.12242. PMC 6849585. PMID 28075073.
- ^ Tabuce, Rodolphe; Clavel, Julien; Telles Antunes, Miguel (2011-02-01). "A structural intermediate between triisodontids and mesonychians (Mammalia, Acreodi) from the earliest Eocene of Portugal". Die Naturwissenschaften. 98 (2): 145–55. Bibcode:2011NW.....98..145T. doi:10.1007/s00114-010-0747-y. PMID 21181109. S2CID 22526630.
- ^ Shelley, Sarah L, Stephen L Brusatte, and Thomas E Williamson (2015). "Resolving_the_higher-level_phylogenetic_relationships_of_Triisodontidae_'Condylarthra'_within_Placentalia". ResearchGate.
{{cite web}}: CS1 maint: multiple names: authors list (link) - ^ Van Valen, Leigh (1978). "The beginning of the age of mammals" (PDF). Evolutionary Theory. 4: 45–80.
