Arctocyonidae
{{Short description|Extinct family of mammals}}
{{Confused|Amphicyonidae}}
{{Automatic taxobox
| fossil_range = Palaeocene–Eocene
| image = Arctocyon DB.jpg
| image_caption = Arctocyon
| taxon = Arctocyonidae
| subdivision_ranks = Subfamilies and genera
| subdivision = * †Karakia Thewissen, Williams & Hussain, 1977
- †Loxolophinae Van Valen, 1978
- †Platymastus Van Valen, 1978
- †Desmatoclaenus Gazin, 1941
- †Baioconodon Gazin, 1941
- †Loxolophus Cope, 1885
- †Mimotricentes Simpson, 1937
- †Deuterogonodon Simpson, 1935
- †Arctocyoninae Giebel, 1855
- †Neoclaenodon Gidley, 1919
- †Claenodon Scott, 1892
- †Colpoclaenus Patterson & McGrew, 1962
- †Mentoclaenodon Weigelt, 1960
- †Arctocyonides Lemoine, 1891
- †Landenodon Quinet, 1968
- †Arctocyon de Blainville, 1841
- †Anacodon Cope, 1882
}}
Arctocyonidae (from Greek arktos and kyôn, "bear/dog-like") is an extinct, possibly polyphyletic family of placental mammals which lived from the late Cretaceous to the early Eocene. They were initially regarded as creodonts, though have since been reassigned to an order of their own, the Arctocyonia. Some have suggested that arctocyonids are ancestral to modern-day artiodactyls, or that they form a sister group. However, more recent phylogenetic analyses suggest that arctocyonids may represent an artificial grouping of extinct ungulates, or that they might be an assemblage of unrelated placentals related to pangolins, pantodonts, and periptychids.
Members of Arctocyonidae are characterised by long skulls, with large sagittal crests and very large canines. In the case of Arctocyon proper, the lower canines especially were large enough to require a diastema on the upper jaw to accommodate them. Arctocyonids varied considerably in size and morphology. Smaller genera, like Chriacus, were about the size of a coati, while larger ones, such as Arctocyon, weighed up to {{Convert|44|kg|lb|abbr=on}} and stood {{Convert|45|cm|in|abbr=on}} at the shoulder. Many arctocyonids have climbing adaptations, suggesting that they were either descended from arboreal taxa, or were arboreal themselves. The North American Anacodon was more robust than other genera, and had adaptations for burrowing as well as climbing. Most genera appear to have been omnivorous, though Anacodon showed signs of an increase in herbivory.
Taxonomy
The family Arctocyonidae was named by Christoph Gottfried Andreas Giebel in 1855, as a carnivoran subfamily that included Arctocyon, the amphicyonid Amphicyon, and the ursid Agriotherium.{{Cite journal |last=Giebel |first=Christoph Gottfried Andreas |author-link=Christoph Gottfried Andreas Giebel |date=1855 |title=Die Säugethiere in zoologischer, anatomischer und palæontologischer Beziehung |journal=Verlag von Ambrosius Abel: Leipzig.}} It was elevated to family level by Scottish zoologist Andrew Murray.{{Cite book |last=Murray |first=Andrew |url=https://www.google.co.uk/books/edition/The_Geographical_Distribution_of_Mammals/rjQBOS7Pt4UC?hl=en |title=The Geographical Distribution of Mammals |date=1866 |publisher=Day and Son |language=en}} At some point thereafter, arctocyonids became a family within Creodonta.{{Cite journal |last=Scott |first=W. B. |date=1892 |title=A Revision of the North American Creodonta with Notes on Some Genera Which Have Been Referred to That Group |url=https://www.jstor.org/stable/4061883 |journal=Proceedings of the Academy of Natural Sciences of Philadelphia |volume=44 |pages=291–323 |jstor=4061883 |issn=0097-3157}} In 1975, Malcolm McKenna erected a new order, Arctocyonia, to accommodate them, placing them within Ungulata.{{Citation |last=McKenna |first=Malcolm C. |title=Toward a Phylogenetic Classification of the Mammalia |date=1975 |work=Phylogeny of the Primates: A Multidisciplinary Approach |pages=21–46 |editor-last=Luckett |editor-first=W. Patrick |url=https://link.springer.com/chapter/10.1007/978-1-4684-2166-8_2 |access-date=2025-01-07 |place=Boston, MA |publisher=Springer US |language=en |doi=10.1007/978-1-4684-2166-8_2 |isbn=978-1-4684-2166-8 |editor2-last=Szalay |editor2-first=Frederick S.}} Since then, Arctocyonidae has largely been treated as a family of its own, though how that family is divided has been another matter. William Diller Matthew, in 1937, divided it into four subfamilies (Arctocyoninae, Chriacinae, Oxyclaeninae, and Triisodontinae);{{Cite journal |last=Matthew |first=William Diller |date=1937 |title=Paleocene Faunas of the San Juan Basin, New Mexico |url=https://www.jstor.org/stable/1005521 |journal=Transactions of the American Philosophical Society |volume=30 |pages=i–510 |doi=10.2307/1005521 |jstor=1005521 |issn=0065-9746}} the latter is now regarded as a family of its own. That same year, George Gaylord Simpson suggested that arctocyonids could instead be divided into Arctocyoninae, Oxyclaeninae, and Triisodontinae.{{Cite journal |last=Simpson |first=George Gaylord |author-link=George Gaylord Simpson |date=1937 |title=The Fort Union of the Crazy Mountain Field, Montana and its Mammalian faunas |url=https://www.biodiversitylibrary.org/part/70808 |journal=Bulletin |volume=169 |pages=1––287}} In 1978, Leigh Van Valen erected a new subfamily, Loxolophinae.{{Cite journal |last=Van Valen |first=Leigh |date=1978 |title=The Beginning of the Age of Mammals |url=https://www.mn.uio.no/cees/english/services/van-valen/evolutionary-theory/volume-4/vol-4-no-2-pages-45-80-l-van-valen-the-beginning-of-the-age-of-mammals.pdf |url-status=live |journal=Evolutionary Theory |volume=4 |pages=45–80 |archive-url=https://web.archive.org/web/20210709151330/https://www.mn.uio.no/cees/english/services/van-valen/evolutionary-theory/volume-4/vol-4-no-2-pages-45-80-l-van-valen-the-beginning-of-the-age-of-mammals.pdf |archive-date=9 July 2021 |access-date=16 August 2024}}
The relationship between arctocyonids and other clades has long been uncertain. Since becoming the sole representatives of their own order, they have been suggested to be either ancestral to artiodactyls{{Cite book |last1=Kenneth D. Rose |first1=Ph D. |url=https://press.jhu.edu/books/title/3329/rise-placental-mammals |title=The Rise of Placental Mammals |last2=J. David Archibald |first2=Ph D. |date=2005 |publisher=Johns Hopkins University Press |isbn=978-0-8018-8022-3 |language=en}} or close to the clade's stem.{{Cite journal |last1=Thewissen |first1=J. G. M. |last2=Cooper |first2=Lisa Noelle |last3=Clementz |first3=Mark T. |last4=Bajpai |first4=Sunil |last5=Tiwari |first5=B. N. |date=2007 |title=Whales originated from aquatic artiodactyls in the Eocene epoch of India |url=https://www.nature.com/articles/nature06343 |journal=Nature |language=en |volume=450 |issue=7173 |pages=1190–1194 |doi=10.1038/nature06343 |pmid=18097400 |bibcode=2007Natur.450.1190T |issn=0028-0836}} The family's monophyly has also been called into question. In 2012, a phylogenetic analysis of Prolatidens waudrae, a traditional arctocyonid, recovered it as a more basal ungulate; Arctocyon, Landenodon and Thryptacodon were recovered as part of a clade sister to triisodonts and mesonychids; and the reminder of tested arctocyonids formed a polytomy basal to that clade and Diacodexis.{{Cite journal |last1=Bast |first1=Eric De |last2=Smith |first2=Thierry |date=2013-07-01 |title=Reassessment of the small 'arctocyonid' Prolatidens waudruae from the early Paleocene of Belgium, and its phylogenetic relationships with ungulate-like mammals |url=https://www.tandfonline.com/doi/full/10.1080/02724634.2013.747531 |journal=Journal of Vertebrate Paleontology |volume=33 |issue=4 |pages=964–976 |doi=10.1080/02724634.2013.747531 |bibcode=2013JVPal..33..964B |issn=0272-4634}} In 2015, Peter E. Kondrashov and Spencer G. Lucas recovered the family as an artificial assemblage of basal ungulates.{{Cite book |last1=Kondrashov |first1=Peter E. |url=https://books.google.com/books?id=au2oDQAAQBAJ&dq=loxolophus&pg=PA107 |title=Fossil Record 4: Bulletin 67 |last2=Lucas |first2=Spencer G. |date=2015 |publisher=New Mexico Museum of Natural History and Science |editor-last=Sullivan |editor-first=Robert M. |language=en |editor-last2=Lucas |editor-first2=Spencer G.}} That same year, a larger analysis by Thomas J. D. Halliday, Paul Upchurch and Anjali Goswami recovered arctocyonids as several entirely unrelated placental lineages. By their unconstrained strict consensus tree, Arctocyon is related to Periptychus and Protolitopterna, Loxolophinae is related to pantolestids and pangolins. By their constrained strict consensus tree, Arctocyon and Loxolophus form a clade related to pantodonts and periptychids, and the rest of Arctocyonidae is recovered close to pangolins.{{cite journal |last1=Halliday |first1=Thomas J.D. |last2=Upchurch |first2=Paul |last3=Goswami |first3=Anjali |date=2015 |title=Resolving the relationships of Paleocene placental mammals |url=http://discovery.ucl.ac.uk/1473028/1/Halliday_et_al-Biological_Reviews.pdf |url-status=live |journal=Biological Reviews |volume=92 |issue=1 |pages=521–55 |doi=10.1111/brv.12242 |pmc=6849585 |pmid=28075073 |archive-url=https://web.archive.org/web/20221223122753/https://discovery.ucl.ac.uk/id/eprint/1473028/1/Halliday_et_al-Biological_Reviews.pdf |archive-date=December 23, 2022}}
Description
Arctocyonidae, if monophyletic, was a morphologically disparate lineage. Some genera, such as Chriacus, were fairly small, and bore adaptations for an arboreal or scansorial lifestyle.{{Cite journal |last=Rose |first=K. D. |date=1987-04-17 |title=Climbing adaptations in the early eocene mammal Chriacus and the origin of artiodactyla |url=https://www.science.org/doi/10.1126/science.3426662 |journal=Science |language=en |volume=236 |issue=4799 |pages=314–316 |bibcode=1987Sci...236..314R |doi=10.1126/science.3426662 |issn=0036-8075 |pmid=3426662}} Others, like Anacodon, were very large and robust, having adaptations for both arboreal and fossorial lifestyles.{{Citation |last=Rose |first=Kenneth D. |title=Postcranial skeletal remains and adaptations in early Eocene mammals from the Willwood Formation, Bighorn Basin, Wyoming |date=1990-01-01 |work=Dawn of the Age of Mammals in the northern part of the Rocky Mountain Interior, North America |pages=0 |editor-last=Bown |editor-first=Thomas M. |url=https://pubs.geoscienceworld.org/gsa/books/edited-volume/378/chapter-abstract/3797098/Postcranial-skeletal-remains-and-adaptations-in?redirectedFrom=fulltext |access-date=2025-01-07 |publisher=Geological Society of America |isbn=978-0-8137-2243-6 |editor2-last=Rose |editor2-first=Kenneth D.}} Arctocyon, particularly A. mumak, appears to have been the most terrestrial,{{Cite journal |last1=Gould |first1=Francois D. H. |last2=Rose |first2=Kenneth D. |date=2014 |title=Gnathic and Postcranial Skeleton of the Largest Known Arctocyonid 'Condylarth' Arctocyon Mumak (mammalia, Procreodi) and Ecomorphological Diversity in Procreodi |url=https://www.jstor.org/stable/24523452 |journal=Journal of Vertebrate Paleontology |volume=34 |issue=5 |pages=1180–1202 |doi=10.1080/02724634.2014.841707 |jstor=24523452 |bibcode=2014JVPal..34.1180G |issn=0272-4634}} though likely descended from arboreal ancestors. Most arctocyonids are fragmentary, making it difficult to determine body size. Chriacus likely weighed {{Convert|5–10|kg|lb|abbr=on}}, and was slightly larger than a modern coati. Arctocyon primaevus may have weighed up to {{Convert|44|kg|lb|abbr=on}}, and had an estimated shoulder height of {{Convert|45|cm|in|abbr=on}}. Remains of Mentoclaenodon suggest a very large body size, though exactly how large it grew to is unclear, and it may have been outsized by Arctocyon mumak.
= Skulls and dentition =
The skulls of arctocyonids were fairly long, with a small braincase and very large sagittal and occipital crests, combined traits of herbivorous and carnivorous mammals.{{Cite journal |last=Stefen |first=Clara |date=1997-09-01 |title=The enamel of Creodonta, Arctocyonidae, and Mesonychidae (Mammalia), with special reference to the appearance of Hunter-Schreger-Bands |url=https://link.springer.com/article/10.1007/BF02988497 |journal=Paläontologische Zeitschrift |language=en |volume=71 |issue=3 |pages=291–303 |doi=10.1007/BF02988497|bibcode=1997PalZ...71..291S }} The zygomatic arch of Arctocyon specifically was very large, with a posterior angle anterior to the mandibular fossa, almost forming a right angle.{{Cite journal |last1=Shelley |first1=Sarah L. |last2=Williamson |first2=Thomas E. |last3=Brusatte |first3=Stephen L. |date=2018-07-18 |title=The osteology of Periptychus carinidens: A robust, ungulate-like placental mammal (Mammalia: Periptychidae) from the Paleocene of North America |journal=PLOS ONE |language=en |volume=13 |issue=7 |pages=e0200132 |doi=10.1371/journal.pone.0200132 |doi-access=free |pmid=30020948 |pmc=6051615 |bibcode=2018PLoSO..1300132S |issn=1932-6203 }} In most genera, the incisors were small and unspecialised, though others, like Thryptacodon, had modified them into grooming teeth convergent with those of lemurs.{{Cite journal |last1=Gingerich |first1=Philip D. |last2=Rose |first2=Kenneth D. |date=1979 |title=Anterior Dentition of the Eocene Condylarth Thryptacodon: Convergence with the Tooth Comb of Lemurs |url=https://www.jstor.org/stable/1379754 |journal=Journal of Mammalogy |volume=60 |issue=1 |pages=16–22 |doi=10.2307/1379754 |jstor=1379754 |issn=0022-2372}} Though the canines likely had a role in feeding in certain genera, in others, like Anacodon, they did not.{{Cite journal |last=Argot |first=Christine |date=2013-06-01 |title=Postcranial Analysis of a Carnivoran-Like Archaic Ungulate: The Case of Arctocyon primaevus (Arctocyonidae, Mammalia) from the Late Paleocene of France |url=https://link.springer.com/article/10.1007/s10914-012-9198-x |journal=Journal of Mammalian Evolution |language=en |volume=20 |issue=2 |pages=83–114 |doi=10.1007/s10914-012-9198-x |issn=1573-7055}} In the case of Arctocyon, the lower canines were longest, to the extent that they were accommodated by a gap (or diastema) between the upper canines and premolars.{{Cite book |last1=Agustí |first1=Jordi |url=https://cup.columbia.edu/book/mammoths-sabertooths-and-hominids/9780231116411 |title=Mammoths, Sabertooths, and Hominids: 65 Million Years of Mammalian Evolution in Europe |last2=Antón |first2=Mauricio |date=2005 |publisher=Columbia University Press |isbn=978-0-231-51633-4}} The cheek teeth were tricuspid (three-cusped) and were often bunodont. The premolars were simple, if fairly sharp, while the molars were blunt and resembled those of bears.{{Cite journal |last=Russell |first=Loris S. |date=1954 |title=Evidence of Tooth Structure on the Relationships of the Early Groups of Carnivora |url=https://www.jstor.org/stable/2405640 |journal=Evolution |volume=8 |issue=2 |pages=166–171 |doi=10.2307/2405640 |jstor=2405640 |issn=0014-3820}} Overall, the dental morphology of arctocyonids suggests that they were omnivorous to varying degrees.
= Postcranial elements =
In Arctocyon, the mammillary processes of the dorsal vertebrae were robust (suggesting powerful musculature), and the caudal vertebrae appear to have been tightly interlocked, suggesting that the tail was fairly rigid. In Chriacus, however, there are signs that the tail may have been prehensile. Arctocyonid limbs were fairly typical in length, with stout ulnar and fibular shafts. In Anacodon, particularly, the limbs were very robust. In Chriacus, the ankle joints were flexible and allowed the hind feet to rotate, enabling them to climb downward. Anacodon
Biology
= Diet and feeding =
Arctocyonid cheek teeth were bunodont, and the carniassials seen in other predatory mammal clades were essentially absent. Overall, the dental morphology of arctocyonids suggests that they were omnivorous, to varying degrees. Anacodon may have been among the least carnivorous, having flat, crenulated cheek teeth while Arctocyon corrugatus was among the most carnivorous.{{Cite web |last1=Kondrashov |first1=Peter |last2=Lucas |first2=Spencer G. |date=2004 |title=Arctocyon (Mammalia, Arctocyonidae) from the Paleocene of North America |url=https://www.researchgate.net/publication/281110906 |archive-url=http://web.archive.org/web/20210509163256/https://www.researchgate.net/publication/281110906_ARCTOCYON_MAMMALIA_ARCTOCYONIDAE_FROM_THE_PALEOCENE_OF_NORTH_AMERICA |archive-date=2021-05-09 |access-date=2025-01-07 |website=ResearchGate |language=en}}