Amphicyon, the bear-dog, is really another giant opossum

This is yet another example of convergence
exposed by a large taxon list, which minimizes the taxon exclusion problem that hampered earlier studies.

Like its sister,
Arctocyon (Fig. 4), the other ‘bear dog’, Amphicyon (Fig. 1), nests at the base of the Marsupialia in the large reptile tree (915 taxa), close to the recently extant, Tasmanian wolf, Thylacinus. All are derived from a sister to the Virginia opossum, Didelphis

Figure 1. Amphicyon major in lateral view along with manus and pes from Argot 2010.

Figure 1. Amphicyon major in lateral view along with manus and pes from Argot 2010. Note the opossum like pollex (thumb) and halux. 

Amphicyon major (Lartet 1836, Blainville 1841; Early Miocene to Early Pleistocene, 17-2mya; 2.5mya; ~1m snout to vent) was traditionally considered a bear-dog and recently considered a lion-dog by Argot (2010), but here nests with Arctocyon at the base of the Marsupialia, despite lacking prepubic bones, which were also vestigial in Thylacinus. Several species are known. Note the posterior extent of the jugal and retention of the septomaxilla (anterior lacrimal).

Figure 2. Amphicyon skull. Note the retention of the septomaxilla/anterior lacrimal and the posterior extension of the jugal to the jaw glenoid.

Figure 2. Amphicyon skull. Note the retention of the septomaxilla/anterior lacrimal and the posterior extension of the jugal to the jaw glenoid. This appears to be a more robust specimen than in figure 1.

Hunt 2003
also found felid traits in the skeleton of Amphicyon. “The North American species of Amphicyon (A. galushai, A. frendens, A. ingens) most likely adopted ecological roles similar to the large living felids (in particular, the lion Panthera leo). Their robust skeleton with powerful forelimbs, massive clawed feet, heavily muscled jaws with large canines, and a composite crushing/shearing dentition suggest a mobile predator that most likely stalked and ambushed prey from cover, overpowering its victims through sheer size and strength.”

Figure 3. Subset of the LRT focusing on marsupials and carnivores. Amphicyon nests with marsupials that also lack prepubic bones.

Figure 3. Subset of the LRT focusing on marsupials and carnivores. Amphicyon nests with marsupials that also lack prepubic bones.

Sure Amphicyon has only three molars
and that converges on members of the Carnivora. And sure it has no marsupial bones (prepubes), and that converges on Placentalia. When all other marsupials are deleted, Amphicyon and Arctocyon do not change their branch. When all taxa except Carnivora and the cynodont Thrinaxodon (to root the analysis) are deleted, Amphicyon and Arctocyon do not change their branch, but remain outside of the Carnivora.

Figure 3. Arctocyon is no longer an ungulate placental, but a carnivorous marsupial, close to Thylacinus.

Figure 4. Arctocyon is no longer an ungulate placental, but a carnivorous marsupial, close to Thylacinus.

Unfortunately,
taxon exclusion is once again the difference in nestings here. There is no indication that prior workers tested Amphicyon with Thylacinus and Didelphis. It takes over 30 steps to shift Amphicyon over to the Carnivora in the LRT.

References
Argot C 2010. Morphofunctional analysis of the pos cranium of Amphicyon major (Mammalia, Carnivora, Amphicyonidae) from the Miocene of Sansan (Gers, France) compared to three extant carnivores: Ursus arctos, Panthera leo, and Canis lupus. Geodiversitas 32(1):65-106.
Blainville HM 1841. Osteographie et description iconographique des Mammiferes récentes et fossiles (Carnivores) 1, 2 Paris.
Hunt RM Jr 2003. Intercontinental Migration of Large Mammalian Carnivores: Earliest Occurrence of the Old World Beardog Amphicyon (Carnivora, Amphicyonidae) in North America. in Bulletin of the American Museum of Natural History. 279: 77–115.
Lartet E 1836. Nomenclature des mammife`res et des coquilles qu’il a trouve´s dans un terrain d’eau douce pre`s de Simorre et de Sansan (Gers). Bulletin de la Socie´te´ Ge´ologique de France 7: 217–220.

wiki/Amphicyon

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