Giant bears of the Mio-Pliocene

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Hello,

Here i would like to start a thread concerning the bears of the Mio-Pliocene period. In fact, we always speak about the bears of the pleistocene because some of them (Arctodus, Arctotherium, Ursus arctos priscus, Ursus ingressus) have reached the mythic mass up to 1000 kg and even more for the largest specimens. I confess it is really impressive for a carnivore but bears are not hypercarnivores like big cats or canids so maybe this is one of the reasons concerning their great size? I don't know.

But, there are other prehistoric bears which lived before the pleistocene period and some of them were probably very big. Here are some of them with measurements and references:

- Agriotherium africanum

Specimen PQ-L 45062 - Condylobasal length of the skull: 420 mm, zygomatic width: 305 mm. The zygomatic width is big, larger than almost all of the extinct big cats. I think that one skull of a very big male panthera atrox has a comparable width, maybe even a little bit larger, i am not sure. But in the same paper there is a skull of Arctodus which has a zygomatic width as much as 364 mm!!! No doubt that these bears could bit with a great power.
Specimen L50446 - Condylobasal length of the skull: 465 mm. The famous specimen of Arctodus PM 24880 had a condylobasal length around 422 mm and probably weighted at least 740kg to give you a brief comparison. And we are talking about bears which are not obese so imagine this bear in a zoo with a lot of food...

- Indarctos oregonensis

With a length of 362.54 mm, the huge mandible from Ft-40, Nebraska represents the largest mandible of all Indarctos and among the largest mandibles of all bears, rivaled only by the largest individuals of Ursus spelaeus (Baryshnikov 2007), Arctotherium angustidens (López et al. 2008) and “Agriotherium schneideri” (personal observation).

Specimen UCMP V22362, Indarctos oregonensis from the Rattlesnake bed: very large humerus (around 50 cm based on the scale bar, measured with Paint 3D) and extremely robust. 
The humerus is enormous, being larger than any living bear and representing the largest known humerus for the genus.
To give you some other comparisons: humerus of Ursus arctos gyas: 44,4 cm. What is the length of the largest humerus of extinct big cats please?

- Huracan

Complete crania permitting measurement of total length and condylobasal length are rare, but those have an average of 490 mm for cranial total length (n = 2) and 440 mm for condylobasal length (n = 4; table 5). However, complete mandibles are abundant, with a mean length of 318.5 mm (n = 8; table 11). Based on the ratios of 1.47 and 1.37 for cranial total length/ mandibular length and condylobasal length/mandibular length, respectively, from an individual associated cranium and mandible of AMNH F:AM 76005, the range of cranial total lengths and condylobasal lengths in this species are estimated at 411–553 mm and 383–515 mm, respectively.
There is a skull about 502.92 mm in length concerning Huracan coffeyi. (table 5). The largest mandible has an impressive size of 376.32 mm! (table 11)
H. coffeyi not only has a small width/length ratio as a result of relative elongation of the humerus, but the epicondyles also are distinctly weaker than those of Ursus, Arctodus, and Ailuropoda. The width/length ratio is distinctly smaller in H. coffeyi than in Ursus, Arctodus and Ailuropoda, but comparable to that of extant Panthera (Felidae) species, a large and relatively cursorial carnivore.
Together, these traits suggest that H. coffeyi was a cursorial animal, more so than any living bear, the extinct giant short-faced bear, and probably extant lions.

Even though H. coffeyi has a huge skull and very large teeth (probably the largest among all known bears, living and extinct, together with Ursus ingressus and Arctotherium angustidens), its postcranial bones are only slightly larger than those of Agriotherium africanum, are smaller than Indarctos oregonensis, and are distinctly smaller than those of the giant short-faced bear Arctodus.

An interesting citation concerning Arctodus: Well-developed humerus epicondyles and a wide range of ulna rotation support an inference that Arctodus was powerful and could subdue large prey, unlike the suggestion of Sorkin (2006)

Indarctos has a variable humerus morphology, although all Indarctos species are clearly distinguished from H. coffeyi in having an entepicondylar foramen. The type specimen of I. oregonensis is very robust, with expanded humerus epicondyles like those of Ursus (Merriam et al., 1916).

I have posted some data about large bears because large bears are popular. But of course the data concerning smaller bears are also interesting and are welcome.

References:

Fossil Bear from South Africa - 1977 - Q. B. Hendey.

References:
Coexistence of Indarctos and Amphimachairodus (Carnivora) in the Late Early Hemphillian of Florida, North America.

References:
New Fossil Giant Panda Relatives (Ailuropodinae, Ursidae): A Basal Lineage of Gigantic Mio-Pliocene Cursorial Carnivores - 2023. Reply