Short-faced bear – Arctodus spp.

[Infinity Blade]

Short-faced bear – Arctodus spp.


A model of Arctodus simus. © @ Blue Rhino Studio

Temporal range: Quaternary; Pleistocene epoch (Blancan-Irvingtonian; 2.5-0.5 Ma[1] for A. pristinus) (Calabrian-Tarantian; 0.8-0.0108 Ma[2][3][4] for A. simus)

Scientific classification:

Life
Domain: Eukaryota
(unranked): Unikonta
(unranked): Opisthokonta
(unranked): Holozoa
(unranked): Filozoa
Kingdom: Animalia
Subkingdom: Eumetazoa
(unranked): Bilateria
Clade: Nephrozoa
Superphylum: Deuterostomia
Phylum: Chordata
Clade: Olfactores
Clade: Craniata
Subphylum: Vertebrata
Infraphylum: Gnathostomata
Clade: Eugnathostomata
Clade: Teleostomi
Superclass: Tetrapoda
Clade: Reptiliomorpha
Clade: Amniota
Clade: Synapsida
Clade: Eupelycosauria
Clade: Sphenacodontia
Clade: Sphenacodontoidea
Order: Therapsida
Clade: Eutheriodonta
Suborder: Cynodontia
Clade: Epicynodontia
Infraorder: Eucynodontia
Parvorder: Probainognathia
Superfamily: Chiniquodontoidea
Clade: Prozostrodontia
Clade: Mammaliaformes
Class: Mammalia
(unranked): Pegasoferae
(unranked): Zooamata
Clade: Holotheria
Superlegion: Trechnotheria
Legion: Cladotheria
Sublegion: Zatheria
Infralegion: Tribosphenida
Subclass: Theria
Clade: Eutheria
Infraclass: Placentalia
Subcohort: Exafroplacentalia
Magnorder: Boreoeutheria
Superorder: Laurasiatheria
(unranked): Ferae
(unranked): Carnivoramorpha
Order: Carnivora
Suborder: Caniformia
Infraorder: Arctoidea
Family: Ursidae
Subfamily: Tremarctinae
Tribe: Tremarctini
Genus: †Arctodus
Species: †A. simus
†A. pristinus

Arctodus ("bear tooth") was a genus of tremarctine bear that lived in North America in from 2.5 million to ~10,800 years ago.[1][2][3][4] There are two species in the genus: A. pristinus and A. simus.

Description & functional morphology:
In a 2010 study, a third of the A. simus sampled approached 1,000 kg, suggesting such specimens were more common than previously suspected. Similarly, two of the specimens were estimated to weigh 300-400 kg, consistent with the marked sexual dimorphism seen in extant ursids.[5] A. pristinus also displayed marked sexual dimorphism; males of this smaller species overlapped in size with females of A. simus and the extinct Tremarctos floridanus.[1]


Skeletal of A. simus. © @ bLAZZE92

Contrary to its popular name, the short faced bear did not actually possess a particularly foreshortened face for a bear, although it did possess a proportionately deeper snout. Its long-legged appearance is sort of an optical illusion created by its rather short back.[5]

Sorkin (2006) analyzed the skeletal remains of A. simus (in addition to those of Agriotherium africanum, which was surprisingly convergent to Arctodus) to test the idea that Arctodus was a hypercarnivore. The anatomical features that served as evidence for hypercarnivory ("sectorial carnassials, position of the mandibular condyle, and development of the angular process") were all found to be present in Tremarctos, a borderline herbivorous bear. Likewise, the buccal cusps present on the upper molars are present in the extant Kodiak bear (Ursus arctos middendorfi). Other things Sorkin noted were that:

- the upper canines were proportionately short (rendering it unadapted to killing akin to a big cat).
- the orbits were small and more laterally placed than those of felids (vision is vital to predation).
- relative to the brown bear, the forelimbs did not seem as adept at grappling with prey ("reduced leverage of deltoid and pectoralis muscles [used to subdue prey], reduced development of the brachialis and brachioradialis muscles [used to grasp prey], reduced pronation of the forearm and the flexation of the wrist and digits [used to grasp prey]”).
- the olecranon process was short and bent (meaning it couldn't crouch very well, reducing ambushing ability).
- the lumbar vertebrae were stiff like those of U. arctos and had limited flexion in the sagittal plane (decreasing its top speed and acceleration).[6]

And at least in Arctodus:

- relative to the proximal limb segments, the distal limb segments of Arctodus were just as short, if not shorter, than those of U. arctos, and certainly far shorter than those of extant cursorial, and even ambush, predators.
- the hind feet were plantigrade, unlike in extant predators that utilize speed for predation.[6]

A 2012 thesis dissertation found that the forelimbs of A. simus feature a number of cursorial adaptations, and that the species represents an early step in the evolution of a cursorial bear. These features include (1) reduced attachment areas for muscles that controlled abduction-adduction and pronation-supination, (2) reorientation of the joints nearer to the parasagittal plane, and (3) relatively gracility of the forelimb bones compared to other bears (these features are consistent with an inferior ability to subdue prey with the forelimbs as asserted in [6]).[7] The laterally compressed ribcage of Arctodus is also consistent with greater cursorial ability, reducing the lateral excursion of the limbs taken during each step.[8] It is also thought that the relatively gracile limbs of Arctodus, combined with its great size, would have rendered it incapable of withstanding the dynamic forces of rapid acceleration or maneuverability that modern mammalian predators exhibit (but capable of moderately fast, straight-line coursing, using energetically efficient paces to traverse through a wide home range[8]), although this needs to be empirically tested with biomechanical analysis.[7]

A 2013 study on the microwear of Arctodus' teeth suggested that it was not a durophagous animal, contrary to popular belief. This study also noted that Arctodus possessed cranial features that herbivorous bears tend to possess. These include "cheek teeth with large surface areas, a deep mandible, and large mandibular muscle attachments". It was also noted that "because herbivorous carnivorans lack an efficient digestive tract for breaking down plant matter via microbial action, they must break down plant matter via extensive chewing or grinding, and thus possess features to create a high mechanical advantage of the jaw".[9] This may explain the broad, deep, and short jaws of Arctodus, which would have evidently had excellent mechanical advantage.

Diet:
Multiple isotope analyses have revealed that A. simus specimens in Beringia and British Columbia incorporated terrestrial meat in their diet, especially caribou. However, some specimens with higher levels of δ¹⁵N indicate that A. simus in the mammoth steppe were also likely preying on muskoxen or even killing other predators for consumption. Kleptoparisitism and scavenging (of both marine and terrestrial carcasses across a wide home range) are also likely. These isotope values also do not mean that the short-faced bear was not eating plants; up to 50% of its diet may have been plant food. Interestingly, A. simus from the mammoth steppe were not consuming a significant amount of mammoth or horse meat, despite these two species composing ~50% of the herbivore biomass in this ecosystem.[10][11]

Bite marks found on the bones of many sloths and young proboscideans found in the Leisey Shell Pit match the canines of A. pristinus. It is unknown if these bite marks were the result of successful predation or scavenging.[1]

Extinction:
The short faced bear went extinct at the end of the Pleistocene epoch about ~10,800 years ago. It is clear that A. simus overlapped with the Clovis culture both temporally and geographically. Because A. simus is known to have consumed a substantial portion of meat, this suggests competition between both humans and Arctodus.[4]


Skeleton of A. simus with Equus occidentalis.

References:

[1] www.flmnh.ufl.edu/florida-vertebrate-fossils/species/arctodus-pristinus
[2] Churcher, C. S., Morgan, A. V., & Carter, L. D. (1993). Arctodus simus from the Alaskan Arctic slope. Canadian Journal of Earth Sciences, 30(5), 1007-1013.
[3] Cassiliano, M. L. (1999). Biostratigraphy of Blancan and Irvingtonian mammals in the Fish Creek-Vallecito section, southern California, and a review of the Blancan–Irvingtonian boundary. Journal of Vertebrate Paleontology, 19(1), 169-186.
[4] Schubert, B. W. (2010). Late Quaternary chronology and extinction of North American giant short-faced bears (Arctodus simus). Quaternary International, 217(1-2), 188-194.
[5] Borja Figueirido, Juan A. Pérez-Claros, Vanessa Torregrosa, Alberto Martín-Serra & Paul Palmqvist (2010) Demythologizing Arctodus simus, the ‘short-faced’ long-legged and predaceous bear that never was, Journal of Vertebrate Paleontology, 30:1, 262-275, DOI: 10.1080/02724630903416027
[6] B. Sorkin (2006) Ecomorphology of the giant short-faced bears Agriotherium and Arctodus, Historical Biology, 18:1, 1-20, DOI: 10.1080/08912960500476366
[7] Lynch, Eric Randally, “Cursorial Adaptations in the Forelimb of the Giant Short-Faced Bear, Arctodus simus, Revealed by Traditional and 3D Landmark Morphometrics“ (2012). Electronic Theses and Dissertations. Paper 1477. dc.etsu.edu/etd/1477
[8] Matheus, P. E. (2003). Locomotor adaptations and ecomorphology of short-faced bears (Arctodus simus) in eastern Beringia. Yukon Palaeontology Program, Department of Tourism and Culture.
[9] Donohue, S. L., DeSantis, L. R., Schubert, B. W., & Ungar, P. S. (2013). Was the giant short-faced bear a hyper-scavenger? A new approach to the dietary study of ursids using dental microwear textures. PloS one, 8(10), e77531. doi.org/10.1371/journal.pone.0077531
[10] Mychajliw, A.M., Rick, T.C., Dagtas, N.D. et al. Biogeographic problem-solving reveals the Late Pleistocene translocation of a short-faced bear to the California Channel Islands. Sci Rep 10, 15172 (2020). doi.org/10.1038/s41598-020-71572-z
[11] Bocherens, H. (2015). Isotopic tracking of large carnivore palaeoecology in the mammoth steppe. Quaternary Science Reviews, 117, 42-71.

[0ldgrizz]

Just how big was Arctodus simus really? I am finding so many conflicting estimates of Arctodus simus and his South American relative, Arctotherium angustidens.

[creature386]

blaze takes credit for the image.
I hope it answers the question concerning Arctodus' size.

[0ldgrizz]

Thank you creature386. Without a doubt, these short-faced bears were taller than any other bear species living or extinct. Would their weight be much higher than that of the living polar bear? I understand that, with prehistoric animals, we can only get educated estimates. I watched a documentary once that used the water displacement method to estimate the weight of a T-rex; but that would entail the use of accurate models of every prehistoric animal tested; perhaps not logical.

[Infinity Blade]

Abstract

The Tremarctinae are a subfamily of bears endemic to the New World, including two of the largest terrestrial mammalian carnivores that have ever lived: the giant, short-faced bears Arctodus simus from North America and Arctotherium angustidens from South America (greater than or equal to 1000 kg). Arctotherium angustidens became extinct during the Early Pleistocene, whereas Arctodus simus went extinct at the very end of the Pleistocene. The only living tremarctine is the spectacled bear (Tremarctos ornatus), a largely herbivorous bear that is today only found in South America. The relationships among the spectacled bears (Tremarctos), South American short-faced bears (Arctotherium) and North American short-faced bears (Arctodus) remain uncertain. In this study, we sequenced a mitochondrial genome from an Arctotherium femur preserved in a Chilean cave. Our molecular phylogenetic analyses revealed that the South American short-faced bears were more closely related to the extant South American spectacled bear than to the North American short-faced bears. This result suggests striking convergent evolution of giant forms in the two groups of short-faced bears (Arctodus and Arctotherium), potentially as an adaptation to dominate competition for megafaunal carcasses.

Reference:

Mitchell, J.K., Bray, S.C., Bover, P., Soibelzon, L., Schubert, B.W., Prevosti, F., Prieto, A., Martin, F., Austin, J.J., Cooper, A. (2016). Ancient mitochondrial DNA reveals convergent evolution of giant short-faced bears (Tremarctinae) in North and South America. Biol. Lett., vol. 12, no. 4, 20160062; doi: 10.1098/rsbl.2016.0062

link

[Infinity Blade]

Reconstructing a monster bear
Mauricio Antón

Several years ago I was approached by the BBC and asked to produce an accurate reconstruction of the giant short-faced bear, Arctodus simus, to be used by a team of 3D artists in order to create life-like animations of the animal. I had drawn Arctodus before but this project required a more in-depth approach, so I set to review all the available information about the anatomy and body proportions of this amazing ursid.
I consulted with paleontologist Paul Mattheus from the university of Alaska in Fairbanks who sent me his Phd thesis along with a wealth of data about the giant bear. With all those figures and measurements I put together a detailed reconstruction of the beast’s skeleton, based mostly on the nearly complete specimen from Fulton County in Indiana, but complemented with bits from other findings. One especially nice skull from Alaska allowed me a detailed restoration of the animal´s head.

In this collage you can see my drawings for the reconstruction of Arctodus simus, from a working sketch of the skeleton to a restoration of the animal’s musculature and soft-tissue outline and a detailed rendering of the head


The creature that emerged from those drawings was quite impressive. With especially long limb bones, it attained a shoulder height of 1.75 meters -while standing on all fours the animal’s eyes would be level to yours! The reconstructed head had a short face indeed, but on top of that the head looked relatively small for the animal’s massive body. Seen from the front, Arctodus also looked tall, narrow and small-headed relative to a modern brown bear.
The next step was to set the animal in motion. The relatively long limbs of Arctodus led some paleontologists to infer that it would be a good sprinter, a very active predator catching speedy prey on the run. But Mattheus’ analysis of the giant bear’s anatomy led him to a different conclusion: Arctodus would move around at moderate speeds with an efficient pace, with more endurance than speed, which in his view was an adaptation for covering huge distances in search of carrion that it would then appropriate using its huge bulk to expel other predators. In other words, the giant short-faced bear would have been a kleptoparasite. Even its great stature when standing on its hind legs would allow it to scan the horizon for any sign of a recent kill.

This second batch of sketches shows the pacing walk of Arctodus and the bipedal posture it would adopt now and then to scan its surroundings. Finally there is a full-color rendering of the living animal


Was that the real ecological niche of Arctodus? Well, if the dentition of an animal is our main guide to inferring its diet, then Arctodus was not a specialized scavenger, for one thing it didn't nearly have the refined adaptations of a hyena for cracking bones. The teeth of Arctodus were, in spite of differences in detail, bear teeth, and that means a broad-spectrum diet. It could certainly steal kills from most predators around it, but it could also consume vegetable matter and kill its own prey now and then. Its wide distribution in North America indicates a considerable habitat tolerance, and probably a high degree of adaptability, but not enough to survive the wholesale extinction of large mammals at the end of the Pleistocene. As a result, now we need to resort to paleobiological reconstruction if we want to have a reasonable idea of what it looked like and how it behaved. Sad, but better than nothing!

chasingsabretooths.wordpress.com/2017/02/06/reconstructing-a-monster-bear/

[Infinity Blade]

Arctodus simus got treponemal disease, either syphilis, yaws, or, getting into speculative territory, an extinct treponeme.

sci-hub.tw/10.1038/329061a0

[dinosauria101]

This bear seemingly has worse grappling than other bears:

[Infinity Blade]

This A. simus metacarpal was found on the Channel Islands. Because there is no evidence for a population of the species on Santarosae, it is thought that an agent, proposed here to be a flying scavenging bird, brought its bone to the Channel Islands post-mortem. This paper also interprets A. simus as an opportunistic scavenger of both terrestrial and marine carcasses.

www.nature.com/articles/s41598-020-71572-z 

[Infinity Blade]

This thesis paper held some interesting insights into the forelimb anatomy of Arctodus simus. While studies before have clearly demonstrated that A. simus was nowhere near as cursorial to the degree of predators like canids or hyaenids, this study concluded that Arctodus seems to represent the first step towards such a locomotor habit.

Here is the abstract.

The paleobiology of the Pleistocene North American giant short-faced bear, Arctodus simus, has eluded paleontologists for decades. Its more gracile form has led past researchers to myriad intepretations of the locomotion and feeding ecology of this species. While earlier studies have focused on craniodental morphology and simple postcranial indices, it is forelimb morphology that represents a direct compromise between locomotor and foraging behavior. The study here uses traditional and 3D landmark morphometrics to more completely compare the 3-dimensional shape of the major forelimb elements and their muscle attachment sites between A. simus, extant ursids, and other carnivorans. Results herein agree well with previous studies and provide additional evidence for reduced abductor/adductor and supinator/pronator musculature, more restricted parasagittal motion, increased stride length, and lighter and more packed distal elements. Forelimb skeletal morphology therefore supports the hypothesis that A. simmus [sic] represents a bear in the early stages of cursorial evolution.

Very frustratingly, however, I can't fully access it anymore because clicking on a link to the PDF just leads you to a 404 page. Trying to download it will just get you the same result. If anyone can get access to the full thesis paper, I would really love it.

dc.etsu.edu/etd/1477/

[elosha11]

^ Interesting abstract. with its developments seeming to be more for cursorial activity, I wonder if the loss of some level of paw flexibility and abductor/adductor muscles, means the short faced bear was better at running but much less proficient at digging or overturning large rocks, like many modern-day bears. Likewise, despite their great size did their comparatively more gracile build and perhaps less powerful limbs substantially hinder their actual grappling ability? Given its enormous size, it wouldn't matter all that much but I do wonder if it had a harder time with gigantic prey, once it caught them.

[Infinity Blade]

Looks like it. The results seem consistent with earlier findings (see dinosauria101's post above) that suggested that Arctodus was less proficient at grappling, and therefore less proficient at restraining large prey to some degree. Granted, this is in comparison to modern bears and big cats, which are excellent at grappling.

Matheus (2003) suggested that the forelimb bones of Arctodus would have been too slender to withstand the forces associated with rapid acceleration and running and maneuvering at high speeds. This hypothesis could probably use some further testing, but if he is correct, then I really wonder how much of Arctodus' diet would have been comprised of adult megafauna it actually killed. At least some populations seem to have been rather omnivorous, meaning A. simus could get a lot of its energy from either plant matter, terrestrial carcasses, marine carcasses, and prey smaller than itself.

Those with more meat-heavy diets, I don't exactly know what they were doing. It depends on what said meat-heavy diet would have been composed of things that weren't large, dangerous, yet fast prey (i.e. large ungulates). If Arctodus did sometimes go after such prey like some modern bears do, then I imagine it might have had a harder time doing it for a variety of reasons in pretty much every regard (tracking, ambushing, pursuing, and killing).

[Supercommunist]

Very frustratingly, however, I can't fully access it anymore because clicking on a link to the PDF just leads you to a 404 page. Trying to download it will just get you the same result. If anyone can get access to the full thesis paper, I would really love it.

PM your email, I just downloaded the thing.

[Infinity Blade]

Done. Thanks in advance.

[Infinity Blade]

I've been wondering what shape this bear's claws were. The claw cores of brown bears seem to be relatively elongated, which makes sense in light of the long claws they possess in life.


image source->

This paper-> shows additional (illustrated) examples.

From what I can make out of reconstructions of Arctodus' skeleton, its claw cores seem relatively shorter and deeper. They don't seem as elongated as brown bear claw cores.


image source->


Bone Clones replica manus-> (admittedly not an ideal view since the terminal phalanges aren't oriented in lateral view here)

I can kind of see the logic behind the Mammoth Site's claw reproduction.


image source->


Obviously one of my more speculative posts, but some food for thought.