Short-Faced Bear, Actodus
July 2007

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Short-faced bear, Arctos

* How Do We Know This? Like living animals, fossil remains of once-living animals are classified and grouped according to their relationships to each other and to their ancestors.

(Agnarsson et al 2010) (Figueirido et al 2010) (Hunt 1997) (Kurtén 1968) (Kurtén & Anderson 1980) (McKenna and Bell 1997) (Tedford & Martin 2001) (Wayne et al 1989) (Li et al 2007)

Describer (Date): E. D. Cope 1879 for Arctodus simus

Kingdom: Animalia
    Phylum: Chordata
        Class: Mammalia
             Order: Carnivora
                    Family: Ursidae (bears)
                          Subfamily: Tremarctinae (the "running bears")
                                    Genus: Arctodus
                                          Species: Arctodus simus (extinct) short-faced bear
                                          Species: Arctodus pristinus (extinct) lesser short-faced bear
                                    Genus: Tremarctos
                                          Species: Tremarctos ornatus - spectacled bear
                          Subfamily: Ursinae (brown, American black, Asiatic black, sloth, sun, polar bears
                                       and many extinct bear species)
                          Subfamily: Ailuropodinae (includes giant pandas)     

Taxonomic History and Nomenclature Phylogeny

* How Do We Know This? Scientists use knowledge of the earth's rocks, global plate movements, and the chemical process of fossilization to make sense of fossil distribution patterns and ancient habitats.

(Gillette & Madsen 1992, 1993) (Kurtén 1967)(Kurtén & Anderson 1980)(Scott & Cox 1993)

Prehistoric Distribution:

* How Do We Know This? Careful study of fossil bone or tooth anatomy yields much exact information about placement and strength of muscles, tendons, ligaments, nerves, and blood vessels. In rare cases, skin and hair impressions or actual skin or hair is preserved. Body weight is more difficult to gauge because fat leaves no impression on the skeleton.

(Christiansen 1999) (Figueirido et al 2009) (Figueirido et al 2010) (Garshelis 2009)(Kurtén & Anderson 1980) (Salesa et al 2006) (Sorkin 2006)

Estimated Body Weight: 1019 kg (2,246 lbs) for A. simus. For comparison, an average male polar bear's weight is about 600 kg (1323 lb) and may be up to 800 kg (1,764 lb); a Brown Bear's weight varies with diet but is generally less than that of a Polar Bear.
Note: Estimates in literature vary widely for Arctodus; this is a highly dimorphic species, as are living bears, with extreme differences between males and females; many estimates based on very small sample sizes
Estimated Height at Shoulder: 1.6-1.7 m (5.25-5.6 ft); 3 m (9.8 ft) estimated standing upright height
Tail Length: vestigial

General Description
  • Arctodus simus is an enormous extinct bear with a domed skull, wide snout, a short back, with tooth and jaw features that suggest omnivorous habits. (Figueirido et al 2010)
    • Recent studies indicate it is not particularily short-faced for a bear its size and its legs are not long compared to other bears as previously asserted.
    • The common name (the "Short-faced Bear") is misleading.
  • Arctodus simus was the largest Pleistocene carnivore on land.
  • A. pristinus was much smaller, with a similar proportions.
  • Cheek teeth are well-developed and relatively tall (Figueirido et al 2009)
  • Canine teeth massive but short compared to some other bears. (Figueirido et al 2009) (Sorkin 2006)
  • Wear patterns on on first molars are not like those of a typical carnivore such as a wolf. (Sorkin 2006)
  • Unknown
Sexual Dimorphism
  •  Distinct sexual dimorphism; males are about 15% larger than females.
Other Physical Characteristics
  • The living tremarctine bear, Tremarctos, has 53 chromosomes; all other bears have 74.
  • Bones are quite light for their size.
  • Toes align forward, rather than inward as in other bears.
  • Feet are plantigrade (walked flat-footed)
  • Fingers have a small bone in the wrist, the radial sesamoid, which is enlarged, forming a "false thumb" somewhat similar to that of the giant panda and the spectacled bear (Salesa et al 2006)
    • This normally small bone is enlarged, acting as an opposable thumb that allows easier manipulation of plant food.
    • This bone can also aid in tree climbing as seen in the giant panda.
    • This bone is small in meat-eating bears.
  • The articulation of the lower and upper jaws falls above the tooth row, as in plant-eating hoofed mammals. (Sorkin 2006)
  • Eye sockets are small and directed somewhat to the side, similar to those of plant-eating hoofed mammals. (Sorkin 2006)

* How Do We Know This? Since direct observation of a fossil animal's behavior isn't possible, paleontologists use comparison and contrast with living animals for guidance. Tracks can sometimes reveal further clues.

(Barnes et al 2002) (Figueirido et al 2009) (Figueirido et al 2010)(Richards et al 2008)

Social Life
  •   Like all bears today, presumed solitary except for females with cubs
  • Not adapted for fast running locomotion as often asserted in literature. (Figueirido et al 2010)
  • Pleistocene tracks of Arctodus may be preserved at a site in Lakeview, Oregon
    • Right legs move forward at the same time, then left legs move forward together.
    • Camels have this same energy efficient gait.
  • Rather than having a waddling, pigeon-toed walking gait seen in other bears, walked with feet pointing frontward, no waddling.
  • A sesamoid bone in the wrist suggests some capacity for tree-climbing (and plant grasping); pandas have this adaptation.
Interspecies Interaction
  • Studies of Pleistocene Arctodus fossils from Beringia (Siberia, Alaska and the Bering Straight) indicate that competition with brown bears may have been a factor in the extinction of Arctodus. (Barnes et al 2002)
    • The two species coexisted for 10,000 years between 45 to 35,000 years ago.
    • Arctodus fossils are mostly found later when brown bears were absent.
    • The last Arctodus fossils date to the same time when brown bears are again present.
  • Arctodus may have competed for carcasses of hoofed animals killed by saber-toothed cats, American lions, dire wolves (Figueirido et al 2009) and possibly humans.

* How Do We Know This? Clues to fossil mammals' diets come from teeth,skull shape, from fossil dung and gut contents, from lab analysis of isotopes in bone and teeth, and by looking at diets of similar modern animals.

(Barnes et al 2002) (Bocherens et al 2006) (Donohue et al. 2013) (Figueirido et al 2009) (Kurtén 1988) (Matheus 1995) (Matheus et al 2002) (Richards et al 2008) (Ruxton & Houston 2004) (Sorkin 2006)
  • Differing interpretations of Arctodus feeding habits
    • A top predator due to its large size and dentition which appears like that of a carnivore. (Kurtén 1988)
    • A hypercarnivore, feeding on a wide variety of herbivores according to stable isotopes in bone of carbon and nitrogen (Matheus 1995, Matheus et al 2002, Bocherens et al 2006)
      • But Sorkin (2006) notes brown bear fossils from the Pleistocene have a similar isotope reading and they are not strictly carnivores - are instead omnivores.
    • A scavenger (Matheus et al 2000)
      • Articulation of bones do not allow flexibility required of a successful predator
      • Pacing gait suggests endurance, not speed that a predator would need
      • Limbs are somewhat slender, not massive for great strength
      • Powerful jaws capable of crushing bone
      • But such a large animal wouldn't have evolved into being primarily a scavenger in an environment with vultures (Ruxton & Houston 2004)
        • Aerial scavengers held too big an advantage in rapidly locating carrion; large earth-bound scavengers need alternate food supplies.
    • Most likely feeding style: an adaptable omnivore (scavenging and hunting hunting animals and consuming plants too) based on new studies of skull shapes of living bears with known food habits (Figueirido et al 2009, 2010)
      • Ate large animal carcasses, plants, and small animal prey
      • Plant-eating (and tree climbing) habits are also suggested by the short-faced bear's somewhat opposable thumb (helps in grasping, pulling and holding plants) and tall molar teeth (which indicate high biting forces typical of herbivorous species)
      • Coarse plants consumed non-discriminately; not a selective browser because of wide muzzle. (Sorkin 2006)
      • Tooth wear patterns are similar to that of the spectacled bear (Tremarctos ornatus), its closest living relative (Donohue et al. 2013)
  • Didn't have the skull or skeletal adaptations for ambush or pursuit of large mammals. (Sorkin 2006)
  • May have occupied the same ecological role, feeding habits, and body shape as the modern striped and brown hyaenas. (Sorkin 2006)

* How do We Know This? Isotope studies of elements present fossil bones and tusks in microscopic quantities give information about timing of reproductive stress, and timing of nursing. Clues to stages of development come from tooth replacement patterns and closure of sutures in skull and limb bones.

(Matheus 1995) (Schubert and Kaufmann 2003)

Life Stages
  • Juvenile specimens are known from La Brea Asphalt deposits.
  • Paleontologists report records of female short-faced bears using use caves for denning (Schubert and Kaufmann 2003)
  • No known predators on this giant carnivore.
  • Life span unknown.
  • Possible competition with Pleistocene black and brown bears leading to extinction. (Matheus 1995)

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