During the Pleistocene (3 million – 11,000 years ago), a genus of bear called Arctodus roamed North America.
Little is known regarding the early history of these short-faced bears. Considered the most common of the early North American bear, they belong to the group known as running bears. About 800,000 years ago it is evident they became widespread in North America and were most numerous in California
Remains have been found as far north as Alaska and south in Mississippi, and a fossil found in California was recorded as one of the largest bear fossils found. At one time, Arctodus simus was possibly the Earth’s largest mammalian, predatorial carnivore. Males would have stood at a height of almost 6ft tall while on all fours and a height of 13ft upright. The South American giant short-faced bear, the closely related genus Arctotherium, was also similar in size. A fossil found in Argentina in 1935 was re-examined in 2011. Estimating the weight of this particular species at 3,500 lb., this species was discovered to be the largest bear ever to exist. The species Acrtodus pristinus, roamed up from central Mexico, east from northern Texas to New Jersey and showing large numbers inhabiting Florida.
Considered highly carnivorous based on the analysis of their bones, adult bears seemed to require approximately 35 lb of flesh per day to survive. High concentrations of the stable nitrogen isotope, nitrorge- 15, revealing no evidence of vegetation ingestion. Researchers do disagree on the bears diet based its size, body-type, and mass. One paleontologist concluded that Arctodus simus moved in a pacing motion like a horse or modern bear would and due to that motion would not be able to attack like a true predator. Some conclude this species to be a kleptoparasite; using their intimidating size to steal the kill from smaller predators, such as American lions and dire wolves. The demise of these smaller predators in conjunction with the improved hunting techniques by humans approximately 12,000 years ago, are both considered to be reasons to the extinction of this species.