Around the Miocene-Pliocene boundary (~5 Ma), tremarctines, along with other ursids, experienced an explosive radiation in diversity, as C4 vegetation (grasses) and open habitats dominated, the world experienced a major temperature drop and increased seasonality, and a faunal turnover which extinguished 60–70% of all Eurasian faunal genera, and 70–80% of North American genera.

[6] The medium sized Arctodus pristinus inhabited a broad range in the North American continent, with Tremarctos floridanus endemic to the Gulf Coast.

[5] Around this time, Tremarctos ornatus, otherwise known as the spectacled bear, starts appearing in the South American fossil record.

[14] Additionally, tremarctine bears' skulls are deeper and more brachycephalic, their zygomatic arches and glenoid fossas are well developed, and they have larger molars in comparison with ursines.

[13] Tremarctines inhabited a wide range of niches- from small and mostly herbivorous bears inhabiting more forested habitat, such as Arctotherium wingei and Tremarctos ornatus, to the colossal Arctotherium angustidens and Arctodus simus; plains adapted omnivores with a penchant for large quantities of meat.

While Arctodus simus had a wide range across North America for 800,000 years, Arctotherium angustidens appears to be limited to the Southern Cone, in open plains habitat.

Furthermore, whereas Arctodus simus varied its diet between quasi-carnivory in Alaska to classic omnivory, Arctotherium angustidens had similar rates of carnivory across specimens, according to isotope studies.

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.