3
* auditory bullae are not enlarged Additionaly, members of this family possess posteriorly oriented M2 postprotocrista molars, elongated m2 molars, and a reduction of the premolars. Modern bears comprise eight species in three subfamilies: Ailuropodinae (monotypic with the giant panda), Tremarctinae (monotypic with the Spectacled Bear), and Ursinae (containing six species divided into one to three genera, depending upon authority). The earliest members of Ursidae belong to the extinct subfamily Amphicynodontinae, including Parictis (late Eocene to early middle Miocene, 38-18 million years (Ma) ago) and the slightly younger Allocyon (early Oligocene, 34-30 Ma), both from North America. These animals looked very different from today's bears, being small and raccoon-like in overall appearance, and a diet perhaps more similar to that of a badger. Parictis does not appear in Eurasia and Africa until the Miocene. It is unclear whether late Eocene ursids were also present in Eurasia, although faunal exchange across the Bering land bridge may have been possible during a major sea level low stand as early as the late Eocene (~37 Ma) and continuing into the early Oligocene. European genera morphologically very similar to Allocyon, and also the much younger American Kolponomos (~18 Ma), are known from the Oligocene, including Amphicticeps and Amphicynodon. The raccoon-sized, dog-like Cephalogale is the oldest-known member of the subfamily Hemicyoninae which first appeared during the middle Oligocene in Eurasia ~30 Ma ago. The subfamily also includes the younger genera Phoberocyon (~20-15 Ma), and Plithocyon (~15-7 Ma). A Cephalogale-like species gave rise to the genus Ursavus during the early Oligocene (30-28 Ma); this genus proliferated into many species in Asia and is ancestral to all living bears. Species of Ursavus subsequently entered North America together
* lacrimal bone is vestigial
* cheek teeth are bunodont and hence indicative of a broad, hypocarnivorous (non-strictly meat-eating) diet (although hypercarnivorous (strictly meat-eating) taxa are known from the fossil record)
* carnassials are flattened
Fossil bears
with Amphicynodon and Cephalogale during the early Miocene (21-18 Ma). Members of living lineages of bears diverged from Ursavus ~20 Ma ago, likely via the species Ursavus elmensis. Based on genetic and morphological data, the subfamily Ailuropodinae (pandas) was the first to diverge from other living bears ~19 Ma ago, although no fossils of this group have been found pre-dating about 5 Ma. The New World short-faced bears (Tremarctinae) differenciated from Ursinae following a dispersal event into the Americas during the mid Miocene (~13 Ma). Their earliest fossil representative is Plionarctos in North America (~10-2 Ma). This genus is probably the direct ancestor to the North American short-faced bears (genus Arctodus), the South American short-faced bear (Pararctotherium and Arctotherium), and the spectacled bears, Tremarctos, represented by both an extinct North American species (T. floridanus), and the lone surviving representative of the Tremarctinae, the South American spectacled bear (T. ornatus). The subfamily Ursinae experienced a dramatic proliferation of taxa ~5.3-4.5 Ma ago coincident with major environmental changes, with the first members of the genus Ursus also appearing around this time. The sloth bear is a modern survivor of one of the earliest lineages to diverge during this radiation event (~5.3 Ma); it took on its peculiar