Several syntheses of bear phylogeny have been done in the past (e.g., Erdbrink 1953, Davis 1964, Kurte'n 1966, 1976, Kurte'n & Anderson 1980). But none is up-to-date.* So I have combined key findings from those sources with more recent information to provide an overview of lineages as a basis for understanding bear evolution. This information is fascinating in its own right. And understanding why some species have already gone extinct may help us prevent remaining ones from sharing their fate (see Owen-Smith 1989).
All living bear species and most extinct ones are grouped into three subfamilies: Tremarctinae, Ailuropodinae, and Ursinae. The only living Tremarctinae and Ailuropodinae are, respectively, the spectacled and panda bears. All other living species are Ursinae. Species names given here for extant species conform to Wozencraft (1989:578).
* Miacids
* Ursavus, Protursus, Indarctos, and Agriotherium
* Ancestors of the Tremarctinae and Ursinae
MIACIDS: THE EOCENE PROGENITOR
All modern carnivores derive from Eocene ancestors known as Miacids (Figs. 1:1 
& 1:2 
). These were small, weasel-like forest dwellers adapted for climbing trees, with a semi-opposable first digit and wide-spreading paws. (Ewer 1973; Eisenberg 1981).
& 1:2 ). These were small, weasel-like forest dwellers adapted for climbing trees, with a semi-opposable first digit and wide-spreading paws. (Ewer 1973; Eisenberg 1981).
MIOCENE ANCESTORS
Ursavus
The warm Miocene era was a time of luxuriant vegetation and rapid evolution of plants, herbivores, and predators. It was then that Miacids gave rise to the first Canoidea, the progenitors of procyonids and later canids, as well as ancestors of the bear-dog Hemicyon and the dawn bear Ursavus (Erdbrink 1953; Ewer 1973; Kurte’n 1976). Ursavus became a distinct genus in mid-Miocene Europe (Ewer 1973; Kurte'n 1976; Colbert 1980), and founded the Ursidae 
(Fig. 2:1).
(Fig. 2:1). Like the canoidea, Ursavus had all 4 sets of premolars--the first 3 sets still adapted for shearing flesh. But its carnassials (P4/M1) had begun to flatten and broaden for chewing, a trend that continued over the next several million years in Ursavus and its descendants, the true bears. Ursavus probably climbed trees and had a diet like that of a modern badger (e.g., Meles meles or Taxidea taxus) (Kurte'n 1976). Ursavus increased in size as much of the Earth's climate became more arid, with savannas, steppes, and deserts encroaching on forests. It was sympatric with apes, crocodiles, hippos, palm trees, and other tropical or subtropcial species.
During the early Miocene, Ursavus split into at least 2 branches. The first led to 2 families, the Agriotherinae and Ailuropodinae. The second led, through Protursus, to 2 more families, the Tremarctinae and Ursinae.
Plionarctos & Protursus
About 7-19 MYA, Ursavus (possibly U. brevirhinus) gave rise to Plionarctos in North America, while another branch of the Ursavus stock led to Protursus (Merriam & Stock 1925; Thenius 1959; Kurte'n 1966, 1967). In the Western Hemisphere, Plionarctos founded the Tremarctinae, giving rise to several species, of which only the spectacled bear remains (Kurte'n 1966, Kurte'n & Anderson 1980). Protursus founded the Ursinae, whose descendants include all other living and many extinct species.
Compared to Ursinae and Ailuropodinae, Tremarctinae had longer legs relative to body length ( 
Fig. 2._). Compared especially to Ursinae, Tremarctine bears had a shorter muzzle, superficially similar to Agriotherium, making the skull a bit similar to that of a felid or bulldog (
Fig. 2:4)
Fig. 2._). Compared especially to Ursinae, Tremarctine bears had a shorter muzzle, superficially similar to Agriotherium, making the skull a bit similar to that of a felid or bulldog ( Fig. 2:4) Each of these 3 subfamilies has a number of unique characteristics, including skeletal features, blood chemistry, and chromosome number. Among living species, 2N = 52 in Tremarctinae, 74 in Ursinae, and 42 in Ailuropodinae (Pocock 1917; Davis 1955, 1964; Hough 1948; Kurte'n 1966; Seal et al. 1970; Ewer 1973; Thenius 1979). Despite those differences, successful interbreeding of captives can occur, as shown by mating of a male Tremarctos to a female Ursus thibetanus; whether their cub was sterile had not been determined (Mondolfi 1983).
Table 2:2. Geographic Radiation Over Time
Chapter 1. PRIMORDIAL BEARS
Part I: PHYLOGENY &
GEOGRAPHIC RADIATION
Table 2:1. Geological & Evolutionary Time Scales
(Tentative Correlations)