In North America, Plionarctos gave rise to 2 more genera: Tremarctos (spectacled bears) and Arctodus
(= Arctotherium = short-faced bears).  It is not yet clear whether these two genera arose separately, or whether Tremarctos arose first, then branched off to form Arctodus (Erdbrink 1953; Fig 1:2).
    At least 1 species of spectacled bear and 2 of short-faced bears inhabited North America.  At least 1 species of spectacled bear and 3 species of short-faced bears colonized South America.  Although one would have expected Tremarctinae in Alaska to have crossed the Bering land bridge to Siberia and possibly farther west, no fossils have been found in the Old World.  Some Eurasian specimens once thought to be Tremarctinae are actually Agriotherinae (e.g., Indarctos) or related to Ailuropodinae (Erdbrink 1953).  
 
 
Figure 3:1. Ancient Distribution of Tremarctinae
 
 
Figure 3:2. Tremarctinae Skeletons
 
(Tremarctos from Kurten 1966.  Arctodus from Emslie & Czaplewski 1985; skeleton partially reconstructed by the authors.)
 
 
 
 
            Spectacled bears
 
           Short-faced bears
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
Figure 3:3.  Tremarctinae Skulls
Chapter 3.  TREMARCTINAE