the Madrean Region. History Although North America and South America are presently joined by the Isthmus of Panama, these continents were separated for about 180 million years, and evolved very different plant and animal lineages. When the ancient supercontinent of Pangaea split into two about 180 million years ago, North America remained joined to Eurasia as part of the supercontinent of Laurasia, while South America was part of the supercontinent of Gondwana. North America later split from Eurasia. North America has been joined by land bridges to both Asia and South America since then, which allowed an exchange of plant and animal species between the continents, the Great American Interchange. A former land bridge across the Bering Strait between Asia and North America allowed many plants and animals to move between these continents, and the Nearctic realm shares many plants and animals with the Palearctic. The two realms are sometimes included in a single Holarctic realm. Many large animals, or megafauna, including horses, camels, tapirs, mammoths, mastodons, ground sloths, sabre-tooth cats (Smilodon), short-faced bears and the American cheetah, became extinct in North America at the end of the Pleistocene epoch (ice ages) in what is called the Quaternary extinction event. Flora and fauna Flora and fauna that originated in the Nearctic Mammals originally unique to the Nearctic include: Order Primates – the first primate/proto-primate, Purgatorius, originated in the Early Paleocene of the Nearctic. Family Canidae – dogs, wolves, foxes, and coyotes. Family Camelidae – camels and their South American relatives including the llama. Now extinct in the Nearctic Family Equidae – horses, donkeys and their relatives. Now only found in the Nearctic as feral horses. Family Tapiridae – tapirs now extinct in the Nearctic. Family Antilocapridae – last survivor of which is the pronghorn. Subfamily Tremarctinae (short-faced bears) – including the giant short-faced bear (Arctodus simus). The only surviving member of the group is the spectacled bear (Tremarctos ornatus) of South America. Flora and fauna endemic to the Nearctic One bird family, the wrentits (Timaliinae), is endemic to the Nearctic region. The Holarctic has four endemic families: divers (Gaviidae), grouse (Tetraoninae), auks (Alcidae), and the waxwings (Bombycillidae). The scarab beetle families Pleocomidae and Diphyllostomatidae (Coleoptera) are also endemic to the Nearctic. The fly species Cynomya cadaverina is also found in high numbers in this area. Plants families endemic or nearly endemic to the Nearctic include the Crossosomataceae, Simmondsiaceae, and Limnanthaceae.