Evolution and systematics

The taxonomy of the Caprinae is complex and several arrangements have been proposed. Three tribes are generally recognized: Rupicaprini, Ovibovini, and Caprini. Molecular analysis in 1997 suggested that Tibetan antelope or chiru (Pan-tholops hodgsonii), previously classified in a tribe of its own or with saiga (Saiga tatarica), was closer to the Caprinae than the Antilopinae and may be a basal member of the Caprinae. As of 2003 its phylogenetic relationship to the rest of the Caprinae had not been definitively assigned. There is more disagreement among taxonomists at species and subspecies levels. IUCN's Caprinae Specialist Group (CSG) adopted a working classification for the Caprinae Action Plan in 1997 that contained 31 species and 81 subspecies. CSG subsequently established a Taxonomy Working Group to examine outstanding problems.

The tribe Ovibovini contains two genera, each with a single species: musk ox (Ovibos moschatus) and takin (Budorcas taxi-color). Musk ox has two listed subspecies and takin has four. The Rupicaprini has four genera and nine species: serows (Capricornis), three species; gorals (Naemorhedus), three species; mountain goat (Oreamnos), one species; and chamois (Rupicapra), two species. Serow (Capricornis sumatraensis) has five subspecies. Some authors include Formosan serow (C. swinhoei) with the Japanese species (C. crispus). Himalayan goral (Nae-morhedus goral) has two subspecies; N. baileyi has two, and N. caudatus has four. No subspecies have been identified in Oreamnos. Seven subspecies are listed for northern chamois (R. rupicapra), but by the end of 2002 the validity of all of these had not been confirmed. Southern chamois (R. pyrenaica) has three listed subspecies. The Caprini has five genera and 20 species:

Barbary sheep (Ammotragus), one species; tahrs (Hemitragus), three species; blue sheep (Pseudois), two species; true goats (Capra), seven species; and wild sheep (Ovis), five species. Ammotragus has six subspecies, but again the validity of all of these is unconfirmed. No subspecies are recognized in the tahrs. Two subspecies of blue sheep (Pseudois nayaur) are listed. The specific status of dwarf blue sheep (P. schaeferi) has been questioned on the ground that it may be only an ecotype. The taxonomy of Capra is complex. Earlier arrangements placed Alpine ibex, Siberian ibex, Nubian ibex, Walia ibex, and west Caucasian tur (C. caucasica) together in a single species, C. ibex, but they are separated here. Markhor (C. falconeri) has three subspecies and wild goat (C. aegagrus) has four. Spanish ibex (C. pyrenaica) has four known subspecies, two of which are already extinct.

The genus Ovis is even more complex, with a variety of arrangements proposed. All recent authors have recognized the distinctiveness of the larger argalis (O. ammon) from the smaller mouflon and urials. The mouflon and urial group is classified as either one species (O. orientalis) or as two (O. orientalis for the mouflons and western forms, and O. vignei for the eastern urials). Twelve subspecies were recognized in the IUCN's Caprinae Action Plan. It has proved difficult to establish a generally agreed classification of the argalis at the subspecies level, with a large number of forms having been named. Eight are listed in the Caprinae Action Plan. Seven subspecies (one now extinct) are listed for the bighorn sheep (O. canadensis) and four for the snow sheep (Ovis nivicola). Thinhorn sheep (O. dalli) has two subspecies. Techniques of molecular genetic analysis are expected to help validate these forms and further clarify relationships within the subfamily.

Bighorn sheep (Ovis canadensis) fighting. (Photo by Bob & Clara Calhoun. Bruce Coleman, Inc. Reproduced by permission.)

The fossil record is incomplete. Musk ox and takin are descendants of a formerly widespread group in Eurasia and both evolved in the late Pliocene or early Pleistocene. Rupicaprini apparently evolved in Asia, with the Pliocene Pachygazella as a possible ancestor, though the earliest fossils of chamois are known only from the late Pleistocene. An ancestral rupi-caprine must have crossed the Bering land bridge into North America and given rise to Oreamnos, which first appeared there during the Wisconsin glaciation. There is general agreement that the Caprini evolved from the Rupicaprini.

The genus Tossunnoria from the early Pliocene of China shows characters intermediate between gorals and Caprini and is regarded as the probable ancestor of modern Ovis and Capra. Nadler and colleagues confirmed in 1973 Geist's 1971 hypothesis that the Caprini evolved via two main lineages: one through a Barbary sheep-like form to true sheep and another goat-like line.

Members of the genus Capra first appeared in the mid-Pleistocene, probably from a tahr-like ancestor. From the fossil evidence, its distribution once reached the Atlantic coast of Europe. There are few fossils of Ovis. The earliest ones known were found in China and Europe and date from the Pliocene. The genus evolved in the mountains of Eura sia from where they radiated northwards and northeast, crossing the Bering land bridge into North America. Earliest evidence of Ovis in North America dates from about 100,000 years ago. Most modern Caprini evolved during the Pleistocene, a time when there were 18 major glaciations and Geist suggested in 1989 that Caprini differ from Rupicaprini as a result of adaptation to colder climates and to open rather than closed habitats.

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