Tribal Origins and Radiations

Bovids have long been classified among a dozen or so higher taxa, typically tribes. Simpson's (1945) classification divided bovids among 13 tribes in five subfamilies. Since that time, work on bovid systematics (morphological, behavioral, and molecular) has revealed a deep bifurcation within Bovidae separating the Bovinae from all the remaining bovids (Kingdon 1982; Lowenstein 1986), and has condensed the number of tribes to around 10 (e.g., Hassanin and Douzery 1999b).

Bovinae (Bovini+Tragelaphini+Boselaphini)

Bovinae is the sister group to all remaining living bovids and is characterized by numerous morphological, behavioral, and molecular synapomorphies (e.g., Kingdon 1982; Hassanin and Douzery 1999b; Vrba and Schaller 2000; Bibi 2009). Eotragus sansaniensis (15.2Ma, France, Sen 1996), E. noyei (ca.18Ma, Pakistan), and an unnamed species from the Vihowa Formation (ca.18Ma, Pakistan) have been tentatively proposed as early or stem Bovinae (Solounias and Moelleken 1992; Azanza and Morales 1994; Solounias et al. 1995). These fossils would suggest that the cladogenetic event separating the Bovinae from the remainder of Bovidae must have occurred in the early Miocene, or certainly by 18Ma.

Stem Bovini were likely present in southern Asia (Siwaliks) by 9Ma, as represented by Selenoportax (Bibi 2007). After this time, early bovines dispersed first to Africa then to Europe. It is not clear when or where exactly the crown clade originated as the fossil taxon closest to the most recent common ancestor of all living bovines has not yet been identified. The earliest African record of a bovine is at Toros-Menalla (Vignaud et al. 2002), Chad, at around 7Ma. By the end of late Miocene, several western Eurasian "boselaphines" begin to exhibit morphology suggestive of the bovine condition but the earliest European record of a bovine is in the early Pliocene (Gromolard 1980). The ability to confirm with certainty a southern Asian origin of Bovini is hampered by the poor African fossil record between 10 and 7Ma, whereby the absence of Bovini from Africa during this time may be a result of preservation bias. Once Bovini had originated, multiple bovine lineages become evident but good hypotheses of between- and within-lineage relationships are lacking. The water buffalo Bubalus most likely emerged from the Pliocene Proamphibos (Pilgrim 1939) and the African Syncerus from a Ugandax (Gentry and Gentry 1978) though the precise limits of the latter genus, along with that of Simatherium, are still unclear. The origins of Bos and Bison and the interrelationships of the many fossil species seemingly relevant to them remain problematic. The hypothesis that Pelorovis gave rise to Bos (Martínez-Navarro et al. 2007) requires further evidence.

The history of the Tragelaphini as known so far is almost entirely African. The earliest tragelaphine that is probably a crown group member (i.e., not a stem representative) is recognized from the Lukeino Formation (Thomas 1980), Kenya, dated to between 6.56–5.72Ma (Deino et al. 2002). Tragelaphini are entirely absent from the fossil record of Chad (Geraads et al. 2001; Vignaud et al. 2002) and not well represented in the North African record (Geraads et al. 1998). The Lukeino tragelaphine already bears the main horn core synapomorphies of the group, and no consensus was reached on the possible existence of any stem group lineages or potential ancestors for Tragelaphini. The stem tragelaphine lineage may have evolved either in Africa or Eurasia. Pheraios chryssomallos (Kostopoulos and Koufos 2006), from the late Miocene of Greece, has been proposed as a possible stem tragelaphine.

Boselaphini is a large, diverse, and, as currently understood, non-monophyletic group. A revision of the systematics of taxa variously grouped as Boselaphini is warranted. The crown clade—i.e. the two living boselaphines, Boselaphus tragocamelus and Tetracerus quadricornis, along with their immediate ancestors—is most likely separate from the majority of Miocene fossil forms generally referred to "Boselaphini." Crown Boselaphini has a poor fossil record, with a handful of Asian localities of imprecise Plio-Pleistocene age yielding fossil Boselaphus and Proboselaphus (Rütimeyer 1878; Matsumoto 1918; Pilgrim 1939). We know of no fossil record for Tetracerus. The Miocene fossils traditionally attributed to "Boselaphini" may be divided among independent clades and stem groups of one or more living bovid tribes (Pilgrim 1939; Moyà-Solà 1983; Thomas 1984b; Bibi 2007). The dominant late Miocene fossil "boselaphines," both in terms of diversity and geographic range, are species of the genera Miotragocerus and Tragoportax. Interrelationships between Tragoportax and Miotragocerus remain unclear, with diagnoses so far proposed to distinguish the two genera being difficult to consistently apply in a wide context. Species of these genera range throughout the late Miocene, and are found across Eurasia from China to Spain, in the Siwaliks, and to the far south of South Africa. Tragoportax, Miotragocerus, and other taxa such as Protragocerus, have been united under the name Tragocerina or Tragocerini (Sokolov 1953; Korotkevich 1981; Thomas 1984a, 1984b). However, by the rules of the International Code on Zoological Nomenclature (Article 39), a family-group taxon name may not be based on a genus found to be a junior homonym—in this case Tragocerus, previously occupied for a beetle. As a result we adopt for this fossil group the name Tragoportacini, appropriately based on Tragoportax—this being the genus in which we now place the old type species T. amalthea, of Tragocerus—for a fossil group uniting Tragoportax, Miotragocerus, and allies.

Antilopinae/Aegodontia

The clade including all bovids that are not Bovinae is known as either Antilopinae (Kingdon 1982; Hassanin and Douzery 1999b) or Aegodontia (Schlosser in Zittel 1911:499, sensu Vrba and Schaller, 2000). The former term is more appropriate in the context of phylogenetic nomenclature but is problematic for the Linnean system, as it would incorporate other subfamilial designations (e.g., Caprinae). 'Antilopinae' (sensu Simpson 1945) has also been previously used to unite the Antilopini + Neotragini, but the non-monophyly of the latter group (see below) means such a designation is now probably unnecessary, leaving the name available for redefinition as the sister-group to Bovinae. Aegodontia has been recently redefined, but is ambiguous in that this group would include bovids that are not 'aegodont' ('goat-toothed') as originally diagnosed. This clade is supported by numerous morphological, behavioral, and molecular synapomorphies (e.g., Kingdon 1982; Hassanin and Douzery 1999b; Vrba and Schaller 2000; Bibi 2009).

Pseudoeotragus seegrabensis from Austria may be an early stem member of Antilopinae (Azanza and Morales 1994 figs. 7-8), with an age of 18–17Ma (MN4, Made 1989; Agustí et al. 2001).

The oldest substantiated African occurrences of Reduncini occur at Lothagam (Harris 2003) and Toros-Menalla (Vignaud et al. 2002). These may prove younger than the earliest occurrence in the Siwaliks, which may be 8Ma or older (Barry et al. 2002). The cranium attributed to Pachytragus aff. solignaci from the late middle or early late Miocene of Ngorora, Kenya (Thomas 1981), deserves re-examination as a possible reduncine. The problem of the relatively poor African late Miocene fossil record is again pertinent here.

In line with recent phylogenetic studies, it was agreed that "Neotragini" is not monophyletic. The majority of "Neotragini" are paraphyletic with respect to Antilopini (Gentry 1992; Rebholz and Harley 1999). The most species-rich and widespread antilopine is Gazella. Fossils attributed to this taxon go back to the late mid-Miocene of Fort Ternan, Kenya (ca. 14Ma, Gentry 1970; Shipman et al. 1981).

The origins of the impala, Aepyceros, remain unknown. The impala is first known from late Miocene African localities, with the oldest secure records coming from Lothagam and Toros-Menalla (Harris 2003; Geraads et al. 2008). A study of impala origins should investigate a possible connection with European and North African spiral-horned antilopines. The living species, A. melampus, is first recorded in the latest Pliocene, at Koobi Fora (Harris 1991) and possibly Omo (Gentry 1985).

The Hippotragini + Alcelaphini + Caprinae together form a monophyletic clade, substantiated by numerous recent phylogenetic studies (e.g., Vrba and Schaller 2000; Ropiquet and Hassanin 2005). Fossil taxa such as Protoryx and Pachytragus, debated to be either caprine or hippotragine (Bosscha Erdbrink 1988; Gentry 2000) may actually lie closer to a common ancestor of both these clades. Taxa such as Tethytragus, Gentrytragus, and Caprotragoides may also lie in various positions in, or close to, the stem group of the larger clade Caprini+Hippotragini+Alcelaphini (Azanza and Morales 1994; Gentry 2000). The relationships of these three genera to one another are still not known, particularly of Tethytragus to the other two. The old name Pseudotragini (sensu Solounias and Moelleken 1992) has been proposed as a paraphyletic group that includes possible stem caprines. Pachytragus solignaci (Tunisia, MN9/10 in Geraads 1989; ca.10Ma in Steininger 1999) has more specifically been proposed as a potential early crown caprine (Solounias and Moelleken 1992). "Ovibovini" is non-monophyletic, with phylogenetic studies partitioning Budorcas and Ovibos among Caprinae, (Bouvrain and de Bonis 1984; Ropiquet and Hassanin 2005).

The earliest confirmed hippotragines are known from Toros-Menalla, Chad, aged to about 7Ma (Geraads et al. 2008). The Chadian fossils demonstrate that hippotragines present a mix of derived and primitive characters that suggestively place them at the stem of the hippotragine clade. The maximal age of the most recent common ancestor of all living Hippotragini (crown clade) is as a result set to around 7Ma.

The oldest Alcelaphini are known from between about 7.5–6.5Ma from Lothagam, Kenya (Harris 2003; McDougall and Feibel 2003). Alcelaphini are virtually absent at Toros-Menalla, and not recorded from Mpesida (Thomas 1980) and Mio-Pliocene Middle Awash deposits (Haile-Selassie et al. 2004; Haile-Selassie et al. 2009). Vrba's (1997) phylogenetic study suggests the origins of crown Alcelaphini are to be found prior to 5Ma.