Evolution of the giraffe

Evolution of the giraffe

In the past Europeans knew about deers, sheep, goats, and cattle in their breeding campaigns and as ibex and chamois in the mountains. Deer have been found in Eurasia, while Bovidae has been concentrated in Africa. The giraffe Giraffa camelopardalis belonged to Giraffids, and in 1901 a second one the Okapi – Okapia johnstoni was discovered in Africa.

The giraffe and okapi are big ruminants with long necks and legs. Both species have somewhat toughened skin over their bony horns, present in both sexes in the Giraffe, but only in males in the Okapi. Giraffes are inhabitants of the countryside or the bush, sometimes in quite arid areas, while the Okapi is a true inhabitant of the forest. It is interesting and instructive that such similar crabapples can be found in such contrasting habitat species.

The earlier relatives of Giraffes go back to early in the Miocene epoch, before 20 mya, in Africa and perhaps elsewhere. Conclusions on their last continent of origin are very much influenced by the molecular phylogenies that they still disagree on which pecoran family is most closely related to the Giraffidae.

The Giraffidae belong to the sous-families of the Canthumerycinae, Bohlininae, Giraffokerycinae, Sivatheriinae, Paleotragines, and the Okapiinae and Giraffinae. The first unequivocal giraffids, animals like Canthumeryxand Injanatherium, have been found as fossils from Africa to Pakistan, also in south-eastern Europe, and their descendants to other parts of Asia.

These early giraffids were large ruminants for their period (17–10 mya) and had widely divergent horns with spreading bases broadened in an anteroposterior direction and flattened mediolaterally. Sometimes they had a small anterior pair of horns in front of the orbits in addition to the main pair, but the nature of the skin covering the horns is unknown.

Coincidental with the arrival of three-toed hipparionine horses from North America between 11 and 10 mya, Old World ruminant faunas changed. Two more advanced giraffe subfamilies emerged: Giraffinae and Sivatheriinae. Animals with branched or bulbously-tined horns, retaining an anterior pair of horns, and re-evolving somewhat shorter cannon bones.

They persisted into the Pleistocene in India and Africa, in the latter until 500,000 years ago or possibly later. They likely went extinct for the same reason that caused the disappearance of large grazing bovids (Pelorovis, Megalotragus) and giant grazing pigs.

The common Eurasian late Miocene giraffines are a separate development from whatever stock gave rise to modern Giraffa. Some of them approach hypsodonty in their teeth, and microwear studies have shown that they must have grazed. In this, they are very unlike Giraffa, as also in that their legs were not invariably lengthened to the same great extent.

After thriving for several million years, they declined and then died out in the Pliocene. The precise phylogenetic position of Okapia is unclear, the key question is whether it split recently from Giraffa or whether it has a long-separate ancestry.

Its most notable difference from Eurasian Miocene giraffes is the proximity of the two horn inserts. This could be considered adaptive for the forest habitat, except the same trait is found in the giraffe. There are big differences between Giraffa and Okapia. Molecular clocks suggest a divergence at 15 mya, while other reviewers have posited 8 mya.

It can be surmised that giraffid ruminants survived by becoming larger than other ruminants, but their Eurasian radiation was curtailed after the end of the Miocene. In Africa, Okapia and Giraffa specialized in high-level sailing ability, one in and sivatheres has not survived to the present day. The giraffe has also been present in India for a long time, but the date of its extinction is unknown.

The Giraffidae are confined to Giraffa camelopardalis, it is only representative of the genus Giraffa. Together with the only other living giraffid, the Okapi – Okapia johnstoni, they represent the extant members of a previously more diverse group. Not long after the description of the species, Levaillant in 1790 described the southern giraffe as a separate species.

This separation between northern and southern species continued through the 1800s. Thomas (1901) separated the reticulated giraffeGiraffa reticulata as a separate species from all other forms of Giraffe, and this was maintained by Lydekker (1904). Subsequent authors followed either classification scheme: Dollman (1929) and Allen(1939) separated the reticulated giraffe as a separate species, while Scott (1959) maintained the northern versus southern species split.

However, for much of the latter half of the twentieth century, only a single living species of Giraffe has been recognized. Brown et al. (2007) identified six genetically distinct lineages, with little evidence of interbreeding between them, and proposed that some may represent distinct species.

The most notable features of the giraffe are the long limbs and the neck. When paired cranial appendages occur in ruminants, they are found on the frontal bones in the lateral supraorbital position. In Giraffa, the pairs of ossiconess attach to the skull further back. These ossicones are straight with rounded ends and vary individually in form.

In nonexistent giraffes, the parietal osicones show strong sexual dimorphism, and the lower incisors and canines are robust and arched. The lower canine istypically bifid and occasionally trifid. The cheek teeth are variable in size between species, with premolars relatively complex. Basicranialand basifacial planes typically are not parallel and skull flexion varies individually in the species.

The genus has an Asian origin, with the earliest known examples of African giraffes coming from fossils dating from the late Miocene and early Pliocene of Kenya and early Pliocene of South Africa. Churcher (1978) described four 1997 – accepted 3 extinct species, with G. gracilis included in G. stillei.

The South African specimens are isolated teeth and ossicones, while the East African material is more substantial. The species is based on an almost complete skull and jaw with a significant proportion of the postcranial skeleton.Giraffa jumae specimens are larger than the G. camelopardalis. While the dimensions of the skull are greater, the teeth are similar in absolute size in comparison between the two species. The ossicones of G. camelopardalis, extend back parallel to the plane of the skull.

Family Giraffidae

No medial horns have been identified in specimens attributed to G. jumae. According to the larger skull, the mandible is more consistent than in Giraffa stillei, from the early Pliocene to early middle Pleistocene of East Africa, which was first described as an Okapi based on dental characters and forelimb anatomy.

Harris (1976) rediagnosed the sespecimens to Giraffa, according to the premolar morphology. Churcher (1978) suggested that the dental characters used to diagnose this taxon were highly individually variable and showed greater affinity to another giraffid genus, Palaeotragus. Churcher(1978) suggested that, without additional material, the affinities ofthe specimens currently assigned to G. stillei remained controversial.

However, Harris (1987, 1991b) and Gentry (1997) accepted G.stillei and subsumed G. gracilis into it. Specimens of Giraffa gracilis have been found in East Africa and possibly South Africa from the late Pliocene to Pleistocene. The limb bones and neck are of a similar absolute length to G.camelopardalis but have a lighter structure and finer proportions in all parts of the skeleton.

The bases of the ossicones are oval in section, smaller than in G. camelopardalis or G.jumae is oriented at the same angles as the modern giraffe. Secondary bone deposition occurs overthe ossicones. The area between the orbits is convex (Harris 1976) and may or may not have a median horn present.

Churcher (1978)suggested that despite generalizations about the relative size of G. jumae, G. camelopardalis and G. gracilis, the skeletal elements overlapped in size between these purported species making size alone an unreliable character for species identification.

A species of giraffe with smaller and more delicate ossicones than those of G. gracilis and flattened on the posterolateral surfaces occurs in the early Pleistocene of East Africa. Giraffa pygmaea shows(presumed) sexual dimorphism with secondary bone deposition occurring in the (presumed) males, increasing the proportions of the ossicones relative to the (presumed) females.

The dental characters are typical of giraffes, but they differ from other species in their small size. Churcher (1978) considered that this species was not sufficiently defined to be recognized as a distinct species. Many species of African giraffe are dependent on relatively little material, and the status of some species can be equivocal. Many fossil specimens were initially assigned to G. camelopardalis and have subsequently been reassigned to the extinct species, bringing forward the earliest recorded fossils of the modern giraffe.

Contemporary giraffes vary individually, sexually and geographically in size while skull morphology is also highly variable within theextant taxon. As many of the described differences (particularly in size) may represent extremes of continuous variation further fossil material is necessary to substantiate the status of the species within Giraffa.

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