Giraffoidea

Reconstruction of Climacoceras gentryi, copyright Apokryltaros.

Belongs within: Pecora.
Contains: Giraffidae.

Africa’s answer to deer
Published 5 June 2023

The word “iconic” gets thrown around far too freely these days but I don’t think I’d be wrong in describing giraffes as one of the iconic animals of the African savannah. Taller by far than any other animal living alongside them, and considerably weightier than most, giraffes simply cannot be easily mistaken for anything else. It should come as no surprise that such a noteworthy animal is attached to an equally noteworthy lineage.

Masai giraffe Giraffa tippelskirchi, copyright Peter Prokosch.

The family Giraffidae to which giraffes belong is represented in the modern fauna by just two genera. Each of these is often recognised as including a single species, the okapi Okapia johnstoni and giraffe Giraffa camelopardalis, though recent authors have argued for the division of giraffes between up to four species (Coimbra et al. 2021). Okapis, which were recently covered in some detail by Darren Naish over at Tetrapod Zoology (see here and here), are found in canopy forests in the north of the Democratic Republic of Congo. They are significantly smaller and shorter-necked than giraffes though it should be pointed out that they are still larger and longer-necked than the average ruminant. Giraffids may be distinguished from other ruminants in the modern fauna by the possession of ossicones, horn-like cranial appendages composed of bone with a covering of skin rather than horn. The ossicones of okapis are sharper than those of giraffes and are not covered at the tips. Conflict between male giraffids involves two individuals standing side-by-side before swinging the head and neck at each other. Though the ossicones presumably add to the impact of such blows, they are not so much offensive weapons in their own right. Even the sharp tips of okapi ossicones are used more in fending off predators than in intra-specific conflict (Churcher 1978).

Okapi Okapia johnstoni, copyright Derek Keats.

The modern Giraffidae represent the surviving component of the broader superfamily Giraffoidea. The exact composition of the Giraffoidea has differed between authors. Depending on the preferred ruminant phylogeny, some would include another living species, the pronghorn Antilocapra americana of North America, within the taxon. There are also various fossil taxa, such as the deer-like Palaeomerycidae and the early Miocene plate-crowned Prolibytherium, that resemble giraffids in the possession of ossicones. However, though the ossicones of giraffids are distinctive in the modern fauna, they may have been less so in an evolutionary context. There has been a fair bit of disagreement down the years over whether the various cranial appendages of ruminants (horns, antlers, ossicones, whatever exactly we should call those things pronghorns have) evolved independently or were modified from a shared ancestral state. If the latter, then ossicones seem likely to be the most primitive form, and their presence in fossils may not evidence a specific giraffid connection. Hou et al. (2023) disputed a direct relationship between palaeomerycids, Prolibytherium and giraffoids.

Ossicone of Climacoceras gentryi, from Hamilton (1978b).

One fossil group whose inclusion in the giraffoids is generally accepted is the Climacoceratidae, deer-like ruminants known from the Miocene of Africa. Climacoceratids and giraffids are united by the presence of a posterior lobe on the lower canine, a feature known from no other ruminant (Hamilton 1978b). Climacoceratids also showed the first stages of the elongation of neck and legs that would come to characterise the giraffids. The ossicones of Climacoceras were remarkable affairs, long and curved with multiple branching tines, resembling the antlers of deer. However, they lack the basal boss of antlers, indicating that they were permanent structures and not shed. True deer have consistently failed to establish a significant presence in Africa, only appearing in the northernmost part of the continent during the Pliocene (Hamilton 1978a). Other climacoceratids may have lacked ossicones though one might question whether such differences were related to species or sexual dimorphism.

Also present in the early Miocene of Africa were species such as Canthumeryx sirtensis that were closer to modern giraffids than Climacoceras, as indicated by features of their dentition (Hamilton 1978b). Giraffids have therefore been an African affair throughout their history though at least two lineages, the true giraffes and the gigantic sivatheres, would later expand into Eurasia. But of them, I think, I will have to speak another time.

Systematics of Giraffoidea
<==Giraffoidea
| i. s.: Andegameryx Ginsburg et al. 1994MSP99
| `--A. andegaviensis Ginsburg et al. 1994MSP99
| LorancameryxHD03
| Progiraffa Pilgrim 1908H78b, H78a
| `--P. exiguaH78b
|--+--GiraffidaeH78b
| `--Canthumeryx Hamilton 1973 [incl. Zarafa Hamilton 1973; Canthumerycidae]H78b
| `--*C. sirtensis Hamilton 1973H78a [incl. *Zarafa zelteni Hamilton 1973H78a, H78b, MSP99]
`--Climacoceratidae [Climacoceratinae, Climacoceridae, Sperrgebietomerycinae]MSP99
| i. s.: InjanatheriumJS88
|--Orangemeryx Morales, Soria & Pickford 1999MSP99
| `--*O. hendeyi Morales, Soria & Pickford 1999MSP99
`--+--Nyanzameryx Thomas 1984MSP99
| `--N. pickfordi Thomas 1984MSP99
`--Climacoceras MacInnes 1936H78b, MSP99
|--C. africanus MacInnes 1936MSP99
`--C. gentryi Hamilton 1978MSP99

*Type species of generic name indicated

References

Churcher, C. S. 1978. Giraffidae. In: Maglio, V. J., & H. B. S. Cooke (eds) Evolution of African Mammals pp. 509–535. Harvard University Press: Cambridge (Massachusetts).

Coimbra, R. T. F., S. Winter, V. Kumar, K.-P. Koepfli, R. M. Gooley, P. Dobrynin, J. Fennessy & A. Janke. 2021. Whole-genome analysis of giraffe supports four distinct species. Current Biology 31: 2929–2938.

[H78a] Hamilton, W. R. 1978a. Cervidae and Palaeomerycidae. In: Maglio, V. J., & H. B. S. Cooke (eds) Evolution of African Mammals pp. 496–508. Harvard University Press: Cambridge (Massachusetts).

[H78b] Hamilton, W. R. 1978b. Fossil giraffes from the Miocene of Africa and a revision of the phylogeny of the Giraffoidea. Philosophical Transactions of the Royal Society of London Series B—Biological Sciences 283: 165–229.

[HD03] Hassanin, A., & E. J. P. Douzery. 2003. Molecular and morphological phylogenies of Ruminantia and the alternative position of the Moschidae. Systematic Biology 52 (2): 206–228.

Hou, S., Q. Shi, M. J. Benton & N. Solounias. 2023. Comment on “Sexual selection promotes giraffoid head-neck evolution and ecological adaptation”. Science 379: eadd9559.

[JS88] Janis, C. M., & K. M. Scott. 1988. The phylogeny of the Ruminantia (Artiodactyla, Mammalia). In: Benton, M. J. (ed.) The Phylogeny and Classification of the Tetrapods vol. 2. Mammals pp. 273–282. Clarendon Press: Oxford.

[MSP99] Morales, J., D. Soria & M. Pickford. 1999. New stem giraffoid ruminants from the early and middle Miocene of Namibia. Geodiversitas 21 (2): 229–253.

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