Ossicones are columnar or conical skin-covered bone structures on the heads of giraffes, male okapi, and some of their extinct relatives. Ossicones are distinguished from the superficially similar structures of horns and antlers by their unique development and a permanent covering of skin and fur.
Giraffe ossicones consist of a highly vascularized and innervated bone core covered with similarly vascularized and innervated skin. [1] They are attached to the skull with vascularized, innervated connective tissue. [1] Ossicones are formed at late gestation, but in early development they are not bony and not fused to the skull yet. Ossicones usually fuse to the skull at sexual maturity. [1] [2]
All male and female giraffes have a pair of parietal ossicones on the parietal bones of the skull. [3] Males also usually have a single median ossicone on the frontal bone that is larger in northern animals and smaller in southern giraffes. [3] Giraffes can also have small additional paired occipital ossicones on the occipital bones, paired orbital ossicones associated with eyes, and azygous ossicones. [3]
In giraffes, male and female ossicones vary in structure and purpose (a manifestation of sexual dimorphism). Males typically have thicker ossicones that become bald on top due to frequent necking. [4] In okapi, the male's ossicones are smaller in proportion to the head, and taper towards their tips, forming a sharper point than the comparatively blunt giraffe ossicone. Whereas female giraffes have reduced ossicones, female okapi lack ossicones entirely.
The morphology of ossicones in the extinct relatives of giraffes and okapi varies widely. Some species had two pairs of ossicones rather than one (e.g. Giraffokeryx ), some had rugged textures (e.g. Shansitherium ), and some had large, flattened ossicones (e.g. male Prolibytherium ).
Similar to species with horns or antlers, male giraffes use their ossicones as weapons during combat, where they use their heads as clubs: the ossicones add weight and concentrate the force of impact onto a small area, allowing it to deliver heavier blows with higher contact pressure. [4] The nerve bundles and large blood supply in the ossicones have led some researchers to speculate that the structures may also play a role in thermoregulation. [1]
Ossicones are only found in some members of the superfamily Giraffoidea, which includes the family Giraffidae (to which giraffes, okapi, and extinct relatives belong) and the entirely extinct family Climacoceratidae. [5] It had been argued that the so-called ossicones known from fossils were actually horns, but later research showed that these structures are consistent with the ossicones of giraffes and okapi. [6] The following is a list of some ossicone-bearing genera:
The giraffe is a large African hoofed mammal belonging to the genus Giraffa. It is the tallest living terrestrial animal and the largest ruminant on Earth. Traditionally, giraffes have been thought of as one species, Giraffa camelopardalis, with nine subspecies. Most recently, researchers proposed dividing them into four extant species due to new research into their mitochondrial and nuclear DNA, and individual species can be distinguished by their fur coat patterns. Seven other extinct species of Giraffa are known from the fossil record.
The okapi, also known as the forest giraffe, Congolese giraffe and zebra giraffe, is an artiodactyl mammal that is endemic to the northeast Democratic Republic of the Congo in central Africa. However, non-invasive genetic identification has suggested that a population has occurred south-west of the Congo River as well. It is the only species in the genus Okapia. Although the okapi has striped markings reminiscent of zebras, it is most closely related to the giraffe. The okapi and the giraffe are the only living members of the family Giraffidae.
Ungulates are members of the diverse clade Euungulata, which primarily consists of large mammals with hooves. Once part of the clade "Ungulata" along with the clade Paenungulata, "Ungulata" has since been determined to be a polyphyletic and thereby invalid clade based on molecular data. As a result, true ungulates had since been reclassified to the newer clade Euungulata in 2001 within the clade Laurasiatheria while Paenungulata has been reclassified to a distant clade Afrotheria. Living ungulates are divided into two orders: Perissodactyla including equines, rhinoceroses, and tapirs; and Artiodactyla including cattle, antelope, pigs, giraffes, camels, sheep, deer, and hippopotamuses, among others. Cetaceans such as whales, dolphins, and porpoises are also classified as artiodactyls, although they do not have hooves. Most terrestrial ungulates use the hoofed tips of their toes to support their body weight while standing or moving. Two other orders of ungulates, Notoungulata and Litopterna, both native to South America, became extinct at the end of the Pleistocene, around 12,000 years ago.
Antlers are extensions of an animal's skull found in members of the Cervidae (deer) family. Antlers are a single structure composed of bone, cartilage, fibrous tissue, skin, nerves, and blood vessels. They are generally found only on males, with the exception of reindeer/caribou. Antlers are shed and regrown each year and function primarily as objects of sexual attraction and as weapons.
The Giraffidae are a family of ruminant artiodactyl mammals that share a common ancestor with deer and bovids. This family, once a diverse group spread throughout Eurasia and Africa, presently comprises only two extant genera, the giraffe and the okapi. Both are confined to sub-Saharan Africa: the giraffe to the open savannas, and the okapi to the dense rainforest of the Congo. The two genera look very different on first sight, but share a number of common features, including a long, dark-coloured tongue, lobed canine teeth, and horns covered in skin, called ossicones.
A horn is a permanent pointed projection on the head of various animals that consists of a covering of keratin and other proteins surrounding a core of live bone. Horns are distinct from antlers, which are not permanent. In mammals, true horns are found mainly among the ruminant artiodactyls, in the families Antilocapridae (pronghorn) and Bovidae. Cattle horns arise from subcutaneous connective tissue and later fuse to the underlying frontal bone.
The northern giraffe, also known as three-horned giraffe, is the type species of giraffe, G. camelopardalis, and is native to North Africa, although alternative taxonomic hypotheses have proposed the northern giraffe as a separate species.
Pecora is an infraorder of even-toed hoofed mammals with ruminant digestion. Most members of Pecora have cranial appendages projecting from their frontal bones; only two extant genera lack them, Hydropotes and Moschus. The name "Pecora" comes from the Latin word pecus, which means "cattle". Although most pecorans have cranial appendages, only some of these are properly called "horns", and many scientists agree that these appendages did not arise from a common ancestor, but instead evolved independently on at least two occasions. Likewise, while Pecora as a group is supported by both molecular and morphological studies, morphological support for interrelationships between pecoran families is disputed.
Prolibytherium is an extinct genus of prolibytheriid artiodactyl ungulate native to Middle Miocene North Africa and Pakistan, from around 16.9 to 15.97 million years ago. Fossils of Prolibytherium were found in the Marada Formation of Libya, Vihowa Formation of Pakistan, and the Moghara Formation of Egypt.
Sivatherium is an extinct genus of giraffid that ranged throughout Africa and Eurasia. The species Sivatherium giganteum is, by weight, one of the largest giraffids known, and also one of the largest ruminants of all time.
Climacoceratidae is a family of superficially deer-like artiodactyl ungulates which lived in the Miocene epoch in Africa. They are close to the ancestry of giraffes, with some genera, such as Prolibytherium, originally classified as giraffes.
Rothschild's giraffe is an ecotype of the Nubian giraffe. It is one of the most endangered distinct populations of giraffe, with 1,399 mature individuals estimated in the wild in 2018.
Giraffokeryx is an extinct genus of medium-sized giraffids known from the Miocene of the Indian subcontinent and Eurasia. It is distinguished from other giraffids by the four ossicones on its head; one pair in front of the eyes on the anterior aspect of the frontal bone and the other behind the eyes in the frontoparietal region overhanging the temporal fossae. It has a brachydont dentition like in other giraffids and its legs and feet are of medium length. Giraffokeryx is considered monotypic by most authors, in the form of G. punjabiensis, but other species have been assigned to the genus:
Honanotherium is a genus of extinct giraffid from the late Miocene of Henan Province, China, and East Azerbaijan Province, northwestern Iran. It was closely related to Bohlinia and was once thought to be ancestral to the modern giraffe. The living animal would have resembled a modern giraffe, but was somewhat shorter, with more massive ossicones.
Bramatherium is an extinct genus of giraffids that ranged from India to Turkey in Asia. It is closely related to the larger Sivatherium.
The Palaeomerycidae is an extinct family of Neogene ruminants belonging to the infraorder Pecora. Palaeomerycids lived in Europe and Asia exclusively during the Miocene, coevolving with cervids, bovids, moschids, and tragulids there as part of a dramatic radiation of ruminants by the early Miocene.
Samotherium is an extinct genus of Giraffidae from the Miocene and Pliocene of Eurasia and Africa. Samotherium had two ossicones on its head, and long legs. The ossicones usually pointed upward, and were curved backwards, with males having larger, more curved ossicones, though in the Chinese species, S. sinense, the straight ossicones point laterally, not upwards. The genus is closely related to Shansitherium. Fossil evidence suggests that Samotherium had a rounded muzzle, which would suggest a grazing lifestyle and a habitat composed of grassland. One common predator of this animal was the Amphimachairodus.
Palaeotragus is a genus of very large, primitive, okapi-like giraffids from the Miocene to Early Pleistocene of Africa and Eurasia.
The reticulated giraffe is a species/subspecies of giraffe native to the Horn of Africa. It is differentiated from other types of giraffe by its coat, which consists of large, polygonal, block-like spots, which extend onto the lower legs, tail and face. These prominent liver-red spots also show much less white between them, when compared to other giraffe species. While the reticulated giraffe may yet still be found in parts of its historic range, such as areas of Somalia and Ethiopia, its population stronghold is primarily within Kenya. There are approximately 8,500 individuals living in the wild.
Giraffomorpha is a clade of pecoran ruminants containing the superfamilies Palaeomerycoidea (Palaeomerycidae) and Giraffoidea, of which the giraffe and okapi of the Giraffidae are the only extant members of the once-diverse clade as a result of a decline in diversity after the Miocene as a result of declines in temperatures. The clade is defined by the presence of ossicones, suggesting that the feature was developed as a result of basal, related evolutions between the two superfamilies rather than as an instance of parallel evolution. Giraffomorpha is also characterized by a lack of articular fossettes (hollows) in metatarsal bones III-IV.
