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 vascularized and innervated skin. [1] The base of an ossicone is 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 were thought to be one species, Giraffa camelopardalis, with nine subspecies. Most recently, researchers proposed dividing them into up to eight extant species due to new research into their mitochondrial and nuclear DNA, as well as morphological measurements. 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. 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 Ungulata which primarily consists of large mammals with hooves. Living ungulates are divided into two orders: the odd-toed ungulates (Perissodactyla) including horses, rhinoceroses, and tapirs; and even-toed ungulates (Artiodactyla) such as cattle, pigs, giraffes, camels, sheep, deer, and hippopotamuses. Cetaceans such as whales, dolphins, and porpoises are also classified as even-toed ungulates, 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.
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 “horned livestock”. 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 giraffids that ranged throughout Africa to the Indian subcontinent. The species Sivatherium giganteum is, by weight, one of the largest giraffids known, and also one of the largest ruminants of all time.
Climacoceras is a genus of extinct artiodactyl ungulates that lived in Africa and Europe during the Miocene. The members of Climacoceras were related to giraffes, and the genus was formerly placed within the Giraffidae, but is now placed in the Climacoceratidae, a sister group within the superfamily Giraffoidea. Fossils of the two best known species of Climacoceras, C. africanus and C. gentryi, have both been found in Kenya. The animals measured about 1.5 m (4.9 ft) tall and had large ossicones resembling antlers. C. africanus had ossicones resembling tall, thorn-covered plant stems, while the ossicones of C. gentryi resembled thorny crescents.
Rothschild's giraffe is a subspecies of the Northern giraffe. It is one of the most endangered distinct populations of giraffe, with 1,399 mature individuals estimated in the wild in 2018.
Protoceratidae is an extinct family of herbivorous North American artiodactyls that lived during the Eocene through Pliocene at around 46.2—4.9 Mya, existing for about 41 million years.
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 are an extinct family of ruminants in the order Artiodactyla. Palaeomerycids lived in North America, Europe, Africa and Asia from 33 to 4.9 million years ago, existing for about 28 million years; one species was also reported from South America, but its identity as a palaeomerycid was subsequently disputed.
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.
Giraffoidea is a superfamily that includes the families Climacoceratidae, Antilocapridae, and Giraffidae. The only extant members in the superfamily are the pronghorn, giraffe, and okapi. The Climacoceratidae are also placed in the superfamily, but were originally placed within the family Palaeomerycidae.
The reticulated giraffe, also known as the Somali giraffe, is a species or subspecies of giraffe native to the Horn of Africa. It lives in Somalia, southern Ethiopia, and northern Kenya. There are approximately 8,500 individuals living in the wild.
