Ossicones are paired weapons 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.
Ossicones are present at birth, located dorsally of the frontal bone. Initially, they are capable of being retroflexed and are not attached to the skull, which permits successful descent from the birth canal. Ossicones fuse to the skull later in life.
Ossicones are similar to the horns of antelopes and cattle, except that they are derived from ossified cartilage or subcutaneous connective tissues rather than living bone, and that the ossicones remain covered in skin and fur, rather than horny keratin. Antlers (as on deer) are derived from bone tissue: when mature, the skin and fur covering of the antlers, termed "velvet", is, unlike ossicones, sloughed and scraped off to expose the bone of the antlers.
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. 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 that possess horns or antlers, giraffids use their ossicones as a weapon by necking and concentrating the force of impact onto a small area. Ossicones add weight to the animal's head, allowing it to deliver heavier, sometimes fatal, blows. The added weight is an evolutionary trait bred from necessity. The addition of ossicones also leads to other evolutionary adaptations such as heavy hides and specialized dermal shields for defence.
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. It has been argued that the so-called ossicones known from fossils are actually horns; however, later research has revealed these structures to be consistent with the ossicones of giraffes and okapi. The following is a list of some ossicone-bearing genera:
- †Bramatherium (2 pairs)
- †Decennatherium (1 pair)
- Giraffa (giraffes; 1 pair)
- †Giraffokeryx (2 pairs)
- †Honanotherium (1 pair)
- †Injanatherium (2 pairs) – Angled horizontally relative to head
- †Mitilanotherium (1 pair)
- Okapia (okapi; 1–0 pairs) – Females lack ossicones
- †Palaeotragus (1–0 pairs) – Some species lacked ossicones
- †Samotherium (1 pair)
- †Shansitherium (1 pair)
- †Sivatherium (2 pairs)
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