Prevalence of Ossicones in Giraffidae (Artiodactyla, Mammalia)

Abstract

Geraads (1986) suggested that extinct Giraffidae differed from the two extant species in having no true ossicones. True ossicones are epiphyseal growths. Geraads (1986) suggested that all extinct Giraffidae have which are, by contrast, apophyseal extensions of the frontal bone. The purpose of this note is to show that extinct giraffids had true epiphyseal ossicones. The okapi (Okapia johnstoni) and the giraffe (Giraffa camelopardalis) are the only extant members of the family Giraffidae, which may have included approximately 25 species during the Neogene. The two living giraffids have horn-like structures termed ossicones (Geraads, 1986; Hamilton, 1978; Lankester, 1907a, 1907b; Spinage, 1968). Developmentally, ossicones appear as separate cartilaginous structures situated dorsal to the frontal bone; they enlarge, ossify, and fuse to the skull later in life (Solounias, 1988; Spinage, 1968), unlike the of the Bovidae that probably are apophyseal extensions of the frontal bone (Janis and Scott, 1987). Until recently horns of extinct giraffids also were assumed to be true ossicones, arising by developmental processes similar to those of the extant species. Geraads (1986) challenged this view by proposing that the Climacoceridae (Giraffoidea), and the Sivatheriinae, Palaeotraginae, and Samotheriinae (Giraffidae) do not have true ossicones, but instead have derived by an upward extension of the frontal or the parietal bone. Thus, the extinct giraffid were apophyseal exostoses rather than epiphyseal growths (Geraads, 1986). Geraads (1986) used the presence of ossicones as a shared derived feature that can be used to unite the two extant species, Giraffa camelopardalis and Okapia johnstoni. According to him, Okapia johnstoni is not a primitive species but a close relative to Giraffa. In his cladogram, all the extinct giraffids have no ossicones, therefore they precede the origin of ossicones. Geraads (1986) argued that if there were true ossicones in Neogene giraffids, they occasionally would be found fossilized before they fused to the skull, which he asserts is a common occurrence with the ossicones in East African Pleistocene Giraffa. For example, there is an undescribed female Giraffa cranium from the Omo deposits with unfused ossicones (J. M. Harris, pers. comm.). However, it is unreasonable to extrapolate from Giraffa for all Giraffidae. In addition, Geraads (1986) asserted that there was a developmental constraint on ossicone form. True ossicones are epiphyseal structures and as such grow downward. Geraads (1986) and Janis (1986) hypothesized that it would be impossible to attain the branching patterns of the horns of Climacoceridae (Giraffoidea) and sivatheres (Giraffidae) with such a developmental process. This assertion implies the presence of a different developmental process for a branching horn. Growth from the base upward would be necessary to create the observed branching patterns in some extinct species. A similar argument has been proposed for sivatheres by Spinage (1968), although he did not question the homology of ossicones between extinct and extant giraffids. Solounias (1988) showed that apophyseal horns are a much more common occurrence among mammals than was supposed. For example, they occurred in mylagaulid rodents, arsinotheres, uintatheres, brontotheres, diceratheriine rhinocerotids, protoceratids, some oreodonts, a suid, antilocaprids, merycodontids, dromomerycids = palaeomerycids, climacocerids, and Triceromeryx. I agree with Geraads (1986) and Janis (1986) that the Climacoceridae and Triceromerycidae probably had horns that were outgrowths of the frontals, whatever their direction of growth might have been. I present evidence that Giraffidae such as Siivatheriinae, Palaeotraginae (which includes only P. rouenii (=microdon) and P. coelophrys (=quadricornis)), and Samotheriinae possessed true ossicones. I also show that sivatheres did not have branching ossicones, despite certain apparently branched conditions. The evidence for true epiphyseal fossil ossicones from extinct giraffid subfamilies comes from the late Miocene fossil-bearing deposits of Samos in Greece, Shanxi deposits of China, Miocene horizons of Russia, and the Siwaliks of India. Samotherium boissieri is an abundant species at Samos (>1,000 specimens) but ossicones are known from only five to eight specimens. Thus, the chance of finding an unfused ossicone of a juvenile male is poor. Nevertheless, there is such a specimen (MGL Samos 712; Fig. 1) figured by Major (1902), who suggested it may be either an ossicone of a juvenile or that of a female. The size of the orbit