Osodobenus
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| Osodobenus Temporal range: Miocene | |
|---|---|
Scientific classification  | |
| Kingdom: | Animalia |
| Phylum: | Chordata |
| Class: | Mammalia |
| Order: | Carnivora |
| Parvorder: | Pinnipedia |
| Family: | Odobenidae |
| Genus: | †Osodobenus Biewer et al., 2020 |
| Species: | †O. eodon |
| Binomial name | |
| †Osodobenus eodon Biewer et al., 2020 | |
Osodobenus is an extinct genus of walrus from the Miocene to Pliocene of California. Osodobenus may have been the first tusked walrus and shows several adaptations that suggest it was a suction feeder, possibly even a benthic feeder like modern species. Three skulls are known showing pronounced sexual dimorphism, with the female lacking the same tusks as the male. Only a single species, Osodobenus eodon, is currently recognized.
Osodobenus is known from three specimens including an adult male, adult female and a juvenile specimen preserving skulls and some postcranial material. All the material was collected from the Late Miocene to Early Pliocene (Messinian to Zanclean) Capistrano Formation, Orange County, California alongside the remains of several other early odobenids. In 2020, Biewer and colleagues published a detailed description of the material, establishing Osodobenus as a new genus while also erecting two new species of Pontolis.[1]
The genus name of Osodobenus is a combination of the genus name for the modern walrus "Odobenus" and the Oso Member, a Messinian strata within the Capistrano Formation. The species name eodon translates to "dawn tooth" or "dawn tusk", a reference to this animal's key role in the evolution of walrus tusks.[1]
Description
Compared to other basal odobenids, the rostrum of Osodobenus is short and robust, bulging notably around the canines and with a forward-projecting premaxilla. The nares are large with thick borders and roofed by the nasal bones. The palate is arched and a pair of infraorbital foramen enlarged. The later suggest that much like the modern species, Osodobenus had a mustache-like collection of whiskeres covering the upper lip. Behind the canines, the teeth are single-rooted with inflated bulbous crowns showing little wear. The first two incisors of Osodobenus are transversely compressed with minor wear of the apices. The third incisor is notably longer and slender and there is no cingulum on the inner side of the jaws as in other odobenids. The prominent canines are robust, conical and recurved. Compared to the width of the skull, these canines are approximately 50% larger than those of other basal walrus genera. The pulp cavity is open which indicates that the canines were continuously growing, something only known from tusked odobenids. This however only applies to the males, while the female has smaller canines more proportional to the skull width. This indicates that the presence of tusks in Osodobenus was sexually dimorphic. The teeth of Osodobenus show a somewhat transitional form between the less specialised odobenids with a tooth specialisation score of 0.4 and lower and the more derived and specialised members of the group (score >0.5). This indicates that odobenids rapidly specialised during the late Tortonian to early Messinian of the Miocene, which included the development of tusks in later taxa. This again corresponds to what is observed in Osodobenus, whose canines form an intermediate form between what has previously been considered tusked and tuskless. Biewer and colleagues argue that the continuous nature of tusk growth in an evolutionary context makes a rigid separation futile and that Osodobenus represents the first walrus with canines that could be considered tusks.[1]
