Delphinida

Amazon river dolphins Inia geoffrensis, copyright Mamíferos de Colombia.

Belongs within: Odontoceti.
Contains: Delphinidae, Phocoenidae.

The whale that looked like a walrus
Published 16 March 2008

Odobenocetops (shown above in an illustration from de Muizon & Domning 2002) was definitely one of the odder forms of whale. It was found off the coast of Peru in the early Pliocene, about 3 or 4 million years ago, where it seems to have converged in a number of details on the modern walrus (Odobenus rosmarus). Or perhaps it is more accurate to say that the modern walrus converged on it. While Odobenus also appeared in the early Pliocene (Berta et al. 2005), Odobenocetops may have had a slight head start on it—the records in the Paleobiology Database suggest that the earliest records of Odobenus (from Belgium and New Jersey) are of somewhat uncertain age.

The walrus feeds on molluscs, particularly soft-shelled bivalves. The thick upper lip bears a strong array of sensitive bristles for finding molluscs buried in the bottom sediment. However, unlike most molluscivores the walrus doesn’t directly crush the shells of its prey. Instead, the shell of the mollusc is held in the thick lips while the tongue is used like a piston to remove the animal from its shell by suction. Odobenocetops had a broadened palate and evidence of a thickened upper lip (probably with similar bristles) like a modern walruses, and probably fed in the same manner. The interesting thing is that Odobenocetops also evolved enlarged tusks like the walrus. Before the discovery of Odobenocetops, most authors had believed that the walrus’ tusks were unrelated to its unusual feeding method, having probably evolved instead through sexual selection. That Odobenocetops also combined suction feeding and tusks suggests that their combination might not be as accidental as previously thought. What exactly the role of the tusks may be in feeding is more uncertain. De Muizon & Domning (2002) suggested that they might function as guides for the sensory bristles, or they may have been used to guide prey animals towards the mouth.

One of the most unusual features of toothed whales (in a group well-provided with unusual features) is a tendency towards a loss of bilateral symmetry, with one side of the skull being more developed than the other. The reasons for this asymmetry are uncertain, but the most popular theory is that it is related to the development of the sonar system. Odobenocetops possessed one of the most dramatic examples of skull asymmetry in the cetaceans. The left tusk of the type specimen of O. peruvianus is estimated to have been about 20 cm in length (because the tusks were quite fragile, fossilised specimens are invariably broken). However, the right tusk was over twice as long, at least 50 cm. The type of the other known species, O. leptodon, is even more dramatic—the left tusk is about the same size as known for O. peruvianus at 25 cm, but the right tusk was over one metre! Oddly enough, the remainder of the skull was rather less asymmetrical than in other odontocetes, as Odobenocetops had lost the melon and therefore the sonar of other toothed whales. Sonar was probably unnecessary for a diet of less mobile animals than fed on by other whales.

White whale Delphinapterus leucas, from Whale Trust.

Relationship-wise, Odobenocetops is included in its own family, but was closely related to the Monodontidae, the family including the narwhal and white whale (also known as beluga). Monodontids also have rather mobile lips compared to other whales. Even more significantly, Odobenocetops and monodontids both have a completely mobile neck with unfused vertebrae, in contrast to other cetaceans which have the first two to five vertebrae fused into a single mass. This extra head motility would have doubtless been critical in allowing Odobenocetops to become an efficient sediment feeder.

Systematics of Delphinida
Delphinida [Monodontoidea]
| i. s.: Albireo Barnes 1984 [Albireonidae]SM93
| `--A. whistleri Barnes 1984BDR85
| Pithanodelphis Abel 1905B76
|--+--+--Parapontoporia [Parapontoporiinae]GS03
| | | `--P. sternbergi Gregory & Kellogg 1927BDR85
| | `--LipotidaeGS03
| | |--Prolipotes Zhou et al. 1984SM93
| | | `--P. yujiangensis Zhou, Zhou & Zhao 1984BDR85
| | `--Lipotes Miller 1918FS15, W81
| | `--*L. vexillifer Miller 1918W81
| `--+--Pontoporiidae [Pontoporiinae, Stenodelphinae, Stenodelphininae]L05
| | |--Pliopontos de Muizon 1983BDR85
| | |--Pontistes Burmeister 1885BDR85
| | `--Pontoporia Gray 1846FS15, W81 [=Stenodelphis Gervais 1847B76]
| | |--*P. blainvillei (Gervais & d’Orbigny 1844) (see below for synonymy)W81
| | `--‘Stenodelphis’ sternbergi Gregory & Kellogg 1927B76
| `--Iniidae [Iniinae, Pontoplanodidae, Saurocetidae, Saurodelphidae]GS03
| |--GoniodelphisHC01
| |--Ischyrorhynchus vanbenedeniGS03
| |--SaurocetesHC01
| |--AnisodelphisBDR85
| |--SaurodelphisBDR85
| |--Proinia True 1910SM93
| `--Inia d’Orbigny 1834W81
| `--I. geoffrensis (de Blainville 1817) (see below for synonymy)W81
| |--I. g. geoffrensisW81
| |--*I. g. boliviensis d’Orbigny 1834W81
| `--I. g. humboldtiana (Pilleri & Gihr 1977)W81
`--DelphinoideaL05
|--+--DelphinidaeFS15
| `--+--PhocoenidaeFS15
| `--+--Odobenocetops Muizon 1993 [Odobenocetopsidae]MD02
| | |--*O. peruvianus Muizon 1993MD02
| | `--O. leptodon Muizon, Domning & Parrish 1999MD02
| `--MonodontidaeMD02
| |--DenebolaMJ11
| |--Monodon Linnaeus 1758FS15, W81
| | `--*M. monoceros Linnaeus 1758W81
| `--Delphinapterus Lacépède 1804 [Delphinapterinae]W81
| `--D. leucas (Pallas 1776) (see below for synonymy)W81
`--KentriodontidaeGS03
|--Atocetus de Muizon 1988SM93
|--Oligodelphis azerbajdzanicus Mchedlidze & Aslanova 1968SB02
|--Macrokentriodon Dawson 1996L05
|--Leptodelphis Kirpichnikov 1954SM93
|--Sarmatodelphis Kirpichnikov 1954SM93
|--Microphocaena Kudrin & Tatarinov 1965SM93
|--Kampholophos Rensberger 1969B76 [Kampholophinae]
| `--K. serrulus Rensberger 1969B76
|--Kentriodon Kellogg 1927 [Kentriodontinae]B76
| `--K. pernixGS03
|--Delphinodon Leidy 1869SM93
| `--D. dividum True 1912B76
|--Liolithax Kellogg 1931SM93
| `--L. kernensis Kellogg 1931B76
`--Lophocetus Cope 1868BDR85, B76 [Lophocetinae]
|--L. calvertensis (Harlan 1842)B76
|--L. pappus Kellogg 1955B76
`--L. repenningi Barnes 1978BDR85

Delphinapterus leucas (Pallas 1776) [=Delphinus leucas; incl. Balaena albicans, Phocoena albus, Catodon candicans, Delphinapterus leucas dorofeevi (Barabash & Klumov 1935), Delphinapterus friemani, Delphinus kingii, Delphinapterus kingii, Delphinapterus leucas marisalbi (Ostroumov 1935)]W81

Inia geoffrensis (de Blainville 1817) [=Delphinus geoffrensis; incl. D. amazonicus, D. rostratus]W81

*Pontoporia blainvillei (Gervais & d’Orbigny 1844) [=Delphinus blainvillei, *Stenodelphis blainvillei]W81

*Type species of generic name indicated

References

[B76] Barnes, L. G. 1976. Outline of eastern North Pacific fossil cetacean assemblages. Systematic Zoology 25 (4): 321–343.

[BDR85] Barnes, L. G., D. P. Domning & C. E. Ray. 1985. Status of studies on fossil marine mammals. Marine Mammal Science 1 (1): 15–53.

[FS15] Faurby, S., & J.-C. Svenning. 2015. A species-level phylogeny of all extant and late Quaternary extinct mammals using a novel heuristic-hierarchical Bayesian approach. Molecular Phylogenetics and Evolution 84: 14–26.

[GS03] Geisler, J. H., & A. E. Sanders. 2003. Morphological evidence for the phylogeny of Cetacea. Journal of Mammalian Evolution 10 (1–2): 23–129.

[HC01] Hamilton, H., S. Caballero, A. G. Collins & R. L. Brownell Jr. 2001. Evolution of river dolphins. Proceedings of the Royal Society of London Series BBiological Sciences 268: 549–556.

[L05] Lambert, O. 2005. Phylogenetic affinities of the long-snouted dolphin Eurhinodelphis (Cetacea, Odontoceti) from the Miocene of Antwerp, Belgium. Palaeontology 48 (3): 653–679.

[MJ11] Meredith, R. W., J. E. Janečka, J. Gatesy, O. A. Ryder, C. A. Fisher, E. C. Teeling, A. Goodbla, E. Eizirik, T. L. L. Simão, T. Stadler, D. L. Rabosky, R. L. Honeycutt, J. J. Flynn, C. M. Ingram, C. Steiner, T. L. Williams, T. J. Robinson, A. Burk-Herrick, M. Westerman, N. A. Ayoub, M. S. Springer & W. J. Murphy. 2011. Impacts of the Cretaceous terrestrial revolution and KPg extinction on mammal diversification. Science 334: 521–524.

[MD02] Muizon, C. de, & D. P. Domning. 2002. The anatomy of Odobenocetops (Delphinoidea, Mammalia), the walrus-like dolphin from the Pliocene of Peru and its palaeobiological implications. Zoological Journal of the Linnean Society 134: 423–452.

[SB02] Sanders, A. E., & L. G. Barnes. 2002. Paleontology of the Late Oligocene Ashley and Chandler Bridge Formations of South Carolina, 3: Eomysticetidae, a new family of primitive mysticetes (Mammalia: Cetacea). Smithsonian Contributions to Paleobiology 93: 313–356.

[SM93] Stucky, R. K., & M. C. McKenna. 1993. Mammalia. In: Benton, M. J. (ed.) The Fossil Record 2 pp. 739–771. Chapman & Hall: London.

[W81] Watson, L. 1981. Sea Guide to Whales of the World. Hutchinson: London.

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