Anyways, the picture on the previous post seems to have been quite straightforward after all. Yes, it is part of a narwhal tusk, so congrats to all who guessed right! Narwhals, which go by the scientific name Monodon monoceros, are odontocetes (toothed whales), distinguished by the long spiraled tusk seen in the males (and occasionally females) of the species, and are thus sometimes called the "unicorn of the sea".
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| Dorsal view of a double-tusked narwhal, on exhibit at the Sant Ocean Hall at the National Museum of Natural History in Washington, DC. Normally, narwhals only have one enlarged tusk (the left one) so this one is an exception. |
The closest relatives of the narwhals are the belugas or white whales (Delphinapterus
leucas); together these two species
comprise the family known as Monodontidae. Among extant toothed whale groups,
monodontids are more closely related to porpoises (Phocoenidae) and to dolphins
(Delphinidae) (e.g. see Geisler et
al., 2011), whereas relationships with extinct groups include Albireonidae
(Barnes, 1984) and Odobenocetopsidae (de Muizon, 1993). Odobenocetops previously featured here,
also has an enlarged tusk-like tooth like Monodon. However, the tusks are located in different bones
within the rostrum (the premaxilla in Odobenocetops, the maxilla in Monodon [see below]), and hence although somewhat similar, they are probably not
homologous.
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| Ventral view of the rostrum of extant monodontids. |
The fossil record of monodontids, is scarce (at
least when compared to that of other living odontocete groups). The oldest monodontid
known is Denebola brachycephala from the
late Miocene of Baja California (Barnes, 1984). Other fossil monodontids are known from early
Pliocene deposits in the North Sea (Lambert
and Gigase, 2007) and the eastern coast of North America (Withmore, 1994;
Withmore and Kaltenbach, 2008; Kazár and Bohaska, 2008)*. All these fossil
forms are more like Delphinapterus,
i.e. they have multiple teeth in the maxilla, whereas Monodon only has an enlarged tusk (usually only in the male) and the females is edentulous (see illustration above). So how did the tusk of narwhals came to be or what is it for? It is
still a bit of a mystery!
*I'll blog more on these North American ones sometime next year, so stay tuned!
For a neat article on the tusk and on monodontids visit this
page over at Tetrapod Zoology v2.
And here if you or someone you know wants to be a marine biologist.
And here if you or someone you know wants to be a marine biologist.
References:
Barnes, L. G. 1984. Fossil odontocetes (Mammalia: Cetacea)
from the Almejas Formation, Isla Cedros, Mexico. PaleoBios 42:1–46.
Geisler, J. H., M. R. McGowan, G. Yang and J. Gatesy. 2011.
A supermatrix analysis of genomic, morphological, and paleontological data from
crown Cetacea. BMC Evolutionary Biology 11:112.
Kazár, E., and D. J. Bohaska. 2008. Toothed whale (Mammalia:
Cetacea: Odontoceti) limb bones of the Lee Creek Mine, North Carolina; pp.
271-324 in C. E. Ray, D. J. Bohaska, I. A. Koretsky, L. W. Ward, and L. G.
Barnes (eds.), Geology and Paleontology of the Lee Creek Mine, North Carolina,
IV. Virginia Museum of Natural History Special Publication 14.
Lambert, O., and P. Gigase. 2007. A monodontid cetacean from
the Early Pliocene of the North Sea. Bulletin de l’Institut Royal des Sciences
Naturelles de Belgique, Sciences de la Terre 77:197–210.
Muizon, C. de. 1993. Walrus-like feeding adaptation in a new cetacean from the Pliocene of Peru. Nature 365:745–748.
Whitmore, F. C., Jr. 1994. Neogene climatic change and the emergence of the modern whale fauna of the North Atlantic Ocean. Proceedings of the San Diego Society of Natural History 29:223–227.
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