Answer to the last post: it was a unicorn...

Well, not really... at least not the horse-type of unicorn, those can only exist in the imagination of people who want to be marine biologist because they think that by doing so, they will get to train and play with dolphins at some theme park...
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".

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.

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.

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.

Whitmore, F. C., Jr., and J. A. Kaltenbach. 2008. Neogene Cetacea of the Lee Creek Phosphate Mine, North Carolina; pp. 181–269 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.