Today two blogposts are published
because they relate strongly to one another. Here is the post on torsioned tenrec/odontocete skulls.
The narwhal (genus Monodon, Fig. 1)
is famous for having one giant spiral tooth sticking out ahead of its skull. Monodon also has one of the most bizarre skulls of all mammals and departs from that of all tetrapods, partly due to the root of the tooth and partly due to the migration of the nares to the back of the skull. Except for its tips, the jugal is missing. The maxilla, lacks teeth (if you don’t count the tusk) and rather than extending below the orbit, extends over the forehead, following the naris on its migration to the back of the skull. The bulbous portion of the skull, the cranium is made of parietals in most mammals, but the parietals are greatly reduced, nearly absent in Monodon.
The sister taxon of the narwhal
is the beluga (genus: Delphinapterus). It helps one understand the narwhal a bit better. At least the beluga has a few traditional teeth. These two taxa nest together in the large reptile tree (LRT, 1087 taxa, Fig. 4).
Less derived and more primitive
is Chonecetus (Fig. 3), which has nares closer to the snout tip, and more teeth, and more cranium. This taxon and its sister, Aetiocetus, have been traditionally considered transitional from toothed whales to baleen whales, like Balaenoptera, but baleen whales have an entirely separate ancestry derived from desmostylians, like Desmostylus.
A recent paper on Monodon tusks (Nweeia et al. 2012)
found “the narwhal tusks are the expression of canine teeth and that vestigial teeth have no apparent functional characteristics and are following a pattern consistent with evolutionary obsolescence.” (See Figs. 5, 6).
In dorsal or ventral view
it is clear that the the tusk (left) side of the skull is longer than the right side due to angling the posterior skull relative to the rostrum.
I wonder about the bending of the skull
toward the left in these two whales. Could asymmetry have anything to do with stereo auditory senses? Asymmetry is also found in owl skulls, another taxon that depends strongly on acute hearing for locating prey.
With 1187 taxa and 231 traits full resolution was recovered in the LRT after running PAUP FOR 16 minutes and 15 seconds. The single best tree has 16,329 steps.
Linnaeus C 1758. Systema naturæ per regna tria naturæ, secundum classes, ordines, genera, species, cum characteribus, differentiis, synonymis, locis. Tomus I. Editio decima, reformata.’
Nweeia MT et al. (9 co-authors) 2012. Vestigial tooth anatomy and tusk nomenclature for Monodon monoceros. The Anatomical Record 295:1006–1016.
Pallas PS 1766. Miscellanea Zoologica.