Monodon: THE weirdest skull of all mammals

Today two blogposts are published
because they relate strongly to one another. Here is the post on torsioned tenrec/odontocete skulls.

Figure 1. Distinct from most narwhals, this skull also has right tusk emerging from the canine position. And yes, that's the maxilla covering most of the skull, even above the orbit! I added an eyeball here to help locate the orbit. The mesethmoid is the red bone that divides the naris (blow hole).

Figure 1. Distinct from most narwhals, this skull also has right tusk emerging from the canine position. And yes, that’s the maxilla covering most of the skull, even above the orbit! I added an eyeball here to help locate the orbit. The mesethmoid is the red bone that divides the naris (blow hole).

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.

Figure 2. The beluga, Delphinapterus, is closely related to, though less derived than the narwhal, Monodon. More teeth of a regular shape were present in the jaws. Those two yellow arrows indicate a misalignment of the centerline of the top of the occiput vs. the bottom. Compare to figure 3.

Figure 2. The beluga, Delphinapterus, is closely related to, though less derived than the narwhal, Monodon. More teeth of a regular shape were present in the jaws. Those two yellow arrows indicate a misalignment of the centerline of the top of the occiput vs. the bottom. Compare to figure 3. The mesethmoid is the red bone in the blow hole. This skull is also bent left, as in the narwhal.

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).

Figure 3. Chonecetus has a more primitive skull with nares closer to the snout tip and no maxilla above the orbit.

Figure 3. Chonecetus has a more primitive skull with nares closer to the snout tip and no maxilla above the orbit. Not a transitional taxon to baleen whales, which have another separate origin. This drawing lacks any indication of torsion, perhaps because the back half was separated from the front half and the artist ‘repaired’ the twist.

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.

Figure 5. Subset of the LRT focusing on the tenrec/odontocete clade with several whales added.

Figure 4. Subset of the LRT focusing on the tenrec/odontocete clade with several whales added.

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).

Figure 4. Image from Nweeia et al. 2012 showing the unerupted right tusk and the root of the left tusk in the male narwhal along with two unerupted tusks in the female.

Figure 5. Image from Nweeia et al. 2012 showing the unerupted right tusk and the root of the left tusk in the male narwhal along with two unerupted tusks in the female. Note the angle of the posterior skull relative to the anterior midline.

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.

Figure 6. CT scans of a female narwhal (Monodon) showing soft tissues and unerupted teeth. Note the angle of the posterior skull relative to the anterior.

Figure 6. CT scans of a female narwhal (Monodon) showing soft tissues and unerupted teeth. Note the angle of the posterior skull relative to the anterior. The left side, the tusk side, is shorter than the right side in figure 5, so the label ‘ventral’ is an error here. This is a dorsal view of the female skull in figure 5. Always test scale bars and labels.

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.

Figure 7. Fetal narwhal skull, here colorized from Nweenia et al. 2012. The jugal disappears in adults.

Figure 7. Fetal narwhal skull, here colorized from Nweenia et al. 2012. The jugal disappears in adults. The asymmetry is already apparent here.

Figure 8. Common bottle nose dolphin skull (genus: Tursiops) also displays a bit of asymmetry in dorsal view.

Figure 8. Common bottle nose dolphin skull (genus: Tursiops) also displays a bit of asymmetry in dorsal view. Note the yellow arrows on the parietal showing the wee bit of torsion here. 

Update:
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.

References
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.

wiki/Narwhal
wiki/Beluga_whale

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