Sensory ability in the narwhal tooth organ system.

The erupted tusk of the narwhal exhibits sensory ability. The hypothesized sensory pathway begins with ocean water entering through cementum channels to a network of patent dentinal tubules extending from the dentinocementum junction to the inner pulpal wall. Circumpulpal sensory structures then signal pulpal nerves terminating near the base of the tusk. The maxillary division of the fifth cranial nerve then transmits this sensory information to the brain. This sensory pathway was first described in published results of patent dentinal tubules, and evidence from dissection of tusk nerve connection via the maxillary division of the fifth cranial nerve to the brain. New evidence presented here indicates that the patent dentinal tubules communicate with open channels through a porous cementum from the ocean environment. The ability of pulpal tissue to react to external stimuli is supported by immunohistochemical detection of neuronal markers in the pulp and gene expression of pulpal sensory nerve tissue. Final confirmation of sensory ability is demonstrated by significant changes in heart rate when alternating solutions of high-salt and fresh water are exposed to the external tusk surface. Additional supporting information for function includes new observations of dentinal tubule networks evident in unerupted tusks, female erupted tusks, and vestigial teeth. New findings of sexual foraging divergence documented by stable isotope and fatty acid results add to the discussion of the functional significance of the narwhal tusk. The combined evidence suggests multiple tusk functions may have driven the tooth organ system's evolutionary development and persistence.

Vestigial tooth anatomy and tusk nomenclature for monodon monoceros.

Narwhal tusks, although well described and characterized within publications, are clouded by contradictory references, which refer to them as both incisors and canines. Vestigial teeth are briefly mentioned in the scientific literature with limited descriptions and no image renderings. This study first examines narwhal maxillary osteoanatomy to determine whether the erupted tusks are best described as incisiform or caniniform teeth. The study also offers evidence to support the evolutionary obsolescence of the vestigial teeth through anatomic, morphologic, and histologic descriptions. Examination of 131 skull samples, including 110 museum skull specimens and 21 harvested skulls, revealed the erupted tusks surrounded by maxillary bone over the entire length of their bone socket insertion, and are thus more accurately termed caniniform or canine teeth. The anatomy, morphology, and development of vestigial teeth in five skull samples are more fully described and documented. Vestigial tooth samples included 14 embedded pairs or individual teeth that were partially exposed or removed from the maxillary bone. Their location was posterior, ventral, and lateral to the tusks, although male vestigial teeth often exfoliate in the mouth lodging between the palatal tissue and underlying maxillary bone. Their myriad morphologies, sizes, and eruption patterns suggest that these teeth are no longer guided by function but rather by random germ cell differentiation and may eventually cease expression entirely. The conclusions reached are that 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.