CONVERGENT EVOLUTION IN THE ENAMEL AND GINGIVA OF THE NIMRAVID POGONODON AND THE FELID SMILODON REVEALED IN NEW MATERIAL FROM JOHN DAY FOSSIL BEDS NATIONAL MONUMENT

Dental enamel with a clear cementoenamel junction (CEJ) is not commonly preserved on the sabres of Nimravid sabertooth cats. Recent material from the John Day Fossil Beds National Monument is so well preserved, and prepared, that this level of detail is present. A Pogonodon, from the Arikareean, has sabers that show remarkable convergence with the Rancholabrean felid Smilodon, despite their extreme temporal and phylogenetic separation. The CEJ in machairodonts is the point of gingival epithelial attachment and, in the case of Smilodon, extends over 25 mm beyond the alveolus on labial and lingual sides of the sabers. In the smaller Pogonodon it extends about 16 mm. Gingiva in this area is ideally situated to provide tactile guidance throughout the bite and more importantly, to cause a reflex release if the cat is jeopardizing its sabers by biting too deeply (risking saber breakage), particularly while learning to bite.

With Pogonodon the preserved lingual CEJ contour is identical to the felid morphology, indicating identical form and function of soft tissue. This establishes an extraordinary degree of convergence between a felid and at least one, but not all nimravid dirktooths. It would also seem that the CEJ has excellent potential as a diagnostic character among sabertooths.

Of the sabertooths, Smilodon is the most recent and best represented. Recent studies using a mechanical Smilodon replica to inflict wounds, on a bison carcass, representing different killing bite hypotheses, have produced some interesting insights into the bite mechanism of sabertooths. Most notable was the extent of elastic movement in the hide and tissue of the bison as the bite progresses. Done properly, in one flowing movement, the sabertooth can sever both the carotid and jugular in fractions of a second and be free of the prey without even reopening the mouth. To do the bite properly requires constant tactile feedback provided by the CEJ, and adjustment of trajectory to avoid too deep or shallow a bite. As the bite progresses and the prey distorts there is risk of hooking the incisors and breaking the sabers.