NEW INSIGHTS INTO THE ONTOGENETIC DEVELOPMENT OF NIMRAVID DENTAL ANATOMY AND IMPLICATIONS FOR THE EVOLUTION OF ‘DIRK-TOOTH’ NIMRAVIDS
BOYD, Clint A., Department of Geology and Geological Engineering, South Dakota School of Mines and Technology, 501 East Saint Joseph Street, Rapid City, SD 57701, WELSH, Ed, Badlands National Park, Interior, SD 57750 and SPEARING, Kurt D., Department of Biology and Chemistry, Morningside College, 1501 Morningside Avenue, Sioux City, IA 51106, email@example.com
Few ontogenetically immature specimens are known for most nimravid taxa, impeding identification of important developmental and evolutionary trends in their dental anatomy. Examination of multiple Nimravus specimens representing various ontogenetic stages provides insight into the pattern of tooth eruption and spacing in this taxon. The only deciduous upper premolars are dP4 and dP3. Eruption of M1 begins before the loss of dP4, and P4 erupts and replaces dP4 just prior to the replacement of dP3 by P3. As the diastema between dC and dP3 elongates, but before P3 fully erupts, the alveolus for P2 opens in this space close to the posteromedial margin of the dC alveolus. As growth continues, a diastema opens anterior to the posterolaterally migrating P2. Eventually the alveolus for P1 (when present) opens in the diastema posteromedial to the dC alveolus. The positions of P1 and P2 continue to migrate posterolaterally relative to the dC alveolus as the muzzle elongates, changing the maxillary tooth row from curved to straight. The position of P3 also migrates from its original position situated tightly between the anterior roots of P4 to a more anterolateral position in line with the crown of P4. Similar tooth eruption and migration patterns are also noted in Dinictis, though fewer growth stages were observed and P1 is usually (but not always) absent. The presence of P1 was also noted in a single specimen of Hoplophoneus (CSC-41-42).
Nimravus and Hoplophoneus deciduous upper canines are similar in morphology to their respective adult upper canines in both general dimensions (‘scimitar-tooth’ versus ‘dirk-tooth’) and in average serration density (~2/mm vs >4/mm). Adult and deciduous upper canines of Dinictis are intermediate in morphology between these two taxa; however, the serration density of dC resembles the ‘dirk-tooth’ condition seen in more derived taxa (>4/mm), while the adult upper canines possess a lower serration density (~3/mm). The current phylogenetic position of the dwarf nimravid Nanosmilus between Dinictis and the ‘dirk-tooth’ nimravids (e.g, Hoplophoneus) suggests a mechanism for the evolution of the ‘dirk-tooth’ canine morphology in nimravids. The dC morphology of Dinictis could have been paedomorphically retained in adults of Nanosmilus, and later accentuated in other hoplophonins.