Kathlyn M. Smith

Evolution of high tooth replacement rates in sauropod dinosaurs.

Background Tooth replacement rate can be calculated in extinct animals by counting incremental lines of deposition in tooth dentin. Calculating this rate in several taxa allows for the study of the evolution of tooth replacement rate. Sauropod dinosaurs, the largest terrestrial animals that ever evolved, exhibited a diversity of tooth sizes and shapes, but little is known about their tooth replacement rates. Methodology/Principal Findings We present tooth replacement rate, formation time, crown volume, total dentition volume, and enamel thickness for two coexisting but distantly related and morphologically disparate sauropod dinosaurs Camarasaurus and Diplodocus. Individual tooth formation time was determined by counting daily incremental lines in dentin. Tooth replacement rate is calculated as the difference between the number of days recorded in successive replacement teeth. Each tooth family in Camarasaurus has a maximum of three replacement teeth, whereas each Diplodocus tooth family has up to five. Tooth formation times are about 1.7 times longer in Camarasaurus than in Diplodocus (315 vs. 185 days). Average tooth replacement rate in Camarasaurus is about one tooth every 62 days versus about one tooth every 35 days in Diplodocus. Despite slower tooth replacement rates in Camarasaurus, the volumetric rate of Camarasaurus tooth replacement is 10 times faster than in Diplodocus because of its substantially greater tooth volumes. A novel method to estimate replacement rate was developed and applied to several other sauropodomorphs that we were not able to thin section. Conclusions/Significance Differences in tooth replacement rate among sauropodomorphs likely reflect disparate feeding strategies and/or food choices, which would have facilitated the coexistence of these gigantic herbivores in one ecosystem. Early neosauropods are characterized by high tooth replacement rates (despite their large tooth size), and derived titanosaurs and diplodocoids independently evolved the highest known tooth replacement rates among archosaurs.

Sexual Dimorphism and Inter-Generic Variation in Proboscidean Tusks: Multivariate Assessment of American Mastodons (Mammut americanum) and Extant African Elephants

Characteristics of social structure, mating strategies, and parental investment can be inferred for mammalian species based on degree of sexual dimorphism, especially when males are substantially larger than females. American mastodons (Mammut americanum) and extant African elephants (Loxodonta africana, Loxodonta cyclotis) both exhibit marked dimorphism in tusk size. To evaluate the hypothesis that this dimorphism might be indicative of similar, and possibly conserved, behavioral patterns in each lineage, we undertook a detailed evaluation of the pattern of tusk growth in these two genera. Separate discriminant function analyses (DFA) of 21 adult mastodon tusks of inferred sex and 48 adult elephant tusks of known sex show that patterns of ontogenetic change in tusk circumference, regardless of genus, effectively discriminate between sexes. Canonical variates analysis (CVA) of tusks from male and female mastodons and male and female elephants shows that male tusks in both genera are larger than female tusks across all measurements, especially for maximum tusk circumference and pulp cavity depth. CVA’s emphasis of inter-group differences in tusk morphology also shows that mastodon tusks are more robust than elephant tusks. Overall, this study illustrates that there is a characteristic male and a characteristic female tusk form shared by elephants and mastodons. This shows that elephants and mastodons display a shared syndrome of traits beyond sex-linked differences in tusk size, supporting the hypothesis that mastodons exhibited behaviors similar to those we observe today in African elephants.