Sauropod dinosaurs, as large herbivores, encountered the same difficulties as mammalian herbivores today, including the need to maintain functional teeth despite the extreme abrasiveness of plant material. While mammals have adapted to this by having continually growing teeth or exceptionally high crowns, sauropods adapted to this by continually replacing their teeth throughout their lives. Some, like Diplodocus, had as many as  five replacement teeth in the jaw for each functional tooth, a number that suggests a very high rate of replacement.

    The sauropod with the most replacement teeth, Nigersaurus, has been proposed to have had a tooth battery, a feature previously seen only in Ceratopsia and Iguanodontia, members of the distantly related dinosaur group Ornithischia.

Two previous studies of dinosaurian tooth replacement rate have been published (Erickson, 1996; Sereno et al., 2007). Erickson (1996) published rates for seven dinosaurs, including rates for both herbivores—some, like Edmontosaurus, with tooth batteries—and carnivores. Later, Sereno et al. (2007) published the only known rate for a sauropod dinosaur, that of Nigersaurus.

    A distinct pattern emerges from the data: herbivores replace their teeth much faster than carnivores. Fastest of all was Nigersaurus, which replaced its teeth every 30 days. This suggests several new questions. Did all sauropods replace their teeth so rapidly? If not, when did rapid replacement evolve? How much variation in rate occurs between sauropod groups?

    To study this, I and several colleagues at the University of Michigan have measured the replacement rate in two sauropods, Diplodocus and Camarasaurus.To do this, we have created thin sections of each tooth in a tooth family (a functional tooth and its replacement teeth). Under light microscopy, each thin section shows bands demarcating daily growth known as incremental lines of von Ebner (ILOvES) which enable us to determine the age of a tooth in days. Determining the difference in age between subsequent teeth enables us to calculate the lapse time between teeth, giving us the replacement rate.

    The replacement rate in Diplodocus is very high, similar to that of Nigersaurus (Whitlock et al., 2009; D’Emic et al., 2009). Camarasaurus replaced its teeth much more slowly, but the recovered rate is still within the range of the fastest-replacing ornithischian dinosaurs. This suggests that rapid replacement evolved early in sauropoda and reached its peak rate within Diplodocoidea. The difference in rate appears to be correlated to tooth shape, which may in turn be suggestive of different strategies to cope with abrasive foodstuffs.

LITERATURE CITED

D’Emic, M.D., J.A. Whitlock, K.M. Smith, J.A. Wilson, and D.C. Fisher. 2009. The evolution of tooth replacement rates in sauropod

        dinosaurs. Presented at the 68th Annual Meeting of the Society of Vertebrate Paleontology, Bristol, UK.

Erickson G. M. 1996. Incremental lines of von Ebner in dinosaurs and the assessment of tooth replacement rates using growth line

        counts. Proceedings of the National Academy of Sciences 93: 14623–14627.

Sereno, P. C., J. A. Wilson, L. M. Witmer, J. A. Whitlock, A. Maga, O. Ide, T. A. Rowe. 2007. Structural Extremes in a Cretaceous

        dinosaur. PLoS ONE 2:e1230.

Whitlock, J.A., M.D. D’Emic, K. M. Smith, J.A. Wilson, D.C. Fisher. 2009. Tooth replacements rates in Mesozoic megaherbivores.

        Presented at the Sixth Annual Michigan Geophysical Union, Ann Arbor, MI.

A tooth of Diplodocus