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Thursday, August 8, 2013

Rapid turnover in sauropod teeth

There are different ways to cope with the dental wear associated with a vegetarian diet. Elephants have only four enormous molars at a time (two on top and two on the bottom). Sauropodsthe largest ever terrestrial herbivores, used a different strategy to process the enormous amount of vegetation they required. According to Michael D’Emic of Stony Brook University and his colleagues, sauropods dealt with the problem of tooth wear by replacing lots of small teeth at a prodigious rate.

The researchers took microscopic sections of sauropod teeth from a number of museums. They were able to count the growth lines laid down in each tooth and thus determine the age of that particular tooth. By comparing the ages of different teeth within one mouth, the scientists were able to figure out how often teeth were replaced.

Below, you can see a chart of the estimated replacement rates for the teeth of a half dozen different sauropod species.

Table 2 Estimated tooth formation times (ages) and replacement rates in several sauropodomorphs.

Estimated tooth formation times (ages) and replacement rates in several sauropodomorphs.

Note that this is the replacement rate per tooth position. This means that these giants would have gained a new tooth somewhere in their mouths every day or two. It’s not clear how many times each tooth could have been replaced, but for many species, each tooth position clearly had at least three replacements waiting in the wings. You can see them behind the primary teeth in the images below.

Figure 2 Tooth replacement in the sauropod dinosaurs Camarasaurus and Diplodocus.

Tooth replacement in the sauropod dinosaurs Camarasaurus and Diplodocus.
Reconstructed skulls (A, D) and premaxillary teeth (B, C) of Camarasaurus (A, B) and Diplodocus (C, D). Premaxillae show replacement teeth in each of the four alveoli adjacent to the symphysis labelled by their position along the tooth row (1–4) and their position in the replacement sequence at each tooth position (i–v). Photographs of thin sections of Diplodocus and Camarasaurus teeth show enamel (en), the pulp cavity (pc), daily-deposited incremental lines of von Ebner (arrowheads mark every other line) in the dentin (den), and the crown-root junction (crj). The 20 mm scale bar is for the premaxilla and tooth images in (B); 10 mm scale bar is for premaxilla and tooth images in (C). 

This rapid tooth turnover makes a lot of sense if you need to balance your head at the end of a long neck. You can keep your head lighter by continuously shedding your blunted, less effective teeth rather than by trying to maintain sturdier and longer lasting teeth. 

Michael D. D’Emic, John A. Whitlock, Kathlyn M. Smith, Daniel C. Fisher, & Jeffrey A. Wilson (2013). Evolution of High Tooth Replacement Rates in Sauropod Dinosaurs PLoS ONE DOI: 10.1371/journal.pone.0069235.