National Geographic

Tooth Turnover Offers Clues About Diplodocus Diet

Every 35 days, an adult Diplodocus got a new tooth. That is a wonderfully precise degree of resolution for the fossil record. Mesozoic time often feels squishy – words such as “about” or “approximately” are workhorses in the vocabulary of anyone who discusses the Age of Dinosaurs – yet, thanks to a new paper by Stony Brook University paleontologist Michael D’Emic and colleagues, the lives of giant, vegetation-scoffing sauropods has come into focus through the continual turnover of their teeth.

As with any other bone in a dinosaur’s skeleton, a fossil tooth is a time capsule. A tooth’s interior – resembling a set of nested ice cream cones in cross section – record its own growth. Microscopic lines inside the tooth track the daily formation of dentin, just as with the teeth of some living vertebrates, and this allowed D’Emic and coauthors a way to see how old dinosaur teeth were when the animal perished. In particular, they focused on two dinosaurian neighbors from the 150 million year old heyday of Jurassic North America’s sauropod giants – the slender, pencil-toothed Diplodocus and the bulkier, spoon-toothed Camarasaurus.

In order to examine tooth formation in these dinosaurs, though, D’Emic and colleagues had to crack a few skulls. The new study itself says – in the classic, passive tone of scientific papers – “Permission was received to access the relevant specimens from museum collections managers.” When I met D’Emic in the collections of the Natural History Museum of Utah to talk about the new study, jaw bones of the charismatic Jurassic carnivore Ceratosaurus laid out on a specimen cart in front of him, he smiled sheepishly as he explained the difficulty in convincing fossil custodians to let him and his collaborators undertake the necessary destructive analysis.

A cross section of a Diplodocus tooth, showing daily lines of growth in the dentine. Image courtesy Michael D'Emic.

A cross section of a Diplodocus tooth, showing daily lines of growth in the dentine. Image courtesy Michael D’Emic.

For the study, D’Emic explained, his team aimed to “mechanically take the bone [of the dinosaur jaws] off and remove every tooth.” The specimens would be cast and CT scanned, thus preserving their form for future researchers, but this limited the researchers to partial jaws of well-known dinosaurs that would not be particularly missed. “I hunted around for a year for jaws that had good teeth but weren’t too nice that I couldn’t section them,” he said. Unique dinosaurs and rare species were out of a question. Hence why the common and well-known Diplodocus and Camarasaurus were good candidates for this kind of study.  So many had been found that D’Emic and coauthors were able to find a few that could be sacrificed for science.

But the destruction of a few fossils allowed the paleontologists to come up with a mathematical way to estimate tooth replacement rates without disassembling additional dinosaurs. By determining the ages of each tooth, the researchers were then able to calculate the time needed for a new tooth to form. In turn, the results gleaned from Diplodocus and Camarasaurus could be applied to other dinosaurs, as the researchers did in the new study for additional narrow- and wide-toothed sauropods from different places and times.

While Diplodocus replaced teeth significantly faster than Camarasaurus – one new tooth every 35 days compared to one every 62 days – both were high-speed tooth-generating champions compared to their archaic relatives. Aside from the evidence of rapid formation within the teeth, D’Emic and colleagues found up to five replacement teeth waiting behind each functioning tooth in Diplodocus and up to three replacement teeth behind erupted teeth in Camarasaurus.

In the much earlier sauropodomorph dinosaur Plateosaurus – often taken as the bipedal archetype from which the more massive sauropods evolved – CT scans failed to reveal any replacement teeth, and the similarly archaic Massospondylus had only one replacement tooth, at best. The hulking sauropods of the Jurassic, by constrast, could generate a great many teeth extraordinarily quickly, cropping and stripping vegetation with practically fresh dentition. Although, of all the dinosaurs examined, the hoover-headed oddity Nigersaurus had the most astonishing tooth replacement rate. Each tooth in the mouth of this broad-snouted grazer was replaced every fourteen days, meaning that this dinosaur was losing and erupting new teeth daily.

Two - of many ways - to be an herbivore: the skull of Diplodocus (top) compared to that of a horse (bottom). Image courtesy Michael D'Emic.

Two – of many ways – to be an herbivore: the skull of Diplodocus (top) compared to that of a horse (bottom). Image courtesy Michael D’Emic.

But why should dinosaurs be specialized to lose teeth so rapidly? As D’Emic explained to me, producing a great deal of low-quality teeth is one evolutionary response to a life of constant consumption.

Years ago, as a graduate student, D’Emic was puzzled by sauropod skulls. “I was struck by how small their heads are compared to their bodies,” he said, and “On top of that, how poorly constructed their teeth seemed.” Sauropods were fueling their mind-boggling growth with plants cropped by “lots of terrible teeth.” This only gave way to a bigger mystery about 80 foot titans such as Diplodocus – “How did this animal get so large with these teeth?”

The ancestry of the dinosaurs yields part of the answer. For sauropods to have highly-resistant, long-lasting teeth – like those of horses or elephants – their teeth would have required a major reorganization in their microscopic construction. The easier evolutionary option was to simply make lots and lots of small, relatively poor-quality teeth that were regularly replaced. Faced with diets of resistant vegetation covered in enamel-scratching grit, the teeth of sauropods were adapted to be use-and-discard structures. The new study follows the rate of that remarkable adaptation, giving us a rare look at one aspect of day-to-day life for Diplodocus and kin.

Dinosaur tooth turnover may also give researchers a new ways to investigate other mysteries of the fossil record.

Some dinosaur-laden sites and formations seem to be replete with dinosaur teeth, but rarely preserve dinosaur bones. This could be a matter of teeth being more resilient than bones, or, D’Emic suggested to me, perhaps those habitats were inhabited by animals that shed a great deal of teeth at a rapid rate. The biology of dinosaurs – rather than the environment – might explain such quirks of burial.

Furthermore, differences in tooth replacement rates among dinosaur contemporaries – such as Diplodocus and Camarasaurus – could provide novel evidence in efforts to understand how similar dinosaurs divvied up habitats. Paired with clues from anatomy, for example, D’Emic and coauthors expect that Diplodocus frequently grazed on low-lying plants that often came covered in tooth-damaging grit and therefore required a higher replacement rate, while Camarasaurus browsed high among the trees and needn’t have blown through teeth so fast. And the fact that rapid replacement of small teeth evolved several times in sauropod history suggests that this adaptation was a common evolutionary workaround for the titanic browsers and grazers. In the miniscule lines locked inside dinosaur teeth, new clues to Mesozoic lives await.

Reference:

D’Emic, M., Whitlock, J., Smith, K., Fisher, D., Wilson, J. 2013. Evolution of High Tooth Replacement Rates in Sauropod Dinosaurs. PLoS ONE. 8, 7: e69235.

 

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