Two new studies reveal the secrets of the early evolution of dinosaur and mammalian teeth

In the first article, Reisz and his colleagues at Jilin University in China examined the teeth of Changchunsaurus parvus, a small herbivorous dinosaur from the Cretaceous period.

Ornithischian dinosaurs ("bird hips") have developed an incredible diversity of teeth, including complex dental batteries derived from ornithopods (like the famous duckbill dinosaurs), but little is known about the how these complex arrangements stem from simple dental arrangements of early dinosaurs. Changchunsaurus parvus belongs to a branch near or at the origin of ornithopods and can therefore give an insight into the early development of ornithopod teeth.

In this study, Reisz and his colleagues discovered a unique method of tooth replacement that allowed Changchunsaurus to recycle teeth without disrupting the continuous shear surface formed by its rows of teeth. The authors also found that the teeth had wavy enamel, a type of tissue that was thought to have evolved only in more modern ornithopods. The authors suspect that these characteristics may have appeared very early when this group of dinosaurs became specialized in the consumption of plants.

<a href = "https://3c1703fe8d.site.internapcdn.net/newman/gfx/news/hires/2018/1-chewonthistw.jpg" title = "A thin section of the lower jaw of the plant-eating dinosaur changchunsaurus, showing teeth at different stages of development Source: University of Toronto, Mississauga ">

The whole tooth: how mammals have developed their unique dental anchoring system

In the second study, published in Acts of the Royal Society BReisz has worked with students from the University of Alberta and British Columbia, as well as collaborators from the University of Washington in Seattle and Unidad Ejecutora Lillo in Argentina.

Anyone who has worn a dental appliance knows that your teeth can be slowly pushed and pulled into the proper places. What you may not know is that this movement is made possible by a special ligament that keeps each tooth in its orbit. This ligament is also used to cushion each tooth when we chew our food. The origins of this ligament, however, have been a mystery.

<a href = "https://3c1703fe8d.site.internapcdn.net/newman/gfx/news/hires/2018/theteethofch.jpg" title = "The skull of the holotype, a specimen of C. parvus (JLUM L0304-j-Zn2). Credit: Chen et al., 2018 ">

This study, led by the former Ph.D. of the University of Toronto at Mississauga student Aaron LeBlanc has solved the mystery surrounding how mammals have developed their complex system. anchoring of teeth. For more than a hundred years, scientists thought that this ligament had evolved with the first mammals at the beginning of their mastication, but this new article shows that this system appeared for the first time in missing relatives of mammals, the therapsids.

By examining CT scans and making thin cuts of fossil teeth and jaws in thermoside for microscopic analysis, LeBlanc and his colleagues found that mammalian teeth were not as unusual as they were in the past. it was thought. "We found evidence of this ligamentous system in several groups of extinct thereasides, telling us that it evolved before the first mammals chewed their food," said LeBlanc.

<a rel = "lightbox" href = "https://3c1703fe8d.site.internapcdn.net/newman/gfx/news/2018/2-chewonthistw.jpg" title = "The wavy appearance of the </ a> enamel of changchunsaurus in thin section and under cross polarized light. Credit: University of Toronto Mississauga ">

The researchers also think they understood how our ancestors Therapsides developed this ligament anchoring system. They found that in many fossil synapsid jaws, the teeth were rapidly fused into place by this invasive bone, but that in some therapsids, the surrounding bone growth was slower.

"We found that some therapies, such as mammals, had to develop this ligament anchoring system not by developing new tissue but by retarding the growth of the surrounding bone," Reisz says. "We have reformulated our vision of the condition of mammals.We do not believe that mammals are more" advanced "than other extinct therapies, but that their teeth are frozen at an earlier stage of development than that of brittle-toothed animals. merged with the jaws. "

<a rel = "lightbox" href = "https://3c1703fe8d.site.internapcdn.net/newman/gfx/news/2018/1-theteethofch.jpg" title = "The wavy appearance of the </ a> enamel of changchunsaurus in thin section and under cross polarized light. Credit: Chen et al., 2018 ">

The two studies undertaken at the University of Toronto in Mississauga involve teeth that are not firmly anchored to the jaw but are held in place by ligaments. There is now clear evidence for the presence of ligaments in carnivorous and herbivorous dinosaurs and mammals, and these ligaments are not necessarily related to the evolution of mastication, as previously thought. Reisz hopes that ongoing research will continue to reveal a better understanding of this interesting riddle.

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More information:
Chen J, ARH LeBlanc, Jin L, Huang T, RR Reisz (2018) Dental Development, Histology and Enamel Microstructure in Changchunsaurus parvus: Implications for Dental Evolution in Ornithopod Dinosaurs. PLoS ONE 13 (11): e0205206. doi.org/10.1371/journal.pone.0205206