Most elementary school kids know that counting the rings on a tree can tell you how old it is, and it’s for the same reason that biologists and forensic scientists count the cementum rings on teeth (microscopic structures that surround the base of the tooth and change color and thickness with the seasons).
Now, scientists have refined the technique and applied it to fossils that are hundreds of millions of years old, using powerful X-ray imaging techniques. This method can not only pinpoint exactly how long fossilized organisms lived, but also reveal how they grew and developed, according to the study. report in Scientific advances.
Revolutionizing fossil analysis with X-ray imaging
The concept came about almost by chance, when Ian Corff was studying shrew teeth at the European Synchrotron Radiation Facility in France (a synchrotron is a high-powered machine that uses a beam of X-rays).
His former boss, Pam Gill, had spent decades studying mammalomorphs, tiny organisms that show the evolutionary link between mammals and their ancestors. Gill thought the synchrotron might show the tooth rings of ancient fossils. So Corfe took a small sample to the synchrotron and, at 4 a.m. (synchrotron time is in high demand and has to be booked months in advance), scanned the Early Jurassic sample. Mammalian type Morganucodon.
“Surprisingly, these increases appear to be preserved after nearly 200 million years!” says Elise Newnham, co-author of the paper with Pam Gill, now a research fellow at Queen Mary University of London and the University of Bonn in Germany.
read more: Did prehistoric mammals coexist with dinosaurs, and what were they like?
Developmental patterns of fossil teeth
Corff reported the data to Gill, who was also Newham’s graduate research supervisor at the time, and they then investigated the possibility of applying the technique to other fossils. They were particularly interested in not only whether the number of cemental rings could indicate an organism’s longevity, but also what the teeth could tell them about developmental patterns and life histories.
Gill booked synchrotron time for Newham, continuing the work started by Corfe’s earlier efforts.
“The exciting results from this experiment formed the basis for starting my PhD project and I’ve been scanning ever since,” says Newnham.
read more: Fossils help explain Jurassic mammal evolution
Evolutionary insights from mammalian teeth
Subsequent scans have revealed fascinating information about these early mammals, with the main finding being that, judging by the size and gaps in the cementum rings, these early mammals were relatively long-lived.
The earliest “true” mammals (not mammalian traits), like modern mammals, grew relatively rapidly during childhood but then slowed during adolescence. However, this slowing occurred much later than mammals of a similar size living today (shrew to mouse size). Because this corresponds to the same life stage at which many modern mammals reach sexual maturity, this discovery suggests that the earliest mammals also took longer to reach sexual maturity than smaller modern mammals.
“This calls into question previous assumptions about the life of early mammals,” says Newnham. “Because early mammals had already developed some of the features we see in modern mammals (single tooth replacement, fur, the mammalian middle ear), it was assumed that early mammals lived like modern small mammals. But we’ve shown that this isn’t the case.”
read more: Study claims existing science on mammalian evolution is wrong
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Prior to joining Discover Magazine, Paul Smaglik spent more than 20 years as a science journalist specializing in U.S. life sciences policy and global scientist career issues, beginning his career in newspapers before moving to science magazines. His work has appeared in publications such as Science News, Science, Nature, and Scientific American.
