When is a tooth a tusk? It’s surprisingly complex.
Tusks likely evolved many times among early mammals.
On a paleontological dig, intact skeletons and fossilized remains are typically the stars of the show, but broken remnants can tell important details about ancient creatures, too. These pieces of tooth and tusk-like structures from early mammals—so old they lived before dinosaurs—are helping researchers tease apart how and when the creatures evolved.
In a study published in Proceedings of the Royal Society B, researchers gathered broken bits of suspected tusks or teeth from dicynodonts, extremely early mammals, that had been housed at multiple institutions in Africa and the US. They brought the tusk-like remains back to the lab to check out their makeup under the microscope. They wanted to figure out whether the fossilized pieces were in fact tusks or whether they were simply teeth. And, if they were tusks, did this new discovery shift our understanding of when tusks evolved in the animal kingdom? In doing so, the researchers realized they had to come up with a formal definition of what a tusk actually is.
“It’s just one of those things that everybody thinks we know,” says Harvard zoology researcher and lead author on the study, Megan Whitney. “In terms of like a true diagnosable anatomical character, there really wasn’t a ton of information out there about [tusks].”
Whitney and her colleagues identified three main characteristics of tusks. A tusk had to be ever growing, protrude from the mouth, and made of dentine, the material inside a tooth, and couldn’t contain enamel, which makes up the outer layer of a tooth.
When the team looked at the broken bits they collected from the field, they found that these fragmented parts were actually great for this kind of research. Whitney and her team had to actually fragment them even further to study them.
“A huge part of our work was collecting a bunch of specimens that we could destructively sample,” Whitney says. To look closer at these specimens, the team used specialized saw equipment to make fine cuts of the fossil material, embedded them in a polyester resin substance for grinding, and ground them down to a consistency that was optimal for viewing under the microscope.
From there, the researchers could identify a whole host of factors, including their age and whether it was dentine or enamel, which were particularly important for this study. And what they did find surprised Whitney.
The team found a number of specimens that met the newly formed definition of what a tusk is. But interestingly, not all of these tusks came from the same time period or from one ancestor. Instead, they seemed to evolve on different timelines for different creatures, what evolutionary biologists call convergent evolution.
“I thought we were going to find more species that had a true tusk,” Whitney says, “and I would say I was shocked to find out that [tusks] evolved convergently at least a few times.”
Further, the research showed that for tusks to evolve, certain factors needed to be present, including flexible tooth ligaments and reduced rates of teeth replacing themselves after falling out, which help researchers understand today’s mammalian tusks.
Classifying these remains as tusks or teeth, and understanding when the creatures they came from existed and how they are related, helps give a more complete picture of the history of dicynodonts, and how tusks in mammals today came to be.
“We’re able to document how this anatomical novelty evolved using the fossil record, and that it’s not this clear cut beautiful story,” Whitney says. “Evolution doesn’t work that way.”