Tusks are a placing characteristic of many animals — elephants, hippos, warthogs, walruses — and are sometimes sought-after by people.
However no birds, fish or reptiles at present sport this excessive and ever-growing little bit of anatomy. Solely mammals do, despite the fact that they weren’t the primary tusked creatures. It’s an historic trait that predates dinosaurs, a brand new examine has discovered.
“We had been capable of present that the primary tusks belonged to animals that got here earlier than fashionable mammals, known as dicynodonts,” stated Ken Angielczyk, a curator at Chicago’s Subject Museum and an creator of the brand new examine, in a information launch. “They’re very bizarre animals.”
Starting from the scale of a rat to elephantine, the dicynodonts lived from about 270 million to 201 million years in the past. Whereas their closest dwelling relations are mammals, they seemed extra reptilian, with turtle-shaped heads.
Dicynodonts had been essentially the most plentiful and numerous vertebrates earlier than the rise of the dinosaurs, and so they all had a pair of tusks protruding from their higher jaws.
Tusks versus enamel
Earlier than digging into how precisely tusks advanced, the researchers needed to outline precisely what a tusk is and the way it differs from a tooth — one thing that had been ambiguous.
They decided {that a} tusk should prolong from the mouth, consist solely of a substance known as dentine and proceed to develop all through an animal’s life — even when it will get broken. Enamel are additionally produced from dentine. Nonetheless, they’re coated in enamel. This, together with their form, makes them sturdy, however as soon as grownup enamel develop in, there’s not a lot that may be finished in the event that they do break. They do not regrow.
“Enamel-coated enamel are a special evolutionary technique than dentine-coated tusks — it is a trade-off,” stated Megan Whitney, postdoctoral fellow at Harvard College’s division of organismic and evolutionary biology. She was the lead creator of the examine.
The researchers then analyzed skinny sections of 19 fossilized tusks of dicynodonts, representing 10 totally different species present in South Africa, Antarctica, Zambia and Tanzania. Additionally they used micro-computerized tomography scans to look at how the fossils had been hooked up to the cranium, and whether or not their roots confirmed proof of steady progress. They discovered that whereas a couple of of the dicynodonts studied had true tusks, with no enamel, the remaining had massive enamel.
The scientists additionally discovered there wasn’t a strict development from non-tusks to tusks. Totally different members of the dicynodont household advanced tusks independently at totally different occasions, and a few by no means advanced true tusks.
“I totally anticipated there to be a single second in dicynodont evolutionary historical past the place tusks advanced as a result of that is the best rationalization. Nonetheless, we discovered convergent evolution of tusks later in dicynodont evolution,” Whitney stated. Convergent evolution is when comparable options evolve independently in several species or totally different intervals in time.
For tusks to evolve, they discovered {that a} versatile ligament attaching the tooth to the jaw was wanted, in addition to decreased charges of enamel alternative — a mix of options that at present is uniquely present in fashionable mammals.
“All of it ladders as much as giving us a greater understanding of the tusks we see in mammals at present,” stated Angielczyk, talking of the analysis, which revealed within the journal Proceedings of the Royal Society B Organic Sciences.
