Animals like humans made an evolutionary investment in durable but hard-to-fix teeth-- once our adult teeth grow in, we’re out of luck if they get broken. Tusks are less durable than our enamel-coated teeth, but they grow continuously, even if they get damaged. It’s like the compromise of getting a car that’s very reliable but very difficult to get repaired when it does have trouble, versus driving a beater that needs frequent repairs but is a model that’s cheap and easy for any mechanic to fix.
The different kinds of teeth animals have evolved can tell scientists about the pressures those animals faced that could have produced those teeth. Animals with tusks might use them for fighting or for rooting in the ground, exposing them to little injuries that would be risky for enamel teeth that don’t grow continuously.
To study whether dicynodonts tusks really were tusks, the researchers cut paper-thin slices out of the fossilized teeth of 19 dicynodont specimens, representing ten different species, and examined their structure with a microscope. They also used micro-CT scans to examine how the teeth were attached to the skull, and whether their roots showed evidence of continuous growth. The scientists found that some dicynodont teeth are indeed tusks, while others, particularly those of some of the earlier species, were just large teeth. It wasn’t a strict progression from non-tusks to tusks, though — different members of the dicynodont family evolved tusks independently.
Whitney says she was surprised by the finding. “I kind of expected there to be one point in the family tree where all the dicynodonts started having tusks, so I thought it was pretty shocking that we actually see tusks evolve convergently,” she says.
“Dicynodont tusks can tell us a lot about mammalian tusk evolution in general,” says Angielczyk. “For instance, this study shows that reduced rates of tooth replacement and a flexible ligament attaching the tooth to the jaw are needed for true tusks to evolve. It all ladders up to giving us a better understanding of the tusks we see in mammals today.”
“Dicynodonts were the most abundant and diverse vertebrates on land just before dinosaur times, and they’re famous for their ‘tusks.’ The fact that in reality only a few have true tusks, and the rest have big teeth, is a beautiful example of evolution we can document. We can see how to build a tusk!” says Brandon Peecook, a curator at the Idaho Museum of Natural History and one of the paper’s authors.
The researchers say that the study, which shows the earliest known instance of true tusks, could help scientists better understand how evolution works.
“Tusks have evolved a number of times, which makes you wonder how — and why? We now have good data on the anatomical changes that needed to happen for dicynodonts to evolve tusks. For other groups, like warthogs or walruses, the jury is still out,” says Christian Sidor, a curator at the University of Washington Burke Museum and one of the paper’s authors.
“Despite being extremely weird animals, there are some things about dicynodonts, like the evolution of tusks, that inform us about the mammals around us today,” says Angielczyk. “Plus, anytime you can say mammals aren’t that special, dicynodonts did it first, that’s a good day.”
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