Fossil dental exams reveal how defenses first develop

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image: Reconstruction of the life of the dicynodon Dicynodon. Aside from the upper jaw tusks, most dicynodonts had a turtle-shaped beak which they used to chew on their food. Image by Marlene Hill Donnelly.
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Credit: Marlene Hill Donnelly

A wide variety of animals have tusks, from elephants and walruses to five-pound guinea pig-like creatures called hyraxes. But one thing that tusk animals have in common is that they are all mammals – there are no known fish, reptiles, or birds with tusks. In a new study in Proceedings of the Royal Society B, paleontologists traced the earliest tusks back to ancient related mammals that lived before dinosaurs, and to do so, they had to define what makes a tusk a tusk in the first place.

“Tusks are this very famous anatomy, but until I started working on this study, I never really thought about how defenses are limited to mammals,” says Megan Whitney, researcher at the Harvard University and lead author of the study.

“We were able to show that the first tusks belonged to animals that predate modern mammals, called dicynodonts,” says Ken Angielczyk, curator at the Field Museum in Chicago and author of the article. “They are very strange animals.”

Most dicynodonts lived before the time of the dinosaurs, around 270 to 201 million years ago, and they ranged from the size of a rat to that of an elephant. Modern mammals are their closest living relatives, but they looked more reptilian, with turtle beaks. And since their discovery 176 years ago, one of their defining characteristics has been the pair of protruding tusks in their upper jaws. The name dicynodont even means “two canines”.

The researchers had the idea to study the origin of the tusks during a lunch break during a paleontological excavation. “We were sitting in the field in Zambia and there were dicynodon teeth everywhere,” recalls Whitney. “I remember Ken picked them up and asked him why they were called tusks, because they had characteristics that tusks didn’t.”

Angielczyk had made a crucial distinction: Not all protruding teeth are technically tusks, and the makeup and growth patterns of teeth tell us if they matter. “For this article, we had to define a defense, because it’s a surprisingly ambiguous term,” says Whitney. The researchers decided that for a tooth to be a defense, it must extend beyond the mouth, it must continue to grow throughout the life of the animal, and unlike most teeth in mammals (including ours), the surfaces of the tusks are rather made of dentin. than hard enamel.

Under these parameters, elephants, walruses, warthogs, and hyrax all have tusks. However, other large teeth in the animal kingdom do not do the trick. For example, rodent teeth, although they sometimes protrude and grow constantly, have an enamel band across the front of the tooth, so they don’t count.

Some of the dicynodont tusks the team observed in Zambia also didn’t seem to fit the definition of a tusk – they were coated with enamel instead of dentin.

The different makeup of teeth and tusks also gives scientists a glimpse into an animal’s life. “Enamel-coated teeth represent a different evolutionary strategy than dentin-coated defenses, it’s a compromise,” says Whitney. Enamel teeth are stronger than dentin, but due to the geometry of tooth growth in the jawbone, if you want teeth that will continue to grow throughout your life, you can’t have a veneer coating. full enamel.

Animals like humans have made an evolutionary investment in durable but hard to repair teeth – once our adult teeth grow we are out of luck if they do break. Tusks are less durable than our enamel coated teeth, but they grow continuously, even if damaged. It’s like the trade-off of getting a very reliable car but very difficult to repair when it has problems, versus driving a drummer that needs frequent repairs but is a cheap and easy to repair model for everything. mechanic.

The different types of teeth that animals evolved can tell scientists about the pressures these animals faced that could have produced those teeth. Animals with tusks could use them for fighting or for rooting in the ground, exposing them to small injuries that would be risky for enamel teeth that don’t continuously grow.

To study whether dicynodont tusks were really tusks, the researchers cut paper-thin slices from the fossilized teeth of 19 dicynodont specimens, representing ten different species, and examined their structure under a microscope. They also used CT scans to look at how the teeth were attached to the skull and if their roots were showing signs of continued growth. Scientists have found that some dicynodont teeth are indeed tusks, while others, especially those of some of the older species, were just large teeth. It wasn’t a strict progression from non-tusks to tusks, however – different members of the dicynodont family developed tusks independently.

Whitney says she was surprised by the discovery. “I kind of expected that there would be a point in the family tree where all the dicynodonts started to have tusks, so I thought it was quite shocking to see the tusks evolve from convergent way, ”she says.

“Dicynodont’s defenses can tell us a lot about the evolution of mammalian defenses in general,” says Angielczyk. “For example, this study shows that reduced rates of tooth replacement and a flexible ligament securing the tooth to the jawbone are necessary for true defenses to evolve. All of this allows us to better understand the defenses we see in mammals today. “

“The dicynodonts were the most abundant and diverse vertebrates on earth just before the time of the dinosaurs, and they are famous for their ‘tusks’. The fact that in reality only a few have real tusks, and the rest have big teeth, is a fine example of evolution that we can document. We can see how to build a defense! Says Brandon Peecook, curator at the Idaho Museum of Natural History and one of the authors of the article.

The researchers say the study, which shows the first known example of true defenses, could help scientists better understand how evolution works.

“The defenses have evolved several times, which makes you wonder how and why? We now have good data on the anatomical changes that had to occur in order for dicynodonts to develop defenses. For other groups, such as warthogs or walruses, the jury is still out, ”explains Christian Sidor, curator at the Burke Museum at the University of Washington and one of the authors of the article.

“Although these are extremely strange animals, there are some things about dicynodonts, like the evolution of tusks, that tell us about the mammals around us today,” says Angielczyk. “Also, whenever you can tell mammals aren’t that special, the dicynodonts did it first, it’s a good day.”

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