Science

Strange Animals That Lived Before the Dinosaurs Reveal the Evolutionary Origin of Tusks

All kinds of animals have ivory, from elephants and walruses to five-pound guinea pigs, called hyraxes. But animals with fangs have one thing in common, that is, they are all mammals-there are no known fishes, reptiles, or birds with fangs. In a new study in Proceedings of the Royal Society B, paleontologists traced the earliest ivory back to the ancient relatives of mammals that lived before dinosaurs. For this, they must first define what makes ivory ivory.

“Tusks are a very famous anatomy, but before I started this research, I never really considered how fangs are restricted to mammals,” Harvard University researcher and lead author of the study Megan Hui Teney said.

“We can prove that the first batch of ivory belonged to animals that appeared before modern mammals, called bidentate animals,” said Ken Angielczyk, curator of the Chicago Field Museum and author of the paper. “They are very rare animals.”

Dicynodont Fossil Skull
Left side of the skull of the dicynodont Dolichuranus (NMT RB554) from Tanzania. The large tusk is visible at the lower left of the specimen. Credit: Ken Angielczyk

Dicynodonts mostly lived before the age of dinosaurs, about 270 to 201 million years ago, ranging in size from mice to elephants. Modern mammals are their close relatives, but they look more like reptiles, with beaks similar to tortoises. Since its discovery 176 years ago, one of its distinctive features is the pair of fangs protruding from its upper jaw. The name Dicynodon even means “two canine teeth”.

Researchers used the lunch break during paleontological excavations to come up with the idea of ​​studying the origin of ivory. “We were sitting in a field in Zambia, with bidentate teeth everywhere,” Whitney recalled. “I remember Ken picking them up and asking why they are called fangs because they have characteristics that fangs don’t have.”

Angielczyk made an important difference: not all protruding teeth are technically canines, and the composition and growth pattern of the teeth tell us whether they are important. “For this article, we have to define a canine, because it is a surprisingly ambiguous term,” Whitney said. Researchers believe that in order for a tooth to become a canine, it must extend beyond the mouth, and it must continue to grow throughout the life of the animal, and unlike the teeth of most mammals (including our own), the surface of the canine is composed of Dentin. Than hard enamel.

Dicynodont Teeth in the Field
Isolated tusk fragments found in Zambia by field teams in 2018. Credit: Ken Angielczyk

Under these parameters, elephants, walruses, wild boars, and hyraxes all have tusks. However, other large teeth in the animal kingdom will not be cut. For example, the teeth of rodents, although sometimes constantly protruding and growing, there is an enamel band in front of the teeth, so they are not counted.

Some bidentate ivory observed by the research team in Zambia does not seem to fit the definition of ivory: they are coated with enamel instead of dentin.

The different composition of teeth and canines also provides scientists with information about animal life. “Enamel-coated teeth and dentin-coated teeth are a different evolutionary strategy, which is a trade-off,” Whitney said. Tooth enamel is stronger than dentin, but due to the geometry of teeth growing in the jawbone, if you want your teeth to continue to grow throughout your life, you will not be able to completely cover the enamel.

Animals like humans have made evolutionary investments in durable but difficult-to-repair teeth; once our adult teeth grow out, if they break, we are unlucky. Teeth are not as durable as our enamel teeth, but they will continue to grow even if damaged. It’s like promising to get a very reliable but difficult-to-repair car when you encounter a problem, rather than driving a model that requires frequent repairs but is cheap and easy to repair by any mechanic.

The different types of teeth that animals have evolved can allow scientists to understand the pressure these animals face that may produce these teeth. Animals with fangs can use them to fight or take root on the ground, causing them to suffer minor injuries, which is risky for discontinuous growth of tooth enamel.

In order to study whether the ivory of the bidentate is really ivory, the researchers cut paper-thin sections from the fossil teeth of 19 bidentate specimens representing ten different species and examined them under a microscope. Structure. They also use micro-CT scans to check how the teeth are attached to the skull and whether their roots show signs of continuous growth. Scientists have discovered that the teeth of some bidentate beasts are actually fangs, while others, especially the teeth of some early species, are just big teeth. However, the transition from cuspid to cuspid is not a strict process: different members of the bidentate family develop cuspids independently.

Whitney said she was surprised by the discovery. “I hope there is a point in the genealogy that all bidentate beasts are beginning to grow teeth, so I think we actually see fangs evolving in a convergent way, which is really shocking,” he said.

“Generally speaking, the ivory of two-dented beasts can tell us a lot about the evolution of mammalian ivory,” Angielczyk said. “For example, this study shows that true canine evolution requires a reduction in the rate of tooth replacement and the flexible ligaments that connect the teeth to the lower jaw. Everything is going up, allowing us to better understand the ivory we see in mammals today. ”

“Before the age of the dinosaurs, the two-dented beasts were the most numerous and diverse vertebrates on the planet. They were known for their fangs. In fact, only a few people had real fangs, and the rest had big teeth. It’s a good example of the evolution that we can document. We can see how to build ivory!” said Brandon Picuk, director of the Idaho Museum of Natural History and one of the authors of the article.

The researchers said that this study shows the first known instance of real canines, which can help scientists better understand how evolution works.

Fangs have evolved many times, which makes you wonder how and why. We now have good information about the anatomical changes that must occur in bidentate mammals to grow canines. For other groups, such as wild boars or walruses, the jury still exists,” said Christian Cido, curator of the Burke Museum at the University of Washington and one of the authors of the article.

“Although it is a very strange animal, there are still some things about the bidentate, such as the evolution of ivory, which tells us the mammals around us today,” Angielczyk said. “Also, whenever you say that mammals are not that special, the two-dented beasts do it first. It’s a good day.”

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