Fossil dental exams reveal how defenses first evolved


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October 27, 2021

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.Marlene Hill Donnelly

Many animals have tusks, from elephants to walruses to hyraxes. But one thing that today’s tusked animals have in common is that they are all mammals – no known fish, reptile, or bird has one. But that was not always the case. In a study published Oct. 27 in the Proceedings of the Royal Society B, a team of paleontologists from Harvard University, the Field Museum, the University of Washington, and Idaho State University traced the earliest tusks of ancient related mammals that lived before dinosaurs. .

“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,” said lead author Megan Whitney. , postdoctoral researcher at Harvard University and former doctoral student at UW.

“We were able to show that the earliest tusks belonged to animals that predate modern mammals, called dicynodonts,” said co-author Ken Angielczyk, curator at the Field Museum in Chicago. “Although these are extremely strange animals, there are certain things about dicynodonts – like the evolution of tusks – that tell us about the mammals around us today.”

The Dicynodonts lived around 270 to 201 million years ago, largely before the so-called “time of the dinosaurs.” 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. One of their hallmarks is a pair of protruding tusks in their upper jaws. The word dicynodont means “two canines”.

An image of the skull of an ancient mammal-like creature, showing its tusks.

Left side of dicynodont skull Dolichuranus from Tanzania. The large tusk is visible at the bottom left of the specimen.Ken angielczyk

Not all protruding teeth are tusks. Their makeup and growth patterns reveal if they matter.

“For this article, we had to define a defense because it’s a surprisingly ambiguous term,” Whitney said.

For a tooth to be a defense, the researchers argued that it must extend beyond the mouth, continue to grow throughout the animal’s life and, unlike most teeth in mammals – including ours – tusk surfaces are made of dentin rather than hard enamel.

Under these parameters, elephants, walruses, warthogs and hyraxes 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 did not meet the definition of a tusk – they were coated with enamel instead of dentin.

The different makeup of teeth and tusks give scientists a glimpse into an animal’s life.

“Enamel coated teeth are a different evolutionary strategy than dentin coated defenses,” said Whitney. “It’s a compromise.”

Enamel teeth are harder than dentin. But due to the geometry of tooth growth in the jawbone, if you want teeth that continue to grow throughout your life, you can’t have a full enamel coating.

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, but they grow continuously, even if damaged. It’s like the trade-off of having a very reliable car that is very difficult to repair, versus driving a drummer that needs frequent repairs but is cheap and easy to repair.

Cross-sectional image of a dicynodont tooth, showing that the outer layer is enamel, which means it is not a real tusk.

The enlarged caniniforms of Diictodon have enamel that makes them more like teeth than tusks. The cross section reveals a ring of enamel around the outside of the tusk which is illuminated by polarized light.Megan Whitney

The different types of teeth that animals evolved tell scientists about the pressures these animals faced that could have produced these 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 of fossilized teeth from 19 dicynodont specimens, representing ten different species, and examined their structure under a microscope. They also used CT scans to examine 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.

They also discovered certain adaptations that dicynodonts needed to develop true defenses, including flexible ligament attachments between tooth and jaw and reduced rates of tooth replacement, according to Angielczyk.

An image of a fossilized dicynodon skull, showing the inside of its tusk.

An example of a real defense in constant growth in the dicynodont Lystrosaurus. When the researchers cut the face of Lystrosaurus, the root of the tusks is composed of a widely open pulp cavity which suggests that dentin has been continuously deposited.Megan Whitney

The study, which shows the first known example of true defenses, could help scientists better understand evolutionary processes.

“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 for dicynodonts to evolve their defenses, ”said co-author Christian Sidor, professor of biology at UW and curator at the Burke Museum of Natural History & Culture from UW. “For other groups, like warthogs or walruses, the jury is still out.”

Most of the dicynodont fossils analyzed in the study were collected during fieldwork in Tanzania and Zambia. These specimens, which are currently kept at the Burke Museum and other American museums, will be repatriated at the end of the project and will become part of the permanent collections of the National Museum of Tanzania and the Livingstone Museum in Zambia. This partnership allows researchers around the world to study the fossils and ultimately bring the specimens back to their countries of origin for further research.

Future studies could examine other species of dicynodonts and how their tusks – or non-tusks – developed. Sidor’s lab at UW is one of the few across the country where fossilized bones and teeth are routinely analyzed on a microscopic scale.

“The fine cut can also provide a lot of useful information, as bones and teeth can capture a record of an animal’s life in their tissues,” Sidor said. “On a constantly growing tusk, dentin records a daily measurement of the animal’s growth rate. Did he grow faster or slower in certain seasons? Stop growing for a while? Creating thin sections of bones and teeth opens up a lot of other interesting things about the animal, almost like analyzing tree rings. These are the types of questions that we can continue to study.

The co-author is Brandon Peecook, former UW doctoral student, assistant professor at Idaho State University, and assistant curator of vertebrate paleontology at the Idaho Museum of Natural History. The research was funded by the National Science Foundation and National Geographic.

Adapted from a press release by the Field Museum and Harvard University.

Tag (s): Burke Museum of Natural History & Culture • Christian Sidor • College of Arts & Sciences • Department of biology • evolution • paleontology

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