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Most people imagine an elephant when they think of animals with tusks. But many other animals have tusks, including warthogs, hippos, Arctic walruses and even a guinea pig-like animal called a daman. Although the size of animals and their canines may vary, they all have one thing in common: they are found only in mammals. Fish, reptiles or birds with fangs are unknown. Despite the fact that this is an iconic feature of modern and fossil mammals, it remains a mystery what evolutionary steps led to the development of this phenomenon and why are mammals the only animals with tusks today?
In a new paper published in Proceedings of the Royal Society B, researchers trace the first tusks back to ancient mammalian relatives who lived before dinosaurs, and shed light on the evolution of mammalian tusks, identifying for the first time what makes a tusk so unique.
"Tusks are a very well-known anatomy, but until I started working on this study, I never really thought about the fact that tusks can only be found in mammals," said lead author Megan Whitney, a researcher in the Department of Organic and Evolutionary Biology at Harvard University.
"We were able to show that the first tusks belonged to animals that appeared before modern mammals, which were called dicynodonts. They are very strange animals."
Dicynodonts, although not mammals, are their distant relatives and are more closely related to mammals than dinosaurs and other reptiles.
Dicynodonts lived between 270-201 million years ago and included a variety of animals from tiny rat-like dicynodonts to huge dicynodonts the size of elephants. They are known for a very peculiar arrangement of teeth.
A distinctive feature of these animals, first discovered 176 years ago, are the protruding canines in their upper jaw. Most of them had two upper canines that went down from the canine position, but they rarely had extra teeth. Instead, dicynodonts had a beak in the front of their mouth, consisting of keratin and resembling a turtle's beak.
Not all protruding teeth are technically tusks. ”We had to define what a tusk is, because it's a surprisingly ambiguous term," the scientists say. Researchers have determined that in order for a tooth to be a tusk, it must come out of the mouth, consist entirely of dentin and it is constantly growing.
The researchers performed paleohistology (study of fossil tissues) on paper-thin sections of fossilized teeth of 19 specimens of dicynodonts representing ten different species. They used micro-CT scans to examine how teeth attach to the skull and to see if there was any evidence of continuous growth.
Some of the tusks of dicynodonts that the team of scientists observed in Zambia did not fit the definition of a tusk — they were covered with enamel instead of dentin.
"There are many different species of dicynodonts, and, apparently, they all have canines, however, if you look at the microstructural details, they differ greatly in these groups," the researchers say. Teeth with enamel are tougher than those with dentin, but because of the geometry of how teeth grow in the jaw, if you want teeth to continue growing throughout your life, you may not have full enamel coverage. Animals, such as humans, have developed strong, but difficult to correct teeth — there is no substitute for the loss of an adult tooth. Tusks are less durable than enamel-coated teeth, but they grow continuously, even if they are damaged. "Enamel-coated teeth are a different evolutionary strategy than dentin-coated canines, it's a compromise," the scientists say.
Analyzing histological thin sections of dicynodonts samples from South Africa, Antarctica, Zambia and Tanzania, the researchers found that, like in humans, these animals apparently had a reduced number of replaced teeth in the canine position and they had soft tissue attachment to the jaw. Interestingly, this combination of properties is unique to mammals. Mammals, like humans, replace baby teeth with adult teeth only once, unlike most other vertebrates — for example, sharks have teeth that are constantly formed. Mammalian teeth are attached to the jaw by gomphosis, which is the attachment of soft tissues or ligaments. However, most vertebrate teeth attach to the jaw as a result of ankylosis, which is the fusion of hard bone tissues with the tooth.
"If you have these two things, fewer replaced teeth and soft tissue attachment, an ever-growing tooth allows the animal to get around the fact that it can't replace a tooth. Instead, it develops to constantly deposit the same tooth tissues, " the scientists say. "And as the animal continues to deposit tissue, the tooth begins to extend beyond the mouth to become functional."
The researchers found that the true evolution of the tusk occurred only at a later stage in the evolution of this group — the early members of this group had a large tooth, not a real tusk. At the end of their evolutionary history, dicynodonts developed a real tusk that was constantly growing, and this is surprisingly convergent in many different species of dicynodonts.
Scientists say the study, which shows the earliest known example of real tusks, may help to better understand how evolution works.
The different types of teeth that evolved in animals can tell scientists about the pressure these animals faced, which could have resulted in these teeth. For example, fangs can act in different ways, including defense, competition, digging, sexual selection, and even help to move around - like a walrus that uses its fangs to climb onto the ice from the water. The ever-growing tusk may have allowed dicynodonts to overcome the difficulties associated with having only one set of replacement teeth throughout their entire life.
"We don't really know yet what functions the tusks of dicynodonts could perform, because we can't observe them and see what they did with them," the researchers say. "This is a long-standing question about dicynodonts."
PHOTO: Reconstruction of the life of Dicynodon Dicynodon. In addition to the canines on the upper jaw, most dicynodonts had a turtle beak, which they used to chew food.
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