Most people picture an elephant when imagining animals with tusks. But many other animals have tusks including warthogs, hippopotamuses, Arctic-dwelling walruses, and even a five-pound, guinea pig looking animal called hyraxes. Though the size of the animal and their tusks can vary they all have one unique thing in common in that they are only found on mammals -- there are no known fish, reptiles, or birds with tusks. Despite being an iconic feature of modern and fossil mammals the mystery remains of what evolutionary steps led to the development of this dental phenomenon and why are mammals the only animals today with tusks?
In a new paper published October 27 in the Proceedings of the Royal Society B researchers trace the first tusks back to ancient mammal relatives that lived before the dinosaurs and shed light on the evolution of mammalian tusks by first defining what makes a tusk a tusk.
"Tusks are this very famous anatomy, but until I started working on this study, I never really thought about how tusks are restricted to mammals," said lead author Megan Whitney, postdoctoral fellow in the Department of Organismic and Evolutionary Biology, Harvard University.
"We were able to show that the first tusks belonged to animals that came before modern mammals, called dicynodonts," said Kenneth Angielczyk, co-author and curator at Chicago's Field Museum. "They're very weird animals."
Dicynodonts, though not mammals, are 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 diverse range of animals from tiny rat-like dicynodonts to huge elephant-sized dicynodonts. They are known for having a very peculiar arrangement of teeth. A defining feature of these animals, first discovered 176 years ago, is the protruding tusks in their upper jaws. Most had two upper tusks that came down from the canine position, but they rarely had additional teeth. Instead, dicynodonts had a beak at the front of their mouths that was made of keratin and resembled a turtles beak.
The researchers were taking a lunch break during a paleontological dig when they got the idea for the study. "We were sitting in the field in Zambia, and there were dicynodont teeth everywhere," recalls Whitney. "I remember Ken picking them up and asking how come they were called tusks, because they had features that tusks don't have."
Not all protruding teeth are technically tusks. "For this paper, we had to define a tusk, because it's a surprisingly ambiguous term," said Whitney. The researchers determined that for a tooth to be a tusk it must extend out from the mouth, be made entirely of dentine -- lacking enamel found on most mammals' teeth, and is ever-growing.
The researchers performed paleohistology (the study of fossil tissues) on paper-thin slices of fossilized teeth from 19 dicynodont specimens, representing ten different species. They used micro-CT to examine how the teeth attached to the skull and to see if there was any evidence of continuous growth.
Some of the dicynodont tusks that the team observed in Zambia didn't seem to fit the definition of a tusk either -- they were coated in enamel instead of dentine. "There are many different kinds of dicynodonts and they appear to mostly all have tusks," said Whitney, "however, when you look at the micro structural details they're very different in those groups." Enamel teeth are tougher than dentine but because of the geometry of how teeth grow in the jaw, if you want teeth that keep growing throughout your life, you can't have a complete enamel covering. Animals like humans evolved durable but hard-to-fix teeth -- there is no replacement for the loss of an adult tooth. Tusks are less durable than enamel-coated teeth, but they grow continuously, even if they get damaged. "Enamel-coated teeth are a different evolutionary strategy than dentine-coated tusks, it's a trade-off," says Whitney.
Analysing the histological thin sections of dicynodont specimens from South Africa, Antarctica, Zambia, and Tanzania the researchers found that, much like human teeth, these animals appeared to reduce the amount of replacement teeth at the canine position and had a soft tissue attachment to the jaw. Interestingly, this is a combination of features that is unique to mammals. Mammals, like humans, replace baby teeth with adult teeth only once unlike most other vertebrates -- for instance sharks have continuous teeth production. Mammal teeth are attached to the jaw by gomphosis which is a soft-tissue, or ligament, attachment. Most vertebrate teeth, however, are attached to the jaw by ankylosis, which is a hard-tissue fusion of bone to tooth.
"If you have these two things, a reduced amount of tooth replacement and a soft-tissue attachment, an ever-growing tooth allows the animal to get around the fact that it cannot replace the tooth. Instead it evolves to continuously deposit the same tooth tissues," said Whitney. "And as the animal continues to deposit the tissue, the tooth begins to move outside of the mouth to become functional."
The researchers found that true tusk evolution only occurred at a later stage of evolution in this group -- early members of this group had a big tooth rather than a true tusk. Late in their evolutionary history dicynodonts evolved a true tusk that was ever growing, and surprisingly did so convergently in multiple different kinds of dicynodonts. "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," said Whitney. "This is a similar story to what we see in elephant evolution in that it mirrors a lot of the patterns that have been studied on how elephants got their tusks."
"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!" said co-author Brandon Peecook, curator at the Idaho Museum of Natural History.
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," said co-author Christian Sidor, curator at the University of Washington Burke Museum.
The various kinds of teeth animals have evolved can tell scientists about the pressures those animals faced that could have produced those teeth. For instance tusks can function in a variety of ways including defense, competition, burrowing, sexual selection, and even assist with locomotion -- as in the walrus which uses its tusks to hoist itself upon to the ice from the water. A continuously growing tusk may have allowed these dicynodonts to overcome the challenges of only having one set of replacement teeth throughout their lives.
"We don't really know what functions the dicynodonts tusks may have had because we can't observe them and see what they were doing with them," said Whitney. "That's a lingering question about dicynodonts, even more so now."
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