Whether elephants, walruses or warthogs – tusks have developed independently of one another in different animal species living today – but only in mammals. But why? Researchers have now clarified this with the help of fossils from primeval relatives of mammals, a group of animals called Dicynodontia. According to this, the prerequisites for the development of the tusks were that tooth changes were rare, the connection between the teeth and the jaw consisted of soft tissue and the teeth grew for a lifetime – characteristics that can only be found in mammals today.
Tusks fulfill very different functions for animals living today: They serve as a defense against enemies, as a tool for digging, can impress potential sexual partners and sometimes even help locomotion, for example in walruses that pull themselves out of the water with their tusks. However, how the tusks developed independently of one another in different animal species, what requirements had to be met and why tusks only occur in mammals, has so far been unclear.
Primeval tusk carriers
A team led by Megan Whitney from Harvard University in Cambridge has now explored the evolutionary origins of the tusks. “To do this, we first had to define the term tusk because it is surprisingly ambiguous,” says Whitney. According to their definition, it is only a real tusk if it protrudes from the mouth, grows for a lifetime and consists only of dentin, i.e. is not covered with tooth enamel. The last point distinguishes tusks, for example, from the incisors of rodents, which also grow back for life, but are coated on the front with tooth enamel.
The researchers selected the oldest known animals in which tusk-like structures were identified as the object of investigation: dicynodontia. Dicynodontia lived 270 to 201 million years ago, so they originated before the dinosaurs. The group comprised a multitude of species, some as big as a rat and some the size of an elephant. Although they weren’t mammals themselves, they are more closely related to today’s mammals than dinosaurs and other reptiles. The dicynodontia (“two canine teeth”) got their name because of an unusual feature: two long, tusk-like canines protruded from the beak-like mouth of most representatives.
Not all tusks were “real”
Whitney and her colleagues examined the fossil teeth of 19 specimens of Dicynodontia from South Africa, Zambia, Tanzania, and Antarctica, representing ten different species. With the help of micro-computed tomography, they analyzed on the one hand how the teeth were attached to the jaw and whether there were any signs of continuous growth. On the other hand, they examined wafer-thin slices of the teeth for their composition.
They found that not all of the teeth examined were real tusks. “Although the dicynodontial canines look morphologically similar, not all of them were tusks in the sense of our definition,” the researchers report. In some specimens, the tooth was fused with the jaw. Continuous growth of the tooth is only possible if the tooth and jaw are only connected by connective tissue. In other specimens, the teeth were covered with tooth enamel – which makes the teeth more resistant, but also prevents renewal, since tooth enamel, unlike dentine, cannot be regenerated. “Enamel coated teeth are a different evolutionary strategy than dentine coated tusks,” explains Whitney.
Unique combination
Only in evolutionarily younger species of the dicynodontia did the researchers discover real tusks. These evidently developed independently of one another in several species. This was accompanied by the fact that the dicynodontia apparently reduced the number of teeth changes – in contrast to other reptiles then and now, who change their teeth regularly. “Fewer tooth changes increased the selection pressure to develop a more durable tooth,” explain the researchers. “A soft tissue connection between tooth and jaw enabled the tooth to grow continuously.”
This combination, unique to mammals today, created the basis for the development of tusks. “This could explain why modern mammals have acquired tusks uniquely and repeatedly,” the researchers said. “Our study shows that dicynodontia developed tusks millions of years before the evolution of the first mammals, likely in similar evolutionary ways and possibly in response to similar selection pressures.”
Source: Megan Whitney (Harvard University, Cambridge) et al., Proceedings of the Royal Society B, doi: 10.1098 / rspb.2021.1670