Stick a turtle's head on a baby hippo. Next, twist the baby hippo's front legs so that they extend to one side. However, keep its back straight. Give it tusks. You now have something that looks somewhat like a mammal-like prototype that walked the earth hundreds of millions years ago.
These animals are known as dicynodonts. This refers to the two canines that protrude from the animal's jaw. These teeth could be the reason you won't find tusks in anything made of feathers or scales.
Megan Whitney, an evolutionary biology researcher at Harvard University, says that tusks are a very well-known anatomy. However, until I began working on this study, I didn't really think about how tusks were restricted to mammals.
Dicynodonts, while not mammals, belong to an extinct family called therapsids.
They varied in their range and diversity between the Permian to their eventual extinction in Triassic around 200 million years ago. Their sizes ranged from small objects in your hands to large elephantine beasts.
They were also very successful in putting their mix of anatomy to use across a variety of ecological niches.
Above: Skull from the Dolichuranus dicynodont Dolichuranus in Tanzania. At the lower left is visible the large tusk.
Since the first dicynodont fossils were examined more than 150 years ago, these protruding teeth have been a focal point of much attention. Richard Owen, the famous paleontologist, gave us the first glimpse into the teeth' construction in 1845. He claimed that they were mostly composed of dentine with thin layers of cementum and enamel, as well as a large pulp cavity.
Studies on other specimens since then have shown a dearth of dentine. This suggests that canines continued to grow, much like rabbit teeth. At least, in certain species.
It is crucial to understand how teeth grow if we want to understand how extinct animals behaved.
It also raises an interesting philosophical question that can have implications for understanding our evolution.
Whitney says, "For this paper we had to define the tusk because it's an amazingly ambiguous term."
Here's what they came up for to make sure we are all on the same page.
When we speak of tusks we usually mean teeth that protrude from the mouth. Think mammoths and walruses.
True tusks have continuous growth to make them stand out from the chompers of crocodiles.
What about rabbits and rats? They might have long, strong teeth that will continue to grow but they are always coated with enamel which is a durable surface.
Tusks aren't bothered. Enamel coating them would make them more durable, but the oblique angles of their bodies make it nearly impossible to apply a consistent coat.
Finally, the ligament keeps tusks in place and allows them to withstand a hard knock.
These features are indicative of the abuse and use tusks have received in comparison to other teeth. They are often used to dig through muck and to fight rivals and predators. Therefore, they must be strong enough to complete the task and also able to regenerate quickly if damaged.
The researchers looked at a variety of dicynodont tooth types and found that there was a lot of variation among them. True tusks were more common in newer species of dicynodonts, and they had evolved independently from other tusks throughout their family tree.
We now know the evolution of teeth in ancient animals. This information, together with the definition of the tusks' characteristic features, gives us a better understanding of why they aren't found in amphibians, fish, and lizards.
"For example, this study suggests that true tusks can evolve by having a lower rate of tooth replacement and a flexible joint attaching the tooth to its jaw," said Ken Angielczyk (a Chicago Field Museum curator).
It all leads to a better understanding about the tusks of mammals today.
This research was published by Proceedings of the Royal Society B Biological Sciences.