The life reconstruction of Dicynodon, a dicynodont. Apart from the tusks on the upper jaw, most Dicynodonts had a turtle-like beak to help them chew their food. Marlene Hill Donnelly. Credit: Marlene Hills Donnelly
There are many animals with tusks. They range from elephants and walruses, to five-pound guinea-pig-looking critters called Hyraxes. One thing all tusked animals share is the fact that they are mammals. There are no fish, reptiles or birds with tusks. A new study published in Proceedings of the Royal SocietyB shows that paleontologists have traced the origins of the first tusks to ancient mammal relatives. To do this, they needed to determine what makes a "tusk" a "tusk".
Megan Whitney, a Harvard University researcher and lead author of this study, 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.
"We were able show that the first Tusks belonged before modern mammals, called Dicynodonts," Ken Angielczyk (a Chicago Field Museum curator and the author of this paper) says. They are very strange animals.
The dicynodonts lived mainly before the time when dinosaurs existed. They were rat-sized up to elephant-sized. Although modern mammals are their closest living relatives they look more reptilian with turtle-like beaks. Their defining feature has been their pair of protruding teeth in the upper jaws since their discovery 176 year ago. Dicynodont literally means "two canine" and is also known as
While taking a break from a paleontological dig, the researchers had the idea of studying the origins of tusks. Whitney recalls, "We were sitting on the ground in Zambia and there were many dicynodonts teeth everywhere." Ken picked them up and asked why they were called "tusks" because they had features not found in tusks.
Angielczyk made a critical distinction. Not all protruding tooth are technically tusks. The teeth's anatomy and growth patterns will tell us if they count. Whitney says, "We had to define a Tusk for this paper because it's an incredibly ambiguous term." Whitney explained that a tooth must extend beyond the mouth to be considered a tusk. Unlike most mammals' teeth, which include ours, tusks are made of dentine, not hard enamel.
Left side of skull of Dolichuranus, a dicynodont (NMTRB554) from Tanzania. The large tusk can be seen at the lower left. K. Angielczyk. Credit: Ken Angielczyk
These parameters dictate that elephants, walruses and warthogs all have tusks. However, there are other large teeth in the animal kingdom that don't make it. Rodent teeth, for example, are not counted because they have an enamel band at the front of their teeth.
The team found that some of the dicynodont-tusks they observed in Zambia weren't quite fitting the definition of a "tusk". They were also coated with enamel rather than dentine.
Scientists can also gain insight into the animal's lives by studying how teeth and tusks are different. Whitney says that enamel-coated teeth have a different evolutionary strategy to dentine-coated Tusks. It's a tradeoff. Although enamel teeth are more durable than dentine due to the way teeth grow in the jaws, it is not possible to have an entire enamel covering if you want teeth that will last throughout your life.
Humans and animals made an evolutionary investment into durable, but difficult-to-fix teeth. Once our adult teeth are in, it's impossible to fix them. Although tusks are not as durable as enamel-coated teeth they can still grow and develop even after being damaged. This is like choosing between a reliable car but difficult to repair when it has problems, or a beater model that can be fixed quickly and cheaply by any mechanic.
Scientists can learn from the different types of teeth that animals have developed about the conditions under which they were able to produce them. Tusks may be used by animals to fight or root in the ground. This could expose them to minor injuries that can cause enamel teeth not to grow normally.
The researchers made paper-thin slices from 19 fossilized dicynodont teeth to examine their structure. The researchers also performed micro-CT scans to check how the teeth were attached and whether the roots had evidence of continuous growth. Scientists discovered that some dicynodont tooth are tusks while others, especially those from earlier species, have large teeth. Although there was no clear progression from non-tusks into tusks it was possible for different members of the dicynodont species to develop tusks on their own.
Field teams found isolated tusk fragments in Zambia in 2018. Photo by K. Angielczyk. Credit: Ken Angielczyk
Whitney said she was shocked by the discovery. Whitney says she was surprised by the discovery.
Angielczyk says that "Dicynodont Tusks can tell us much about mammalian evolution in tusks," For true tusks evolution, it is necessary to have a flexible ligament connecting the tooth to your jaw and lower rates of tooth replacement. This all leads to a better understanding about the tusks that we see today in mammals.
"Dicynodonts were among the most diverse and abundant vertebrates on earth just before dinosaurs, and are well-known for their 'tusks. We can see the beauty of evolution in the fact that only a few species have true tusks and all others have large teeth. It is possible to see how to make a tusk! Brandon Peecook is a curator at Idaho Museum of Natural History, and one of the paper’s authors.
Researchers claim that this study, which shows the first instance of true tusks in human history, could help scientists understand the mechanisms behind evolution.
"Tusks have changed a lot over the years, making it difficult to understand how and why. Now we have data about the anatomical changes required for dicynodonts' evolution of tusks. Christian Sidor is a University of Washington Burke Museum curator and one of the paper’s authors.
Angielczyk says that despite being very strange animals, there are still some things we can learn about dicynodonts like the evolution and tusks. "Plus, whenever you can say that mammals aren’t very special, dicynodonts made it first, that’s a good day."
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