Researchers Trace Tusks Back to Mammals’ Toothy Ancestors

Have you ever noticed that only mammals have tusks? No bird, reptile, or amphibian has developed this distinct feature, indicating that the tusk is solely a mammalian characteristic. A recent study conducted by Harvard University examines tusks, their distinction among mammals, and uncovers the origins of tusk evolution.

What Are Tusks?

We all know which animals have tusks: elephants, warthogs, walruses, and babirusas. Yet, until recently, “tusk” has been a very undefined and ambiguous scientific term. To trace tusk evolution, researchers needed a more concrete definition to distinguish tusks from teeth and other external bone growths in fossils.

For a tooth to be a tusk, researchers defined the feature as a bone made entirely of dentine that extends from the mouth and is ever-growing. This definition helps distinguish tusks from other tusk-like structures, such as teeth. Teeth are usually covered in enamel, which tusks, being made of dentine, do not have.

In addition, teeth do not regrow if removed or damaged. Tusks, however, do. Researchers came up with this definition while working in Zambia, excavating dicynodont fossils.

Dicynodonts and Tusk Research

Dicynodonts lived around 250 million years ago. They are among the only large animals from the Permian and Triassic periods closely related to mammals. These creatures were herbivores and ranged from the size of a rat to the size of an elephant. They had no teeth, but instead had two protruding tusks and a beak made of keratin.

While working at a dicynodont excavation site, researchers were surrounded by dicynodont “tusks”  and realized many of the tusks didn’t actually fit their definition of tusks. Researchers then observed thin slices of dicynodont tusks and teeth under a microscope. The specimens, primarily from South Africa, Zambia, Tanzania, and Antarctica, appeared to have evolved tooth attachment anatomy similar to mammals today. In addition, they grew tusks. However, true tusks were only found in later species of dicynodonts.

The discovery of when dicynodonts acquired true tusks answers some researchers’ questions about how tusks evolved. They realized that tusks evolved in parallel to teeth. As teeth became enamel-covered and irreplaceable, dicynodonts grew tusks to compensate for the shortcomings of teeth. Eventually, this evolution led to the modern tusk.

What Does This Research Mean For Tusked Mammals?

The discovery of how and when dicynodonts grew true tusks allows researchers to begin pinpointing the reasons for tusk evolution in modern-day mammals. In their time, dicynodonts were among the most widespread and diverse species on Earth. Today, they are the closest known relative of their era to modern mammals. By discovering the parallel evolution of teeth and tusks, researchers can better understand the evolution of elephants and hippopotamuses.

Researchers can also begin to understand why species diverged on the evolutionary path: some evolving with only enamel-covered teeth, and others with a combination of teeth and tusks. This study could help pinpoint the challenges early mammals faced and the pressures that determined tusks versus teeth.

Finally, this study shows that for tusks to grow, tusked mammals must have also evolved reduced rates of tooth replacement and a flexible ligament attachment. These traits are not found in reptiles, amphibians, or birds, which explains why only mammals have tusks.

Though it is still unknown whether tusked mammals such as walruses or warthogs evolved tusks for the same reasons as elephants, understanding the evolutionary origins of tusks will inform future research on these mammals as well.