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Have you ever noticed that only mammals have tusks? Neither birds, nor reptiles, nor amphibians had this distinctive feature – rather, the tusk is an exclusive property of mammals. New research from Harvard University looks at tusks and their characteristics in mammals and reveals the beginning of the evolution of tusks.
What are tusks?
We all know which animals have tusks: elephants, warthogs, walruses and babiruses. However, until recently, canine teeth were a very vague and ambiguous scientific term. To trace the evolution of the tusk, researchers had to come up with a more specific definition to distinguish tusks from teeth and other external bone structures in the fossils.
For a tooth to be a tusk, researchers have determined that it is a bone made entirely of dentin that comes out of the mouth and grows constantly. This definition helps distinguish tusks from other canine-like bones, such as teeth. The teeth are usually covered with enamel, which should not be on the canines if they are made of dentin.
In addition, teeth are more difficult to replace than tusks. Teeth will not grow back if removed or damaged. There are tusks, however. Researchers came up with this definition while working in Zambia to excavate fossils of dicynodonts.
Study of dicynodonts and canines
Dicynodonts lived about 250 million years ago. They are one of the few large Cretaceous animals closely related to mammals. These creatures were herbivores, ranging in size from a rat to an elephant. They had no teeth, but had two protruding fangs and a keratin beak.
While working at the dicynodont excavation site, the researchers were surrounded by the “fangs” of the dicynodonts and realized that many of the “fangs” did not actually match their definition of canines.
The researchers then observed thin sections of the canines and teeth of the dicynodont under a microscope. Specimens, mainly from South Africa, Zambia, Tanzania, and Antarctica, appear to have developed anatomy of tooth attachment similar to that of modern mammals. In addition, they have grown tusks. However, true tusks were found only in later species of dicynodonts.
The discovery of when the dicynodonts acquired real tusks answers some of the researchers’ questions about how the tusks evolved. They realized that the tusks developed parallel to the teeth. When the teeth became enamelled and irreplaceable, the dicynodonts grew tusks to make up for tooth deficiencies. Eventually, this evolution led to the modern tusk.
What does this research mean for fanged mammals?
The discovery of how and when true tusks grew in dicynodonts allows researchers to begin to determine the reasons for the evolution of tusks in modern mammals.
At one time, dicynodonts were the most extensive and diverse species on Earth. Today they are the closest relatives of their time to the modern mammal. By discovering the parallel evolution of teeth and canines, researchers can better understand the evolution of elephants and hippos.
Researchers may also begin to understand why species separate along the evolutionary tract: some develop with only enamel teeth, and others with a combination of teeth and canines. This research could help identify the problems faced by early mammals and the pressure that determined the tusks compared to the teeth.
Finally, this study proves that in order for the tusks to grow, canine mammals also had to develop with a reduced rate of tooth replacement and the flexibility of ligament attachment. These traits are absent in reptiles, amphibians or birds and explain why only mammals have tusks.
While it is still not known whether canines evolved in fanged mammals such as walruses or warthogs for the same reasons as elephants, understanding the evolutionary partners of tusks will also help in future research on these mammals.
Find out more about Tusks And Tusk Evolution!
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