Figure 1: Tusk of a dicynodont
(Source Credit: The Harvard Gazette)
A group of researchers that published a new study decided that a tooth be a tusk only if it amplifies out from the mouth, is made exclusively of dentine (a major tissue that builds the structure of teeth), and develops persistently to regenerate the damaged areas.
Therefore, not all mammals have tusks. Homo sapiens, for instance, have their teeth coated with a substance called enamel that prevents the growth of teeth. Unlike tusks, which can repair themselves, enamel-covered teeth are sturdy but irreplaceable once destroyed. On the other hand, elephants, water deer, narwhals, and walruses are just a few that do have tusks. But did they have tusks from the beginning, and why do only a handful of mammals have these features?
According to the study, led by Megan Whitney, a postdoctoral researcher at Harvard University, these mammals evolved to have tusks. She and her colleagues detected the first authentic tusks from several dicynodonts, mammal-like reptiles that existed approximately 270 to 201 million years ago. These reptiles belong to therapsids, an extinct group. Dicynodonts had beaks and a pair of teeth similar to the appearance of tusks—large and projecting from their heads.
The team was able to detect how the first tusks were attached to the skulls of dicynodonts through the examination of micro-CT scans. Researchers utilize micro-CT scans to primarily explore the internal structure of material and biological specimens without literally cutting them so that these samples are preserved for use in future research. Through observation, they found that several of the 19 dicynodont specimens tested did not meet the requirements of what should be considered a tusk. Researchers found that many dicynodont “tusks” were coated in enamel—not dentine—making them large teeth. Only a few dicynodonts started to develop tusks. These reptiles that formulated tusks also began to form soft tissue ligaments that connected the tooth to the jaw. The tissue ligaments decreased the rates of tooth replacement.
Previously, Dr. Whitney hypothesized that all dicynodonts came to have tusks at a certain point in the family tree. So, she was astonished to discover that the dicynodonts developed their tusks convergently. In other words, these reptiles independently evolved similar traits to adapt to similar environments. An incredible discovery, as such, allows scientists to further probe through our (mammal) lineage of evolutionary processes.
Q&A:
Sunmin: You mentioned that this discovery leads to further exploration through the mammal lineage, so what are some examples? What other aspects can now be explored?
It wasn't a while back when scientists discovered that dicynodonts were closely related to mammals. Recently, they have found how dicynodonts developed to form tusks. Scientists can conclude how mammals came to have teeth by exploring the origin of tusks that evolved in such ancient reptiles.
Jennah: The article mentions that other species, such as the narwhal, water deer, and walrus also have tusks for a similar function as the elephant. What are some similarities and differences that the tusks of those mammals have with the elephant?
Elephants use their tusks for many purposes, such as foraging, digging, defense, and lifting objects. Similarly, water deer use their tusks for defense; their tusks help them fight off mates and predators. Walruses and narwhals use their tusks for slightly different purposes. Walruses use their features to pull themselves out of the water. Narwhals use their tusks to detect temperature and water pressure. However, all of these mammals use their tusks for survival.
Xavier: The article explains how tusks came to be, but not exactly why. Why did animals evolve to have tusks? How were they a favorable trait for survival?
As mentioned in the previous question, tusks are essential for the survival of mammals. Tusks allow mammals to defend themselves from predators, attack their prey agilely and feasibly, and attract mates. Mammals with tusks have been deemed favorable within changing environments; they essentially were naturally selected.
Hannah: What exactly is the significance of this discovery? How can we benefit from learning about the mammal lineage?
There are essentially many more homo sapiens in this world than any other mammal. And as dicynodonts are reptiles very closely related to mammals, researchers may use the results of this study to relate them to humans. Furthermore, by learning about mammal lineage, we are learning more about ourselves—essentially how we evolved to become who we are today.
Works Cited
Elbein, Asher. “How Did Elephants and Walruses Get Their Tusks? It's a Long Story.” The New York Times, The New York Times, 28 Oct. 2021, www.nytimes.com/2021/10/28/science/elephant-walrus-tusks-evolution.html.
Siliezar, Juan. “Harvard Researchers Show How Tusks Evolved.” Harvard Gazette, Harvard Gazette, 2 Nov. 2021, news.harvard.edu/gazette/story/2021/11/harvard-researchers-show-how-tusks-evolved/.
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