One of the most exciting things for any fossil lover is finding just that: a fossil! Shark teeth are often the only links we have to ancient sharks due to their cartilaginous skeletons not being able to withstand the test of time. While many believe all sharks look the same, this isn’t the case today nor was it the case thousands of years ago. This same train of thought applies to shark teeth, each sporting a unique shape based on the shark’s diet.
The scissor tooth shark, Edestus, goes for a bite.
Jesse Pruitt and Evelyn Vollmer, Idaho Museum of Natural History.
Particularly odd sharks are those in the genus Edestus, known as scissor tooth sharks, which had some species with spiral tooth whorls. Sharks of Late Paleozoic oceans evolved unique dentition for catching and eating soft-bodied animals. A diverse but poorly preserved clade, edestoids like Helicoprion and Edestus are noted for developing biting teeth at the midline of their jaws. Since 2012, researchers Dr. Leif Tapanila and Dr. Jesse Pruitt have worked with the remaining bits of these ancient sharks with teeth in the midline of their jaws, specifically the buzzsaw and scissor tooth sharks. “Back in 2012 Jesse was a student at Idaho State and wanted to work in paleontology, so I sent him over to our museum. He found our collection of Helicoprion. We worked together to solve some long-standing problems about that fossil; how the teeth fit in the mouth, how it fed, and how many species belonged to the group,” said Tapanila in an e-mail. Tapanila, Pruitt and others previously showed that their individual serrated teeth crowns grasped, sliced, and pulled prey items into their esophagus. “After finishing that project, we turned our attention to a cousin of the buzzsaw shark, Edestus (the scissor tooth shark). As a fossil, Edestus had many of the same mysteries that we helped solve for Helicoprion: how did the teeth fit in the mouth, how did it feed, and how many species were there. We described the first skull of Edestus earlier this year, and in our most recent paper we address the diversity of Edestus and its geographic history.” In fact, the researchers reevaluated the tooth morphology used to define species within the genus Edestus and published the findings.
“In Edestus, the tooth is shed at the end of the conveyor belt out the front of the mouth; and the animal has both upper and lower midline teeth. They used these teeth to split their prey in half,” explained Tapanila. Tapanila and Pruitt combined linear measurements and geometric coordinate approaches to identify species. “We took over 200 Edestus teeth and use these measurements to quantify the shape of the tooth and then compare all the teeth with each other. Doing this allows us to see how tooth shape changes with size (or growth of the animal), which is expected. The growth curve of tooth shape often allows us to distinguish species and even tell the difference between a tooth that formed in the upper vs lower jaw.”
By studying the teeth, the intact spiral tooth whorls demonstrate that teeth from the upper and lower whorls differ in shape and ontogeny. And the most surprising find of the study? Tapanila says: “Math kills species! We find over and over that too many species get named than can be justified by the fossils. In this new study we condensed Edestus from thirteen species to four. For our previous Helicoprion study, we went from ten to three. Why people name so many fossil species (without good reason) is partly human nature — over-exuberance of finding a new fossil from a new place and wanting to name it — and limited information from the fossil. A lot of species have been named from a single specimen so there’s no context of variation, growth series, etc.”
Liam Hourihan inspects a model of a Helicoprion – a strange toothed shark – whose vertical teeth … [+]
AFP via Getty Images
Edestus fossils are recovered from coastal marine to estuarine deposits spanning roughly six million years and Edestus first appears in England during the latest Bashkirian (313 Ma, Carboniferous), a few million years after its most closely resembling genus Lestrodus. Diversification and range expansion of Edestus coincides with the Moscovian transgression that flooded Laurentia and the Russian platform.
Thanks to this new research, the science community now has an idea as to what Edestus looks like, how it used to feed, what kind of variation was within the group, and even where they first started! According to the research, Edestus fossils are recovered from coastal marine to estuarine deposits spanning roughly six million years and this ancient animal first appears in England during the latest Bashkirian (313 Ma, Carboniferous), a few million years after its most closely resembling genus Lestrodus. Its range then expanded to the USA and Russia as sea level rose. According to Tapanila, Edestus thrived in the coastal estuaries of what is now coal-country in the American midwest!
When asked if he thought that a species with this type of dentition (the spiral tooth whorl) would be successful in today’s oceans, Tapanila replied, “Edestus would probably be at home in coastal Louisiana, chomping happily on fish that got too close to its wonderfully sharp teeth.” Is Edestus still around? Unfortunately, no. But the modern ratfish and chimaeras are the remnants of this once mighty group of cartilaginous fish.