Describing a Mysterious Shark

How do you identify an ancient shark with just a few partial fossils? It’s a tricky task that a team of scientists set out to do in a study published today in PLoS ONE.

In the study, they describe a shark discovery from Fort Worth, Texas. This seems like an odd place for a shark, but 100 million years ago, the Western Interior Seaway covered a large swath of our continent, stretching from the Arctic Ocean to the Gulf of Mexico.

This ancient shark appears to have been very large, comparable to today’s great white shark. The authors of the paper report that the shark was likely 6.3 meters in length (over 20 feet!), and that’s a conservative estimate. And most remarkably, the scientists came up with that measurement from only three vertebrae.

The three vertebrae were discovered at the Duck Creek Formation, and the team says the fossils belong to one individual shark. No other fossils were discovered, but fortunately, vertebrae can tell scientists a lot about a shark. In fact, several years ago, scientists came up with a formula that uses vertebrae diameter to estimate a shark’s length. With the current find, the scientists are unsure if the vertebrae they found were the largest for this particular shark—which means the estimated length is conservative, and the shark could have been even larger.

While vertebrae are a good indicator of shark size, it’s really the teeth that allow scientists to identify shark species. This shark was identified as a lamniform—whose relatives include the great white and megalodon—making it one of the largest of that period. The researchers also compared it to specific shark species from the same time and place, including Cretoxyrhina mantelli, Cardabiodon ricki, Carcharias amonensis, and Leptostyrax macrorhiza. Vertebrae fossils from the first two differ enough that the authors suspect they cannot be from those species. The latter two sharks have been found at or near the Duck Creek Formation, making them worthy candidates for the mysterious new find. While no vertebrae are known from C. amonensis, its teeth indicate it was much smaller than the new fossil. Finally, while vertebrae are also missing for L. macrorhiza, its teeth suggest it was larger than the new find—8.3-9.8 meters (27-32 feet).

So the scientists are closer to understanding what the new finding is not… The team does note that the new discovery can tell us more about the ancient ecosystems where these sharks resided, and in fact, they would have been among the largest marine predators in that environment. “Fossil tooth marks on dinosaurs, mosasaurs, plesiosaurs, teleost fishes, and turtles indicate that large lamniform sharks of the Late Cretaceous occupied the ecological position of generalist predator and scavenger, much as they do today,” they write.

Image: New shark (OMNH 68860) reconstruction, doi:10.1371/journal.pone.0127162.g005