An excavation in the Lower Cretaceous Duck Creek Formation of Texas has yielded three vertebrae that belonged to a gigantic 6.3-meter-long shark (that’s over 20 feet). Researchers haven’t been able to identify the species yet, but whatever it was, it was one of the largest sharks ever documented in the Early Cretaceous of North America 100 million years ago, according to findings published in PLoS ONE this week.
The disarticulated vertebrae (OMNH 68860 – see image below) were discovered in Albian-age limestone rocks near Fort Worth by members of the Paleontology Club of the University of Wisconsin–Milwaukee in 2009. The largest one was about 34 millimeters wide and had a diameter of 110 millimeters. Fossilized shark vertebrae are incredibly useful for helping researchers to estimate total body length, which then helps clarify the biology and ecology of prehistoric sharks.
To get at a size estimation, a trio led by Joseph Frederickson from the University of Oklahoma compared the vertebrae to more complete Cretaceous shark fossils – assuming that the relationship between vertebral size and total body length is consistent among sharks who live in the open ocean. Additionally, by assuming that the largest of the three vertebrae was the largest vertebra of the whole animal, they estimated the smallest hypothetical length. This means that the shark was 6.3 meters long at minimum.
Compared with shark teeth, however, isolated vertebrae aren’t really very helpful for determining species. This large animal does compare pretty well to a contemporaneous shark (KUVP 16343) that was unearthed from the Kiowa Shale of Kansas. Vertebrae from this specimen had a diameter of between 144 and 170 millimeters, and the animal was likely 8.3 to 9.8 meters long. While this shark is bigger, the two may have been of the same species.
Unfortunately, neither of these specimens were recovered with associated teeth. But here’s something interesting: one shark that has been found in both Duck Creek Formation and Kiowa Shale is Leptostyrax macrorhiza, the largest of the common lamniform sharks. (This group includes many familiar sharks, such as the great white, as well as unusual ones like goblin sharks.) In the figure at the top, both KUVP 16343 and OMNH 68860 are reconstructed as Leptostyrax macrorhiza.
As for what they ate, probably "whatever fits in their mouth," Frederickson tells Live Science. This means large fish as well as marine reptiles like baby pliosaurs, or maybe even fully grown ones.
Images: 2015, Frederickson et al., PLoS ONE