Sixty-eight million years ago, a Tyrannosaurus rex died in a small river in what is now South Dakota, USA. Along with "Sue", the T. rex's skeleton, many other animals had pieces of their skeleton preserved in the same section of the river. The most recently discovered of these species is a new type of shark named Galagadon nordquistae.

Galagadon nordquistae is a name derived from two important aspects of the shark fossils. First, Galagadon is a reference to the 1980's video game Galaga, because when one looks at the teeth in various perspectives they resemble the space ships from that video game. The second part of the name, nordquistae, is a Latin version of the surname Karen Nordquist, who patiently searched through the washed rock to find all of the teeth.

Because the fossils of Galagadon were found right alongside the bones of "Sue", we know that T. rex and this new shark lived near one another; however, it is uncertain if they ever interacted with one another. All that is currently known of Galagadon are 24 teeth, which is the way that most fossil sharks are known. This is because the skeletons of sharks are not made of the same hard material found in mammal or bird bones, therefore, the process of turning into a fossil destroys their bones except in rare instances.

The teeth used to name Galagadon were all found in the dirt right alongside Sue's bones, yet unlike the giant bones of the dinosaur, Galagadon's teeth are only 1 mm across. Once the Field Museum of Natural History in Chicago, IL, USA received the T. rex specimen, they began cleaning the fossils, but also washed all of the surrounding dirt through screens with tiny openings that trapped many fascinating fossils best seen under a microscope.

Determining that Galagadon was a new species took many days of specialized photography under a microscope to get accurate pictures for comparison. We then set out looking at all of the possible fossil shark teeth that we could find and began the detailed work of defining which characteristics were present on our specimen compared to ones already named. Turns out that we were able to find three features unique to Galagadon, including the way that the top (also called the crown) of the tooth attached to the root, the different patterns of enamel that rose from the flat part of the tooth crown, and the edges of the tooth crown that have a distinct change in angle instead of being a smooth transition from tooth top to bottom.

One difficulty in working with shark teeth is that many species have teeth that change shape from the front to the back of the jaw. There are certain things that give hints as to where in the jaw a tooth might go, such as teeth towards the back will have the pointed part of the tooth inclined to one side or the other, or teeth at the very rear of the jaw will usually be squat and stunted in comparison to the other teeth. Luckily, we found features consistently across all of the teeth that helped us determine we were looking at the teeth of a single species and not teeth from different shark species. For instance, if you look at the broad side of the Galagadon tooth crown, everyone has bumps, folds, and ridges, they are not just flat as is typical among sharks.

Galagadon does not help us understand the physical environment that "Sue" the T. rex that was buried. In quite the opposite way, "Sue" and the other vertebrates in the fossil assemblage help us to understand Galagadon more. Our new shark was swimming with a variety of freshwater fish like gar and bowfins, there were frogs and salamanders, turtles and crocodilians, which together only occur in freshwater, not saltwater like most other sharks. Looking from the Mesozoic to today, we know that once there were many different shark species closely related to Galagadon that lived in rivers and streams across the world. At some point between the end-Cretaceous extinction and today, all of the carpet sharks living in these freshwater ecosystems went extinct. Today's bamboo shark is known to go into water that is a mixture of fresh and saltwater, but does not live solely in freshwater.

Our study was also the first study to attempt a family tree for fossil sharks by combining genetic information from modern sharks with tooth shapes from modern and fossil carpet sharks. From the new family tree data, we then determined that carpet sharks (the group of sharks to which Galagadon belongs) once lived everywhere across the world. However, now they are restricted to the warm waters of the South Pacific Ocean and the Indian Ocean.

Next read: Sea otters: how welcome should a recovering top predator be? by Edward Gregr

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