On a typical day in 2011, Chicago Field Museum volunteer Karen Nordquist was undertaking her usual task: sifting ancient mud through a mesh screen that captures small fossils. This particular batch of mud had been found around the excavation site of the museum's most famous resident—Sue the T. Rex, the best preserved and most complete example of the species.

As if that wasn't cool enough, Nordquist accidentally discovered a new species of shark. described today in the Journal of Paleontology.

Scientists study such mud samples because they can provide clues to the environment in which an ancient creature lived and died. The mud found around Sue the T. rex had previously yielded a few bird bones and dinosaur fossils and limited evidence of a swamp-like environment via a crocodile tooth. It had been largely thought that Sue lived in a mostly terrestrial environment. Then came that day in 2011.

"Kind of late in the sorting, all of a sudden … it was kind of hard to tell what it was, but when I turned it around I saw a little flash of enamel," Nordquist says.

On further inspection, the fossils from the South Dakota site yielded something unexpected: shark teeth. She had stumbled upon a new type of shark, which in turn suggested that the ancient sea once covering Nebraska had tributaries reaching up into the Dakotas, where this mud was found.

Now, don't expect a great white. The new species is a type of carpet shark, which are typically small-mouthed bottom feeders. This particular species, the Galagadon nordquistae, was probably only a foot long, with one-millimeter teeth that looked a bit like the aliens in Galaga, hence the name (the other part coming from Nordquist.)

Nordquist, who has been unable to volunteer for a few months because of illness, found out about a month ago that not only had she found a new species through her volunteer work, but also that it had been named for her. "I was really surprised and honored that they would do that," she says. "That doesn’t happen very often."

Peter Makovicky, a curator at the Field Museum and author on the paper, paints the picture of the world the Galagadon lived in. The shallow inland ocean had a series of connected waterways, and the particular one in the Dakotas was likely to be freshwater rather than salt water, or a sort of brackish mix in between.

It may have been an upstream breeding spot for the fish, an ancestor of modern-day carpet sharks which are now mostly found in the islands in and around Indonesia. "It’s possible given the very small size of the teeth that we’re dealing with a shark nursery situation," Makovicky says. Sue and the Galagadon, despite their proximity to each other, likely interacted very little.

The actual soil came with the Field Museum's purchase of Sue in 1997. It's not unusual to keep the sediment found around a fossil in order to sort through for smaller bits later, but the dirt around Sue sat in storage for another ten or so years before anyone thought to do anything with it. "It was just standing around in our collections for a while, and then about ten years later, Bucky Gates, who's the first author on this paper, was here in Chicago starting a project to screen-wash and go through all this material," he says.

No other material was found from the sharks save the teeth, but that's by the design of the shark. At some point, Makovicky says, sharks likely had bones more like other vertebrates, but eventually evolved a skeleton made out of soft cartilage. This means that only hard parts like teeth and occasional tips of fins survive. "Getting anything else from the skeleton is very rare and requires very rare circumstances," he says.

This means that the millimeter-size teeth may be all we'll ever know of the Galagadon. And it might not have been discovered at all had not someone decided to sift through a little ancient dirt nestled in a much larger discovery.

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