Prehistoric Saltwater Shark Nursery Fossils Found in Illinois.

Not far from Chicago, in a region now dominated by cornfields and whitetail deer, scientists say they've found fossil evidence of a “shark nursery” where prehistoric predators hatched.

The finding, which challenges long-held notions about ancient marine life, highlights a collection of prized local fossils preserved for more than 310 million years by a rare geologic process and then brought to the surface in recent decades by coal mining.

Like salmon in reverse, long-snouted "Bandringa Rayi" sharks (henceforth; Bandringa) migrated downstream from freshwater swamps to a tropical coastline to spawn 310 million years ago, leaving behind fossil evidence of one of the earliest known shark nurseries.

Photo of a fossil impression left by a juvenile Bandringa Rayi shark. These long-extinct sharks are known for their extremely long spoonbill snouts, which resemble those of modern-day paddlefish. This individual measures about 4 inches from snout to tail and was found in marine sediments at the Mazon Creek deposit in Illinois.

Fossils of Bandringa was discovered in 1969 in Will County as strip mining altered the landscape south of Chicago. Coal companies would discard piles of dirt rich with fossils, said Paul Mayer of the Field Museum, and allow people to pick through the churned earth.

Michael Coates, a University of Chicago evolutionary biologist, said the Bandringa sharks likely spent most of their adult lives in the rivers that run through present-day Ohio and Pennsylvania, citing fossils found in those states in recent decades. But Coates and a colleague suggest that the long-snouted critters laid their eggs and spent the early part of their lives in shallow coastal waters, such as the sea that once covered Illinois' Mazon Creek area and much of the Midwest.

Fossils of Bandringa Rayi were discovered in Will County, Illinois in 1969.

Amateur archaeologists and experts alike combed through the piles, and many of the fossils they brought home ended up at the Field Museum, which has the two Illinois samples studied by Coates, a public display about Mazon Creek, and thousands of the region's specimens in storage.

Mazon Creek's fossils began forming hundreds of millions of years ago when flowering plants and grass were nonexistent and when dinosaurs — not to mention humans — had yet to roam the Earth, Mayer said.

“When these sharks died, they fell into the mud of an estuary or even freshwater ponds in a little delta-like area,” said Mayer, who oversees about 40,000 Mazon Creek specimens as the Field's fossil invertebrate collections manager. “They were buried in the mud, and, for whatever reason, iron came in and cemented the rock around them.”

The process preserved many organisms that would have simply decomposed elsewhere, allowing today's scientists to study ancient jellyfish, worms, and the soft-bodied Tully Monster. It also led to a fuller picture of sharks.

The Tully Monster is found only in Illinois and is the state's official fossil.

“The preservation of Mazon Creek allowed us to reconstruct this animal in detail using the fossil record,” said Lauren Sallan, a University of Michigan evolutionary biologist, who started the research as a graduate student rotating through Coates' Chicago lab.

Coates and Sallan's analyzed two Bandringa samples from Mazon Creek originally identified by scientists as separate species. The juvenile sharks, just 4 to 6 inches long, had pronounced spoon-billed snouts that stretched half as long as their bodies and, Coates said, “looks a little bit like the things you see today in sturgeon, paddlefish.” Their findings suggest that the two sets of fossils are in fact members of the same bottom-feeding species, and a juvenile version of the adult sharks found fossilized in Ohio and Pennsylvania. 

After reevaluating 24 fossils, including latex “peels” of Bandringa’s scale-covered skin, it was concluded that Bandringa was a single species that lived, at various times during its life, in fresh, brackish water, and salt water.

Young Bandringas — but not adults — have turned up in Illinois and adult ones — but not their offspring — were found farther east suggests that the sharks thrived in freshwater but used saltwater havens (like the one south of present-day Chicago) as a “shark nursery” to lay eggs and allow young animals to live safely.

Although no sharks living today are known to travel from freshwater to saltwater to lay eggs, most sharks do use shark nurseries.

An artist’s rendering of a 310 million-year-old Bandringa Rayi shark, originally found in fossil deposits from Mazon Creek in Illinois.

The physical differences between the two purported species were due to different preservation processes at marine and freshwater locations, Coates and Sallan concluded. The freshwater sites tended to preserve bones and cartilage, while the marine sites preserved soft tissue.

By combining the complementary data sets from both types of fossil sites and reclassifying Bandringa as a single species, Coates and Sallan gained a far more complete picture of the extinct shark’s anatomy and discovered several previously unreported features. They include downward-directed jaws ideal for suction-feeding off the bottom (getting their nutrients from algae and other plant material), needle-like spines on the head and cheeks, and a complex array of sensory organs (electroreceptors and mechanoreceptors) on both the extended snout and body, suited for detecting prey in murky water.

It’s also the earliest evidence for segregation, meaning that juveniles and adults were living in different locations, which implies migration into and out of these nursery waters. Adult Bandringa sharks lived exclusively in freshwater swamps and rivers, according to Coates and Sallan. Females apparently traveled downstream to a tropical coastline to lay their eggs in shallow marine waters, a reverse version of the modern-day salmon’s sea-to-stream migration. At the time, the coastline of the super-continent Pangaea ran diagonally between the Mazon Creek freshwater and marine sites.

All the Bandringa fossils from the Mazon Creek marine sites are juveniles, and they were found alongside egg cases -- protective capsules that enclose eggs of the next generation -- belonging to an early species of shark. Adult Bandringa fossils have been found only at freshwater locations, including several in Ohio and Pennsylvania.

An artist’s rendering of a 310 million-year-old, bottom-feeder, Bandringa Rayi shark.

Coates and Sallan said that the juvenile Bandringa sharks hatched from the Mazon Creek egg cases and that the deposit’s marine sites represent a shark nursery where females spawned and then departed, returning upstream to freshwater rivers and swamps.

“This is the first fossil evidence for a shark nursery that’s based on both egg cases and the babies themselves,” Sallan said. “It’s also the earliest evidence for segregation, meaning that juveniles and adults were living in different locations, which implies migration into and out of these nursery waters.”

The findings, both scientists say, were possible only because of the fossils found south of Chicago, which are renowned in scientific circles even if they're unknown to many locals. Coates said he learned about the fossils as a graduate student in the United Kingdom and was eager to study them when he arrived here. “The Mazon Creek fossils are world-famous,” he said, “and it's on Chicago's doorstep.”

Compiled by Neil Gale, Ph.D.