When Midwesterners say they are going to the beach, they are generally referring to the sandy shores of one of the Great Lakes. However, in northeastern Illinois, it is possible to spot a shark, well a shark fossil anyway.
In the late Paleozoic ice age, the ancient coastline ran right through the Midwest, and prehistoric sharks swam into the swamps of Illinois to lay their eggs, according to new research conducted by Lauren Sallan, a University of Michigan professor of ecology and evolutionary biology.
When coal miners began extracting coal from the area known as Mazon Creek, they dug up nodules of siderite, a reddish-brown mineral often found embedded within layers of shale. Hidden within those rocklike nodules were some of the most well-preserved fossils in the world. When left out over the winter and allowed to thaw, those nodules crack open, revealing the remains of mollusks, crustaceans, and yes, prehistoric sharks.
While the idea that sharks swam in these ancient waters is nothing new, Dr. Sallan is the first to discover that the sharks used the area as a nursery before moving on to live out the remainder of their life in fresh water. That behavior is quite different from what we know about modern sharks, which typically live out their lives in marine environments and find slightly less salty estuaries along the coastline to lay their eggs.
“Most sharks lay their eggs at the mouths of rivers along the shore, but they are mostly marine sharks,” Sallan says. “There’s only one shark that will go into rivers, but they only go there to hunt. Today there are freshwater rays that spend their whole lives in fresh water, but they don’t use that nursery behavior either.”
When surveying the fossil record found along the ancient coastline, paleontologists originally identified two kinds of fossils. In Illinois's Mazon Creek, they found skeletal remains of miniature four- to six-inch sharks, which they named Bandringa herdinae. In another coal pit in Ohio, they identified preserved soft tissue of sharks 150 times larger than those found in Mazon Creek and dubbed them Bandringa rayi. Paleontologists cataloged the fossils as two separate species and moved on.
Sallan is the first scientist to examine these fossils in detail. Her research, which was published in the Journal of Vertebrate Paleontology on Tuesday, indicates that these were not two separate species, but the remains of the same species from different stages of life. She refers to the single species simply as Bandringa.
By comparing the fossils found in several locations throughout Illinois and Ohio, Sallan has been able to piece together a relatively complete snapshot of Bandringa.
While most people think of fossils as skeletal remains, “in the marine water soft tissues were preserved really, really well, which is very rare in the fossil record, especially for sharks,” she says. “For those in the swamp, we have only skeletal remains because of the differences of the water chemistry. When we put the two together, we get a full picture.”
That full picture is quite extraordinary. With a protruding rostrum covered in scales and electro-receptors, long needle-like spines that extended out from the top of the head, and a back covered in pointed scales, Bandringa struck a very different profile from what we think of sharks today.
Sallan also has been able to determine much about Bandringa’s behavior and diet thanks to the exceptionally complete fossil record.
Bandringa appears to be one of the earliest sharks capable of suction feeding. Most early sharks caught their prey by snapping. But Bandringa more likely sucked its prey up from the murky river bottom and swallowed it whole. Sallan has found complete crustaceans preserved within the sharks’ stomach cavities, providing additional clues about their diet.
For the sharks to find those crustaceans buried in the murky bottom took some specialized equipment. Bandringa probably relied on electro-receptors to detect prey buried in the sediment – similar to those used by paddlefishes that live in the Mississippi River today. Tiny hairs similar to those found in the human inner ear probably added another dimension to Bandringa’s “view” of the bottoms.
Sallan hopes that this new image of Bandringa is just the beginning in terms of understanding prehistoric shark communities.
“What’s next is looking at the other sharks that are found in other sites, especially in Ohio and Pennsylvania, and figure out what else is there and what else might be migrating from the fresh water,” she says.