January 7, 2014
Study Shows Ancient Shark Spawned Like Salmon In Reverse
[ Watch the Video: Evidence Of Ancient Shark Migration In Reverse ]
Brett Smith for redOrbit.com - Your Universe Online
The study of the long-snouted Bandringa provides the earliest known evidence of shark migration – which took place 310 million years ago in a region now known as the American Upper Midwest.
"This pushes migratory behavior in sharks way back," said study author Lauren Sallan, an ecology and evolutionary biology professor at the University of Michigan. "These sharks bred in the open ocean and spent the rest of their lives in fresh water. No shark alive today is known to do that."
Discovered in 1969, the long-extinct Bandringa is thought to be one of the earliest close relatives of today’s sharks. The fish had a spoon-billed snout up to half its body length, much like modern sawfish and paddlefish. Young Bandringa were 4 to 6 inches long and grew up to 10 feet long.
For their research, the study team reanalyzed all known specimens of Bandringa. Scientist had believed that the genus Bandringa included two species, one freshwater and one saltwater. However, the study team found after examining 24 individuals that Bandringa was a single species that lived in both freshwater and marine ecosystems. The confusion seemed to arise from different preservation processes that occurred in either environment, with freshwater sites tending to preserve bones and marine sites preserving soft tissue.
By considering the genus a single species, the researchers were able to reveal a more complete picture of the shark's anatomy. They found that the animal’s downward-directed jaws were probably ideal for bottom feeding. Additionally, Bandringa’s needle-like spines on the head and cheeks as well as a complex array of sensory organs were well-suited for finding prey in murky waters.
The researchers also found that females swam downstream to a tropical coastline where they would place their eggs in shallow marine waters. While all of the fossils found at the marine sites, such as the well-known Mazon Creek deposits in northern Illinois, have been juveniles – freshwater locations, including several in Ohio and Pennsylvania, have produced adult fossils.
The research team concluded that the sharks hatched from the Mazon Creek egg cases, and that the site represents a shark nursery where females spawned and then left, traveling 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 Mazon Creek deposit is famous for its diverse, well-preserved fossils – with over 320 animal species found there, according to the Illinois State Museum. While many of those animals lived in shallow marine bays, other species lived in swampy areas near rivers that emptied into the bay. When the decaying organic matter from these plants and animals sank to the bottom of the bay, they were quickly covered and preserved by mud washing in from the river.