In its heyday (230 million years ago), the nursery was home to two types of sharks who spawned in freshwater.
Tiny shark teeth found next to egg capsules provide evidence for a 230 million-year-old shark nursery.
Hybodont and xenacanthid female sharks deposited their eggs at the nursery, located in Kyrgyzstan.
Modern egg-laying sharks never spawn in freshwater environments, so researchers are curious why prehistoric sharks did this.
The discovery of dozens of tiny sharp teeth together with egg capsules proves the existence of a 230 million-year-old shark nursery, according to a paper published in the latest Journal of Vertebrate Paleontology.
The discovery represents the first occurrence of egg remains and young shark teeth in one area.
"It allows the direct correlation between those eggs and the young and the interpretation of the area as a shark nursery," lead author Jan Fischer explained to Discovery News.
Fischer, a researcher at the TU Bergakademie Freiberg's Geological Institute, and his colleagues found the shark remains at the Madygen Formation in southwestern Kyrgyzstan. The region is well known to paleontologists for its exquisite preservation of insects and plants from the Late Triassic, a time when the earliest dinosaurs walked the earth.
The excavated eggs and teeth represent two different types of sharks: hybodontids and xenacanthids.
"Hybodontid sharks are the extinct sister group of modern sharks that superficially resemble modern sharks in body shape except for small horns on the heads of males," Fischer said. Hybodontids were the dominant sharks during the Triassic and Jurassic periods.
Xenacanthids, on the other hand, died out around 210 million years ago.
"They are characterized by an eel-like body shape in contrast to the 'normal' shark shape, and a noticeable spine that protrudes backwards from their head," he said.
The two types of sharks co-existed for quite a while, with females of both apparently depositing their eggs in freshwater streams near ample shoreline vegetation. Modern sharks aren't exactly known for their loving parental care, so the researchers believe females returned to nearby lakes or other streams while their young hatched on their own, staying in the nursery area until they grew and then later joined adults.
"We may speculate that they formed schools, especially in the first months, but this cannot be proven by the available fossil data," Fischer said.
In its shark heyday, the nursery area was full of nutrient-rich flora and fauna found within the bottom sediments. On the downside (from the shark's perspective), the large predatory fish Oshia, hungry amphibians, reptiles, and other animals also lived in the area and may have snacked on young shark eggs and infant sharks. There is no evidence that dinosaurs consumed the small sharks, but Fischer said that it is "possible, especially in lake environments."
Modern shark nurseries resemble this prehistoric one, except the one at Madygen was comprised of freshwater.
"There is no modern egg laying shark known to deposit egg capsules in a freshwater environment," he said.
Gilles Cuny of The Natural History Museum of Denmark, pointed out that this difference is significant, proving that sharks have really changed over the years.
"For all the people who think that modern sharks are some kind of living fossils that did not evolve since hundreds of millions of years, there is something to think about!" Cuny exclaimed. He's curious how the egg capsules could survive in a freshwater environment, and if all hybodont sharks laid their eggs in this kind of setting.
John Long, vice president of research and collections at the Natural History Museum of Los Angeles County, was also interested to learn about the freshwater nursery. "Well-preserved shark egg capsules are extremely rare, and they show how sharks first evolved hard, fibrous egg capsules to protect the unborn young," Long said.
Andrew Heckert of Appalachian State University's Department of Geology, told Discovery News that such evidence is important, particularly because "shark skeletons are largely cartilaginous, so the preservation of additional material like the egg capsules is very exciting."
"In my own work in deposits of similar age in North Carolina and the American West," Heckert continued, "we often find these tiny shark teeth, so certainly it will cause us to look even more carefully at the rocks these fossils come from to see if we can find similar egg capsules and other fossils."