London: In popular imagination, sharks are fearsome predators looking for their next unfortunate victim. But researchers say many shark species were relatively small, harmless fishes living in lakes and rivers more than 200 million years ago.
Scientists have suggested that these ancient sharks bred in the shallows of freshwater lakes, forming nurseries for their hatchlings, the Journal of Vertebrate Paleontology reports.
A team of German paleontologists from Geologisches Institut, TU Bergakademie Freiberg, supports this claim with spectacular 230 million-year-old fossil egg capsules and tiny teeth from Kyrgyzstan.
The Madygen Formation in southwestern Kyrgyzstan is well known to paleontologists for its preservation of insects and plants from the Late Triassic period (some 200 million years ago), according to a Geologisches Institut statement.
"Today, this amazing fossil site is one of the farthest points on land from any sea -- quite similar to the situation during the Late Triassic," said Sebastian Voigt, study co-author.
It was, therefore, something of a surprise when fossil shark eggs and babies were recently discovered in this area.
Study leader Jan Fischer remarked: "The chemistry of the tooth enamel indicates that the Madygen nursery was unequivocally created in freshwater, which is in sharp contrast to all modern egg laying sharks, which spawn exclusively in the sea."
The team found the tips of dozens of tiny teeth together with egg capsules representing two different species of shark.
Almost all of the tiny teeth represent small juveniles. Only a very small number of adult teeth have been discovered. This suggests that just like modern sharks, these freshwater cousins spawned in shallow waters.
The young sharks lived in these more protected areas before moving away from the lake shoreline as they matured.