3 min readLondonSep 12, 2011 04:18 PM IST
First published on: Sep 12, 2011 at 04:18 PM IST
Sharks usually conjure images of fearsome sea predators. But,researchers have now claimed that many shark species were relatively small and harmless fish living in lakes and rivers more than 200 million years ago.
A team at Geologisches Institute,TU Bergakademie Freiberg in Germany says that these ancient sharks bred in the shallows of freshwater lakes,forming nurseries for their hatchlings,the ‘Journal of Vertebrate Paleontology’ reported.
They have based their findings on 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 exquisite preservation of insects and plants from the Late Triassic — a time when the earliest dinosaurs walked the Earth.
Today,this amazing fossil site is one of the farthest points on land from any sea — quite similar to the situation during Late Triassic, said Sebastian Voigt,a team member.
It was therefore something of a surprise when fossil shark eggs and babies were recently discovered in this area.
Team leader Jan Fischer said “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. One species is based on both teeth and egg capsules and is considered to be member of a shark family called hybodontids. The second species is based solely on egg capsules and probably is a type of shark known as a xenacanthid. The hybodontids became extinct at about the same time as the dinosaurs 65 million years ago,while the xenacanthids failed to survive beyond the Triassic,200 million years ago.
Michael Buchwitz,a team member,said: “The fossil record of sharks is no laughing matter; a spine here,a tooth there,or three miniscule denticles small spines of the skin picked from a 10 kilogramme sample. Therefore,dozens of egg capsules alongside juvenile teeth in one deposit is a dream come true!”
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.
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