Sunday, 11 September 2011

Tiny Teeth Indicate The Site Of An Ancient Shark Nursery

The classic image of a shark is of a large, mindless predator that cruises the seas, preying on innocent surfaces, an image made immortal by films such as Jaws. They are not often thought of as small, peaceful river dwellers which prey only upon small fish. Yet Palaeoxyris alterna fits this description perfectly. This 230 million year old species of shark was a peaceful river dweller that lived in the shallow, wide, freshwater rivers and lakes of Triassic Pangaea.

A recent discovery in south-west Kyrgyzstan is shedding light on the lifestyle of this unusual shark. Reporting in the most recent issue of the Journal of Vertebrate Palaeontology, a team of German scientists have discovered a fossil site which they believe was the shark nursery for Palaeoxyris alterna. A shark nursery is a large, shallow and safe area of water where young sharks are raised in large groups by adult members of the species.

The fossils and a computer reconstruction of the egg cases of Palaeoxyris alterna
They can reveal many things about the lifestyle of a species. This prehistoric nursery dates to 230 million years old. The site, known as the Madygen Formation, was known previously for well preserved late Triassic insects and plants. The site is now as far from the sea as is possible for a once tidal pool. However the team found the tips of dozens of tiny teeth alongside egg cases. Almost all of the tiny teeth represent juveniles which suggests that the creatures spawned in shallow waters where the young remained until they were fully mature.

Fossil sharks are generally rather rare. This is partly due to their cartilaginous skeleton, which usually is not preserved as a fossil. Consequently, painstaking searches of large quantities of rock often yield very little reward. As Michael Buchwitz, an author of the report, noted, '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 kilogram sample. Therefore, dozens of egg capsules alongside juvenile teeth in one deposit is a dream come true!'