Palaeontologists have discovered evidence of how an extinct shark attacked its prey, reconstructing a killing that took place 4 million years ago.

Such fossil evidence of behavior is incredibly rare, but by careful, forensic-style analysis of bite marks on an otherwise well-preserved dolphin skeleton, the research team say they have reconstructed the events that led to the death of the dolphin, and likely determined the identity of the killer: a 4 m shark called Cosmopolitodus hastalis.

The evidence, published in Palaeontology, comes from the fossilized skeleton of a 2.8 m long dolphin (Astadelphis gastaldii) discovered in the Piedmont region of northern Italy.

Bite marks on the ribs, vertebrae and jaws tipped off researchers to the possibility that Cosmopolitodus hastalis was responsible for the attack. But confirming the identity of the predator required more evidence than that provided by the initial bite marks.

"The smoothness of the bite marks on the ribs clearly shows that the teeth of whatever did the biting were not serrated, and that immediately ruled out some possibilities. We simulated bite marks of the potential culprits and, by comparing them with the shape and size of the marks on the fossils, we narrowed it down to Cosmopolitodus hastalis," says lead author Giovanni Bianucci.

Circumstantial evidence also supports this verdict: fossil teeth from Cosmopolitodus are common in the rock sequences that the dolphin was found in. "From the size of the bite, we reckon that this particular shark was about 4 m long" says fossil shark expert Walter Landini.

Detailed analysis of the bite pattern allowed the researchers to go even further. "The deepest and clearest incisions are on the ribs of the dolphin" says Bianucci, "indicating the shark attached from below, biting into the abdomen.

"Caught in the powerful bite, the dolphin would have struggled, and the shark probably detached a big amount of flesh by shaking its body from side to side. The bite would have caused severe damage and intense blood loss, because of the dense network of nerves, blood vessels and vital organs in this area. Then, already dead or in a state of shock, the dolphin rolled onto its back, and the shark bit again, close to the fleshy dorsal fin."

The study is significant because of the rarity of such 'fossilized behavior'. According to Dr Kenshu Shimada, fossil shark expert at DePaul University, "studies like this are important because they give us a glimpse of the ecological interactions between organisms in prehistoric seas...Fossil remains of prey species with shark bite marks, like those described by Bianucci and his team, provide direct evidence of what each prehistoric shark ate and how it behaved."