Jaws, From 4 Million Years Ago

Paleontologists have performed a forensic style examination of the fossilized remains of a 4-million year old dolphin in order to find its killer. (Giovanni Bianucci)

Four million years ago, a shark took a mortal bite out of a dolphin abdomen from the rear and right, just as living white shark do today. It followed up with a second, less strong bite to the dorsal area as the dolphin, mortally injured, rolled to the left.

Then the shark released its prey, dead or dying, and other sharks or fishes scavenged the torn body of the dolphin.

All this and more scientists have gleaned from the careful, forensic-style analysis of a four-million-year-old dolphin fossil found off the coast of Italy. As paleontologists reconstructed the killing, they uncovered an abundance of evidence of just how those ancient shark attacked and killed their prey.

The careful forensic analysis of the bite marks was possible despite the extreme age and rarity of the fossil, a 9-foot long dolphin skeleton. The analysis is published in the latest issue of the journal Palaeontology.

The research team, based in Pisa, Italy, have identified the probably killer: a 13-foot shark by the name of Cosmopolitodus hastalis.

According to Giovanni Bianucci, who led the study: "the skeleton lay unstudied in a museum in Torino for more than a century, but when I examined it, as part of a larger study of fossil dolphins, I noticed the bite marks on the ribs, vertebrae and jaws. Identifying the victim of the attack was the easy part -- it's an extinct species of dolphin known as Astadelphis gastaldii -- working out the identity of the killer called for some serious detective work, as the only evidence to go on was the bite marks."

The overall shape of the bite indicated a shark attack, and Bianucci called in fossil shark-expert Walter Landini to investigate further. "The smoothness of the bite marks on the ribs clearly shows that the teeth of whatever did the biting were not serrated," explains Landini, who went on to simulate bite marks of the potential culprits in order to narrow the killer down to Cosmopolitodus hastalis.

Circumstantial evidence also supports this verdict, Landini explained, noting that 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 meters long" said 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," said 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 and the Sternberg Museum of Natural History in the U.S., studies like this are important because they give us a glimpse of the ecological interactions between organisms in prehistoric seas.

"Shark teeth are among the most common vertebrate remains in the fossil record, yet interpreting the details of diet and feeding behavior of extinct sharks is extremely difficult," said Shimada.

"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."