Earth's Early Cannibals Caught in the Act

* One of the most cuddly early animals might have been a pack-hunting cannibal.
* Signs of violence on agnostid trilobites found in Cambrian rocks suggest they were attacking each other.
* Other possible interpretations of the fossils include these were sexual behaviors.

Some of the cutest little critters of the early Earth may have been roving packs of cannibals, according to new evidence on trilobites.

Teeth marks, attack positions, shredded relatives and other signs of cannibalism have been found in the fossilized remains of tiny agnostid trilobites, affectionately called "bugs" by paleontologists and fossil hunters.

"I was always very puzzled by these tiny trilobites," said paleontologist Mark McMenamin of Mount Holyoke College in Massachusetts. "It was never very clear what their role in the Cambrian ecosystem was."

Then last summer a donor gave McMenamin's school 44 slabs of what's called the Wheeler shale -- rocks full of Cambrian era agnostid trilobites.

"I had them spread out all over my laboratory," said McMenamin. "The pattern really starts to emerge from these rocks."

That pattern includes larger trilobites atop smaller ones, bite marks and even a case where it looks like one trilobite has torn another in half.

"This is not scavenging but predation," McMenamin told Discovery News. And not just predation, but cannibalism.

"It's possible that they may have attacked other organisms and may have done it in roving packs," he added.

The discovery is bound to stir up debate when it is presented on Nov. 1 at the annual meeting of the Geological Society of America in Denver.

"It'll be a ping-pong match," said paleontologist Jessica Whiteside of Brown University.

Among the other possible interpretations of the slabs is that the trilobites are mating. There are precedents of scarring and violent behaviors while mating in crabs and even in cats, she said.

If, however, there is agreement that the trilobites are predators and cannibals, it has important implications for the evolution of animals with hard parts and other defenses, since it shows how predators got more sophisticated in the early days of animal life, Whiteside said.

The evolution of hard exoskeletons, as in these trilobites, is generally interpreted as a defensive adaptation against predators. That then caused predators to evolve pincers or other means of cracking the exoskeletons. Next the prey started burrowing and hiding in the ground, and so on and so forth, Whiteside explained.

"It's an arms race, if you will," said Whiteside. And the result is pressure to evolve all kinds of new lifestyles and species, which is exactly what happened.

For now, however, anyone with a fossil of one of these very common kinds of agnostid trilobite should give them another look and perhaps a little more respect.

"They seem like such innocuous little guys," said McMenamin. "There may be a dark side of these little creatures."