Was This Earth's First Predator?

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Another source of evidence about ancient hunting is what sated hunters have left behind. Every organism excretes material it consumes but can't digest. Fossilized pellets of fecal matter, called copralites, store critical information about the diets of the animals that produced them.

Some copralites from between 520 and 545 million years ago contain biomineralized body parts of animals that clearly reflect the aftermath of successful hunting. Unfortunately, however, these "fossil turds" generally don't preserve enough anatomical detail to reveal what animal got eaten, and also shed little light on what animal did the eating, Hagadorn said.

Caught in the Act

On rare occasions, paleontologists discover the fossilized remnants of one organism inside the fossilized carcass of another—evidence of a hunter that died with its stomach full. Such findings provide a third—and especially highly valued—kind of information about ancient hunting, Hagadorn said, because they reveal anatomical details of both the predator and the prey.

In fossils 510 to 520 million years old, scientists have found evidence of worms that preyed on clam-like brachiopods and on hyolithids, which were small animals that lived in cone-shaped shells.

Last in paleontologists' book of clues, Hagadorn said, are trace fossils. These show footprints, burrows, tracks, or other marks made during the course of an organism's lifetime. Sometimes, these etchings in stone record ancient moments of high drama—like the climax of a successful hunt.

Worms weren't always the hunters in these ancient games of cat and mouse. Trilobites—ancient and extinct relatives of modern horseshoe crabs—produced distinctive scratch marks as they moved about their watery world. Peculiar conditions on the seafloor occasionally preserved these traces, leaving them to be discovered by paleontologists.

In one well-preserved example, said Hagadorn, the path of a trilobite intersects the burrow of a worm, with the latter trace ending where the two paths converge.

The evidence might be too circumstantial to incriminate the voracious trilobite in the paleontological equivalent of a courtroom. But the traces nevertheless provide valuable evidence for scientific sleuths on the trail of the first hunter.

This story is featured in The Shape of Life, an eight-part television series produced by Sea Studios Foundation for National Geographic Television and Film in association with PBS.

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