Was This Earth's First Predator?
By Ben Harder
for National Geographic News
|April 9, 2002|
Deep beneath an ancient ocean, a flatworm stalked its unsuspecting prey. Slithering along the seabed, the predator wrapped itself around some sea creature and perhaps delivered an immobilizing dose of venomous toxins before gradually digesting its meal.
The only incriminating clues the worm left behind, impressed in the muddy stage, were the marks of its movements and those of its prey in the final throes of death.
Among the earliest hunters, flatworms were simple but effective killing machines. Their efficiency has paid off. Today, flatworms occupy virtually every habitat on Earth and number about 25,000 species in all. And the hunting behavior they employed so long ago has become one of the predominant occupations of the animal kingdom.
But what animal was the very first hunter? And what did it hunt? If anyone knows the answers to these questions, it's likely to be marine paleontologist James Whitey Hagadorn of the California Institute of Technology.
According to Hagadorn, assembling a line-up of prime suspects isn't too hard: It was some sort of simple marine organism.
That conclusion is easy to deduce because the first animals evolved in the sea, fashioning many of the anatomical and behavioral adaptations that modern predators as diverse as ospreys, lions, and killer whales still use today.
But Hagadorn doesn't hold much hope of tracking down a single culprit beyond a reasonable doubt. Science may never know for certain what the first hunter was, he said. But by reading fossilized clues that early hunters left behind, "we [can] go play detective in deep time."
Paleontological detectives are piecing together several kinds of evidence in their attempts to home in on the identity of the first hunters.
Some clues are left by the damage that hunters wrought on prey that had shells, exoskeletons, bones, or teeth, which may survive long after the soft-bodied organisms that produced them have perished.
Scientists have found shells as old as about 550 million years with marks suggesting that predators had attempted to drill through them. These finds would represent the oldest direct evidence of predation, but some paleontologists propose that the damage could have resulted from some other process, Hagadorn said.
Such drilling by predators became widespread only hundreds of millions of years later, according to evidence in the geological record.
Whatever the age of the first drilling predator, the earliest predator is probably even older. Because biomineralized body parts such as shells and exoskeletons often exist to protect an organism from predation, hunters must have existed before organisms developed these defensive structures to foil attackers.
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 anotherevidence of a hunter that died with its stomach full. Such findings provide a thirdand especially highly valuedkind 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 dramalike the climax of a successful hunt.
Worms weren't always the hunters in these ancient games of cat and mouse. Trilobitesancient and extinct relatives of modern horseshoe crabsproduced 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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