Who You Callin' Shrimp?

25 May 2011 1:30 pm

Esben Horn (left); Peter Van Roy/Yale (right)

Jumbo indeed. The fossil of a giant, Ordovician “killer shrimp” (right), which probably looked like its Cambrian cousin Laggania, restored as a model (left).

Here’s a finding that will gratify monster-crazed children everywhere: Roughly 505 million years ago, giant killer shrimp—ok, shrimplike creatures called anomalocaridids—prowled the ocean gobbling up any soft-bodied prey that wandered across their path. Now, a new fossil discovery shows that these bizarre invertebrates grew far bigger—to more than a meter in length—and persisted millions of years longer than previously thought. The find may force paleontologists to rethink one of the greatest extinctions in the history of life.

The newfound predators were part of a horde of new life forms that evolved about 505 million years ago, when all animal life was confined to the ocean, in a sudden proliferation called the Cambrian explosion. The segmented, stalk-eyed anomalocaridids sat at the top of the food chain, ruling the weird Cambrian reefs. Scientists once thought they were all wiped out 488 million years ago, along with many other bizarre creatures, in a mass extinction that ended the Cambrian period. Then in the next geological period, the Ordovician, new animals evolved to repopulate the seas.

But now, Peter Van Roy and Derek Briggs, paleontologists at Yale University, have found a fossil of a gigantic anomalocaridid dating to between 488 million and 472 million years ago—a slice of time from the Early Ordovician. Dug up in southeastern Morocco, the huge predatory invertebrate was part of an ecosystem that almost seemed to be transplanted from an earlier age. Previous digging at the same site had yielded strange creatures akin to those found in the famous 505-million-year-old rock of Canada’s Burgess Shale, proof that some of the odd Cambrian animals had survived for about 30 million years longer than scientists had thought.

The new, as-yet-unnamed anomalocaridid is the latest member of the strangely persistent community to be unearthed. Equipped with grasping, raptorial appendages, these Ordovician hunters plucked up soft-bodied prey and fed it into their camera-shutter mouths. The fossils weren't in great shape. Some were small, flattened specimens. Others—larger individuals preserved inside giant balls of mudstone—had lost body parts, including the front of the head, to imperfect preservation. But Briggs is sure better ones will turn up. “We are confident that we will find more specimens that will allow a full description of the animals in due course,” he says.

The discovery shows that anomalocaridids likely remained major players in the evolution of marine life for much longer than previously understood. “Clearly, anomalocaridids persisted as important predators at later times,” Briggs says. “They played a role as marine communities became more complex and the number of animal genera in the seas increased nearly four times during the Great Ordovician Biodiversification Event,” a major flowering of new types of marine organisms that occurred beginning 488 million years ago.

It might be time for paleontologists to rethink what happened to life on Earth between the last days of the Cambrian and the early part of the Ordovician, says Allison Daley, a paleontologist at the Natural History Museum in London. “The extinction event at the end of the Cambrian, which was thought to have wiped out all the Burgess Shale-type soft-bodied taxa, appears not to have had a dramatic effect on these marine communities,” she says, noting that the caprices of preservation in the fossil record may have hidden the persistence of the Cambrian lineages. Within this apparent gap in the history of life, the remains of other strange creatures undoubtedly wait to be found.

Posted In: