By Will Dunham
WASHINGTON, March 26 (Reuters) - The so-called Cambrian Explosion - a critical juncture in the history of life on Earth when a dizzying array of animals first burst onto the scene half a billion years ago - may have been even more explosive than previously known.
Scientists on Wednesday identified a weird sea creature dating from 520 million year ago that, like today's baleen whales and whale sharks, evolved from an apex predator to become a filter feeder - sifting the tiniest animals from the water.
The creature, called Tamisiocaris borealis, was a primitive relative of the arthropods - the group that includes crustaceans, insects and spiders. Nothing like it exists today.
Its fossilized remains were unearthed in 2009 at the northern-most tip of Greenland, the researchers said.
They recognized that it belonged to a group typified by the famous Cambrian Period creature Anomalocaris, known from a rich Canadian fossil site called the Burgess Shale, that were the first large predators ever to appear on Earth. They also were among the most bizarre-looking creatures on record.
While the size of Tamisiocaris - roughly 28 inches (70 cm) long - may not sound impressive, it was among the biggest animals alive at the time, said paleontologist Jakob Vinther of the University of Bristol in Britain, who led the study.
"It was a gentle giant," said Vinther, whose findings were published in the journal Nature. "Even though this thing was not a whale or a whale shark, it evolved to become the equivalent."
Like its predatory cousin Anomalocaris, Tamisiocaris possessed a pair of spiny, grasping appendages up front to catch food, a pair of insect-like compound eyes on stalks and a circular mouth. Its body boasted a series of flaps down the sides that could be used for swimming, but it had no legs.
Spines on the grasping appendages of Anomalocaris were developed to spear or grab sizable prey. But in Tamisiocaris, these two appendages had a comb of long, slender, finely spaced spines that could be swept through the water to trap tiny creatures in the water like zooplankton, the researchers said.
"It would sweep its net-like appendages through the water, and then suck up whatever it caught," said University of Bath paleontologist Nicholas Longrich, another of the researchers.
It lived alongside primitive jawless fish that were the earliest vertebrates, horseshoe crab-like trilobites, primitive shellfish, relatives of starfish, jellyfish, sponges and others.
'NEW MODES OF LIFE'
The Cambrian Period, from about 542 to 488 million years ago, was a pivotal point in the history of life when many major animal groups first appeared. The relatively short span of time in which this unfolded inspired the term Cambrian Explosion.
"It was a very active period of evolution. We were seeing the appearance of complex animals for the first time - things with eyes, brains, jaws, legs and fins," Longrich said.
"Animals were trying out new modes of life - burrowing, swimming and crawling. Previously, before the Cambrian Explosion, there were probably animals but they would have been minute, simple little worm-like things," Longrich added.
The presence of a large, free-swimming filter feeder like Tamisiocaris indicates the Cambrian oceans were rich with life and, in particular, loaded with plankton, the researchers said.
Several times in Earth's history, large marine predators have given up active predation - attacking sizable prey - in favor of the more passive approach of filter feeding.
Baleen whales like today's enormous blue whale - which have plates of material called baleen on the upper jaw that act like a sieve to filter plankton, small fish and crustaceans - evolved from toothed whales that were active predators. Filter-feeding sharks like today's whale shark, basking shark and megamouth shark arose from sharks that were fearsome hunters.
The newly identified creature's genus name, Tamisiocaris, means "sifter shrimp," and its species name, borealis, means "northern," for the northerly locale where it was found.
(Reporting by Will Dunham)