Pre-dinosaur fossils reveal pattern of animal evolution on Earth

Fossil remains of trilobites that went extinct some 250 million years ago, reveal the pattern of early animal evolution on Earth, according to a new study published in todays issue of the journal science.

Washington, July 27 : Fossil remains of trilobites that went extinct some 250 million years ago, reveal the pattern of early animal evolution on Earth, according to a new study published in today's issue of the journal science.

Trilobites were among the creatures that emerged 500 million years ago, during what paleontologists call "the Cambrian explosion," or "the Cambrian radiation." Before this time, life on Earth was limited mostly to bacteria, algae, single-celled organisms and only the simplest animal groups.

But during the Cambrian Period, more complex creatures with skeletons, eyes and limbs emerged with amazing suddenness. Trilobites ranged in size from nearly microscopic to more than a foot long, though most of the 17,000 known species measured from one to four inches.

"They're segmented little creatures, very beautiful to look at. They catch the eye of a lot of amateur collectors, and professionals like myself tend to get hooked on them very easily," said Mark Webster, assistant professor in geophysical sciences at the University of Chicago.

As part of his study, Prof. Webster combed through 68 previously published studies of trilobites, searching for descriptions of evolving characteristics that could be incorporated into his analysis.

After eliminating studies that were inappropriate for inclusion, 49 still remained.

He focused on actively evolving characteristics and found that approximately 35 percent of the 982 trilobite species exhibited some variation in some aspect of their appearance that was evolving.

More surprising was that while more than 70 percent of early and middle Cambrian species exhibited variation, only 13 percent of later trilobite species did so.

"From an evolutionary perspective, the more variable a species is, the more raw material natural selection has to operate on. There's hardly any variation in the post-Cambrian. Even the presence or absence or the kind of ornamentation on the head shield varies within these Cambrian trilobites and doesn't vary in the post-Cambrian trilobites," said Prof. Webster.

"Various studies have approached questions pertaining to it-but this is the first to convincingly document it in any group," said Nigel Hughes, Professor of Earth Sciences at the University of California, Riverside.

"The paper is relevant to the big question of what fuelled the Cambrian radiation, and why that event was so singular. It appears that organisms displayed "rampant" within-species variation "in the 'warm afterglow' of the Cambrian explosion, but not later. No one has shown this convincingly before, and that's why this is so important," Prof. Hughes added.

ANI