Fossils reveal evolution pattern-Health/Science-The Times of India

WASHINGTON: 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 the latest 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.

As part of his study, Mark Webster, assistant professor in geophysical sciences at the University of Chicago, 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 focussed on actively evolving characteristics and found that approximately 35% of the 982 trilobite species exhibited some variation in some aspect of their appearance that was evolving.

"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 Webster.