Giant Trilobites in Portugal Could Be Biggest

May 7, 2009 -- Giant trilobites have been discovered in a rock quarry in northern Portugal.

The finding, published in the journal Geology, adds a new chapter to the story of some of the most successful creatures that ever lived, and may even challenge current specimens for the largest the planet has ever seen.

The current record holder for the largest trilobite still officially stands at the 72 centimeter (28.3 inch)-long Isotelus rex, a fossil recovered in Manitoba, Canada in the year 2000.

Last year, Artur Sa of the University of Tras-os-Montes and Alto Douro and a team of colleagues unearthed a slew of similar giants between 50 and 70 centimeters (19.7 and 27.6 inches) long in a slate quarry in Arouca, Portugal.

They also found two fossilized tail sections which they believe belonged to 90 centimeter (35.4 inch)-long animals, the biggest in the world.

"Normally trilobites in the Iberian Peninsula and throughout Spain don't get bigger than 10 centimeters (3.9 inches) long," Sa said. "In the quarry, they are normally above 30 centimeters in length."

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In their heyday, more than ten thousand known species of trilobites blanketed the seas. They drifted in the water column, and crawled on the ocean floor like horseshoe crabs, their modern day descendants. Throughout their 300-million-year stay on the ancient earth, adults generally grew to between 2 and 5 centimeters (0.8 and 2.0 inches) long.

But some ballooned to almost a meter long, and scientists are at a loss to explain why.

"We did a lot of speculation when we found the Isoletus rex specimen," David Rudkin of the Royal Ontario Museum said. "The part of Canada where they were discovered was in the tropics during the Ordovician period. We thought it could have something to do with large nutrient supply, or high productivity in tropical waters. But these new ones are very different."

The largest Portuguese fossils, Ogyginus forteyi and Hungioides bohemicus lived 465 million years ago, in a shallow ocean near the South Pole. Sa speculates that the cool waters might have been oxygen-deprived, and the creatures immense size allowed them to slow their metabolisms.

"It's like King Crabs today," he said. "Polar gigantism is common among modern marine arthropods."

The rocks of Arouca also reveal trilobites as social beasts; clusters of hundreds to thousands of animals are common. Sa believes they congregated as protection during molting when their shells were soft an vulnerable to predators. They also appear to have gathered as an efficient way to mate with one another.

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