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Trilobites teemed on the Earth for approximately 300 million years, and split off into about 20,000 distinct species over that huge timeframe.

They were also evolutionary pioneers. Fossils from a quarry in southernmost China from more than 500 million years ago shows at least two of the species evolved a stomach and digestive tract 20 million years earlier than had previously been believed, according to a new study in the journal PLoS One.

“The anterior part of the glabella in several of the trilobite specimens is enlarged and characterized by dark brown or red staining, which we interpret to be the remains of an expanded stomach, or crop,” write the researchers, from two Chinese institutions as well as the American Museum of Natural History in New York.

The fossils hail from the early Cambrian, about 514 million years ago.

A total of 270 species were found and analyzed.

A total of 118 belonged to the genus Palaeolenus, and 19 contained soft-tissue traces of guts. A group of 152 belonged to Redlichia, and eight showed traces of a gut.

Two of them showed the “crop” along with glands lining a digestive tract, they report. The timing shifts the organs’ development to millions of years earlier, they add.

“This combination of digestive structures has also never been observed in trilobites this old, and is rare in general,” they conclude.

Melanie Hopkins, the AMNH assistant curator who is one of the study’s authors, said in a statement released by the museum that the finds should start a reconsideration of the trilobite record.

“Trilobites are one of the first types of animals to show up in large number in the fossil record,” she said. “Their exoskeletons were heavy in minerals, and so they preserved really well. But like all fossils, it’s very rare to see the preservation of soft tissues like organs or appendages.”

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