"Fool's gold" fossils reveal trilobites breathed through their legs
Trilobites are one of the most common creatures found in the fossil record, but now some particularly well-preserved specimens have revealed some intriguing new details. It turns out that trilobites breathed oxygen – through their legs.
Trilobites were ancient marine arthropods with rounded heads and segmented bodies, and they may have been among the most successful animals that ever lived, thriving for around 270 million years. That, along with their easily-fossilized outer shell, means the critters turn up in the fossil record in such abundance that you can pick them up cheap in museum gift shops.
We know quite a lot about trilobites, but plenty of secrets are still lurking in the soft body parts that don’t fossilize as well. A few rare specimens, however, have managed to capture those squishy bits in exquisite detail, thanks to the medium in which they were preserved – pyrite, commonly known as fool’s gold.
Researchers at University of California, Riverside (UCR) used a CT scanner to map the specimens in three dimensions at high resolution. The pyrite provides a starker contrast between the animal itself and the surrounding rock in the CT image, highlighting much finer detail than can usually be seen.
“It allowed us to see the fossil without having to do a lot of drilling and grinding away at the rock covering the specimen,” says Melanie Hopkins, an author of the study. “This way we could get a view that would even be hard to see under a microscope – really small trilobite anatomical structures on the order of 10 to 30 microns wide.”
With such a clear instrument, the researchers were able to spot never-before-seen structures on the upper sections of the trilobites’ legs. These looked an awful lot like the gills of its modern arthropod descendants like crabs and lobsters. They could even make out the way blood would have passed through the structures to transport oxygen.
“Up until now, scientists have compared the upper branch of the trilobite leg to the non-respiratory upper branch in crustaceans, but our paper shows, for the first time, that the upper branch functioned as a gill,” says Jin-Bo Hou, lead researcher on the study.
The work helps fill out our understanding of the evolution of these common creatures, and what may have triggered the booms and busts of species diversity throughout history. For instance, most types of life we know today got their start about 540 million years ago, during a sudden increase in evolutionary experimentation known as the Cambrian explosion.
“We’ve known theoretically this change must have been related to a rise in oxygen, since these animals require its presence,” says Nigel Hughes, co-author of the study. “But we have had very little ability to measure that. Which makes findings like these all the more exciting.”
The research was published in the journal Science Advances.