Yes, the ancient sea creatures indeed took in oxygen through their legs

Barkha Kumari

Scientists suspected it for a long time – that trilobites breathed through their legs. But now, CT scans of fool’s gold, in which the fossils were embedded, show some of the fine structures involved in respiration.

Trilobites crawled the planet long before the first dinosaur raised its reptilian snout. The first modern scientific discovery of one occurred in 1698, courtesy of Reverend Edward Lhuyd. That naturalist was also responsible for identifying the first fossil identified as belonging to a dinosaur.

Trilobites were arthropods. That’s the same group as spiders, centipedes, shrimp, insects and crabs and lobsters. They are only distantly related to the horseshoe crabs they resemble, and which have been around for 300 million years. But trilobites have been around longer – harking as far back as the Cambrian Period, about 520 million years ago. While some researchers assumed it may be far older, at least one recent study has put a damper on those ideas.

Famous trilobites from the Middle Devonian-aged Silica Formation of northeastern Ohio. Courtesy James St. John / Wikimedia. CC 2.0

Think of trilobites as long-lost, ancient relatives of spiders and horseshoe crabs, as this report puts it. They ranged between 1 cm (0.4 inches) and 72 cm (29 inches). Trilobites owe their moniker not to the three regions of their body – the head, thorax and tail – but because they were divided into three along their lengths.

Speculation existed whether the legs, which had two branches, could help the animals breathe, thus supplying oxygen to the hemolymph, which acts like blood in invertebrates. Many took that as a given, despite the dearth of clinching evidence. Skeptics said that was unlikely, since trilobites walked the sea floor. They argued that the eddying sediments could clog the thin, feather-like gills if they were on the arthropod’s legs.

Yunnanocephalus yunnanensis, one kind of trilobite
Yunnanocephalus yunnanensis, one kind of trilobite. Courtesy Jin-Bo Hou

“The researchers felt that the primary gills must still lie on the ventral surface [the underbelly],” Jin-Bo Hou, a doctoral student at the University of California, Riverside, explained to Truly Curious in a call.

Hou was lead author in a paper in Science Advances on March 31, 2021, in which he set the matter to rest along with his mentor Nigel Hughes, a geology professor; and Melanie Hopkins, the curator of invertebrate paleontology at the American Museum of Natural History.

For his work, Hou relied on CT scans, which so far had been used only to study microscopic trilobite eggs. He scanned exquisitely preserved trilobites in pyrite (iron disulfide, better known as fool’s gold for its resemblance to a vastly more expensive metal). Such scans can expose the fine structure within the pyrite.

CT scan of a trilobite
CT scan of a trilobite. Courtesy Jin-Bo Hou

The fossils form in pyrite when bacteria involved in decomposing dead animals had enough carbon and sulfur to work with, but not oxygen. In the oxygen-deprived sediments that the dead trilobites settled, the bacteria relied on Plan B. They shifted to anaerobic – or airless – respiration. Under these conditions, the bacteria converted iron sulfide in the sediments into iron disulfide, which replaces living tissue, thus preserving the structure of the creature.

The team confirmed the expert view that the gills were on the upper legs, which were always known to be split into two along their lengths. Trilobites had 28 gills located along the sides of the body, in pairs on each thoracic segment.

The team found the primary gills wedged between the exoskeleton and the lower gills, so they never touched the ocean floor and got choked.

A trilobite fossil that is about 450 million years ago and found in New York state
A trilobite fossil that is about 450 million years ago and found in New York state. Courtesy Jin-Bo Hou

“There must be some mechanism to keep the gills clean but that’s another topic of discovery,” Hou explained.

The biggest challenge, however, was finding well-preserved fossils.

Hou goes over the work of five years.

“I first browsed through publications, many publications, to find the specimen I wanted to study,” he says. “Then came the task of finding the fossil that was the right size — neither too big nor too small. To our luck, we found a few well-preserved fossils at the Yale Peabody Museum of Natural History, Yale University, and the Smithsonian National Museum of Natural History in the U.S. and the Hunterian Museum at the University of Glasgow in the U.K. It took us over two years to get to that.”

One trilobite fossil used in the study
One trilobite fossil used in the study. Courtesy Jin-Bo Hou

So far, so good. But why does his work matter?

Hou smiles and says, “The information about early animals helps us to understand how the structures we see in modern animals today were gradually formed. That’s evolution. In the age of those trilobites, the atmospheric oxygen concentration was about 10% of the present atmospheric level. Yet, they were able to grow from small to big. How? Because they took advantage of breathing oxygen from water, which is required to release energy and grow their bodies. Now we know they breathed through the gills on their legs.”

Hou admits that he was an inquisitive kid while growing up in China.

“I would see grasshoppers flying in the fields and wonder, ‘They are so tiny. How can they jump like human beings?'” While we are not sure if Hou knows that, he certainly knows how the trilobites used to breathe.

Barkha Kumari is a freelance journalist based in Bengaluru, India. You can follow her on Barkha2803

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