A new study has found the first evidence of sophisticated respiratory organs in sea creatures 450 million years old. Contrary to what was previously thought, trilobites breathed their legs, with gill-like structures hanging from their thighs.
Trilobites were a group of marine animals with half-moon shaped heads that resembled horseshoe crabs, and they were extremely successful in terms of evolution. Although they are now extinct, they have survived for over 250 million years – longer than dinosaurs.
Thanks to new technology and an extremely rare set of fossils, scientists at UC Riverside can now show that trilobites breathed oxygen and explain how they did it. Published in the newspaper Scientific progress, these findings help piece together the puzzle of early animal evolution.
“So far, scientists have compared the upper branch of the trilobite’s leg to the non-respiratory upper branch in crustaceans, but our article shows, for the first time, that the upper branch functioned as a gill,” said Jin-Bo Hou, a UCR paleontology doctoral student who led the research.
Among the oldest animals in the world, this work helps locate trilobites on the evolutionary tree more reliably between older arthropods, a large group of animals with exoskeletons and crustaceans.
The research was made possible, in part, by exceptionally preserved fossil specimens. There are over 22,000 species of trilobites that have been discovered, but the soft parts of the animals are only visible in about two dozen.
“These were kept in pyrite – the fool’s gold – but it’s more important to us than gold because it’s the key to understanding these ancient structures,” said Nigel Hughes. , professor of geology at UCR and co-author of the article.
A CT scanner was able to read the density differences between the pyrite and the surrounding rock and helped create three-dimensional models of these rarely seen gill structures.
“It allowed us to see the fossil without having to do a lot of drilling and crushing the rock covering the specimen,” said paleontologist Melanie Hopkins, a member of the American Museum of Natural History research team.
“That way we could have a view that would be even difficult to see under a microscope – very small anatomical trilobite structures on the order of 10 to 30 microns wide,” she said. For comparison, a human hair is about 100 microns thick.
Although these specimens were first described in the late 1800s and others have used CT scans to examine them, this is the first study to use the technology to examine this part of the animal. .
Researchers were able to see how blood would have filtered through the chambers of these delicate structures, picking up oxygen along its path as it moved. They closely resemble the gills of modern marine arthropods like crabs and lobsters.
Comparing the pyrite specimens to another species of trilobite gave the team additional details on how the filaments were arranged relative to each other and relative to the legs.
Most trilobites swept the ocean floor, using spikes on the bottom of their legs to catch and crush prey. Above these parts, on the upper limb branch, were these extra structures that some believed were meant to help with swimming or digging.
“In the past, there was debate over the purpose of these structures because the upper leg is not a great place for breathing apparatus,” Hopkins said. “You would think it would be easy for these filaments to get plugged with sediment where they are. It’s an open question why they evolved the structure there on their body.”
The Hughes Lab uses fossils to answer questions about how life developed in response to changes in Earth’s atmosphere. About 540 million years ago, there was an explosive diversification in the variety and complexity of animals living in the oceans.
“We knew theoretically that this change had to be related to an increase in oxygen, since these animals need its presence. But we had very little ability to measure this,” said Hughes. “Which makes such discoveries all the more interesting.”