Trilobites had well-developed gill-like structures in their upper leg branches, according to a new imaging study led by the University of California, Riverside.
Trilobites are extinct marine arthropods that dominated the ecosystems of the Paleozoic era.
They appeared in ancient oceans in the Early Cambrian period, about 540 million years ago, well before life emerged on land, and disappeared in the mass extinction at the end of the Permian period, about 252 million years ago.
They were extremely diverse, with about 20,000 species, and their fossil exoskeletons can be found all around the world.
“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,” said lead author Dr. Jin-Bo Hou, a doctoral student in the Department of Earth and Planetary Sciences at the University of California, Riverside.
Triarthrus eatoni lived approximately 450 million years ago (Ordovician period); Olenoides serratus lived during the Cambrian period, about 500 million years ago.
“These were preserved in pyrite — fool’s gold — but it’s more important than gold to us, because it’s key to understanding these ancient structures,” said co-author Professor Nigel Hughes, a paleontologist in the Department of Earth and Planetary Sciences at the University of California, Riverside, and the Geological Studies Unit at the Indian Statistical Institute.
Using a CT scanner, the researchers created 3D models of dumbbell-shaped filaments in the upper limb branches of both Olenoides serratus and Triarthrus eatoni.
“It allowed us to see the fossil without having to do a lot of drilling and grinding away at the rock covering the specimen,” said Dr. Melanie Hopkins, a paleontologist in the Division of Paleontology at the American Museum of Natural History.
“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.”
The researchers could see how blood would have filtered through chambers in these delicate structures, picking up oxygen along its way as it moved.
They appear much the same as gills in modern marine arthropods like crabs and lobsters.
“In the past, there was some debate about the purpose of these structures because the upper leg isn’t a great location for breathing apparatus,” Dr. Hopkins said.
“You’d think it would be easy for those filaments to get clogged with sediment where they are. It’s an open question why they evolved the structure in that place on their bodies.”
The findings appear in the journal Science Advances.
Jin-bo Hou et al. 2021. The trilobite upper limb branch is a well-developed gill. Science Advances 7 (14): eabe7377; doi: 10.1126/sciadv.abe7377