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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.

Trilobite fossil preserved in pyrite. Image credit: Jin-Bo Hou / University of California, Riverside.

Trilobite fossil preserved in pyrite. Image credit: Jin-Bo Hou / 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.

Dr. Hou and colleagues examined the pyritized remains of two trilobite species: Olenoides serratus from the Burgess Shale and Triarthrus eatoni from the Beecher’s Beds.

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.

Dumbbell-shaped filaments of Triarthrus eatoni: (A) dorsal view; (B) posterior view of the truncated filaments in stacked (A); (C) same area of (B) with nonstack function; (D and E) the sixth and seventh filaments showing dumbbell-shaped outline, tilted about 40° to the dorsal view; (D) high-contrast backscattered electron (BSE) image; (E) high-contrast, gaseous secondary electron (GSE) image; (F and G) the eighth filament showing dumbbell-shaped outline, tilted about 40° to the dorsal view; (H and I) top view of the eighth and ninth filaments showing dumbbell-shaped outlines; yellow dotted lines mark the cross section of the filaments (E and G); arabic numbers are references for locating the cross section of filaments in (A); asterisks locate the top and bottom inflated marginal bulbs of dumbbell-shaped filaments; small white arrows indicate the narrow central region of dumbbell-shaped filament. Abbreviations: ar - article of shaft, lob - lower branch of the limb, upb - upper branch of the limb. Scale bars - 500 μm in (A), 100 μm in (B and C), and 50 μm in (D to I). Image credit: Hou et al., doi: 10.1126/sciadv.abe7377.

Dumbbell-shaped filaments of Triarthrus eatoni: (A) dorsal view; (B) posterior view of the truncated filaments in stacked (A); (C) same area of (B) with nonstack function; (D and E) the sixth and seventh filaments showing dumbbell-shaped outline, tilted about 40° to the dorsal view; (D) high-contrast backscattered electron (BSE) image; (E) high-contrast, gaseous secondary electron (GSE) image; (F and G) the eighth filament showing dumbbell-shaped outline, tilted about 40° to the dorsal view; (H and I) top view of the eighth and ninth filaments showing dumbbell-shaped outlines; yellow dotted lines mark the cross section of the filaments (E and G); arabic numbers are references for locating the cross section of filaments in (A); asterisks locate the top and bottom inflated marginal bulbs of dumbbell-shaped filaments; small white arrows indicate the narrow central region of dumbbell-shaped filament. Abbreviations: ar – article of shaft, lob – lower branch of the limb, upb – upper branch of the limb. Scale bars – 500 μm in (A), 100 μm in (B and C), and 50 μm in (D to I). Image credit: Hou et al., doi: 10.1126/sciadv.abe7377.

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.

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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

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