For more than 270 million years, trilobites were among the most successful and diverse creatures on Earth, with over 22,000 known species spanning the Paleozoic Era. Yet, despite their abundance in the fossil record and their presence on every continent, one of the most fundamental questions about their survival has remained a subject of intense scientific debate: how did they breathe?
A new study led by Sarah R. Losso, postdoctoral researcher in the Department of Organismic and Evolutionary Biology (OEB) at Harvard University, provides the most definitive evidence yet, confirming that the feather-like structures attached to their limbs functioned as sophisticated gills. The study, published in Biology Letters, used advanced 3D modeling and compared fossil data to modern marine life. The research aids in settling a long-standing controversy over the respiratory capabilities of these ancient ocean-dwelling arthropods.
"Respiration is one of the most important functions for any animal as it's required to produce energy," said Losso. "These structures need to be large enough to get enough oxygen into their bodies to support their metabolism."
Trilobites possessed "biramous" appendages—limbs with two distinct branches. The inner branch, known as the endopodite, was a walking leg used for locomotion and feeding. The outer branch, or exopodite, featured a series of thin, hair-like filaments called lamellae. While some researchers argued these exopodites were used for swimming or to create water currents for ventilation, others proposed they were primary respiratory organs. The debate centered on whether these structures provided enough surface area to support the oxygen needs of the animal. Previous research on the mid-Cambrian Olenoides serratus suggested the surface area was too low compared to modern arthropods, casting doubt on the "gill" theory, while other research showed that the Late Ordovician Triarthrus eatoni had exopodites with a similar surface area to gills of modern arthropods.
To resolve this, the international team created anatomically correct 3D models of exopodites from two well-preserved species of Olenoides serratus and Triarthrus eatoni. Using software such as Shapr3D and Ansys, they calculated the precise surface area of the lamellae and explored how that area related to the animal's overall biomass. O. serratus, measuring 67.8mm in length, had a total lamellar surface area of 16,589mm². The smaller T. eatoni, at 36.3mm in length, possessed 2,159mm² of surface area.
The team expanded their analysis to nine additional trilobite species, spanning a broad range of sizes and ages from the Cambrian to Silurian Periods. They discovered that the surface area of trilobite lamellae increases exponentially with overall body size, matching the growth patterns seen in modern aquatic species.
"Interestingly, this increase in breathing capacity was not achieved by adding more filaments," Losso noted. "The number of lamellae and limbs didn't correlate with body length; instead, larger trilobites grew significantly longer filaments to meet their higher energy demands." For example, the giant Redlichia rex possessed lamellae up to 11.02 mm long, vastly larger than those of its smaller relatives.
To put these numbers into context, the researchers compared the fossil data to modern aquatic euarthropods, including the Atlantic horseshoe crab (Limulus polyphemus). The results were striking: trilobites showed surface-area-to-biomass ratios ranging from 174.62 to 759.48 mm²/g. These figures overlap significantly with modern species like thalassinid shrimp, which range from 256 to 1,043 mm²/g. This suggests that trilobites were just as capable of extracting oxygen from seawater as many crustaceans living today. The surface area calculations of trilobites fall along the same trendline as modern crabs and horseshoe crabs, demonstrating that these structures were large enough to carry out respiration.
"Our findings also provide a glimpse into the diverse lifestyles of these ancient animals," said Losso. "Even though Triarthrus eatoni lived in low-oxygen environments, it appears to have maximized its lamellar surface area in order to thrive, while Redlichia rex had a lower surface area for its size, suggesting it may have had lower metabolic requirements or supplemented its oxygen intake through other parts of its body, such as the underside of its exoskeleton."
"Gill morphology and these metrics can inform us of how respiration has changed over 508 million years," reflected Losso, "even informing some of the most fundamental biological requirements of extinct giants."
Understanding how trilobites breathed means understanding the energetics of Paleozoic life. By confirming that exopodites functioned as gills, scientists can better explore how these animals moved, fed, and dominated the oceans for hundreds of millions of years.
"The study shows that although the gills of extant and extinct arthropods are very different in terms of their overall appearance, both follow predictable patterns that help inform their broader function as key evolutionary adaptations," said co-author Professor Javier Ortega-Hernández, also in OEB. "It is a remarkable example of the present being the key to the past."
