Ancient trilobites grew like shrimp: slowly

Researchers have shown that trilobites, which roamed the world’s oceans 450 million years ago, may have grown to a similar size and age as current crustaceans.

Trilobites are marine arthropods that existed from about 520 million years ago until they went extinct 250 million years ago, at the end of the Permian period. Just like modern arthropods whose growth is limited by having gill-like appendages rather than lungs, trilobites lived in low-oxygen environments and so, exhibited low growth rates.

In a paper published in the journal Paleobiology, researchers from the University of British Columbia (UBC) and Uppsala University show that the Ordovician trilobite Triarthrus eatoni reached a length of just above four cm in about 10 years, with a growth curve very similar to that of today’s shrimp.

“T. eatoni lived in low-oxygen environments and, similarly to extant crustaceans exposed to hypoxic conditions, exhibited low growth rates compared with growth under more oxygenated conditions,” said UBC marine biologist and fisheries expert Dr. Daniel Pauly, lead author of the study.

“Low-oxygen environments make it more difficult for water-breathers to grow, and add to the difficulties of breathing through gills, which, as two-dimensional surfaces, can’t keep up with the growth of their three-dimensional bodies. Thus, under hypoxic conditions, they must remain small if they are to maintain the rest of their body functions.”

In the case of trilobites, their exopods—external branches on the upper part of their limbs—functioned as gills which caused similar growth constraints to those of their modern counterparts.

To reach these conclusions, Dr. Pauly and his colleague from Uppsala University, paleontologist Dr. James Holmes, resorted to the analysis of length-frequency data, a method developed within fisheries science and marine biology for studying the growth of fish and invertebrates lacking the physical markings that indicate their age.

“These findings provide the first reasonable estimates of absolute growth in early animals using methods known to accurately characterize growth in comparable living species,” Dr. Holmes said. “They show us that nearly half a billion years ago, growth in marine arthropods like trilobites was similar to modern examples like crustaceans living in today’s oceans.”

The information to perform their analysis was obtained from an earlier publication with information of the length-frequency distribution of 295 exceptionally-preserved trilobite fossils collected in the 1830s at Beecher’s Trilobite Bed in New York State.

After estimating the parameters of a growth model widely used in fisheries science, the von Bertalanffy growth function, the researchers compared their results with published data on the growth of modern crustaceans. They found that the growth parameters they estimated for Triarthrus eatoni were well within the range of recent, slow-growing crustaceans such as shrimp.