Researchers Count Body Segments of 1,500 Trilobite Species

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Trilobite fossil specimen with eight segments in thorax and many more segments in tail. A fossil of the Ordovician trilobite Pseudogygites latimarginatus, which has eight segments in its thorax and many more in its tail.
M. Hopkins/© AMNH

A new study charts the number of exoskeletal segments for more than 1,500 species of trilobites, finding that although the location of the segments in these extinct marine animals changed over their 250-million-year tenure on Earth, the average total number of segments stayed mostly the same.

“For 200 years, scientists have described segmentation patterns that they have seen across the evolution of trilobites, but this was only done with a handful of species. No one had followed up on this in a more extensive way,” said the study’s lead author Melanie Hopkins, curator and chair of the Museum’s Division of Paleontology.

The first trilobites evolved about 520 million years ago, during the Cambrian Period, and went extinct at the end of the Permian about 250 million years ago. Trilobites have three distinctive sections of their bodies: the cephalon (head), thorax (body), and pygidium (tail). Each of these sections contains segments: the segments in the cephalon and pygidium were fused, but those in the thorax were flexible, allowing the trilobite to roll into a ball to protect its softer underside when threatened.

Trilobite specimen with fifteen segments in thorax and few in the tail.
A fossil of the Cambrian trilobite Bolaspidella housensis, which has 15 segments in its thorax and just a handful in the tail.
M. Hopkins/© AMNH

All arthropods, a large group that includes insects, crustaceans, arachnids, and myriapods like millipedes and centipedes, have segments. Some add segments throughout their lives. For example, trilobites would add new segments as they grew and molted for a period of time. As it aged, a trilobite would continue to grow and molt without adding extra segments. This process is seen today in copepods as well as some myriapods and crustaceans.

To determine how trilobite segmentation changed throughout time, Hopkins teamed up with University of Michigan student Rebecca To, a participant in the Museum’s Research Experiences for Undergraduates (REU) program. The pair tabulated the number of segments in the thorax and pygidium for more than 1,500 species of trilobites.

Their results are consistent with previous observations: Across species, there was a decrease in the number of segments in the thorax over time and an increase in the number in the pygidium. While the number of segments in each species can vary greatly, from seven to 77 segments in this study, overall, the average total number of trilobite segments stayed the same—an average of 17—despite changes in where they were located on the body. There was also a decrease in the variation in number of segments over time: only some of the earliest species had very many or very few segments. 

Why would more segments be advantageous in a trilobite’s fused tail rather than in its flexible body? The researchers ruled out two possible explanations—one tied to the appearance and extinction of entire subclades of trilobites and another tied to the trilobite’s “roly poly” ability—so more investigation is needed.

“Our study is only one of many that could be pursued with this dataset, which provides, for example, the basis for new research on trilobite growth and body size evolution. And by making the dataset easily available to other scientists, we hope to inspire additional research that we haven’t even thought of yet,” said Hopkins.

Research on this ancient and long-lived group could also help scientists understand more about segmentation in living arthropods.