The Trilobites were a diversified group of former marine arthropods. Many trilobites lived on the seabed but some occupied open doors. New research has resolved the evolutionary history of a bizarre group of trilobites called cyclopygids, named after the legendary Borgne Cyclops monster, while their eyes have gradually merged into a single large visual area as they have evolved.
Cycloplopygid trilobite Stevaninae symphysops. Image credit: Francisco J. López-Soriano and Joan Corbacho, https://www.elfosil.com, via Battle.
The Trilobits were former frightening aquatic crawlies (arthropods) who lived between 521 million years (Ma) at 252 mA.
About 21,000 species are known and they occupied many ecological niches; Most live on the seabed (benthic) but some lived in the open (pelagic) ocean.
Despite their diversity, we know relatively little about their global evolutionary history.
A bizarre group of mainly ordovicians (485 to 444 mA) is called cyclopygids, with 5 to 7 bodily segments (thoracic) and no gene spines in their heads (cephalon).
The cyclopygids are named after the mythical (Greek) cyclops because their eyes have merged into a large visual area with a wide view of view as they have evolved.
Cyclopygids lived in the deep and dark parts of the ocean (Mesopelagic) – swimming at the lower limit of the photic zone – the so -called twilight zone, actively chasing small animals (zooplankton).
Certain cyclopygids, like prickopyge, probably swam backwards and had light emitting organs on their third thoracic segment.
New research has resolved the evolutionary history of 47 cyclopygid genres using cladistic analysis, a method that brings together organisms based on their shared derivative characteristics.
A family called Taihungshaniidae, mainly known in China, is basal to Nileidae and Cyclopygidae families, the latter united by their enlarged eyes (hypertrophied) and include IllaenopsisA supposed Nileid.
As cyclopygids have evolved, they have developed adaptations increased to a pelagic lifestyle, including larger eyes which have gradually moved to the edge of their cephalon, a reduction in the number of thoracic segments, a decrease in size and improved rationalization.
The cyclopygids that lived higher in the water column tend to extend closer to the shore.
When we map the depth they are interpreted as having lived on our evolutionary tree, we see that cyclopygids near the base of the tree tended to live in less deep waters.
Thus, cyclopygids have gradually evolved into deeper waters as well as their increased pelagic adaptations.
This document appears in the newspaper Historical biology.
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SJ Brady, La Dale and H. Wang. 2025. The phylogeny of cyclobenic trilobites. Historical biology. DOI: 10.1080/08912963.2025.2501330
