Twilight Zone: The Trilomus Enigma

Trilobites were a varied group of ancient marine arthropods. While many inhabited the seabed, some took to more mobile lifestyles. Recent research has clarified the evolutionary lineage of a peculiar group of trilobites known as cyclopizids, named after the legendary one-eyed creature, Cyclops.

Mirobiaceae was an ancient aquatic creature (arthropod) that existed from 521 million years ago (MA) until around 252 MA.

Approximately 21,000 species are known, representing numerous ecological niches. Most thrived on the seabed (benthic), while some ventured into the open ocean (pelagic).

Despite their diversity, our understanding of their comprehensive evolutionary history remains limited.

Strange groups of Ordovician trilobites (485-444 MA) are referred to as Cyclopizids. They possess 5-7 body (thoracic) segments, and the cephalon lacks typical spines.

The name Cyclopizid is derived from the mythical Greek cyclops, as their eyes evolved to merge into a single large visual surface, enhancing their field of vision.

Cyclopygids inhabited the deep, dark zones of the ocean (mesopelagic), swimming at the lower edge of the illuminated region, known as the Twilight Zone, where they actively hunted small organisms (zooplankton).

Some Cyclopygids, such as Pricyclopyge, may have swum upside down and featured bioluminescent structures on their third thoracic segment.

Recent research has elucidated the evolutionary history of the 47 cyclopygid genera through cladistic analysis, a method that categorizes organisms based on shared derived traits.

The family predominantly found in China, Tai Hungshaniidae, is connected to the Nileidae and Cyclopidiidae families. Illaenopsis is expected to be included.

As cyclopizids evolved, they adapted to a more transparent lifestyle, which included larger eyes shifting towards the edges of the thorax, a reduction in the number of thoracic segments, size decrease, and enhanced streamlining.

These cyclopizids, which occupy higher positions in the water column, usually reside closer to the coasts.

Mapping the depths they are interpreted to inhabit on our evolutionary trees reveals that cyclopizids positioned near the base tend to inhabit shallow waters.

Consequently, cyclopizids gradually transitioned into deeper waters, adapting more to a pelagic lifestyle.

This paper will be published in the journal Historical Biology.

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SJ Braddy, La Dale & H. Wang. 2025. Phylogenesis of cyclopide trilobites. Historical Biology. doi:10.1080/08912963.2025.2501330