Based on multiple analytical techniques applied to two well-preserved soft-bodied specimens of trilobites, the Late Ordovician species Triarturus Eatonii and Middle Cambrian species Polygonum gracilis, paleontologists claim that there was an additional pair of cephalopods just behind the antennae, indicating that the trilobite had five pairs of cephalopods and six segments.

Trilobites are extinct arthropods that dominated the marine fauna of the Paleozoic Era.

During their lifetime on Earth, which lasted much longer than the dinosaurs, they survived two major extinctions and dominated undersea ecosystems.

They appeared in the ancient oceans of the Early Cambrian period about 540 million years ago, long before life appeared on land, and disappeared during the mass extinction at the end of the Permian period about 252 million years ago.

They are incredibly diverse, with around 20,000 species, and fossils of their exoskeletons have been found all over the world.

Like other arthropods, trilobites have a body made up of many segments and a head made up of several fused segments.

Like the rest of the trilobite’s body (the thorax and tail), these segments are associated with appendages whose functions range from sensing to feeding to locomotion.

“The number of these segments and how it relates to other important features, such as eyes and legs, is important for understanding how arthropods relate to each other and how they evolved,” said Dr. Melanie Hopkins, curator and head of the Department of Palaeontology at the American Museum of Natural History.

The segments on a trilobite’s head can be counted in two different ways: by looking at the grooves (called sulci) on the top of the trilobite fossil’s hard exoskeleton, and by counting the pairs of antennae and legs preserved on the underside of the fossil.

However, trilobites’ soft appendages are rarely preserved, and when looking at trilobite head segments, researchers frequently find a mismatch between these two methods.

In the new study, Dr. Hopkins and Dr. Hou Jinbo of Nanjing University studied Triarturus Eatonii.

These fossils are known for their golden glow from well-preserved pyrite replacements, and show that there are additional, previously undescribed legs beneath the head.

“This incredible preservation method allows us to view the 3D appendages of hundreds of specimens directly from the ventral side of the animals, just like grabbing an appendage from a horseshoe crab on the beach and turning it upside down to view it,” Dr. Hou said.

Exceptionally well preserved compared to other trilobite species, Polygonum gracilis based on the fossil, which was discovered in the Burgess Shale of British Columbia, the authors propose a model for how the appendages may have been attached to the head in relation to grooves in the exoskeleton.

“This model resolves apparent inconsistencies and shows that the trilobite head contained six segments: the anterior segment associated with the origin of eye development, and five additional segments each associated with a pair of antennae and four pairs of walking legs,” the researchers explained.

Their paper published in the journal Paleontology.

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Jin-bo Hou & Melanie J. Hopkins. 2024. New evidence for five cephalopods in trilobites and its implications for trilobite head segmentation. Paleontology 67(5):e12723; doi:10.1111/pala.12723