Trilobite Fossils Discovered in Upstate New York Reveal ‘Extra’ Pair of Legs

A new study reveals that a species of trilobite whose fossils are exceptionally well preserved in upstate New York has an extra pair of legs under its head. The research, led by the American Museum of Natural History and Nanjing University in China, suggests that the presence of a fifth pair of head appendages may be more common among trilobites than previously thought. Published today in the journal Paleontology, The study helps researchers better understand how trilobite heads are segmented.

Trilobites are a group of extinct arthropods whose living relatives include lobsters and spiders. Like other arthropods, trilobites have many segments in their bodies, with the head region composed of several fused segments. As with other parts of the trilobite body (the thorax and tail), these segments were associated with appendages, whose functions ranged from sensing to feeding to locomotion.

“The number of these segments and how they are associated with other important traits, like eyes and legs, are important for understanding how arthropods are related to each other and, therefore, how they evolved,” said Melanie Hopkins, curator and chair of the museum’s division of paleontology.

Trilobite head segments can be counted in two different ways: by looking at the grooves (called furrows) on the upper side of the trilobite fossil’s hard exoskeleton, or by counting the pairs of antennae and legs preserved on the underside of the fossil. The soft appendages of trilobites are rarely preserved, however, and when examining trilobite head segments, researchers often find a discrepancy between these two methods.

In the new study, Hopkins and his colleague Jin-Bo Hou of Nanjing University examined recently recovered specimens of the exceptionally preserved trilobite Triarthrus eatoni from upstate New York. These fossils, known for the golden sheen of the pyrite substitute that preserves them, have an additional, previously undescribed leg beneath the head.

“This fantastic style of preservation allows us to observe the 3D appendages of hundreds of specimens directly from the ventral side of the animals, just as if we were looking at the appendages of horseshoe crabs on a beach by grasping and turning them over,” Hou said.

By comparison with another trilobite species, Olenoides serratus, exceptionally preserved from the Burgess Shale of British Columbia, Hopkins and Hou propose a model of how the appendages were attached to the head in relation to the grooves in the exoskeleton. This model resolves the apparent discrepancy and indicates that the trilobite head consisted of six segments: an anterior segment associated with the origin of eye development and five additional segments, associated with a pair of antennae and four pairs of walking legs, respectively.

This study builds on analysis that Hou and Hopkins did on Triarthrus eatoni, which showed that the walking legs carry micron-sized respiratory structures (gills) and that the function of some of the spines on the walking legs was to keep these gills clean.