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

"The number of these segments and how they are associated with other important traits, like eyes and legs, is important for understanding how arthropods are related to one another, and therefore, how they evolved," said Melanie Hopkins, curator and chair of the Museum's Division of Paleontology.

The segments in the trilobite head 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 preserved antennae and legs on the underside of the fossil. The soft appendages of trilobites are rarely preserved, though, and when looking at the segments in the trilobite head, researchers regularly find a mismatch between these two methods.

In the new study, Hopkins and colleague Jin-Bo Hou from Nanjing University examined newly recovered specimens of the exceptionally preserved trilobite Triarthrus eatoni from upstate New York. These fossils, known for the gold shine of the pyrite replacement preserving them, show an additional, previously undescribed leg underneath the head.