An extremely well-preserved fossil of Triarthrus eatoni from upstate New York. Daniel Kim /© AMNH
Because of their hard exoskeletons and ability to molt, trilobite fossils are a relatively common find, but rarely do they preserve the soft parts of the animal. Now, newly recovered, exceptionally well-preserved trilobite fossils from upstate New York have led to a new discovery: an extra pair of legs.
Examining these fossils, which are described today in the journal Palaeontology, is “just like looking at the appendages of horseshoe crabs on a beach by grabbing them and turning them upside down,” said lead author Jin-Bo Hou from Nanjing University. Hou conducted the study with coauthor Melanie Hopkins, curator and chair of the Museum’s Division of Paleontology.
Trilobites are a group of marine arthropods that lived for almost 300 million years until 250 million years ago, when Earth experienced the largest mass extinction in its history. Their closest living relatives today include lobsters and spiders.
Like other arthropods, the bodies of trilobites were made up of many segments. The segments were associated with appendages, which range from antennae used for sensing to legs that moved the animal along the sea floor and assisted with feeding.
“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,” Hopkins said.
Counting those segments, especially in the trilobite’s fused head, has been a challenge. To infer the number of segments, researchers look at the grooves on the upper side of the trilobite fossil’s hard exoskeleton and then compare it to the pairs of preserved antennae and legs on the underside of the fossil. But because the soft appendages of trilobites are rarely preserved, there often is a mismatch between these two methods.
In the new study, Hopkins and Hou examined exceptionally well-preserved specimens of T. eatoni from upstate New York, finding an additional, previously undescribed leg underneath the head, no doubt, one of a pair. By making comparisons with another well-preserved trilobite species, Olenoides serratus from British Columbia, the researchers propose a model for how appendages were attached to the head in relation to the grooves in the exoskeleton. This model resolves the apparent mismatch and suggests that the trilobite head included six segments: one associated with the developmental origin of the eyes and five additional segments, associated with one pair of antennae and four pairs of walking legs.
Hou and Hopkins have previously shown that the walking legs of T. eatoni 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. This study expands upon that work.