Trilobite

Rhode Island designated the trilobite as its official state fossil in 2023, one of the last states in the country to make such a designation. Unlike most state fossil laws, Rhode Island's does not name a specific genus or species: it recognizes trilobites as a group. Trilobites were hard-shelled marine arthropods that dominated the world's oceans for nearly 270 million years during the Paleozoic era before vanishing in the largest mass extinction in Earth's history.

Trilobites were ocean-dwelling invertebrates with a rigid, segmented exoskeleton divided lengthwise into three lobes, which is where the name comes from. The body also had three sections front to back: a head shield (cephalon), a segmented middle section (thorax), and a fused tail plate (pygidium). Many species had compound eyes built from calcite crystal lenses, some of the earliest complex eyes in the fossil record.

Species ranged from under half an inch to over two feet long. Most lived on the seafloor and fed on organic material, though some could swim and some could burrow into mud. Many species could roll into a tight ball for protection, much like a pill bug today. Trilobites appeared in the Cambrian period about 521 million years ago and died out about 252 million years ago when the Permian mass extinction wiped out over 90 percent of marine species.

Rhode Island was among the last states in the country without an official state fossil when the General Assembly passed the 2023 trilobite designation. The legislation named trilobites broadly rather than a single genus or species, a choice that reflects the range of trilobite types found across New England's Paleozoic rock record.

Kentucky used a similar approach in 1986, designating brachiopods as a group rather than one species because so many different types appear in its Paleozoic rocks. Rhode Island's 2023 trilobite law follows comparable reasoning. The designation came 56 years after Nebraska and North Dakota became the first states to designate official fossils in 1967.

Much of Rhode Island's bedrock is metamorphic rock altered by heat and pressure during ancient mountain-building events that deformed much of southern New England. Metamorphic rock does not preserve fossils, which limits where trilobite material turns up in the state.

Trilobite fossils in New England occur where Paleozoic sedimentary or lightly metamorphosed rocks are still exposed. The northern part of Rhode Island, near the Massachusetts border in the Cumberland and Woonsocket area, sits close to Cambrian and Ordovician outcrops that have produced trilobite material in the broader region. Rhode Island does not have a single famous trilobite site comparable to Ohio's Cincinnati limestone beds or Pennsylvania's Devonian quarries.