Fossil Arthropods in National Parks: Trilobites, Ostracodes, Insects, Spiders and More

Well-preserved trilobites in Great Basin National Park. NPS photo by Groden Bell

Among the most striking invertebrate fossils in national parks are arthropods. Trilobites may be the best-loved marine invertebrate fossils, and fossil insects fascinate because they are so delicate and so rare. The parks' arthropod fossils span from the earliest Cambrian to the Pleistocene and Holocene. Trilobites lived only in the Paleozoic; the first insects appeared in the Devonian.

What makes an arthropod

Arthropod means "jointed leg." Arthropods also have segmented bodies and an external skeleton made of chitin, a tough fibrous polymer. Many shed that skeleton as they grow — which shows in the fossil record: many trilobite fossils are discarded molts, not dead trilobites.

Today arthropods are everywhere — in the sea, fresh water and on land:

A lopsided record

The fossil record doesn't reflect how diverse arthropods really were:

• Land animals are far less likely to be buried and preserved than sea animals, so marine arthropods such as trilobites are much more common as fossils.
• Some marine arthropods, trilobites among them, had tougher hard parts; some arachnid and insect exoskeletons are very delicate.
• Many trace fossils in parks were probably made by arthropods — especially crustaceans and insect larvae — but a trace can only rarely be pinned to a particular genus or species.

What the parks hold

Trilobites and ostracodes are the most common marine arthropod fossils in the parks. Some parks also have eurypterids (sea scorpions), barnacles, crabs and lobsters.

Part of a Silurian eurypterid (sea scorpion) from Delaware Water Gap National Recreation Area, held for the National Park Service by the New Jersey State Museum. NPS photo by Justin Tweet

Land arthropods such as insects and spiders are rare in park fossils — with one big exception, Florissant Fossil Beds National Monument, whose unusual depositional setting preserved them, some exceptionally well. Caves have also yielded many land arthropods, mostly from the Holocene and late Pleistocene, including fragments of chitin in bat guano.

Trilobites

Cambrian trilobites from Yellowstone National Park. NPS photo by Megan Norr

Trilobites are probably the most famous fossil arthropods. Their hard exoskeletons fossilized readily — all the more because they molted often as they grew.

Their name describes three lobes running lengthwise: a central axial lobe with a pleural lobe on each side. Their bodies also had three sections from front to back:

Pygidia and partial cephalons are common fossils because they stay together after molting. Trilobites also had antennae and many legs with feathery gills, but these are rarely preserved. They lived throughout the Paleozoic until the Permian–Triassic extinction, and were most abundant and varied in the Cambrian.

A well-preserved trilobite from Mojave National Preserve. Image from the NPS page on fossil arthropods

Trilobites are known from at least 42 units of the National Park System. Notable finds, including many type specimens, come from:

• Death Valley National Park, California;
• Grand Canyon National Park, Arizona — where one species, Glossopleura mckeei, is named for Eddie McKee, the canyon's second park naturalist;
• St. Croix National Scenic Riverway, Minnesota and Wisconsin;
• Yellowstone National Park, Wyoming;
• Yukon-Charley Rivers National Preserve, Alaska.

In Great Basin National Park, Nevada, trilobites are among the most abundant and varied fossils; in Mojave National Preserve, California, they are common in the lower Cambrian Latham Shale.

Glossopleura mckeei, Grand Canyon National Park, Arizona. NPS photo by Anne Miller

Ostracodes

Ostracodes, Lake Mead National Recreation Area. NPS photo by Aubrey Bonde

Ostracodes, or seed shrimp, are a major class of marine and freshwater crustaceans: small, bean-shaped and usually microscopic, though larger ones reach about 1 cm. They help geologists read ancient environments and work out the relative ages of rock formations, so scientists seek them out with other microfossils — which is why their record is well known even though most can't be seen with the naked eye.

Ostracodes are known from at least 59 park units. Many species have been named from:

Great Basin National Park has unusually large ostracodes in abundance in Ordovician rock, and at Lake Mead National Recreation Area they occur in the Jurassic Kayenta Formation, in limestone beds interpreted as freshwater.

Insects and spiders

Insects or spiders are known from at least 34 parks. Mostly land animals with fragile exoskeletons, they are rarely preserved. Most insect fossils in the parks are Quaternary, from caves, packrat middens and lake deposits.

A Permian insect wing from the Hermit Formation, Grand Canyon National Park. Image from the NPS page on fossil arthropods

• One of the oldest insect fossils in the park system is that wing, from a Permian dragonfly-like insect at Grand Canyon.
• Most of the parks' insect and spider fossils come from one site: Florissant Fossil Beds National Monument, Colorado, where Eocene paper shales preserve delicate wasps, ants, beetles, dragonflies, flies, butterflies and moths — and even spiders, which are very rare as fossils.
• Remarkable insects also come from Fossil Butte National Monument, Wyoming, and Glacier National Park, Montana — by coincidence, also in Eocene rock.

A fossil spider from Florissant Fossil Beds National Monument, Colorado. Image from the NPS page on fossil arthropods

A beetle, Fossil Butte National Monument, Wyoming. Image from the NPS page on fossil arthropods

Other arthropods

An Eocene freshwater crayfish, Fossil Butte National Monument. Image from the NPS page on fossil arthropods

• Freshwater shrimp and crayfish in the Green River Formation, laid down in Fossil Lake, at Fossil Butte National Monument, Wyoming.
• Triassic crayfish in the Chinle Formation of Canyonlands National Park, Utah, and Petrified Forest National Park, Arizona.
• Crabs in several parks — including three species of Cretaceous and Paleocene crabs named from Santa Monica Mountains National Recreation Area, California.

Featured parks

More: Grand Canyon's trilobites · An inventory of trilobites from National Park Service areas (PDF) · Body fossils · previous in the series: Fossil mollusks · next: Fossil echinoderms

Sources

Based on "Fossil Arthropods - Trilobites, Insects and Spiders, and Others," part of the series "Invertebrate Fossils in National Parks," National Park Service; a work of the United States government in the public domain. The article's caption names "Delaware Gap National Recreation Area"; it is Delaware Water Gap, as the article's own list of parks has it.