Fossil Arthropods - Trilobites, Insects and Spiders, and Others

Well-preserved trilobites in Great Basin National Park.

NPS photo by Groden Bell.

Introduction

Some of the most striking invertebrate fossils present in national parks are those of arthropods. Trilobites are probably the most beloved marine invertebrate fossils, and fossils of insects are a source of wonder because of their delicate features and rarity.

Arthropod fossils in parks are diverse and range in age from the earliest Cambrian to Pleistocene and Holocene fossils. Trilobites lived exclusively in the Paleozoic, and the first insects appeared in the Devonian.

Phylum Arthropoda

The word “arthropod” means “jointed leg” and describes one of the key features of these animals. Other key features of these invertebrate animals include segmented bodies and external skeletons. Their exoskeletons are made of chitin, a tough fibrous polymer. Many arthropods molt their exoskeleton when they grow, and this is reflected in the fossil record. For example, many fossils of trilobites represent discarded molts rather than deceased trilobites.

Today, arthropods are diverse and numerous, inhabiting marine, freshwater, and terrestrial environments. They include:

• Arachnids (spiders, scorpions, ticks, etc.)

• Crustaceans (barnacles, crabs, lobsters, shrimps, etc.)

• Horseshoe crabs

• Insects

• Myriapods (centipedes and millipedes)

The fossil record of arthropods does not reflect what their true diversity was. Marine arthropods like trilobites are much more common in the fossil record than terrestrial ones because organisms that live in terrestrial environments are much less likely to be buried and preserved than marine organisms. Additionally, some marine arthropods, such as trilobites, have more durable hard parts than other arthropods. The exoskeletons of some arachnids and insects may be quite delicate. It is likely that many invertebrate trace fossils known from parks were made by arthropods, particularly crustaceans and insect larvae, but trace fossils can only rarely be attributed to a specific genus or species.

Arthropod Fossils in National Parks

Trilobites and ostracodes are the most common marine arthropod fossils in national parks. Other marine arthropods such as eurypterids (sea scorpions), barnacles, crabs, and lobsters have been documented in some national park areas.

A fragment of a Silurian eurypterid (sea scorpion) from Delaware Gap National Recreation Area. New Jersey State Museum holds this specimen in repository for the National Park Service.

NPS photo by Justin Tweet.

Overall, fossils of terrestrial arthropods like insects and spiders are quite rare in national parks. The major exception is Florissant Fossil Beds National Monument, where unique depositional environment led to the fossilization of these arthropods, some having exceptional preservation. Fossils of a wide variety of terrestrial arthropods have also been found in caves, 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.}

^{Well-preserved trilobite from Mojave National Preserve.}

^{Glossopleura mckeei, Grand Canyon National Park, Arizona.
NPS photo be Anne Miller.}

Trilobites from St. Croix National Scenic Riverway. Courtesy of the Cincinnati Museum Center.

NPS photo by Justin Tweet.

Trilobites are perhaps the most famous form of fossil arthropod. These marine animals had a hard exoskeleton that was prone to fossilization, especially since trilobites molted frequently as they grew. Many trilobite fossils likely are of molted exoskeletons.

True to their names, trilobites had three-lobed bodies: the axial lobe running along the midline of the body, with pleural lobes on either side. They also had three main divisions of the body from end to end: the head region (cephalon), the segmented thorax, and the tail region (pygidium). Pygidia and partial cephalons are common trilobite fossils because they stay together after molting.

Trilobites also had a pair of antennae and numerous limbs with feathery gills, but these structures rarely preserve. Trilobites lived throughout the Paleozoic until the Permian–Triassic extinction event, but were most abundant and diverse during the Cambrian.

Trilobite fossils have been documented in at least 42 units of the National Park System. Notable trilobite fossils in the National Park System, including many type specimens, occur at Death Valley National Park in California, Grand Canyon National Park in Arizona, St. Croix National Scenic Riverway in Minnesota and Wisconsin, Yellowstone National Park in Wyoming, and Yukon-Charley Rivers National Preserve in Alaska. One species (Glossopleura mckeei) from Grand Canyon was even named for Eddie McKee, the canyon’s second park naturalist.

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

Ostracodes

Ostracodes. Lake Mead National Recreation Area.

NPS photo by Aubrey Bonde.

Ostracodes are a major class of marine and freshwater crustaceans that have left a significant fossil record in national parks. Commonly known as seed shrimp, they are small bean-shaped organisms that are usually microscopic. Larger ostracodes can be up to approximately 1 cm in length. Ostracodes are useful for understanding ancient depositional environments and determining the relative ages of geologic formations. Because of this, some scientists will specifically seek them out, along with other similar microfossils, so their fossil record is well-understood even though most of their fossils are hard to see with the naked eye.

Ostracode fossils have been identified in at least 59 park units. Many fossil species have been named from some parks, including Cape Hatteras National Seashore in North Carolina ( at least 22, mostly from Cretaceous rocks recovered from deep cores), Mississippi National River and Recreation Area in Minnesota (possibly as many as 37 from Ordovician rocks), and Yukon-Charley Rivers National Preserve in Alaska (at least 17 Devonian species). At Great Basin National Park in Nevada, unusually large ostracodes have been found in abundance Ordovician rocks. In Lake Mead National Recreation Area in Arizona, ostracodes are present in the Jurassic Kayenta Formation, in limestone beds that have been interpreted as freshwater.

Insects and Spiders

^{Permian insect wing from the Hermit Formation in Grand Canyon National Park.}

^{Fossil spider from Florissant Fossil Beds National Monument, Colorado.}

Beetle, Fossil Butte National Monument, Wyoming.

Insect or spider fossils are known from at least 34 parks. As predominantly terrestrial organisms with delicate exoskeletons, their remains are rarely preserved. Most of the known insect fossils in national parks are of Quaternary age, and include fossils found in caves, packrat middens, and lacustrine (lake) deposits. One of the oldest insect fossils in the National Park System is a wing from a Permian dragonfly-like insect from the Grand Canyon National Park in Arizona.

Most insects and spider fossils in a national park come from one site: Florissant Fossil Beds National Monument in Colorado where Eocene paper shales contain a wide variety of delicate fossils, including those of wasps, ants, beetles, dragonflies, flies, butterflies, and moths. Fossil spiders are very rare in the fossil record, but some were preserved at Florissant. Remarkable Eocene insect fossils are also known from Fossil Butte National Monument in Wyoming and Glacier National Park in Montana, coincidentally also in Eocene rocks.

Other Arthropods

Eocene freshwater crayfish. Fossil Butte National Monument.

A wide variety of other arthropod fossils have been documented in other national parks. For example, fossils of freshwater shrimp and crayfish have been found in the Green River Formation that was deposited in Fossil Lake in Fossil Butte National Monument in Wyoming. Triassic crayfish are known from the Chinle Formation of Canyonlands National Park in Utah and Petrified Forest National Park in Arizona. Crabs are known from several parks. For example, three species of Cretaceous and Paleocene crabs have been named from Santa Monica Mountains National Recreation Area in California.

Coloring Book Pages

Featured Parks

• Canyonlands National Park (CANY), Utah—[CANY Geodiversity Atlas] [CANY Park Home] [CANY npshistory.com]

• Cape Hatteras National Seashore (CAHA), North Carolina—[CAHA Geodiversity Atlas] [CAHA Park Home] [CAHA npshistory.com]

• Death Valley National Park (DEVA), California and Nevada—[DEVA Geodiversity Atlas] [DEVA Park Home] [DEVA npshistory.com]

• Delaware Water Gap National Recreation Area (DEWA), New Jersey and Pennsylvania—[DEWA Geodiversity Atlas] [DEWA Park Home] [DEWA npshistory.com]

• Florissant Fossil Beds National Monument (FLFO), Colorado—[FLFO Geodiversity Atlas] [FLFO Park Home] [FLFO npshistory.com]

• Fossil Butte National Monument (FOBU), Wyoming—[FOBU Geodiversity Atlas] [FOBU Park Home] [FOBU npshistory.com]

• Glacier National Park (GLAC), Montana—[GLAC Geodiversity Atlas] [GLAC Park Home] [GLAC npshistory.com]

• Grand Canyon National Park (GRCA), Arizona—[GRCA Geodiversity Atlas] [GRCA Park Home] [GRCA npshistory.com]

• Great Basin National Park (GRBA), Nevada—[GRBA Geodiversity Atlas] [GRBA Park Home] [GRBA npshistory.com]

• Lake Mead National Recreation Area (LAKE), Arizona and Nevada—[LAKE Geodiversity Atlas] [LAKE Park Home] [LAKE npshistory.com]

• Mississippi National River and Recreation Area (MISS), Minnesota—[MISS Geodiversity Atlas] [MISS Park Home] [MISS npshistory.com]

• Petrified Forest National Park (PEFO), Arizona—[PEFO Geodiversity Atlas] [PEFO Park Home] [PEFO npshistory.com]

• Santa Monica Mountains National Recreation Area (SAMO), California—[SAMO Geodiversity Atlas] [SAMO Park Home] [SAMO npshistory.com]

• Saint Croix National Scenic Riverway (SACR), Minnesota and Wisconsin—[SACR Geodiversity Atlas] [SACR Park Home] [SACR npshistory.com]

• Yellowstone National Park (YELL), Idaho, Montana, and Wyoming—[YELL Geodiversity Atlas] [YELL Park Home] [YELL npshistory.com]

• Yukon-Charley Rivers National Preserve (YUCH), Alaska—[YUCH Geodiversity Atlas] [YUCH Park Home] [YUCH npshistory.com]

Related Links

• Grand Canyon—Trilobites

• An Inventory of Trilobites from National Park Service Areas [PDF]

Body Fossils