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Introduction
Invertebrates are animals without backbones, that is, organisms that lack interlocked vertebrae to form a backbone or segmented spinal column. Parks from Texas (e.g., Big Bend National Park) to the tundra (e.g., Katmai National Park in Alaska) preserve invertebrate fossils, primarily ammonites in marine limestone that was deposited in the Cretaceous Interior Seaway. Abundant invertebrate fossils (e.g., trilobites, crinoids, and brachiopods) from older Paleozoic limestone occur at Natchez Trace National Scenic Trail in Mississippi and Chesapeake and Ohio Canal National Historical Park in Maryland.
Types of Invertebrates
Select a type of invertebrate to learn more:
Cnidarians are a varied group of organisms with cells organized into definite tissues. This group includes corals, jellyfish, and sea anemones. They have a stomach and a mouth surrounded by tentacles. Moreover as their Greek name “cnidos”—meaning stinging nettle—implies, all cnidarians have specialized cells that can inject poison into their prey or hapless passers-by.
Surprisingly soft-bodied cnidarians do have a fossil record. However, the fossil record of cnidarians without mineralized skeletons is quite sparse and restricted to unusual sites with excellent fossil preservation. On the other hand, cnidarians that possessed hard skeletons, particularly corals, have left a significant legacy of their existence. Corals are essentially sea anemones that support their bodies by building skeletons of calcium carbonate. Corals appeared for the first time in the geologic record in tropical Ordovician environments. Soon after their inception, corals started to live in colonies forming large, wave-resistant structures—coral reefs. Some of these reef structures have been preserved in the rock record at Delaware Water Gap National Recreation Area in Pennsylvania and Death Valley National Park in California. While modern, soft-bodied animals build the present coral reef at Biscayne National Park in Florida, fossil corals occur in the rocks of the islands there.
Sponges are among the simplest of animals; they have no true tissues (i.e., muscles, nerves, and internal organs). They are primitive feeders that pump water through their internal structure to filter out particulates of food matter. Sponges are highly variable in shape: many are shaped like cups or flasks; others are like spheres or cauliflower; some form flat plates folded together; others are encrusting. Not all sponges contain hard parts capable of being fossilized. Those that do have slender, pointed elements called “spicules,” which are composed of silica and serve as supportive skeletons. Sponges are marine organisms today and are assumed to have been so in the past.
Sponges have a long fossil record from the Precambrian onwards. In many places they were abundant enough to form widespread rock formations. They also have been prominent reef-building organisms, often in association with bryozoans. One group popularly known as “glass sponges” because 90% of their dry weight is “glass” (silica) are preserved complete in the Permian rocks at Guadalupe Mountains National Park in Texas and Silurian rocks at Glacier Bay National Park in Alaska.
Mollusks are one of the most varied, successful, and numerous invertebrates. Thousands of living species occupy every marine habitat from the shallowest shores to the deepest abyss. They have been truly successful in making the transition from sea to land (e.g., snails and slugs), and inhabit freshwater environments. The term mollusk means “soft-bodied,” and most primitive mollusks lacked shells; however, gastropods (snails), bivalves or pelecypods (clams), and cephalopods (e.g., squids, octopuses, and chambered nautilus) have well-developed hard parts, which readily fossilize.
Today, mollusks such as gastropods, bivalves, and cephalopods inhabit the seas. Extinct mollusks include ammonoids, which existed from the Devonian to the Cretaceous, and the squid-like belemnites, which existed from the Devonian to the Cretaceous Period. The Cretaceous cliffs along the Rio Grande in Big Bend National Park in Texas are studded with ammonites, and the ammonite fossils at Fossil Point in Lake Clark National Park in Alaska reach 18 inches (46 cm) in diameter. The Cretaceous Pierre Shale at Badlands National Park in South Dakota yield lovely ammonites with mother-of-pearl coatings. A nice juxtaposition at Cabrillo National Monument on the California coast has modern marine organisms washing onto rocks containing older Cenozoic marine invertebrates.
Brachiopods possess a distinctive feeding and respiratory organ called a lophophore—a ring of tentacles with the mouth inside. The tentacles move, creating a current that brings water and food to the animal. Brachiopods generally have two-part shells that differ in size and shape. As such they have a superficial resemblance to bivalve mollusks; however, in most bivalve mollusks (except oysters) the plane of symmetry runs through the hinge, so the two shells are mirror images. In contrast, the plane of symmetry in brachiopods runs at right angles to the hinge, so their left and right sides are mirror images. This difference reflects fundamental differences in internal anatomy and feeding habits. Brachiopods lack the ligament that helps open a bivalve shell.
Appearing in the Cambrian Period, brachiopods have one of the longest histories and one of the best fossil records of any invertebrate group. They became dominant during the Silurian and still survive today, though they are presently quite inconspicuous and rare and only found in very cold (deep or polar) water. Brachiopod fossils, like many invertebrate groups, occur in many national parks with Paleozoic marine rocks, including Arches in Utah, Death Valley in California, Denali in Alaska, and Guadalupe Mountains in Texas.
Arthropods are the most numerous invertebrate group today. They inhabit marine, freshwater, and terrestrial habitats. Arthropods, meaning “jointed leg,” are those animals with an external skeleton and characteristic legs and feelers with joints to give them flexibility. Today arthropods include:
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crabs,
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lobsters,
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shrimps,
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crayfish,
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spiders, and
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insects.
With certain exceptions, the fossil record of arthropods, which began during the Cambrian Period, is not as rich as their diversity today (and their probable existence in the past). One notable exception is trilobites, which did leave a good record. Trilobite fossils are particularly abundant in Cambrian rocks, though they lived until the Permian. Notable trilobite fossils in the National Park System occur at Fort Necessity National Battlefield in Pennsylvania and Carlsbad Caverns National Park in New Mexico.
Investigators have described a huge diversity of fossil arthropods, particularly insects, from what is now Florissant Fossil Beds National Monument. Fossil Butte National Monument in Wyoming also has numerous insect fossils: beetles, moths, flies, and caterpillars. Fossil spiders are very rare in the fossil record, but some were preserved at Florissant. The legs on these specimens stayed splayed during preservation; hence, eight tiny legs on these fossils are readily distinguished and studied—a unique occurrence indeed! Finally, one of the showiest arthropod fossils in the National Park System is a freshwater shrimp (Astacidae) from Petrified Forest National Park (Amati et al. 2004); it is also one of the earliest shrimp in the fossil record. This rare specimen is beautifully preserved with its legs and antennae clearly distinguishable.
The first echinoderms—“spiny skinned” animals—appeared in the Cambrian Period. Living members of this group include starfish, sea urchins, sand dollars, and the less familiar sea cucumbers. Three groups of echinoderm fossils are common: blastoids, crinoids, and sea urchins. While blastoids are extinct, crinoids and sea urchins are still living today. Parks with extensive Paleozoic and Mesozoic marine deposits throughout the Midwest preserve echinoderm fossils.
Blastoids had bud-like skeletons of fused plates rich in calcium carbonate; food-gathering grooves bisected five distinct, radial, depressed areas. Most blastoids were attached to segmented stalks that anchored to the seafloor.
Crinoids, also known as “sea lilies,” have cup-like skeletons that contain plates like blastoids, but the plates are more numerous and circular. Branched appendages extend upward. Like blastoids, most crinoids have stalks, which are found as fossils. The five-sided stems of crinoid fossils (Pentacrinus) at Zion National Park in Utah are quite amazing. Crinoid fossils also occur in the Mississippian and Pennsylvanian rocks at Fort Necessity National Battlefield in Pennsylvania and on the western slope of the Grand Teton in Grand Teton National Park in Wyoming.
Both living and extinct sea urchins, also called “sea hedgehogs,” have globe-like, heart-shaped, or disk-like skeletons that are rich in calcium carbonate. Spines, some of which are sharp and break easily (often into an unwary wader’s foot!), protect many living sea urchins. Unlike the related blastoids and crinoids, sea urchins lack stalks or any arm-like appendages.
Graptolites are an extinct group of colonial organisms with a geologic record extending from the Cambrian to the Mississippian. They were either planktonic (floating) or benthonic (bottom dwelling) in habit and were probably filter-feeders. Their current classification is hemichordates—a group of tube-dwelling organisms—because of their affinity to a modern group of hemichordates, the pterobranchs.
In the fossil record, graptolites appear as long, dark, carbon films in fine-grained clastic and carbonate rocks. They generally resemble narrow, saw blades with teeth on one or both sides. The saw blades may occur singly, in groups, or as nets. The teeth on the blades (or branches) were in reality tubes or cups in which tiny animals lived and, together, formed colonies. The colonies varied widely in shape: some are shrub-like with numerous slender branches; others had only a few or even a single branch (Fortey 1991). Graptolites have been found in rocks near Delaware Water Gap National Recreation Area in Pennsylvania. Investigators have documented 26 graptolite genera from more than 20 localities near Delaware Water Gap.
Bryozoans are colonial animals that commonly live primarily in marine environments. Because of their resemblance to mosses, they are sometimes called “moss animals,” but also “sea mats” or “lace corals.” They generally prefer warm, tropical waters but are known to occur worldwide. In their aquatic habitats, bryozoans may be found on all types of hard substrates: sand grains, rocks, shells, wood, kelp, pipes, and ships may be heavily encrusted with bryozoans. However, some colonies form on soft sediment.
About 5,000 species of bryozoans live today. The earliest bryozoans are from the Early Ordovician Period. Bryozoan fossils occur in many national parks with Paleozoic marine rocks, such as the Permian reef of Guadalupe Mountains National Park in Texas. The Permian reef is composed of the Capitan limestone—a massive, fine-grained fossiliferous limestone that formed by growth and accumulation of invertebrate skeletons of algae, sponges, and tiny bryozoans. Delaware Water Gap National Recreation Area in Pennsylvania also has numerous bryozoan fossils.
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Featured Parks
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Badlands National Park (BADL), South Dakota—[BADL Geodiversity Atlas] [BADL Park Home] [BADL npshistory.com]
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Big Bend National Park (BIBE), Texas—[BIBE Geodiversity Atlas] [BIBE Park Home] [BIBE npshistory.com]
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Biscayne National Park (BISC), Florida—[BISC Geodiversity Atlas] [BISC Park Home] [BISC npshistory.com]
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Cabrillo National Monument (CABR), California—[CABR Geodiversity Atlas] [CABR Park Home] [CABR npshistory.com]
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Chesapeake and Ohio Canal National Historical Park (CHOH), Washington DC, Maryland, and West Virginia—[CHOH Geodiversity Atlas] [CHOH Park Home] [CHOH npshistory.com]
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Death Valley National Park (DEVA), California and Nevada—[DEVA Geodiversity Atlas] [DEVA Park Home] [DEVA npshistory.com]
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Delaware Water Gap National Recreation Area (DEWA), New Jersey and Pennsylvania—[DEWA Geodiversity Atlas] [DEWA Park Home] [DEWA npshistory.com]
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Fort Necessity National Battlefield (FONE), Pennsylvania—[FONE Geodiversity Atlas] [FONE Park Home] [FONE npshistory.com]
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Fossil Butte National Monument (FOBU), Wyoming—[FOBU Geodiversity Atlas] [FOBU Park Home] [FOBU npshistory.com]
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Grand Teton National Park (GRTE), Wyoming—[GRTE Geodiversity Atlas] [GRTE Park Home] [GRTE npshistory.com]
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Guadalupe Mountains National Park (GUMO), Texas—[GUMO Geodiversity Atlas] [GUMO Park Home] [GUMO npshistory.com]
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Katmai National Park and Preserve (KATM), Alaska—[KATM Geodiversity Atlas] [KATM Park Home] [KATM npshistory.com]
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Lake Clark National Park and Preserve (LACL), Alaska—[LACL Geodiversity Atlas] [LACL Park Home] [LACL npshistory.com]
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Petrified Forest National Park (PEFO), Arizona—[PEFO Geodiversity Atlas] [PEFO Park Home] [PEFO npshistory.com]
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Zion National Park (ZION), Utah—[ZION Geodiversity Atlas] [ZION Park Home] [ZION npshistory.com]
Last updated: May 10, 2024