Invertebrate Fossils

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Invertebrates, simply and broadly defined, are animals without backbones. This includes a large, abundant, and diverse group of organisms that represent the earliest forms of multicellular life that appeared in the fossil record in the Late Precambrian and went through explosive episodes of diversification that led to their conquering of the sea, freshwater, and the air. There are few environments on earth where invertebrates are not present. Many invertebrates are familiar to us, such as clams, snails, and, of course, insects. They have adapted to many modes of life from sedentary, attached filter feeders, to sediment processors, to active predators. Some invertebrate groups, such as trilobites, are long extinct but abundantly represented in the fossil record.


Arthopods (Phylum Arthropoda) are perhaps the most successful group of multicellular animals and include insects, spiders and scorpions, centipedes and millipedes, lobsters, crabs, crayfish, ostracodes and barnacles, and trilobites. Arthropods are characterized by jointed legs, a body divided into segments, and a hard external covering (exoskeleton), that is periodically shed to accommodate the growth of the organism. They are an ancient group, dating to the late Precambrian and all of the above groups are known from the fossil record.

Trilobites were an exclusively marine group of arthropods that appeared in the Cambrian Period and persisted throughout the Paleozoic, eventually becoming extinct during the Permian Period. Most were small, bottom-dwelling organisms that fed on organic matter in sediment on the sea bottom. The body is divided in three segments: head (cephalon), middle (thorax), and tail (pygidium). Many species had large, compound eyes.

Trilobites are among the most desirable of fossils for the fossil collector and Ohio has an abundance of trilobites in the Paleozoic rocks of the state. This importance was recognized in 1985 when an Ordovician trilobite, Isotelus, was named the official state fossil of Ohio. Specimens of Isotelus have been found that are nearly two feet in length. Although no Cambrian rocks crop out at the surface in Ohio, a Cambrian trilobite, Cedaria, was found in a drill core from Warren County. Trilobite fossils are particularly abundant and well-preserved in Ordovician rocks that crop out in southwestern Ohio. Common species in these rocks are Cryptolithus, Flexicalymene, and Isotelus.

Trilobites were abundant and diverse in the seas that covered Ohio during the Silurian Period but many of the specimens have been destroyed or altered by the process of limestone changing to dolomite. However, excellent examples of Silurian trilobites are occasionally found. Common species are Bumastus, Calymene, and Dalmanites.

Devonian rocks in Ohio are well known for trilobite fossils. The Columbus Limestone yields several species, the best known and most common of which is Coronura, a large species that reached nearly a foot in length. Most specimens are not complete. The best-known trilobites from Ohio Devonian rocks come from the Middle Devonian Silica Shale of northwestern Ohio, where Eldredgeops rana (formerly known as Phacops rana) is exceedingly abundant and well preserved. Eldredgeops specimens from the Silica Shale are known worldwide. They are only a few inches in length and are noted for their large, frog-like compound eyes. Many are found rolled into a ball shape, a protective behavior employed by a number of trilobite species. The Silica Shale is exposed only in quarries, which are now closed to collecting.

A major, worldwide extinction event near the end of the Devonian Period significantly reduced the abundance and diversity of Late Paleozoic trilobites. Although several species are known from Mississippian rocks in Ohio, they are small and uncommon. By Pennsylvanian time, only three species are known from marine rocks in Ohio and they are small and uncommon. No marine rocks of Permian age are present in Ohio to mark the final extinction of the trilobites.


Ohio rocks have produced fossils of crustaceans, including ostracodes, millipedes, spiders and eurypterids, and insects. Ostracodes are microscopic crustaceans that have a two-part shell (carapace) made of calcium carbonate. In general, the shell looks like a tiny bean and many of them have intricate ornamentation on the shell surface. They are known from the Cambrian Period to the Recent and can be found in modern freshwater lakes and ponds. Most fossil species were marine and they are exceedingly abundant in some Paleozoic rocks in Ohio, ranging from Ordovician through Pennsylvanian. A microscope is needed to collect and study them.

Phyllocarids are crustaceans with a two-part shell and a tail. Some reached two or more inches in length and they are found in marine rocks in Ohio ranging from Devonian through Pennsylvanian age. The most abundant genus is Echinocaris, known from Devonian rocks. A decopod crustacean, Palaeopalaemon, related to shrimps and lobsters, is known from Devonian rocks in northeastern Ohio. Eurypterids were aquatic predatory arthropods that looked superficially like scorpions. Most had large pincers and, although most were small, some reached lengths of several feet. They were Paleozoic creatures and became extinct by the end of the Permian. They are rare fossils in Ohio being known from Ordovician, Silurian, and Devonian rocks.

Millipedes are segmented, terrestrial arthropods with a long, cylindrical body. Each body segment has two pairs of legs, thus leading to the name “thousand-leggers” for modern species. Rare specimens are known from shales associated with coal beds in Pennsylvanian rocks of eastern Ohio.

Fossil insect remains are known from Pennsylvanian rocks in eastern Ohio. Although they are rare fossils, some of them are spectacular because of their large size and preservation. Cockroaches nearly four inches in length have been found. Insect wings found in these rocks are commonly confused with fossil leaves. Perhaps the rarest arthropod fossils are those of spiders. A few specimens are known from Pennsylvanian rocks in eastern Ohio. The preservation of these specimens is remarkable and they look superficially similar to some modern spiders.


Brachiopods (Phylum Brachiopoda) are bottom-dwelling marine organisms with two shells, or valves, made of calcium carbonate. However, unlike clams, the shells differ in shape, and sometimes size. Although brachiopods range from Cambrian to Recent, they were dominant marine invertebrates in the Paleozoic. Many Paleozoic limestones and shales, particularly those of Ordovician age in Ohio, are literally a fossil hash made up mostly of brachiopod shells. Most species were small, less than an inch, but some were up to two inches in longest dimension. Although most species have ribbed shells of various forms, some brachiopods of the Mississippian and Pennsylvanian developed long, thin, sharp spines on their shells, probably as a defense against many species of shell-crushing sharks that became abundant at that time.

Ordovician and Devonian marine rocks, particularly limestones and limy shales, have a great diversity and abundance of brachiopod remains. Silurian carbonates have locally abundant brachiopods but in many areas dolomitization of Silurian limestones has destroyed the calcium carbonate shells. Sandy marine shales and some limestone beds locally have abundant brachiopods, although in some case preservation is not ideal. Marine limestones and shales of Pennsylvanian age have yielded brachiopod fossils. Brachiopods are easy to find and collect and are a good starting point for the beginning collector. Their diversity makes them of interest to the advanced collector as well.


Bryozoans (Phylum Bryozoa) are colonial, filter-feeding animals that are mostly marine but a few live in freshwater. They range from Ordovician to Recent and are common in marine limestones and shales in several geologic systems present in Ohio. They are particularly abundant in Ordovician rocks in southwestern Ohio.

Bryozoans consist of a skeletal structure of calcium carbonate that has numerous tiny holes or openings dotting the surface. These holes once housed individual bryozoan animals that derived their nutrients from the seawater. Bryozoan colonies range from small encrusting forms on shells of other invertebrates to branching, twig-like forms, to fan-shaped forms. Most colonies were only a few inches in diameter but a colony of an Ordovician form found in the Cincinnati region (Florence, Kentucky) is more than 26 inches in diameter and is one of the largest known bryozoan colonies.

Although many bryozoan species are common fossils in some rocks, and some can easily identified, many species are only identifiable using microscopic techniques. Commonly, branching forms are broken into many pieces.


Sponges (Phylum Porifera) are simple, filter-feeding animals that range from Precambrian to Recent. They are mostly marine, but some forms live in freshwater. Sponges are best known as fossils from microscopic needle-like to geometric skeletal structures called spicules. These structures are found in marine rocks in Ohio ranging from Ordovician to Recent. Flint or chert deposits, particularly in Devonian and Pennsylvanian rocks in Ohio are thought to have derived their silica from siliceous sponge spicules. The extensive bed of flint in the Pennsylvanian-age Vanport limestone at Flint Ridge in Licking and Muskingum Counties is thought to be silica derived from a massive, long-existing bank of sponges.

Complete sponges are relatively rare in Ohio rocks, although this may be in part because they are difficult to recognize. However, a group known as stromatoporoids is well represented and easily recognizable in Ohio Ordovician, Silurian, and Devonian rocks. These organisms were originally thought to be related to corals but are now recognized as an informal group of calcareous sponges. They form hemispherical shapes that may be as large as a soccer ball.


Corals (Phylum Cnidaria, formerly called Coelenterata) are solitary or colonial, filter-feeding and predatory animals that build a skeleton of calcium carbonate. The cnidaria also includes such soft-bodied forms such as jellyfish. Corals range from Ordovician to Recent and are prominent fossils in Ordovician through Devonian rocks in Ohio. They are somewhat rare in Mississippian and Pennsylvanian rocks. In particular, Silurian and Devonian carbonate rocks in Ohio contain an abundant and diverse coral fauna. At least 70 species are known from Devonian rocks in the state.

Corals in Paleozoic rocks in the state are of two forms. Solitary, or rugose corals are cone-shaped or irregular tube-shaped and contained a single coral animal. The pointed end of the coral was anchored to the sea bottom and the animal's food-gathering tentacles were present on the open end. Sometimes rugose corals are called horn corals because of their resemblance to a cow horn. Most specimens are an inch or two in length.

Colonial, or tabulate, corals consist of many identical coral animals that build a skeletal structure of calcium carbonate. Some individual coral “heads” can be up to three feet in diameter. “These organisms were major reef builders during the Silurian and Devonian and can be found in abundance. Common genera are Favosites, which is sometimes called a honeycomb coral because of its resemblance to honeycombs, and Halysites, which is commonly called a chain coral because the individual corallites are linked in chain-like fashion.


Echinoderms are a diverse group of marine organisms with radial symmetry that first appeared in the Cambrian Period. Most people are familiar with starfish but several other groups are important in the fossil record and are prized by collectors. The most abundant group of echinoderms found in Ordovician through Pennsylvanian marine rocks in Ohio is the crinoids. These animals resemble plants in that they have a long stalk that was anchored to the sea floor and a flower-like cup that housed the filter-feeding body parts of the animal. The skeletal material is calcium carbonate. Upon death of the animal, the stalk-like portion broke up into a series of circular plates and the body (calyx) broke up into individual plates. These disarticulated remains are very common in some marine rocks and sometimes are a major constituent of the rock. Rarely, a calyx is found intact with all, or a portion of the stalk attached.

Starfish (asteroids) and brittle stars (ophiuroids) are known from Ordovician, Silurian, Devonian, and Mississippian rocks of Ohio. They are rare fossils but an outcrop in the Cuyahoga Formation (Mississippian) of Cuyahoga County contained approximately 4,500 individuals of Strataster ohioensis per square meter.

Blastoids were echinoderms with a hickory-nut sized and shaped body attached to a long stalk, similar to that of a crinoid. They are known from Silurian, Devonian, and Mississippian rocks. They are common in some units.

Among several groups of rare echinoderms known from Ohio are edrioasteroids, which are small, circular animals with starfish-like arms on the dorsal surface. They are usually found attached to the shell of a brachiopod or other shelled invertebrate.


Mollusks (Phylum Mollusca) are a diverse group of organisms that include such familiar animals as clams and snails but other, less familiar ones as well. They range from Cambrian to Recent and, although they are primarily marine organisms, some groups have conquered freshwater and terrestrial environments.


Clams, sometimes called pelycopods (Class Pelecypoda) are filter-feeding animals with two identical shell halves. Some burrowed in bottom mud, whereas others rested on the sea bottom and a few were capable of swimming. Marine species are abundant and diverse in Ordovician through Permian rocks in Ohio and are commonly well preserved. Pennsylvanian and Permian rocks yield species that lived in fresh or brackish waters. Clams are common in some sediments deposited in lakes or streams during the latter part of the Pleistocene Ice Age.

Particularly interesting to fossil collectors are the large clams found in Middle Silurian rocks that are called “beef heart” clams because of their size and resemblance to the heart of a cow. This species, Megalomoidea canadensis (formerly called Megalomas) can reach almost 10 inches in length. They are commonly preserved as internal molds of the shell as the original calcium carbonate of the shell has been leached away. Clams from Mississippian and Pennsylvanian rocks have rarely been found with the original color patterns preserved on the shell. These occurrences remind us that in life many of the fossil organisms we find in Ohio rocks were not dull gray in color as they appear as fossils but had bright color patterns.


Snails (Class Gastropoda), or gastropods, are familiar to most everyone who has collected shells along a seashore or observed a terrestrial snail in the garden. They have been a very successful, diverse, and long-ranging group, having originated in the Cambrian. Shells of snails are well-represented in Ordovician through Pennsylvanian marine rocks and freshwater species are known from Pennsylvanian and Permian rocks and from deposits of the Pleistocene Ice Age. Most specimens are small, less than an inch long.

Snails are characterized by a single shell that is coiled, either in a flat plane (planisprial) or in a cone shape (conispiral). They have a foot that is used to crawl across the bottom in search of food and a mouth with a rasp-like tongue. Some modern species are predatory and are noted for boring circular holes in clam shells in order to get to the soft parts.

Many Ordovician and Silurian snail shells are internal molds of the shell (steinkerns) as the original calcium carbonate shell has been removed. These specimens are sometimes difficult to identify, as the external shell ornamentation is absent.


Cephalopods (Class Cephalopoda) are probably familiar to most people as the octopus and the squid. However, in the fossil record, shelled cephalopods known as nautiloids and ammonoids were the dominant forms. Today, the chambered nautilus survives as the only surviving representative of these once diverse groups. Cephalopods are relatively common in some marine rocks in Ohio, ranging from Ordovician to Pennsylvanian.

Cephalopods were swimming, predatory marine animals that had either a straight or coiled, chambered shell. The chambers were divided by walls (septa) and in life filled with gas. This enabled the cephalopod to move up and down in the water column by changing the pressure in the chambers. The last chamber housed the body of the cephalopod. These organisms had tentacles for capturing prey, well-developed eyes, and a beak used to kill, dismember, and ingest prey. They were also capable of swimming by aid of a tube for jet propulsion.

Many cephalopods were comparatively large animals. Ordovician strait-shelled (orthocone) cephalopods nearly seven feet in length have been found in the Cincinnati area that are. Most coiled forms were a few inches to perhaps 10 inches in diameter. In many examples the outer shell of calcium carbonate has been leached away, revealing the surface of the septa that divide the chambers. Those cephalopods that have straight-walled septa are called nautiloids whereas those that have convoluted or wiggly septal walls are called ammonoids. Nautiloids occur throughout the marine rocks in Ohio whereas ammonoids are known from Devonian and later rocks.


Several other distinct groups of invertebrates are found in Ohio ’s Paleozoic marine rocks. Most of them are rare but some are locally common.

Conularids (Phylum Conulariida) are conical-shaped, extinct marine invertebrates that have been placed, after much debate, in their own phylum. They are found in Silurian through Middle and Upper Paleozoic marine rocks in Ohio. The exoskeleton is phosphatic and characterized by numerous, closely spaced, horizontal ridges. Most specimens are small but some reached lengths of nearly 12 inches. They are most common in rocks of Mississippian age.

Rostrochonchs are a class of mollusks that until fairly recently were classified as clams, which they resemble. They range from Cambrian to Permian and are found in Ordovician, Devonian, Mississippian, and Pennsylvanian rocks.

Conodonts (Phylum Conodonta) were worm-like marine animals that are best known from their microscopic, phosphatic tooth-like jaw elements that occur in considerable abundance in many marine rocks. Conodonts range from Cambrian to Triassic. They have been important index fossils for correlating strata. Limestones and calcareous shales treated with acetic acid yield residues of conodont jaw elements, along with microscopic vertebrate remains. They are found in Ordovician through Pennsylvanian marine rocks in Ohio.

Graptolites (Phylum Hemichordata, Class Graptolithina) were colonial marine organisms that are most common in rocks that represent deposition in deep water. Most graptolite colonies were floating forms. Although some of these colonies may have reached nearly three feet in diameter, most graptolite fossils are represented by fragmentary, black-to-gray pieces that look like small saw blades or pencil marks on bedding planes. Although they range from Cambrian to Pennsylvanian, they are found in Ohio in Ordovician through Devonian rocks and are very rare, with a few exceptions.

Tentaculitoids are a poorly known group of invertebrates that had a calcium carbonate, conical shell, that is has horizontal ribs. These marine organisms ranged from Ordovician to Devonian and are common in the Columbus and Delaware Limestones of Middle Devonian age. Most specimens are an inch or so in length.

Trace fossils (ichnofossils) consist of burrows, trails, track borings, and footprints found in marine and nonmarine rocks. Although the remains of the animal that made the particular trace fossil may not be present, they preserve a record of inorganic activity. Trace fossils are found in all of Ohio ’s geologic systems and are very abundant in some rock units. Many burrows were probably made by soft-bodied marine worms that fed on organic detritus in the sediment. Bedding planes in some rocks have plant-like impressions that represent feeding activities by marine worms that probably swept the nearby sea floor with feeding structures. Tracks of trilobites have been identified. Non-marine rocks of Pennsylvanian age in eastern Ohio have yielded footprints of amphibians.

See Also


  1. Feldmann, R. M., 1996. "Introduction," in Fossils of Ohio, edited by R. M. Feldmann and Merrianne Hackathorn. Ohio Division of Geological Survey Bulletin 70, p. 1-25.