Fossils of the Inland Basin



The Inland Basin region primarily contains the story of the evolution of marine and coastal plant life superimposed on the story of mountain-building events, associated sediments deposited in the inland ocean, and changes in relative sea level. The earliest fossils in the Inland Basin region are stromatolites, formed from cyanobacteria in the warm shallow Iapetus Ocean. Stromatolites are preserved in late Cambrian rocks, found to the southwest and southeast of the Adirondacks. Abundant marine fossils are found in Ordovician through Mississippian rocks formed in the inland ocean that existed through most of the Paleozoic. Pennsylvanian-age rocks preserve an excellent record of plant material.

Ordovician to Mississippian

Ordovician, Silurian and Devonian marine fossils of the Inland Basin, especially in New York, are world-famous for their quantity and quality. Ordovician-to-Devonian fossil assemblages are nearly always dominated numerically by brachiopods, and may also contain trilobites, sea lilies, corals, clams and other mollusks, and many other less common organisms. What is perhaps the most striking is the differences in fossil assemblages from different types of paleoenvironments. The type of environment determined the types of organisms that lived there, and thus the fossils that are preserved in the rock.

Clear, shallow marine environments, generally preserved as limestones, often have abundant corals (Figure 4.3), bryozoans (Figure 4.2) and sea lilies (Figure 4.4). Corals, bryozoans and sea lilies are all filter feeders, collecting fine particles from the water. These environments form in places and at times when there is little sediment settling in the water. Western New York and Pennsylvania, far from the Taconic and Acadian highlands where sediment was being eroded into the inland ocean, preserve rocks recording this environment. Also, throughout the Inland Basin, and relatively undeformed sections of the Appalachian/Piedmont, rocks formed in-between mountain-building events record clear, shallow marine conditions because there was no highland to erode sediment into the basin.

Figure 4.2: Bryozoan, mid-Devonian fenestellid (4 cm wide).


Many animals that are not easy to study, and in some cases even recognize without the aid of a microscope, have a long and exemplary fossil record. One such group is the bryozoans, colonial marine animals that have evolved a wide variety of skeletal shapes and textures. one of the more common Paleozoic varieties looks like fine mesh cloth with numerous tiny holes in which the individual animals in the colony lived. Small tentacles on the animals captured food particles from the water.


Corals have been important and common elements of clear, shallow marine waters since the Ordovician. Ordovician, Silurian and Devonian rocks of the Inland Basin region have numerous examples of reefs or other shallow environments in which colonial 'tabulate' corals are common. Even more abundant in these rocks is the solitary 'rugose' or horn coral. Both tabulate and rugose corals became extinct at the end of the Permian. Soon after, a new type of coral had appeared which are present today: the scleractinians. Though scleractinians look somewhat similar to rugose and tabulate corals, each group possesses distinctive features in the shape of the skeletal cup holding the individual animals.

Figure 4.3: Corals were abundant in clear, shallow marine Paleozoic seas. Above: tabulate coral, mid-Devonian Favosites, cross-section (11 cm wide); Below: rugose coral, mid-Devonian Heliophyllum halli (7 cm long).

Sea Lillies, Blastoids and Cystoids

Several groups of stemmed echinoderms appeared in the early Paleozoic, including crinoids, blastoffs and cystoids. All have in common 5-fold symmetry and a head (calyx) held off the sea floor by a stem, where ti collected organic particles from the water. The stems, which are the most often preserved part, are made of a series of stacked discs that look like Cheerios. Upon the death of the organism, the stems often fall apart and the individual disks are seen separated in the rock. Feathery arms radiated from the head of crinoids, looking something like a lily flower on a stem. Thus, crinoids are commonly called 'sea lilies', though they are not actually plants.

Figure 4.4: Stalked echinoderms, common in clear, shallow marine environments. Upper left: crinoid stems, mid-Devonian (largest is 4 cm long); Upper right: crinoid cup, mid-Devonian Dolatocrinus (4 cm diameter); Lower left: blastoid theca, mid-Devonian Devonoblastus (2 cm wide); Lower right: cystoid theca, Silurian Caryocrinites ornatus (5 cm diameter).

Muddy, well-oxygenated environments, generally preserved as gray shales, often have abundant brachiopods (Figure 4.5), trilobites (Figure 4.8), cephalopods and small clams (Figure 4.7). Small or flattened brachiopods that are not likely to sink into the mud, such as Mucrospirifer , are common in this environment.


Brachiopods look somewhat similar to clams you might find at the beach today. However, from the soft parts of modern brachiopods, we know that they are completely unrelated to the animals that make 'shells' that are common today (bivalves); brachiopods are rare today and are unlikely to wash up on shore. Brachiopods are the most common fossil in Paleozoic sedimentary rocks and are therefore very common in the Inland Basin region where these rocks are preserved.

Figure 4.5: Left: brachiopod, mid-Devonian Mucrospirifer (5 cm wide); Right: brachiopod, Ordovician Rafinesquina (2.5 cm wide).

Brachiopod or bivalve?

Brachiopods and bivalves both have a pair of hinged shells ('valves') to protect themselves while feeding. To tell the difference between a brachiopod and a bivalve, look for symmetry on the surface of the shells. Brachiopods are symmetrical across the shell, like your face. Bivalves are asymmetrical (Figure 4.6). The exception would be a deformed brachiopod, which might be found in the relatively more compressed rocks of the Appalachian/Piedmont. The size of the valves also helps to identify to which organism the shell belongs. Bivalve values are of equal size and mirror image shapes. Brachiopods bottom valves, however, are slightly bigger and often have a different shape.

Figure 4.7: Mollusks found in muddy, well-oxygenated environments. Upper left: bivalve, mid-Devonian Modiomorpha (5.5 cm); Upper middle: gastropod, mid-Devonian Platyceras (4 cm); Upper right: ammonoid, mid-Devonian Tornoceras (5.5 cm); Bottom: nautiloid cephalopod, early-mid-Devonian (20.5 cm long).


These marine organisms were bottom dwellers, present in a variety of environments and in paleozoic rocks in the Inland Basin, Appalachian/Piedmont, and a few locations in the Exotic Terrane. Trilobites had a well-defined head, often with large eyes that had multiple lenses usually visible with the naked eye. A primitive arthropod distantly related to horseshoe crabs, trilobites have been extinct since the end of the Paleozoic.

Figure 4.8: Trilobites were abundant in muddy, well-oxygenated environments in the early to mid Paleozoic. Upper left: Isotelus , Ordovician (20 cm long); Upper right: Dalmanites , Silurian (6 cm long); Lower left: Cryptolithus , Ordovician (1.5 cm); Lower right: Phacops , mid-Devonian (7 cm long).

Figure 4.9: graptolites, Ordovician Didymograptus (2 cm long).

Muddy, oxygen-poor marine environments are preserved as black shales, which often are completely lacking fossils, though plankton such as graptolites may be found (Figure 4.9). This environment forms in stagnant basins and areas where there is abundant organic material settling to the bottom; sometimes it is apparently associated with basin deepening due to down-warping crust during stages of rapid mountain building.


Graptolites are a group of extinct, puzzling planktonic organisms found in dark shales. No clear soft parts have been found, though they appear to be related to a minor group of modern colonial invertebrate organisms known as pterobranchs. They are relatively common fossils in the Ordovician rocks of the Inland Basin.

Figure 4.10: Glass sponge, upper Devonian Hydnoceras (16 cm long).

Silty to sandy marine environments, preserved as siltstone and sandstones, may contain abundant rugose corals, large thick-shelled brachiopods, sea lilies, and bryozoans and lesser amounts of many other organisms such as sponges (Figure 4.10). These organisms were robust filter feeders.


Technically known as poriferans, sponges come in a variety of shapes and body forms, and have been around at least since the Cambrian. Entire sponges are rarely preserved, but their tiny skeletal pieces, called 'spicules,' are common in sedimentary rocks. Glass sponges (with skeletons make of silica) are a particular group of sponges that existed from the Cambrian to the present. Though now largely found in deep water environments, they were sometimes part of shallow marine environments in the Paleozoic. The best-known glass sponge fossils are from New York Devonian sedimentary rocks of the Inland Basin.

Figure 4.11: Eurypterid, Silurian Eurypterus remipes (13 cm long).

Hypersaline marine environments are preserved as evaporite deposits. Because hypersaline environments have higher than normal levels of salt, most organisms cannot survive. Unusually tolerant organisms generally inhabit these environments. Eurypterids, or sea scorpions, were able to withstand the salty water and are well-preserved fossils in rocks of this environment (Figure 4.11).


Eurypterids are an extinct group of arthropods, the group that also includes horseshoe crabs. Though known by the name ‘sea scorpions’, they were not actually scorpions. Eurypterids were apparently one of the great predators of their time, reaching as much as 3 meters in length. The largest complete eurypterid in the world, about 1.3 meters long, was found in New York State and is on display at the Paleontological Research Institution in Ithaca, New York. Paleontologists believe that eurypterids lived in near shore environments, including salty, shallow sea environments like the Silurian inland ocean.

Intertidal and river environments are often preserved as coarse grained sandstones and conglomerates. Rocks preserving these environments commonly contain plant fossils. When land plants first evolved in the Silurian, they were non-vascular, relatively small plants with only very tiny, hair-like roots, if any. Gradually plants began to evolve and diversify. Vascular plants became more common, leading to taller plants and larger, more extensive root systems. By the Devonian, woody matter from vascular plants is commonly found in the fossil record of the Northeast. The Gilboa forest, in mid-Devonian shales of Scoharie Creek Forest in central New York, contains fossilized tree stumps and is the oldest preserved forest. More commonly, though, Devonian plant material is restricted to thin carbonized sticks.

Early forests were composed of quite different types of plants than today’s forests. For example, progymnosperms are a group of plants with spores rather than true seeds; those with two different forms of spores probably were the ancestors to gymnosperms. An important progymnosperm was Archaeopteris, a leafy tree of the late Devonian.


There exits a formal Latin name even for enigmatic fossil groups: Problematica. Most Problematica are late Precambrian or Paleozoic organisms, all of which have become extinct and so provide no modern organism that would enable us to clarify their anatomy and genetics. Two commonly seen Problematica fossils in the Inland Basin sedimentary rocks are hylithids (Figure 4.12), conical tubes with a shell covering; and tentaculitids (Figure 4.13), small, cone-shaped, ribbed shells. Both have been considered mollusks in the past, though hyolithids are believed by some to be a distinct phylum. 

Figure 4.12: Hyolithid, mid-Ordovician (2 cm).

Figure 4.13: Tentaculites, lower Devonian (2 cm).

Mississippian and Pennsylvanian

Pennsylvania preserves one of the best-known Pennsylvanian-age plant communities in the world (Figures 4.14-4.17). Large amounts of sediment were being rapidly eroded from the Acadian Mountains to the east, quickly burying plant material in coastal floodplain environments and creating oxygen-poor conditions that prevented the decomposition of organic matter. Plant and other non-marine fossils from the Mississippian and the early Permian are also present in Pennsylvania, but are far less extensive. Common Mississippian and Pennsylvanian plants include horsetails, ferns, seed ferns, and scale trees. These plants formed extensive forests in swampy areas along the edge of the inland ocean that led to the formation of coal deposits found in Pennsylvania and Maryland. Plants are not the only fossils recorded in the Pennsylvanian and Mississippian rocks of the Inland Basin, as the inland ocean still existed in much of the basin at this time. The plant fossils represent typical ferns, seed ferns, and horsetails, while the marine fossils represent typical Inland Basin brachiopods, cephalopods, clams, corals, and snails. 

Figure 4.14: Annularia leaf, Pennsylvanian (9.5 cm wide).

Pennsylvanian-age horsetails reached over 30 meters high. Their stems are known as Calamites and their leaves are called Annularia. The modern horsetail equivalents are known as sphenopsids (Figure 4.14).

Figure 4.15: Left: Plant root, Stigmaria , Pennsylvanian (14 cm long); Right: Lycopod, Lepidodendron, bark with leaf scars, Pennsylvanian (10.5 cm wide).

Scale trees (lycopsids) grew up to 45 meters high in Mississippian and Pennsylvanian forests. The roots of a scale tree are called Stigmaria. The entire tree is known as Lepidodendron. Themodern scale tree equivalents are known as lubmosses or ground pines (Figure 4.15).

Figure 4.16: Seed fern plant, Neuropteris, Pennsylvanian (5.5 cm long).

Figure 4.17: Seed fern plant, Alethopteris, Pennsylvanian (8 cm long).

Seed ferns (pteridosperms) lived from the Mississippian to the Jurassic. The leaves (Neuropteris) resemble ferns, but have seeds instead of spores (Figures 4.16 and 4.17).