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Earth Science Themed Essays


Earth Science Themed Essays

Depositional Environments and Fossils of the Late Devonian Catskill Delta


By: David
Grade: 12
State: New York

About 390 million years ago, during the Middle Devonian period, a warm, shallow inland sea, known as the Catskill Sea, covered New York State. It extended from the present Hudson River westward across New York State and into Ohio and Indiana. It also extended southward into West Virginia and Kentucky. This sea was home to many invertebrates, especially coral, along with many jawless, shark-like fish. The corals in the Catskill Sea built many reefs in different areas. Along with corals there were many species of brachiopods and pelecypods, which were shelled organisms similar to modern clams and scallops. They survived and prospered in this warm climate because at this time this region of Proto-North America was located at 3 degrees south latitude (Isachsen et al. 1991).


During the Early Devonian, the microcontinent of Avalon collided with Proto-North America. This collision produced the Acadian Mountains, which rose in present-day New England and the Canadian Maritime Provinces. During and after the collision, erosion gradually stripped sediment from the Acadian Mountains. Streams and rivers carried huge quantities of sediment westward to the Catskill Sea. All this occurred throughout the Middle and Late Devonian. The figure below shows the general location of the major river systems in the Late Devonian.

Arrows show inferred Late Devonian river systems. Adapted and redrawn from Isachsen et al (1991).

We can tell where these rivers existed because of the patterns of alluvial sediment, which is the type of sediment deposited by rivers and streams. The coarser sediment was deposited closer to the mountains, while the finer sediment was deposited closer to the shoreline. This process built alluvial plains between the foothills of the Acadian Mountains and the Catskill Sea shoreline. As the rivers changed course, the plains became wider and overlapped each other. As the rivers met the sea, they made huge deltas that grew out into the Catskill Sea. These rivers started filling the eastern edge of the sea with coarser sediment, such as sand. Finer sediment of silt and mud was deposited farther out in the sea. With sediment filling in the eastern edge of the basin, the shoreline began moving westward, and the sea gradually retreated from New York State (Isachsen et al. 1991).

This deposition continued throughout the Middle and Late Devonian. By the end of the Devonian period, the sea's eastern shore was in western New York. This great deposit of sediment formed the layers of sedimentary rock seen today in New York State (Isachsen et al. 1991). It is in these layers that I found the fossils for this project.


The Gowanda Shale outcrops on the shore of Lake Erie.

Depositional Environments of  the Shale Layers Investigated in this Project
In this investigation, fossils were collected from the Cashaqua, Gowanda, and Ellicott Shales, all of which are Late Devonian deposits. The Cashaqua Shale outcrop is at the mouth of Eighteen Mile Creek, on the Lake Erie shore, ten miles south of Buffalo, New York. It is a soft, olive-gray colored shale that weathers to gray. The Cashaqua Shale has interbedded zones of flattened concretions, some of which coalesce to form irregular siltstone beds. There are also a few thin, interbedded black shale layers. They were deposited in the deeper regions beyond the shelf and slope of the Catskill Sea during the Late Devonian (Buehler 1963).

Outcrop of Cashaqua Shale near Buffalo, New York.

The Gowanda Shale is found at an outcrop on the shore of Lake Erie in the town of Brocton, New York. The thickness varies from 120 feet to 230 feet. It consists primarily of a light gray and dark gray shale. Some light gray arenaceous (quartzitic) shale and some gray siltstone bands that average about 3-inches thick also occur. Silty, calcareous concretions that range from 1-inch to 1-foot in diameter can be found as well. The Gowanda Shale was deposited on the slope of the sea floor of the Catskill Sea during the Late Devonian (Tesmer 1963).

The Ellicott Shale is exposed in Chautauqua Gorge of northwestern Chautauqua County, New York. It is a gray shale that averages about 150-feet thick. The Ellicott Shale was deposited on the shelf of the Catskill Sea during the Late Devonian (Tesmer 1963).


Fossils Collected for This Project

Phylum Brachiopoda
Brachiopods are simple two-shelled organisms similar to modern mussels. Brachiopods are one of only seven phyla that have a geologic record that spans the entire Phanerozoic eon, which began 540 million years ago. There are brachiopod fossils in rocks dating back to the earliest part of the Cambrian period. Brachiopods were particularly abundant during the Paleozoic era and within many layers are the most common fossils (Rowell and Grant 1987).

Brachiopods are solitary animals. They did not form colonies, although many clustered together. They secreted a shell that was made up of two halves that enclosed most of the animal. Each shell, or valve, had bilateral symmetry. All brachiopods are marine, but some of them have wide salinity tolerances (Rowell and Grant 1987).

Brachiopods were incapable of actively pursuing food. Their sessile, or motionless, way of life put great limits on their feeding ability. They were filter feeders, meaning they pumped water through the cavity between the valves and extracted the nutrients. Some species are able to absorb nutrients directly into their body tissues (Rowell and Grant 1987).

Most brachiopods live below the low tide level in seawater with normal salinity. However, during the Paleozoic era, a larger percentage of the phylum was probably able to colonize the tidal zone (Rowell and Grant 1987).

Mucrospirifer is one type of brachiopod found in the Cashaqua Shale and Ellicott Shale. It lived during the Middle and Late Devonian. Mucrospirifer is much wider than it is long. The hinge has extremities that extend into sharp points. The Mucrospirifer's valves are convex. Mucrospirifer also has a very deep sulcus. It also shows fine concentric growth lines, and radial ribs (Thompson 1982).

Traces of calcite can be found on these fossils. They fossilized with both valves intact. The hinge line on these fossils is very well preserved. Unfortunately, its location exposed it to erosion; therefore it does not have much detail. The growth lines, however, can be seen, as well as the ribs on the shells.

The Mucrospirifer sample was found with both halves intact. Unfortunately, the extremities known as "wings" were broken off, probably due to erosion. This fossil has a very well defined hinge line. The ribs are very well preserved in some areas but not in others. Traces of calcite can be found on this fossil as well.

The brachiopod fossil I found is known as Athyris. Athyris existed from the Early Devonian through the Mississippian period. It is an abundant fossil in the Cashaqua Shale. Usually it is found with both valves intact. Athyris is a round to oval shaped fossil. It has valves that are convex. It is a very well- preserved fossil, usually found in the shale above the level where waves erode. Traces of rust-colored iron oxide can be found on these fossils along with white and gray calcite. The growth lines on the fossil sample are very well preserved. The thickness of the lines shows how much it grew during a particular growth period. Athyris ranges in size from nickel-size to half-dollar size fossils. It has a very narrow hinge line.

Rhipidomella was a brachiopod that existed during the Early Silurian through the Late Permian (Thompson 1982). It was found in the Cashaqua Shale. The growth lines on the fossil were very well preserved. There are ribs that start at the center of the hinge and radiate out to the edges, but they are difficult to see because of their fineness. There is also a slight depression, known as a sulcus, that runs down the center of the ventral valve. The hinge line on the fossil is well preserved. The sample has the dorsal valve preserved and attached to the larger ventral valve and also has well preserved growth lines and ribs.

Class Pelecypoda
Pelecypods are in the phylum Mollusca, class Pelecypoda. Pelecypods, like brachiopods, are shelled organisms, having a left and right valve. Most pelecypods were sediment feeders. They would either burrow into the mud and eat organic material, or stay on the top of the mud and eat. Some were carnivorous, eating arthropods and small worms. Some of the active pelecypods would swim short distances by clapping their valves together. Except for those few active species, pelecypods were mainly a stationary organism (Pojeta 1987).

Pelecypod valves are bilaterally symmetrical to each other: in other words, the right and left valves are symmetrical. An individual valve, however, is not symmetrical like that of a brachiopod (Poleta 1987).

Ellicott Shale exposed in Chautauqua Gorge

The pelecypod Leptodesma was found in the Ellicott Shale of Chautauqua Gorge. It is a clam-like fossil that is obliquely oval to almost triangular in shape and lived from the Middle Silurian through the Late Permian. The rear of Leptodesma, called the "wing," is much wider than its smaller front, known as an auricle (Thompson 1982). The left valve is more convex than the right. The hinge line has a few large teeth.

The Leptodesma found in this study was just a single valve without the wing. The hinge line is poorly preserved. The teeth cannot be seen on it. The growth lines on the Leptodesma are preserved well enough to see how much it grew in a specific growth period.

Class Cephalopoda 
Cephalopods are in the phylum Mollusca, class Cephalopoda. These creatures were among the most intelligent of mollusks. They also were very agile and had a structure that was more complex than that of any other type of unsegmented invertebrate (Pojeta and Gordon 1987).

Paleozoic era cephalopods, like modern-day cephalopods, were carnivorous. Like modern shelled cephalopods, they have a problem with buoyancy. Some filled parts of their shell with mineral deposits such as calcite and used it for ballast. Others had poise adaptations, which are differences in the distribution of hard and soft parts of the animal. The head-foot of the animal was kept horizontal, and the shell of straight-shelled cephalopod stayed horizontal. They also used hydrostatic adaptations. Hydrostatic adaptations adjusted the buoyancy of the animal to prevent it from floating uncontrollably to the top. This was done by adding or subtracting liquid from the hollow chambers of the shell (Pojeta and Gordon 1987).

Most living cephalopods have an ink sac that emits a cloud of very dark fluid, allowing them to escape from predators. Fossilized ink sacs can be found as far back as the Paleozoic era (Pojeta and Gordon 1987).

Tornoceras Concentricum
Tornoceras Concentricum is one of two cephalopods collected in this project. It was found in the Gowanda Shale, on the Lake Erie shoreline, near Brocton, New York. It lived only in the Middle and Late Devonian. The fossil was completely pyritized, meaning the cavity left behind filled in with the mineral pyrite instead of mud. Tornoceras is a coiled, disc-shaped fossil. It has shell suture lines on it that are known as whorls, which are very well pronounced.

Orthoconic Nautiloid
An Orthoconic Nautiloid, another cephalopod, was found in the Gowanda Shale. It is a 5.5-centimeter-long cone-shaped fossil. The anterior (front) of the fossil is much wider than the posterior (end), as can be seen by the tapering to a point at the posterior end. The wide end contains the chamber where the soft body of the animal was located. The cone-shaped shell contains fossilized chambers where the organism stored its minerals for ballast. The nautiloid that was collected is completely pyritized, however; there is not enough detail to tell its genus and species.

Phylum Cnidaria, Stereolasma
Stereolasma is a coral. It is in the phylum Cnidaria, order Rugosa. It was a solitary coral, meaning it did not make reefs by growing together with other corals. Solitary rugose corals range in size from a couple of millimeters in diameter and in length to 14 centimeters in diameter and close to 1 meter in length. Rugose corals lived from the Middle Ordivician to the Late Permian. They fed by using tentacles to capture and sweep organisms into their mouths (Oliver and Coates 1987).

The sample collected was very well preserved and has well defined horizontal growth lines on it. The growth lines on the coral span its length from the calice (top) to the base. It also has vertical lines, called septal grooves, that run from the base to the calice (Oliver and Coates 1987).

Phylum Echinodermata, Crinoid Stem

Crinoid Stem

Crinoid stems are pieces of organisms that belong in the phylum Echinodermata. Echinoderms have a very long fossil record. There are more extinct fossil species than there are living species today. Fifteen extinct classes of fossil echinoderms have been identified. These are different from the five living classes of today. Since fossil echinoderms are considered so interesting, the fossil record, even though it is not complete, is almost as well studied as any other group of fossil invertebrates. Echinoderms can colonize many different environments ranging from intertidal zones to deep ocean trenches (Sprinkle 1987).

Crinoids are suspension feeders in the subphylum Crinozoa, class Crinoidea. They have a geologic record spanning the Middle Cambrian to the Holocene epoch. A crinoid is attached to the sea floor by a long stem-like structure. At the top of the stem is the thecae, where the mouth is located. It is also where food gathering appendages, known as brachioles, are attached. These brachioles trap microorganisms in the water and sweep them into the mouth (Sprinkle 1987). The crinoid stem sample was found in the Cashaqua Shale and was very well preserved. Small spine stubs can be see on the stem along with disc-shaped segments.

Class Gastropoda 
Gastropods live in both marine and terrestrial environments and belong to the phylum Mollusca. They are single-shelled organisms. The shell is usually coiled into a corkscrew helix, as was the sample found for this project. A gastropod feeds using its radula, which is similar to a tongue extending from its mouth. In primitive gastropods there were thousands of teeth on the tongue, which it used to scrape algae from sediment surfaces or to eat cells of other living animals (Peel 1987).

The fossil sample was found in the Cashaqua Shale. There is not enough detail to identify its genus and species. The gastropod's coiled shell is completely pyritized and has a thin line that travels down the center. This line may represent a shell suture.

Miscellaneous: Fossil Wood 
The fossil wood was found in the Gowanda Shale. It was most likely a piece of driftwood washed into the Catskill Sea. It became waterlogged and sank to the bottom. Once at the bottom, pressure from overlying sediment caused partial carbonization, and partial pyritization also occurred. There is not enough detail to identify the type of tree from which the wood came. This lack of detail makes it impossible to determine whether the specimen is fossilized bark or if it is from an internal section. When dry, the fossil is a dull gray, but when wet, the pyrite is easily seen.



Isachsen, Y.W. Geology of New York: A Simplified Account. Albany, New York: The State Education Department, Educational Leaflet Number 28. 1991.

Oliver Jr., W. A. and Coates, A. G. "Class cnidarian fossil invertebrates" Fossil Invertebrates. Palo Alto, California: Blackwell Scientific Publications, 1987.

Peel, J. S. "Class gastropoda in fossil invertebrates" Fossil Invertebrates. Palo Alto, California: Blackwell Scientific Publications, 1987.

Pojeta Jr., J. "Class pelecypoda in fossil invertebrates" Fossil Invertebrates. Palo Alto, California: Blackwell Scientific Publications, 1987.

Pojeta Jr., J. and Gordon Jr., M. "Class cephalopoda in fossil invertebrates." Fossil Invertebrates. Palo Alto, California: Blackwell Scientific Publications, 1987.

Rowell A. J. and Grant R. E. "Phylum brachiopoda in fossil invertebrates." Fossil Invertebrates. Palo Alto, California: Blackwell Scientific Publications, 1987.

Sprinkle, J. "Phylum echinodermata in fossil invertebrates." Fossil Invertebrates. Palo Alto, California: Blackwell Scientific Publications, 1987.

Tesmer, I. Geology of Chautaqua County, New York. Stratigraphy and Paleontology. New York State Museum and Science Service, Bulletin Number 391. Albany, New York: State Education Department, 1963.

Thompson, I. National Audubon Society Field Guide to North American Fossils. New York: Alfred A. Knopf, 1982.

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