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Mahantango Formation: Fossil Identification Page

Devonian Reef Fossils

Click on the type of fossil or scroll down to browse:

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Invertebrates

Click on the type of fossil to go to that fossil ID section.

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Trilobites

Trilobites are a group of extinct arthropods (similar to crustaceans) with a hard shell. They are common in Devonian rocks in the Mid-Atlantic. In the Mahantango Formation, almost all are found as fragments. Occasionally whole ones are found and are usually enrolled. Different layers of the Mahantango contain different types of trilobites.

The most commonly found trilobites are Eldredgeops, Greenops, and the large Dipleura trilobites.

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Dipleura dekayi Trilobites

Dipleura trilobites from the Phacopida order are the largest trilobites found in the Mahantango and also the most sought after. They are large carnivorous trilobites that patrolled the sea floor. They have large eyes and an elongated cephalon. A notable characteristic of Dipleura is the structure of their shell, the shell is not smooth but covered by tiny bumps. This bumpy pattern makes identifying Dipleura fragments fairly easy.

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Bivalve Fossils

Bivalves are two shelled filter feeders. Common examples are Oysters, Clams, and Mussels. Bivalves are some of the most successful animals on Earth. They first appear in the early Cambrian over 500 million years ago and are still abundant today!

An outdated name for these animals are Pelecypods, which is an outdated Class of Bivalves that is no longer used but still lingers in some amateur fossil hunting circles.

There are many beautiful examples of Bivalves preserved in the Mahantango reef.

Mahantango Bivalves shown on this page include: Orthonota, Grammysioidea,Grammysia . Click on any one of these genera or simply scroll down to browse.

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Orthonota Bivalve
Razor Clam

Orthonoa bivalves are, in my opinion, the most beautiful fossils in the Mahantango. They often have a shiny sheen to them, and just simply more visually striking than the rest of the fossils found in the formation. They range in size from around 1 to 3 inches across.

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Grammysioidea Bivalve

Grammysioidea Bivalves look like the Little Neck Clams you can purchase at the seafood counter. They are very flat fossils.

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Grammysia Bivalve

Grammysia Bivalves look just like Grammysioidea, but have a line running down the center of their shells.

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Bivalve Internal Molds

Internal molds of bivalves are fairly common in the Mahantango formation. Unfortunately, the internal molds lack the external shell detail and exact shape, making them difficult to identify.

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Brachiopod Fossils

Brachiopods look like little clams, however, they are actually very different.
Brachiopods are one of the most abundant fossils in the Paleozoic, and this shows at Mahantango, as they are the most abundant fossils in the formation.
Some Brachiopods are still alive today, but Bivalves (clams and such) far outnumber them.
There are many different types of Brachiopods in the Mahantango Formation. Many look nearly identical. Below are a few of the more common ones that can be found.

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Goniatite
Mucrospirifer Brachiopods

These Spirifered type brachiopods are some of the more common ones found in the Mahantango formation. Well preserved ones have long "wings" coming from them.

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Chonetes Brachiopods

These type of brachiopods have a shell that resembles a half oblate circle. They are a larger type of Strophomenida brachiopod found in the Mahantango formation.

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Cephalopod Fossils

A cephalopod is a type of mollusk that includes squid, cuttlefish, octopus and the nautilus. In the Devonian, there were a wide varaiety of cephalopods that do not exist today including many types of coiled and straight-shelled cephalopods (see illustration below). The Mahantango reef system was home to both types of cephalopods. Below are examples of Cephalopods from the Mahantango formation.

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Goniatite
Coiled Cephalopods

Goniatites are coiled cephalopods that resemble Ammonites. They are somewhat difficult to find in the Mahantango. They have also not been studied very much, so identifying one to the genus level is tricky at best. There appear to be two common genera in the literature: Tornoceras and Agoniatites

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Straight-Shelled Cephalopods

Straight-shelled Cephalopods have a long, straight external shell instead of a coiled shell. No straight-shelled cephalopods exist today. In the Mahantango formation, two groups are found, the straight-shelled Nautiloids that have the shell divided into chambers, and the straight-shelled Bactritids, with the shell having no divisions. Common Nautiloid genera include Spyroceras and Michelinoceras while common Bactritid genera include Bactrites.

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Coral Fossils

Coral (Cnidaria) fossils are common in most Devonian fossil bearing formations. The Mahantango is no exception. This ancient reef has many types of coral preserved. The most common type of coral is the tabulate Pleurodictyum, the branching Thamnopytchia (Trachypora) Corals, and the horn coral Heterophrentis.

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Pleurodictyum Tabulate Coral

Pleurodictyum is a type of tabulate coral. Tabulate corals are an extinct type of colonial coral. The conolies creat a group of hexagonal cells. In other formations, like the ones in Western New York, the Pleurodictyum fossils are almost always external molds (looking like golf balls cut in half). However, in the Mahantango, certain layers are full of internal molds of Pleurodictyum corals. Both inernal molds, and external casts are shown below.

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Crinoid Fossils

Crinoids, or sea lillys, look like a little flower. They have a root (holdfast) that attaches to an object on the sea floor, they have a stem (columnal), then an array of arms (crown) that filter feed.

Usually only small stem fragments, or individual disks from the stems are found.

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Gastropod Fossils

Gastropods are snails. Think escargot! Usually, in the Mahantango formation, the inside of the shell are preserved when makes an internal fossil mold. These internal molds look coiled, like in the image below.

There are many Genera of gastropods from the Mahantango. Ones in the sparse literature include Bembexia, Cyclonema, Glyptotomaria, Loxonema, Platyceras, and others.

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Recommended Trilobite and Fossil Hunting Books:

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Link to another Mahantango Site:

The following website: Views of the Mahantango is great blog about the Mahantango fossils and other Devonian formations. It is well written and goes into much more detail than this page.

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