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Fossil Identification Mahantango

Mahantango Formation: Fossil Identification Page

Devonian Reef Fossils

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


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


Dipleura Trilobite from the Mahantango Formation - Found and prepped by another fossil hunter.

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.

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.

This is a large enrolled Dipleura triobite from the Mahantango. It's 3 1/8" across (around 7" if not enrolled).
It was found on This Fossil Hunting Trip.

This is a large enrolled Dipleura triobite as it was found. It looks like it tumbled down the talus slope.

This is a smaller enrolled Dipleura triobite from the Mahantango.

This is a complete prone Dipleura triobite from the Mahantango. It was found and prepped by another Fossil Hunter.

This is how Dipleura are usually found, as partials. This is the Pygidium (tail) and part of the Thorax.

Fragments are often found. This is a large Dipleura Cephalon (head).

Fragments are often found. This is a large Dipleura Pygidium (tail) with some Thorax segments.

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An outdated name for these animals are Pelecypods, which is an outdated Class of Bivalves that is no longer used but still lingers in some amateur fossil hunting circles.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

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.

This Razor Clam (Orthonota bivalve) fossil has both sides still attached. This fossil specimen is also strikingly contrasts from the matrix.

This Orthonota bivalve fossil is 3D with both sides attached in a life like position.

This Orthonota bivalve fossil has both sides still attached.

This Orthonota bivalve fossil also has both sides still attached.

This Orthonota bivalve fossil has the top side visible, and a tiny pieces of the other half.

Grammysioidea Bivalve

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

This is a Grammysioidea bivalve fossil from the Mahantango Formation.

This is a Grammysioidea bivalve fossil from the Mahantango Formation.

Grammysia Bivalve

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

This is a Grammysia bivalve fossil from the Mahantango Formation. Notice the line running down the shell. This is how one can tell the difference between Grammysia and Grammysioidea.

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.

Internal mold of a bivalve fossil, it's proably a Modiomorpha bivalve. Both halves are present.

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.

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.

Mucrospirifer Brachiopod fossil from the Mahantango Formation

Mucrospirifer Brachiopod fossil from the Mahantango Formation

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.

Chonete Brachiopod fossil from the Mahantango Formation.

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.

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

Coiled Cephalopod from the Mahantango Formation partially hidden in the martrix - Gonaitite.

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.

Bactrites sp. Straight-Shelled Cephalopod fossil from the Mahantango Formation.

Bactrites sp. Straight-Shelled Cephalopod fossil from the Mahantango Formation. This one is about 3 inches across.

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.

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.

Pleurodictyum tabulate coral from the Mahantango formation. This is an external cast of the fossil. One can see the numerous hexagonal cells that make up the colony. This one is pretty weathered as it was found at the bottom of a rock pile.

Pleurodictyum tabulate coral from the Mahantango formation. This is an internal mold of the fossil.

Pleurodictyum tabulate coral from the Mahantango formation. This is another internal mold of the fossil.

Pleurodictyum tabulate coral from the Mahantango formation. This is the backside of an internal mold of a larger coral fossil. It looks like the Pleurodictyum was attached to a large bivalve at one point. Three is also a smaller Orthonota bivalve fossil to the left of the coral.

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.

This is a small section of a crinoid stem. They are usually less than an inch in size.

This is an individual disk of a crinoid stem.

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.

This gastropod internal mold might be cyclonema.

Another internal mold of a Gastropod fossil from the Mahantango formation.

Another internal mold of a Gastropod fossil from the Mahantango formation.

Recommended Books for Devonian Fossils:

101 American Fossil Sites You've Gotta See
By Albert B Dickas, 2018
This is a great updated fossil sites book with at least one fossil site in each state. Each site is broken into 2 pages. One has detailed information, such as directions, GPS coordinates, formation information, etc... The other is dedicated to images of the site and the fossils found there. It also gives information on fossil 'viewing' sites such as dinosaur trackways, museums, and active excavations.

This book is great for both beginning and expert fossil collectors. Beginners will find fossil hunting much easier with this book and experts will find it to be a great reference.
Plus, my fossil photos are peppered throughout this book!

Here is a link to my Review of the book.

Fossil Collecting in the Mid-Atlantic States: With Localities, Collecting Tips, and Illustrations of More than 450 Fossil Specimens
This book is a must for West Virginia! It's also a classic!
Out of the 45 fossil collecting sites, 22 are from West Virginia, and another 10 or so are just across the border! Although the book is getting old, many of the sites are roadcuts and are still accessable. Many of these sites are roadcuts that expose the Mahantango Formation.

What makes this book a classic is Jasper Burns incredible sketches of the locations and the fossils found at each location. It is a very descriptive and useful guide book. Even after all these years, I still find myself referencing it!

Fossil Collecting Locations in this book are from Virginia, West Virginia, Pennsylvania, Maryland, and Delaware.

Trilobites: Common Trilobites of North America (A NatureGuide Book)
by Jasper Burns, 2000

I love Jasper Burns. His Fossil Collecting in the Mid-Atlantic book is still one of my favorite fossil books. His drawings are spectacular and the books are well laid out. This book serves as a field guide and identification guide to North American trilobites. If you fossil hunt for trilobites in North America, you should have this book!

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.

Trilobite Fossils:

Trilobite fossils are some of the most beautiful and collectible fossils in the world! There are countless species and countless colors of trilobites. They make beautiful display pieces and conversation pieces. Common ones make very affordable for gifts to fossil and paleontology enthusiasts. Fossil Era has a huge selection of top quality trilobites from many states and many countries. It's fun just to browse through the inventory and look at all the different types!

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