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A Journey into the Devonian Mahantango Formation in Search of Dipleura Trilobites

Central PA Fossil Hunting Trip Report
April 2014

view of the fossil site containing the mahantango formation in central PA
This is a panoramic view of the fossil site in central PA containing the Mahantango formation.

A Hunt for Dipleura Trilobites in the Mahantango of Central PA

Our first fossil hunting trip of the year was to an outcropping that exposes one of the largest Devonian formations in Pennsylvania, the Mahantango formation. To learn about the Mahantango formation, go to the Mahantango page. The Mahantango covers about half of Pennslyvania, but it is usually deeply buried under younger sediments. However, in central pa it is often exposed in roadcuts and quarries. We took a trip with the DVPS to one of these places where the Mahantango is exposed.

One interesting aspect of this particular Mahantango exposure is the layers show evidence of a more silty bottom; ideal habitat for the large Dipleura trilobites. So instead of the usual Eldredgeops (Phacops) and Greenops trilobites, this side has Dipleura as the most common trilobite. Unfortunately, the Mahantango shale and siltsones are very fissile, meaning the rock tends to shatter into many small shards. Extracting whole specimens is very difficult.

Hoping the incredibly harsh winter and ensusing freeze thaw cycle eroded specimens out, we started the day surface hunting. We found a mangled pygydium and a few thorax segments of a large Dipleura, and some broken up brachiopods. After having little success, we hunkered down and tried to pry out large chunks of the Mahantango, which, as stated before, is very difficult. After doing this for a couple hours and still not finding much, we decided to try and surface hunt again. We managed to find a section with lots of Dipleura bits and pieces. Deciding this was a good spot, we chiseled through some of the larger rocks in search of a whole fossil specimen. In the end I managed to find a really beat up and small enrolled Dipleura. However, it was complete (although part of the head is chipped off). We also ended up with an assortment of brachiopods and pelecypods, including these neat looking pelecypods called Orthonota that look like modern day razor clams.

Overall it was great to go out and collect with the nice fossil hunters that make up the DVPS!

Below are the fossils found from the trip:

A Mucrospirifer Brachipod fossil
Here is a common Mucrospirifer brachiopod. These appear to be quite abundant in the Mahantango.

An orthonota pelecypod fossil
This is one of the more common pelecypods found here. It's an Orthonota pelecypod. Often, both halves of the shell can be found.

A Dipleura trilobite head
Dipleura trilobites are fairly common in this section of Mahantango. Unfortunately, there are almost always pieces, like this large Dipleura head. Notice the Tubercles all over the surface, these are little bumps that are common inside the exoskeleton of larger Dipleura

An enrolled Dipleura trilobite
This is a smaller enrolled Dipleura trilobite fossil. Unfortunately, the cephalon is damaged.

A Dipleura trilobite pygydium
This is the pygidium (tail) of a large Dipleura. a few thorax segments are also visible. Unfortunately, it's kind of banged up.

A Dipleura trilobite head
Here is another nice Mucrospirifer brachiopod.

A Dipleura trilobite head
Here, Amy is chiseling away at rock, looking for fossils