Mahantango/ Juniata 1 - Pearly Shells - Preserved Shell Material

[hitekmastr]

This is the first in a series of fossils from our Sept. 16 trip to an exposed 380 million year old Devonian site in the Mahantango Formation in Juniata County, PA (we'll do a trip report in the coming week or so).

Most of our samples raised ID questions that we hope some of our friends and colleagues can help answer. The first two samples are what I call "pearly shells."

This raises the issue of what can be learned from original shell material that is preserved?

These first two samples are shells that have quite a bit of the original shell (white color) attached.

Pearly Shell 1 - The best ID I can find online suggests that this is a brachiopod called Devonochonetes. The white shell is especially clear and well preserved.

Pearly Shell 2 - Squalicorax identifies this as Tropidoleptus and I included a link to a paper that I found on this species. The shell on this specimen is much more "pearly white" than the photographs suggest - the color is actually bright, pearly white and the lighting/camera angle distorted the colors a bit. The shell is shiny and gleaming with a pearlescent quality and much whiter than it looks in the pictures. This shell bears some faint markings that may indicate the original pattern. Sometimes (but rarely of course) the original patterns show up in the fossil, or the original unmineralized shell material is preserved, which makes fossil shell collecting especially interesting.

Here is some additional information on fossil shells that I recently found: There are two broad types of fossils - ones composed of the actual material the original creature was composed of, and ones where the original material has been replaced by some mineral after the original material completely decayed or dissolved (technically a "fossil" is the remains of an organism at least 10,000 years old. Some fossil shells are actual shells, even with the delicate aragonite material intact. Plain aragonite is chalky (think of the exterior of a clam shell). In a complex arrangement with calcite and protein (called nacre), aragonite takes on the mother-of-pearl appearance seen on the inside of mollusk shells. Aragonite is unstable over geologic time and inverts to calcite. [source: Various websites including: "Fossil Preservation" - http://www.csus.edu/indiv/k/kusnickj/Geology105/pres.html]

Edited September 19, 2012 by hitekmastr

[squalicorax]

My guess would be Tropidoleptus for the last one. I agree with Devonochonetes

[hitekmastr]

My guess would be Tropidoleptus for the last one. I agree with Devonochonetes

Thanks - everything fits for that ID - here is a paper on Tropidoleptus: http://archive.org/details/cbarchive_133941_thetropidoleptusfaunaatcananda9999

Edited September 18, 2012 by hitekmastr