Chesapecten sp.

[PegCo98]

I have 2 fossils I recently obtained near Yorktown VA, within a mile of the York river near the Chesapeake Bay. I'm quite the novice fossil collector but I believe they're either C. madisonius or C. middlesexensis. They were in very close proximity to one another (a couple inches). I've attached 3 pictures of each shell. I've looked at a couple resources (like this image https://legacy-cdn-assets.answersingenesis.org/images/articles/2007/08/old-earth-time-scale/shell-evolution.jpg and this report http://pubs.usgs.gov/pp/0861/report.pdf), but I'm still unsure. I'd really appreciate if someone could explain how to distinguish some of these species!

Thanks!

Edited December 5, 2015 by PegCo98

[FossilDAWG]

MikeR would be the best person to explain the differences. I'll just comment that C. madisonius is found in Zone 2 of the Yorktown Formation, which is exposed in the area you collected these from. C. middlesexensis is an older species from the St. Marys formation. Based on the locality, I think C. madisonius is the most likely correct ID.

Don

[PegCo98]

Thanks. I'm leaning toward C. madisonius because it doesn't look like it has as many ribs as C. middlesexensis should... but I can't find a definite guide to number of ribs for anything besides C. jeffersonius

[FossilDAWG]

I don't think rib counts is sufficient to separate these species, as C. middlesexesnsis is quite variable in this regard and the range overlaps with C. madisonius. I think the distinction has more to do with internal features around the hinge.

Don

[mbeyer747]

That looks a lot like C. madisonius. Check out the back of the shell - see how the ribs are strongly expressed only in the upper edge of the valve? That's classic C. madisonius, the ribs in C. middlesexensis are strongly expressed throughout the interior. Apparently the terminal Chesapectens like C. madisonius and C. sepetenarius have heavy calcite deposits on the interior and that makes the ribs on the back of the shell less prominent throughout. It also makes the shells feel heavier and stronger.

Don is right, rib count is not the best way to ID Chesapectens, esp. C. middlesexensis. Some C. middlesexensis specimens I've collected have 12-14 ribs, some have 20+!. The best way to be sure is to identify the formation first, then collect directly from the exposure.

Also some shells have this transitional overlap thing going on. A long time ago I found some really nice blue and white striped Chesapectens on the beach at Cobham Wharf. The color is so crisp the shells look brand new! I'm inclined to think they are C. madisonius bc of heavy calcite deposition on the interior and "fuzzy" shell sculpture similarities. However, they also look a lot like C. Jeffersonius bc the ribs are more elevated, in C. madisonius ribs tend to be flatter. Since I didn't pull them directly from the cliffs, I feel like they will always remain a mystery!

Nice shells!