Ostrea what???

[I_gotta_rock]

Okay, here's a stumped for the detail-oriented. The first picture is Ostrea falcata, one of the more common finds in the Mount Laurel Formation. It is curved like a hook, with ruffles radiating out from the hinge. The second one is O. panda. (same size, left out the penny.) It's more or less circular, with ruffles only at the edgesThe other two are from the same spot at the same site, on the same day, but are clearly not the same species. The third one is rather fan-shaped. the fourth has a depression dividing the raised center from the the ruffled edge. I can't find them in my DE or NJ field guides. Web search turned up nothing. Anyone recognize them?

 

[Plax]

oysters are highly variable. Often their shape is influenced by the substrate they're attached to in life. Last one looks like your falcata.

[I_gotta_rock]

Agreed, Plax.  As a matter of fact, I had chalked every one of my ruffly ostrea shells up to the same species for probably 12 years, as have others on this site, until this month. 

 

I was poking through Richards, et al's 1963 work and found exactly 20 members of the genus identified, many of which look exactly like what I had chalked up as deformed O. falcate when I found them myself. O. tectcosta has a totally flat center to the valve with the classic ruffles around it. (I haven't photographed my own specimen yet. It's in another batch.) Another two species look like all the ruffles have broken off (I threw a couple of those back in the sand because they were "broken!") but in fact they only formed rippled edges to begin with. Now I am smacking my head and pulling things out for comparisons, looking for other variations, looking up other Ostrea members, etc. 

 

I'm not a paleontologist, but I do love a good mystery, especially when it involves a site at which it's just been the same old finds every time for everyone for quite awhile.

[Plax]

I was much like you in '76 when I first went there. I also got the Richards books at about that time. They are still an excellent reference for the area. Are none of the other sites available to collecting? There were spoils just west of St Georges on the north bank that had nice crab concretions. Also the deep cut on the north bank was good. Guess you could check on google earth or ask the Corps if there are fresh dredgings anywhere else along the canal.

[I_gotta_rock]

The other sites have been so overgrown for so long that it would require some digging just to get down to the sand layer. I was out at another site poking around a few weeks ago. There were a few exogyras eroding down of the slope on one side, but no sign of a fossiliferous outcrop and walking on the plateau it's all grass over dirt laid down from earlier plants. Several of the other sites have been messed up due to construction of a bike trail recently, but I may go poking around again to see if I can find any signs. The Corps of Engineers is now depositing their dredge spoils son an inaccessible island in the Delaware River. Was hoping that I could get access through work (Delaware Nature Society), but so far no luck. Even if they let me be there, I lack a boat.  

 

However, with the occasional removal of a few feet of sand layer for road beds from this site, it is periodically refreshed so the weeds don't put down the dirt and fresh material is within reach. I found a really sweet spot when I was leading a trip out there for DNS in October, and I'm still working it slowly. That's where all my recent mysteries are coming from. Just about everything else form the last 12 years is easy to identify.

 

Speaking of identification, here's a throwback I just figured out from Richards this week: Ostrea tecticosta. Clearly, this one was not a swimmer. I know I've seen in the boxes somewhere. O. monmouthensis probably got thrown back in the sand in October. doh!

[Plax]

I was thinking a very young exogyra costata attached to a flat shell.

[I_gotta_rock]

15 minutes ago, Plax said:

I was thinking a very young exogyra costata attached to a flat shell.

Looks like, but the picture here is a dead ringer for Richards' photo and the type description notes the plications extend from a large, central attachment point, making it rather distinct from its cousins with very small attachment points. The attachment point on this, if you look closely, is convex. E. costata does have similar plications on the domed valve, but it would have to be really pathological to be concave where it is usually convex. It is odd, however, that Richards does not list this formation as a place they have been found. 

[Plax]

Exogyra start out like many oysters attached to bits. If this is an exogyra it would have been the big shell you see at maturity with a flat bit near the beginning of the first whorl. the shell we are looking at is upside down. Spat fall wherever and will take the shape of their substrate until they grow out of it. This could well be monmouthensis. Am just suggesting that it may be a juvie Exogyra because that is what it looks like to me. No judgement just opinion (a smiley face should go here!)

[I_gotta_rock]

Oh, and I finally found one O. mesenterica in the box. It is distinguished, among other things that match this specimen, by the center. "Central area of the shell marked only by yje concentric lines of growth." -Richards, et al, 1958