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Concretion? Exogyra?


Grimboots

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These are two of three "fossils" I found on my trip to Big Brook in New Jersey last week, the stream runs through sediments that date back to the late Cretaceous.(the third one I will post later today hopefully).

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After having been really excited about this find, and then having done a decent amount of research I think I am safe in assuming that this is just concretion, or a psuedofossil if I am using those terms correctly. Even though this is the conclusion I have come to, this is my first fossil hunting trip and I would love some second opinions to either confirm my theories or offer some new insight. This one is about three inches long, and two inches thick.

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These two images are what I believe to be extinct clams, Exogyra. It doesn't seem like it is the shells, but just a fossil of the clam itself (if that makes sense). Again, I would love to hear what anyone in the community has to say.

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those are the lids to the oyster exogyra, there is also a deep dish valve on the bottom, googling exogyra should give you a good idea of what you have,

your concretion may have fossils in it

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Thank you for the confirmation Plax. As far as the oyster exogyra, while I have to admit that they are not my favorite type of fossil (nor are they complete), there is something humbling about holding and examining something that lived almost 66 million years - it is very exciting!

As for are concretion goes - I was reading a forum on here that suggested the best way to break open concretions (at least they type they were refering to) was by submerging in water - freezing, thawing out and repeating until it breaks open to check.

Does anyone think that seems appropriate?

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Both are found in quantity at Big Brook. One is a concretion, the other a bottom from an exogyra.

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here is a complete Exogyra from Tenn. A larger pix is in my Cretaceous gallery tn_gallery_2520_1339_113238.jpg

"Absence of evidence is not evidence of absence"_ Carl Sagen

No trees were killed in this posting......however, many innocent electrons were diverted from where they originally intended to go.

" I think, therefore I collect fossils." _ Me

"When you have eliminated the impossible, whatever remains, however improbable, must be the truth."__S. Holmes

"can't we all just get along?" Jack Nicholson from Mars Attacks

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Jpeva and Herb thank you again. And a Herb that looks like a really nice specimen you have there. This just adds the encouragement to search for a more complete one!

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Most of the Exogyras at Big Brook are coming out of two layers that occur high in the banks upstream but crop out closer to the stream as you follow the down dip east. Since digging in the banks is verboten you need to get there after big storms and early in the season for the best finds.

Another aspect of Exogyras that has not been mentioned is that they can be an index fossil throughout the Cretaceous. There are numerous species that change size and ornament over time. Some species overlap but generally if you have E. costata you are in a different time zone that if you have E. ponderosa.

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Erose, thank you very much for the hunting tips! I did see that there were many different types of exogyra...thanks for the additional information!

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At Big Brook you should just be finding E. costata. Other species will be found in older and younger strata around NJ. The other large bivalve at BB will be Pycnodonte convexa. It will have a smooth shell.

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Here's better shots of Pyncodont and Exogyra from the K of Tenn.post-2520-0-14215300-1400022242_thumb.jpgpost-2520-0-38456900-1400022255_thumb.jpgpost-2520-0-08624800-1400022267_thumb.jpgpost-2520-0-10467100-1400022278_thumb.jpg

"Absence of evidence is not evidence of absence"_ Carl Sagen

No trees were killed in this posting......however, many innocent electrons were diverted from where they originally intended to go.

" I think, therefore I collect fossils." _ Me

"When you have eliminated the impossible, whatever remains, however improbable, must be the truth."__S. Holmes

"can't we all just get along?" Jack Nicholson from Mars Attacks

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  • 3 weeks later...

I live fairly close to Big Brook and visit the site occasionally. Colts Neck nature preserve, about two or three miles east of Boundry Road, is a good and somewhat less collected location. Walk about 1/4 mile up from the entrance and you'll find the most concentrated outcropping of fossil I've seen; all the exoyra and belemnites a person would ever want and other fossils including sharks teeth.

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