Hunting Close to Home

[erose]

Finally got out and enjoyed some of the great weather we have had so far. This is one of the road cuts within five minutes of my home where I often suggest folks go when visiting Austin. Exposes the Bee Cave Member of the Walnut Formation, Fredericksburg Group (Albian) Lower Cretaceous. As you will see it is a mix of limestone and clay and it is PACKED with fossils. Bivalves and gastropods are the majority, but corals, ammonites, crustaceans, annelids, shark/fish teeth and echinoids are also present. I have most of the inverts already in my collection so when I go I am looking for one or two rare echinoids, crabs and "better" specimens.  Here are photos of some of the urchins and other bits I decided to record. I left many behind but brought back one odd bivalve and a handful of echinoids including Loriolia rosana, Leptosalenia mexicana and a rather nice plump Coenholectypus planatus.  Haven't cleaned 'em up yet so stand by for an update in a day or two.

 

[erose]

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[erose]

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[erose]

[Manticocerasman]

wow, now that is a productive outcrop

I wish i had something like this in my area. 

nice echinoids

 

[Fossildude19]

Very cool, Erich!

Thanks for posting the report!

 

[Jeffrey P]

Really nice echinoids there. I'll try to make that a stop next time I visit Texas. Thanks for sharing. 

[LiamL]

Nice to have a great location so close!

[Max-fossils]

Very cool! Looks like a fun place to hunt at, I would love to go there one day. I love that oyster in the 2nd last picture; or is it a jewel box clam? 

[Innocentx]

4 hours ago, erose said:

PACKED with fossils.

I see what you mean from looking at your in situ photos, totally impressive. Nice finds for you, pretty echinoid.

[erose]

51 minutes ago, Max-fossils said:

Very cool! Looks like a fun place to hunt at, I would love to go there one day. I love that oyster in the 2nd last picture; or is it a jewel box clam? 

That is a rather nice specimen of the oyster Ceratostreon texanum. It was attached to another small oyster which I believe is Pycnodonte wardii or possibly Texigryphaea mucronata.  All are common at this local.

 

[Max-fossils]

36 minutes ago, erose said:

That is a rather nice specimen of the oyster Ceratostreon texanum. It was attached to another small oyster which I believe is Pycnodonte wardii or possibly Texigryphaea mucronata.  All are common at this local.

 

3 different oyster genera common in one location is quite special! What other bivalves can you find there?

[erose]

42 minutes ago, Max-fossils said:

3 different oyster genera common in one location is quite special! What other bivalves can you find there?

 Too many to easily list*. Probably at least another dozen. But except for maybe 2-4 other genera the majority are only preserved as steinkerns.  Same for the gastropods. Out of easily a dozen species present only two ever have shell preserved.  And ammonites from this local are almost always badly preserved and only as internal casts.

 

*of all our Texas Cretaceous formations the Walnut is known for diversity.

[Max-fossils]

1 hour ago, erose said:

 Too many to easily list*. Probably at least another dozen. But except for maybe 2-4 other genera the majority are only preserved as steinkerns.  Same for the gastropods. Out of easily a dozen species present only two ever have shell preserved.  And ammonites from this local are almost always badly preserved and only as internal casts.

 

*of all our Texas Cretaceous formations the Walnut is known for diversity.

Interesting. 

"But except for maybe 2-4 other genera the majority are only preserved as steinkerns"

Let me guess, are those genera limited to oysters, mussels and scallops (not clams)?

[erose]

6 minutes ago, Max-fossils said:

Interesting. 

"But except for maybe 2-4 other genera the majority are only preserved as steinkerns"

Let me guess, are those genera limited to oysters, mussels and scallops (not clams)?

Yes, exactly.  Neithea, a scallop-like genus, and Lima, a mussel I think, are the two others that show mineralized shell.  All the clams are internal mud casts.

 

The two gastropods that preserve shell are like Neritina.

[OwlEyes]

Really nice.  I hope to be able to visit Austin again soon

[Darktooth]

Nice finds!

[Max-fossils]

1 hour ago, erose said:

Yes, exactly.  Neithea, a scallop-like genus, and Lima, a mussel I think, are the two others that show mineralized shell.  All the clams are internal mud casts.

 

The two gastropods that preserve shell are like Neritina.

Aha, thought so. It's because oysters, mussels and scallops have a different shell structure (aragonitic if I recall) than the other bivalves (normal calcitic I think). Although the elements are the same, I think that the two compositions are structural isotopes of each other, meaning that the shells fossilize well or not in certain sediments. Clearly the Walnut Fm sediments preserve aragonitic shells much better than calcitic shells. It's the same with gastropods, some groups have aragonitic shells, and some calcitic, but I don't know which have which for those. 

Another example of a formation like this that springs to my mind is the Olhos de Agua Fm in the Algarve of Portugal (Miocene), where you'll find steinkerns of most shells but the scallops (which are very abundant) and the oysters are fossilized with the shell. 

[Al Dente]

 

 

28 minutes ago, Max-fossils said:

Aha, thought so. It's because oysters, mussels and scallops have a different shell structure (aragonitic if I recall) than the other bivalves (normal calcitic I think). Although the elements are the same, I think that the two compositions are structural isotopes of each other, meaning that the shells fossilize well or not in certain sediments. Clearly the Walnut Fm sediments preserve aragonitic shells much better than calcitic shells. It's the same with gastropods, some groups have aragonitic shells, and some calcitic, but I don't know which have which for those. 

Another example of a formation like this that springs to my mind is the Olhos de Agua Fm in the Algarve of Portugal (Miocene), where you'll find steinkerns of most shells but the scallops (which are very abundant) and the oysters are fossilized with the shell. 

You are almost correct but have the aragonite and calcite mixed up. Aragonitic preservation in the Cretaceous is rare but calcitic preservation is more common. Oysters and scallops (and Echinoids) are calcitic.

[Monica]

Beautiful urchins! 

 

Congrats on a great hunt! 

[Max-fossils]

16 hours ago, Al Dente said:

 

 

You are almost correct but have the aragonite and calcite mixed up. Aragonitic preservation in the Cretaceous is rare but calcitic preservation is more common. Oysters and scallops (and Echinoids) are calcitic.

Ah god, I always do that mistake 

Thanks for correcting me though  

[Nimravis]

Thanks for posting, that is s great road cut and the fossils are beautiful.

[masonboro37]

Great report, pictures and I love the echinoids!

 

Libby

[Al Tahan]

I’m currently in Austin till Sunday  .  How far is it from downtown Austin? I found some outcrops on a stream that I saw a partial ammonite. I’m gonna try to go to it today and find a decent one. I’d like to check that roadcut out! I just visited Leon in oaklahoma yesterday.

 

Wanna meet up perhaps?  @erose

[erose]

2 hours ago, Al Tahan said:

I’m currently in Austin till Sunday  .  How far is it from downtown Austin? I found some outcrops on a stream that I saw a partial ammonite. I’m gonna try to go to it today and find a decent one. I’d like to check that roadcut out! I just visited Leon in oaklahoma yesterday.

 

Wanna meet up perhaps?  @erose

The cut is only about 20 minutes from downtown depending on traffic. Send me a PM for directions and I may possibly have time to meet you there.