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MSirmon

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Found an interesting rock just east of Alpine TX. When cleaning it noticed an overlapping triangle pattern on the bottom. The area is cretaceous but am not sure of the formation. Any ideas?

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Not sure about the impressions but am thinking the item is a Holaster echinoid. Probably a Holaster Simplex? Here are some reference photos 

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Echinoid for sure. It shows the typical by cacite replaced shell. The "impressions of crystals" are outbreaks of the crystallised calcit shell material. That is very typical for echinoids all over the world.

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Thank you. I was wondering about the "rock" layer on the outside and if I should try to clean off but now know to leave it alone.

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The pattern of triangular sections of released calcite prisms is typical of how echinoid tests weather.  I see this most often in the Texas desert.  Look at a geological map of the area.  Holaster simplex will be restricted to areas mapped as Washita undivided or Boracho fm, Levinson member.  Based on condition, I can't really make out enough diagnostic detail to key it out from photos.   

Grüße,

Daniel A. Wöhr aus Südtexas

"To the motivated go the spoils."

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Hi,

 

Perhaps we can see ambulacra (um ?) here

 

11uvygy.jpg

 

Coco

----------------------
OUTIL POUR MESURER VOS FOSSILES : ici

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Ma bibliothèque PDF 2 (Animaux vivants - sans poissons ni sélaciens) : ici
Mâchoires sélaciennes récentes : ici
Hétérodontiques et sélaciens : ici
Oeufs sélaciens récents : ici
Otolithes de poissons récents ! ici

Un Greg...

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I agree with echinoid;

and with regard to the “triangle pattern”, when rhombohedral calcite crystals grow interpenetrating and overlapping each other (geminated), they might resemble a tetrahedral-like structure with triangular facets

 

https://www.google.it/search?q=calcite+geminata&rlz=1C1AKJH_enIT733IT734&espv=2&tbm=isch&tbo=u&source=univ&sa=X&ved=0ahUKEwiyvqTro4DTAhVBoBQKHXnLCiEQsAQIGQ&biw=1280&bih=863#tbm=isch&q=rombohedral+calcite+fointambleu&*

 

ciao

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