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Thecosmilia Trichitoma

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I found this mystery fossil eroding out of the Bay Point Formation at a site near Point Loma, San Diego, California in a very densely packed shell layer. I thought it was just an interesting shell fragment when I picked it up, but realized it was something else during prep. I am absolutely clueless to what it is. Any ideas? (Sorry for the bad lighting, I couldn’t get it to be any better.)

Top view

IMG_0462.thumb.jpeg.45576b19d70393d278bb86715558044c.jpeg

 

Bottom View

IMG_0463.thumb.jpeg.7ba658340ddbddd4e811e1d5987c5020.jpeg

 

Side ViewIMG_0465.thumb.jpeg.ae1512ac646ad8f19c411cf76737940a.jpeg

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Very cool. It is a segment of a chiton. They still exist today.

https://en.m.wikipedia.org/wiki/Chiton

5DAF5322-789E-4D9E-B0E5-640F55ADC032.jpeg

Edited by DPS Ammonite
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That is indeed very cool!  We don’t get to see chitons on here very often and they are one of my favorite fossils. Unfortunately, I am mostly familiar with Paleozoic forms and you have one that is much more recent. But I took a look through the Treatise and I believe you have some species of the genus Ischnochiton. There are well over 100 species of this genus and numerous are found along the west coast of the US. It also makes species ID a bit more difficult with a single isolated head valve which is what you have. @Coco May like to see this as well. Thanks for showing this!

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Thank you for the help with this! I knew about chitons (we have them everywhere in the tide pools here) but didn’t know about the little “teeth” that hold their plates together. Fascinating!

It is better to keep your mouth closed and let people think you are a fool than to open it and remove all doubt

 

-Mark Twain

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I am very aware of the locality in which your chiton plate came from as I have collected it in past.  It is different from other Bay Point sites as it has a tide pool fauna.  I posted the pic below before, but it seems relative to post once more.  It is a composite of chiton plates from that locality that I purchased a couple of years ago from the Frank Peska collection.  I have not researched it well enough however to determine its accuracy.

 

Mike

 

983267236_MR3567-456.thumb.jpg.51960d447f98f1dbbc5c6f70585790ec.jpg

"A problem solved is a problem caused"--Karl Pilkington

"I was dead for millions of years before I was born and it never inconvenienced me a bit." -- Mark Twain

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2 hours ago, Thecosmilia Trichitoma said:

but didn’t know about the little “teeth” that hold their plates together. Fascinating!

Yes, those are called the insertion plates and the number of slits in them is one (of many) key diagnostic features that are used to separate and identify species. Yours has about 11 and between that, the general shape of the valve and the ornamentation on it, with the proper reference material or comparative collection (I have neither for this age/formation), you could probably ID this with some certainty. Obviously, having an associated intermediate and/or tail valve would also be very helpful. It is a common problem in working with fossil chitons that if you have multiple species present you get a bunch of disassociated valves and matching up which heads go with which tails, etc presents a challenge. 

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Hi,

 

15 hours ago, ClearLake said:

 @CocoMay like to see this as well. Thanks for showing this!

Oh yes ! :wub:

 

I think I remember that only the plates of the extremities, or the posterior plate alone has "teeth".

 

Coco

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

Ma bibliothèque PDF 1 (Poissons et sélaciens récents & fossiles) : ici
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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