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Daughter found cool fossil on NorCal beach- what is it?


fay_fay

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My daughter found this really cool fossil yesterday on Bolinas Lagoon. Any clues as to what it could be? We find lots of sand dollar fossils around here but this looks more like a bone from a large animal. 

 

It's about two inches in length & an inch wide. Very heavy. 

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Welcome to TFF!

Sorry but this is not a fossil bone. It could be a borow cast (ichnofossil) or it could be a concretion (not fossil).

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FossilDudeCO

@Boesse or @Harry Pristis

 

are dugong rib sections known there?

 

If not I like @ynot idea of a burrow or cast of some sort.

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I'd stick to concretion, remeinds me of a septerian nodule without the cracks.

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Here's some more loot from our morning walk. These are all sand dollar fossils from what we can see with our untrained eye.

 

the second shot is one that I don't believe is a fossil but my boyfriend does. My hunch is fossils aren't generally that brittle (this broke easily when it dropped on the counter). I am sure this forum will clear this one up! 

 

I'll read up a bit more on ID'ing these things and see if we find anything else interesting during the next low tide. 

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They all look fossil to Me. Extant sand dollars tend to be white. and most fossils are brittle.

Look in the rock at the head of the beach and You may find some whole ones in matrix (rock).

 

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Here's another one we're not sure about. We found most of these up at "the point", which I think is the spot Ynot is referring to. 

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Sorry but the last one does not look like a fossil.

The head of the beach is the furthest away from the ocean, usually bordered by cliffs. The point is where the cliffs project out from the shore line.

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https://californiafossils.org/2016/07/03/bolinas-lagoon/ has links to papers on the local geology that could be helpful. The complete sand dollar and bigger pieces appear to be dendrastrids. They're probably examples of the (still extant) species Dendraster excentricushttps://inverts.wallawalla.edu/Echinodermata/Class Echinoidea/Dendraster_excentricus.html has photos and diagnostic features.

http://online.sfsu.edu/bholzman/courses/Fall02 projects/sandollar/sanddollar.html describes the evolutionary history.

 

 

The last rock does appear to have a linear inclusion in the chipped edge, might be something there. A large number of small cone or ring-shaped structures are also visible on that surface in the full size photo, not sure if they're embedded or encrusting organisms. Can you take more photos of that side?

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So, not sure what the tube-shaped doodad is. However, I can shed light on the sand dollars: those are Scutellaster oregonensis which have eroded out of the Merced Fm., exposed on the west shore of Bolinas Lagoon. The Santa Cruz Mudstone is immediately west, and is the unit that produces whale bone. There are giant dugongids (Dusisiren dewana) from the Santa Cruz Mudstone but their ribs are the size of small whale mandibles (~10 cm in diameter) and not very common. I suspect that thing is a concretion.

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