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Need ID help for Purisima fossils


sagacious

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I found these items years ago in Capitola, CA, where the Purisima Formation is exposed in seaside cliffs. They were found in the gravel 'float' at the base of a cliff. Both objects measure approximately 20x15x7mm, and are nearly identical in form. The concave side retains traces of the sediment typical of the Purisima at that location, and the convex side shows rounded ridges approximately perpendicular to the object's major axis. They have a somewhat similar color and density to bone fragments also found that location.

 

It may take someone who has experience with the Purisima, or a trip to UCMP and/or SCMNH to get these objects identified, but I'm hopeful that someone here can shed some light on what they are.

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The bottoms look like they could be echinoids, but then the tops are unlike anything I have seen before.

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Excellent find! You're one of a half dozen folks who have collected these and thought "is this a fossil?"

 

Frank Perry of the Santa Cruz Museum puzzled over these things; between he and I, we've collected dozens of these. He heard different things from different folks; plant experts said they were a kind of nut, invertebrate paleontologists identified them as a crayfish gastrolith. However, crayfish gastroliths are calcium carbonate, plants are woody/carbonaceous, and these are all preserved black and shiny (like vertebrate bones/teeth) and the hole in the middle is a hollow cavity under an enameloid tipped cap/cusp. Paleoichthyologists Doug Long and/or Bruce Welton identified these as dermal bucklers from some kind of skate - Raja clavata has similar bucklers (giant denticles) but they have big hooks on them. There'd be a few pairs of these, and a bunch down the midline.

 

If you want to read more about them, here's the paper I wrote on the Purisima vertebrate assemblage from Halfmoon Bay: http://www.palarch.nl/wp-content/Boessenecker_RW_2011_A_New_Marine_Vertebrate_Assemblage_from_the_Late_Neogene_Purisima_Formation_in_Central_California_PJVP_8_4.pdf

Raja.jpg

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Right on Boesse! Thanks very much for the insights and background!

 

I read your paper above while searching for an ID for these objects -- thank you for providing the link. These objects first reminded me somewhat of the bleached-white, hemispherical crayfish gastroliths found anywhere the large Signal Crayfish Pacifastacus leniusculus lives. However, as you noted, the softish calcium carbonate of crayfish gastroliths weathers and erodes fairly quickly, and these have the appearance and 'feel' of a more durable bonelike tissue -- and specifically, the appearance of a dermal structure, and not an interior structure. And yup, the thin blackish enameloid on some of these objects is also a tip-off to some sort of external structure.

 

I turned my focus to some sort of dermal denticle or buckler, as I later found a few of these that had grown together, and some that showed evidence of having been crowded during growth. Ray or skate bucklers seemed like a good bet, but I haven't found any of these objects with a sharp, or even rough, cusp. I couldn't find a match with any known Purisima elasmobranch (that I'm aware of).

 

So, is it known which skate has smooth bucklers like these, or does the Long/Welton ID simply assign these as bucklers of an unknown skate? Could they possibly be from a guitarfish?

 

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6 minutes ago, doushantuo said:

Crustacean gastroliths & predation:

 

BIR_2012_3_Mortimer_etal.pdf

NB:and good illustration of the gastroliths themselves

An articel with perhaps taphonomical implications

 

Thanks very much, doushantou! And look, great photos of gastroliths from Signal Crawfish.

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22 minutes ago, doushantuo said:

Right on! Now this is very interesting. Something from the Pacific would be spot-on. This is a guide to skates of the North Sea, using the distinctiveness of dermal bucklers to diagnose identification.

 

The rub here, as it were, is that skates lack the barbed, poisonous tail stinger that stingrays have, and so the dermal bucklers of skates tend to be more numerous and viciously sharp -- as seen in the ID guide above. These particular bucklers from the Purisima are smoothly ridged and not viciously sharp at all. This fact should narrow down the suspects and help diagnose identification.

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No identification key - to my knowledge - yet exists for equivalent elements from the eastern North Pacific. I tentatively lumped all of the skate material into Raja sp. cf. R. binoculata; look long enough and you'll find mandibular & palatoquadrate cartilages as well, preserved as if they were bones.

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1 hour ago, Boesse said:

No identification key - to my knowledge - yet exists for equivalent elements from the eastern North Pacific. I tentatively lumped all of the skate material into Raja sp. cf. R. binoculata; look long enough and you'll find mandibular & palatoquadrate cartilages as well, preserved as if they were bones.

 

Thanks very much, that's about what I figured, in terms of a specific identification. I've found tooth-plates from two species of Purisima rays, but I suspect that I'm overlooking the mineralized cartilage.

 

I've examined R binoculata twice. The first was a very large, fresh-dead mature specimen that washed ashore near Netarts Bay, OR.  The second was a likely immature 40lb individual caught as bycatch near Halfmoon Bay, CA. All the dermal bucklers -- all that I saw -- on both animals were as sharp as thorns. Since these fossil smooth-topped bucklers are evidently unworn, the Purisima may be hiding a large unknown elasmobranch.

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I would not have guessed batoid dermal denticles but then at least some on the ventral surface of the body would be expected to be smoother either worn by abrasion with the seafloor (and its many obstacles) or perhaps smooth in their original shape like natural skids.

 

Agreeing with Bobby, I have also not seen any publication formal nor informal identifying skate denticles of the Pacific.  The North Sea pub is therefore something every collector/researcher should have if only to let someone say "Mine doesn't look like any of these."  Not many pubs on denticles in any case.  

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