Friday, March 27, 2015

Giant oysters of Miocene age from California

With a height (from left to right) of 21 cm (8 in.) and a width (from top to bottom) of 11.5 cm (4.5 in.), the above-pictured oyster is a "giant" and large and heavy enough to be an effective door stop. It is a complete specimen of Crassostrea titan of late Miocene age (about 10 million years ago) from the Coalinga area, central California. Specimens about twice as big as the one shown above are commonly found in this area.

Unlike many oysters, Crassostrea has two almost equal-sized valves. This genus can reach sizes up to 60 cm in length. Crassostrea has a geologic range of Early Cretaceous (about 120 million years ago) to recent.

These are exterior views of two valves of another, but-same sized specimen, of C. titan. The one on the left is the left valve, and the other one is the right valve.
These are interior views of the same two valves shown above. The one on the left is the left valve, and it is 21 cm (8 inches) in height (from top to bottom). It has the ligamental area (a prominent groove) at its top, and that is where the ligament was located. The ligament consisted of a band of elastic material that helped open and close the valves. 

The other valve (right valve) has a projecting massive structure called the buttress, which fitted into the corresponding ligamental area. The adductor muscle scar is readily visible near the bottom edge of the right valve. A corresponding scar on the other valve is covered by rock matrix. The adductor muscle was the very strong muscle that helped keep the two valves tightly held together.

For most oysters, the left valve (always referred to as the "lower" one) rests horizontally on the ocean floor. Specimens of C. titan found in living position in the upper Miocene Santa Margarita Formation in the Coalinga area (see picture above), however, are found with both valves oriented vertically. Their specimens, which rest closely against each other, are massive and heavy, and it seems that they must have used gravity to help slightly open their valves. The rock hammer in the picture is 12.5 in. long. 

Wednesday, March 18, 2015

The Poundstone Clypeus ponti

The fossil Clypeus ponti is the so-called "poundstone." This term was used for this species because during Medieval times in western Europe, people would use these fossils as counterweights for measuring one pound of butter or of cheese (i.e., a single specimen = one pound). 

The species is of Middle Jurassic age (Bajocian Stage = 170 million years old). This specimen shown above is 7 cm (2.75 in.) in diameter, and the photograph shows its top (dorsal = aboral) side. The specimen is from France.

There are several very informative articles on the web that discuss the history of the usage of this fossil, but, so far, all the ones I have read refer incorrectly to this species as a "sea urchin." Clypeus ponti does belong to phylum Echinodermata, as do sea urchins, which are regular echinoidsClypeus ponti, however, belongs to the irregular echinoids. Unlike sea urchins, which live on the ocean floor and have long spine, irregular echinoids (like sand dollars, etc.) burrow into the sea floor and have very short spines.
This is the same specimen illustrated above but shows the bottom (ventral or oral) side. Notice the five-ray symmetry of its feeding grooves
Again, this is the same specimen, but the view is of the front end with its depressed central area. I thank the Natural History Museum of Los Angeles County for allowing me to photograph this specimen of C. ponti.  

Monday, March 9, 2015

Cambrian mystery trace fossil

I have been puzzled by this Early Cambrian trace fossil ever since I collected it about 35 years ago while on a field trip with my paleontology class. The trace consists of two parallel grooves that show up on the bottom of a bed of slightly metamorphosed sandy siltstone. What you see is the sediment that infilled the grooves (the sediment constitutes a cast). The trace is from the Inyo Mountains, California. I wonder what kind of animal could have made this trace. Did a single animal make it while crawling over the surface of the muddy sediment? Or, did two animals crawling alongside each other make the grooves?

I have tentatively identified this trace fossil (i.e., technically speaking, an ichnofossil) as Scolicia sp.

If anyone has a better identification, please let me know via the comments option on this blog site.