“An Ancient Skate of Monterey Bay”

 

This is WT744, the *upper jaw*, or palatoquadrate, from the skull of a 2-3 million year old skate, related to stingrays, from the ancient Monterey Bay. Its scientific name is Raja sp cf. R. binoculata. It was discovered in one of the now well-known and most favorite layers, or shell beds, in the upper part of the Purisima Formation of Santa Cruz, CA. The other creatures that this skate was preserved alongside of include Princess Slipper snails (Grandicrepidula princeps), Arc Clams (Anadara trilineata), Giant Barnacles (Balanus aff. B. proxinubilus), and many others, and all represent a nearshore environment for this creature. In point of fact, when Chaco and Yoshi and I are out in the water on the modern reef here we can still disturb living skates resting on the bottom; they then gently flap their flippers up into the floating kelp fronds and it’s always an exciting meeting for all of us.

The rock that it came from had what we call “rip clasts” in it: chunks of angular mudstone that were broken off, or ripped, from the layer of mudstone below it during an erosional event, or regression, representing a period where there was a lowering of sea level. We have now mapped several dozen of these erosional events, called unconformities, preserved in our study area. All together they represent the dynamic geological history of our bay, itself mirroring the dynamic changes happening Globally over deep time and thus influencing and creating a dynamic biological evolutionary history as a result.

This upper jaw is not actually bone, but rather the cartilage precursor of bone found in all sharks, skates, and rays. Being soft rather than hard like bone, cartilage is less likely to be preserved in the fossil record and more rare. In fact, body cartilage of sharks, skates, and rays is even rarer and not found here at all as far as I know; presumably because mouth cartilage had to be slightly more calcified and hard structurally to do its job, and thus preserved more commonly. You will notice in the beginning of the video the hexagonal pattern of the fossilized jaw tissue that is a key feature of fossilized cartilage.

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