Sunday, December 28, 2014

Cryptochiton stelleri

                              Dorsal view of a modern Cryptochiton stelleri, length 24.5 cm.

This post is the second part of the subject of chitons. The previous post dealt with what is a chiton and how it lives. This second part concerns the chiton
Cryptochiton stelleri Middendorff, 1847. This species, which is also known as the "Giant Pacific" chiton or the "Giant Gumboat" chiton, is the world's largest chiton and can grow to 36 cm in length and weigh over 2 kilograms. Today, it is a cool-water mollusks that is found in Japan, Kamchatka, and Alaska to northern California. It lives in the low intertidal zone, where it feeds mostly on algae that it scrapes off of rocks.

Ventral view of same specimen shown above.

Side view of same specimen shown above.

Cryptochiton stelleri also has a fossil record that extends as far back as, at least, 120,000 years. For over 35 years, I and my paleontology students have been visiting a 47,000 year-old, emergent marine-terrace locality near Goleta, west of Santa Barbara, southern California. During that time, we have found only three plates of C. stelleri.  To my knowledge, no one has reported before that this chiton occurs in these particular marine-terrace deposits. One of the best preserved valves (width 5.3 cm) is shown below.

The marine terrace near Goleta formed during the Wisconsin Glacial Stage, which was the fourth and last stage of the Pleistocene Ice Age. Relative to the present, sea level was lower during the Wisconsin Glacial Stage. The ocean temperature was cooler, and is the reason why C. stelleri occurred as far south as southern California during that time. 

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