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. 

Chitons, bioerosion

This post consists of two parts. The first concerns what a chiton is and how it obtains its food. The second part (i.e., the next blog) concerns the rare presence of the world's largest chiton in a Pleistocene marine-terrace deposit in southern California. 

Chitons are animals that belong to phylum Mollusca. This phylum includes gastropods, bivalves, and cephalopods (nautiloids, squids, and octopus). Chitons live on hard surfaces and cling tightly to them (even on golf balls--see above--that find their way into the intertidal zone of the ocean), but the animal is capable of a very slow creeping type of locomotion. 

The morphology of a chiton is shown above. Notice that the shell (exoskeleton) part consists of eight valves (plates). The mouth has a scraping device called a radula, which consists of small teeth with hardened caps for scraping its food (algae) off of hard surfaces.

Top side of limestone (19 cm long) found in intertidal waters. 

The above picture and the one following it show how effective chitons can scrape a rock-hard surface.  The pictures show two sides of the same chunk of rock. The unscraped side never had a chiton living on it, whereas the scraped side shows how effective the grazing of a chiton can be. This is an excellent example of bioerosion. This chunk of rock came from Palau, in the South Pacific, and the scrape marks were made by another genus of chiton (chitons?), other than Cryptochiton.

Chiton-scraped, bottom side of rock shown above.

Grandicrepidula

                       length 8 cm, width 5 cm, Pleistocene, Los Angeles, southern California
The extinct gastropod Grandicrepidula princeps Conrad, 1856 is a common fossil in relatively young shallow-marine rocks of California. This gastropod belongs to the family Calyptraeidae, which is commonly referred to as the "slipper shells" because of the distinctive wide shelly shelf inside the shell. This shelf serves as a support for the soft digestive gland.

interior of same specimen shown above

The shell also has a noticeable muscle scar. Grandicrepidula cannot move much in search for food; hence, it modified its manner of feeding to become a filter feeder. In that sense, it resembles the filter-feeding manner of oysters. Food is trapped by mucus in the mantle (a layer of tissue that covers the soft parts) of the gastropod.

                       length 8 cm, height 5 cm, late Pliocene, Simi Valley, southern California
This unusual shape of a fossil is an internal cast of C. princeps. Complete internal casts of shells like this are some of the most puzzling fossils for new collectors.

Grandecrepidula princeps, like other species in this family, commonly forms vertical stacks ("chains") of individuals. This is done for reproductive purposes.

The geologic range of C. princeps is early Miocene through middle Pleistocene.