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.

Gastropod operculum

                              apertural view of Megastraea undosa, height 5 cm, diameter 7 cm.

This post concerns the operculum ("trap door") that closes the aperture of certain shells, like that of the turbinid shallow-marine gastropod Megastraea undosa (W. Wood, 1828) shown above. The reason that I picked this topic is because when beginning fossil collectors find a gastropod operculum they commonly do not have a clue as to what kind of animal made it.

Exterior side of the operculum of M. undosa. The width is 3 cm. 

Interior side of the operculum shown above.

Same specimen of Megastraea undosa as shown above but now with its operculum, which very effectively seals the aperture from predators.

Megastraea undosa is found today in central California and southward along the outer coast of Baja California, Mexico. The species is restricted to hard substrates in the intertidal zone and grazes on algae attached to rocks. The species has a fossil record extending back to the approximately 10 million years ago (i.e., late Miocene).

Tentaculites

Tentaculites is a genus of a problematic group of Paleozoic fossils called tentaculids. The shells of Tentaculites consist of conical tubes (5 to 20 mm long) that are  straight or slightly bent and have transverse rings (ribs) on the surface. The tubes are closed at the small pointed end and open at the opposite, larger end called the aperture. The three specimens shown above are incomplete Tentaculites sp. from the same locality in Devonian rocks of New York. The specimen on the left is 5.8 mm in length; the middle specimen is 6 mm in length; and the specimen on the right is 4 mm in length.

One of the interesting features of most shells of Tentaculites is the presence of chambers in the early stage of growth. A photomicrograph (i.e., taken through a microscope) of a cross-section of a Tentaculites shell, 7 mm in length, shows these chambers. Jasmyn Nolasco took the photomicrograph, and James Ingraffia facilitated the process.

This is a cluster of Tentaculites sp. from the Manlius Limestone of Devonian age near Ravenna, New York. The largest specimens are approximately 10 mm in length. 

Tentaculites ranged in geologic age from the Early Ordovician to the Late Devonian (see time diagram below) (a span of 128 million years).

Rudistid bivalve

          Coralliochama orcutti White, 1885, height (incomplete) 12.7 cm, width 7.7 cm

Rudistid bivalves (also known as rudists) are among some of the more unusual (aberrant) bivalves (clams). They are of Late Jurassic to Late Cretaceous in age (see time diagram at bottom of this post) and had widespread distribution in equatorial (tropical) areas. They were gregarious, and their shells were closely packed together. At many locales, their abundant shells commonly formed thick beds (biostromal accumulations).

As shown above, the lower valve (right valve) of a rudistid belonging to genus Coralliochama has the form of an elongate thick-walled cone. The upper valve (left valve) is reduced to a domed structure. The commissure is the line of junction of the two valves.  

The following picture is of the same specimen as above but shows the front of the bivalve. This specimen is of Late Cretaceous age (approximately 72 million years old) and is from northwestern Baja California.

Ammonite sutures in interior of shell much simpler than you would expect

      Late Mesozoic ammonite whose maximum diameter is 10.5 cm (4 in.);

       the specimen is from Madagascar

This blog is a follow-up on my previous blog, which also deals with ammonite sutures. What I want to show you now is something that I noticed years ago about suture lines. I have looked through many textbooks for a similar depiction, but I have never found any illustration of what I am showing here. The photograph above is of the exterior (partly worn) surface. Along the upper part of the photograph, you can see complex ammonitic sutures.

This second photograph shows one half of the interior of the same specimen that is illustrated above (i.e., the specimen was sliced in half). The bright yellow material on the right side is secondary foreign mineral material that replaced some of the chambers. Notice that in the upper part of the photograph, the partitions that form the chambers are very simple curves. They are not like the complex sutures lines show in the first photograph. The suture lines are complex ONLY at the junction between the chamber partitions and the outer shell wall.

The presence of complexity of the sutures at the junction with the wall, but not in the main parts of the chambers, begs the question "why." The answer is not clear because ammonites are extinct organisms. Some experts believe that the complex suture lines are related to the strengthening of the outer shell wall, so as to resist being crushed by hydrostatic pressure as the ammonite slowly swan (descended) into relatively deep depths in the ocean environment. The main parts of the chambers must have not needed the extra reinforcement.

Ammonite sutures

Ammonite shells are some of the most popular fossils for collectors. For a link that shows representative pictures of ammonites, click HERE. Ammonites were cephalopods (e.g., squids and octopus) that resemble the modern-day Nautilus. Like Nautilus, ammonites have curved partitions (septa), which divide the shell into chambers. The lines (sutures), which formed where the septa made contact with the inside wall of the shell, consist of distinctive curves that characterize different groups of ammonites. Sutures are only visible when the outer wall is removed or has been nearly stripped away.

Ammonite sutures gradually changed (see diagram above) during the 250 million years through which they ranged; namely, from the Middle Paleozoic (Devonian Period) to the end of the Mesozoic (Cretaceous Period) (see diagram below). 

The ammonite sutures gradually became increasingly more complex, and these changes enable the paleontologist to use ammonite shells as "geologic clocks" for helping to determine geologic time.

The gonitatic shells were the earliest (oldest) and the most simple; the ceratitic shells were intermediate in age and complexity; the ammonitic shells were the latest (youngest) and the most complex. The following pictures show a representative specimen for each type of sutures.

This is an example of deeply undulating goniatitic sutures. It is a partial specimen of Gonioloboceras coniolobum of Late Paleozoic (Pennsylvanian) age from Texas. The maximum dimension of this specimen is 6 cm.

This is questionably a specimen of Uddenoceras of Late Paleozoic age. It is an example of ceratitic sutures. The maximum dimension is 9 cm. 

This is Hoploscaphites brevis? of Late Cretaceous age from Montana and is an example of ammonitic sutures. The maximum dimension is 3.25 cm. The shell shows impressions of its straight ribs, which must not be confused with the sutures. This specimen displays "mother-of-pearl" luster, which indicates that the preservation of the shell material is very good. 

Oldest fossil shells in California

The oldest known fossil shells in California are poorly preserved hollow tubes of Wyattia reedensis Taylor, 1966.  They are found in uppermost Precambrian to possibly lowermost Cambrian (see the time diagram in my last post) carbonate rocks in the Inyo Mountains of eastern California. A representative tube, as indicated in the above image by the short black arrow, is approximately 10 mm in length and 3 mm in diameter. The rock surrounding this tube contains many other, but less distinct, tubes of W. reedensis. 

Wyattia is a probable molluscan fossil and is pre-trilobite in age. Wyattia and other small-sized shells are important because they represent the oldest skeletonized faunas on Earth. These fossils occurred during an interval of geologic time between when only impressions of soft-bodied organisms have been found versus when the first shells of larger animals (e.g., trilobites) have been found.

Late Precambrian to early Cambrian Wyattia-like fossils have also been found in Esmeralda County Nevada and near Caborca in northern Sonora, Mexico.

Wyattia was named for J. Wyatt Durham (deceased), who was a professor of paleontology (study of fossils) for many years at the University of California, Berkeley. 

"Blind" trilobite Itagnostus from Utah

Trilobites like the one shown above are well known to most collectors of fossil invertebrates (animals without backbones). This trilobite, which is 4 cm long, has a cephalon or head (with eyes), a thorax (body with many segments), and a short pygidium ("tail"). This particular specimen is an Elrathia kingi of Middle Cambrian age (see time diagram below) from the Wheeler Shale near Delta, Utah.

Another type of trilobite found alongside E. kingi is Itagnostus interstictus, until recently known as Peronopsis interstrictus, which belongs to one group of so-called "blind" trilobites. Technically speaking, "blind trilobites" were not blind because they never had eyes. Itagnostus is an agnostid trilobite, which are characterized by having a thorax consisting of only two segments and a cephalon and pygidium of approximately the same size.

This is a "cluster" of two specimens of Itagnostus interstices. The largest specimen is nearly 1 cm  long. 

Scaphopod

This post concerns scaphopods, which are mollusks that belong to the same phylum as snails, squids, octopi, etc. Scaphopods are tusk-shaped shells that are partially infaunal, that is to say they burrow into the ocean floor but do not burrow deep enough to cover the top part of their shell.

The drawing shows how the shell projects into the sediment. The mouth is surrounded by feeding tentacles which bring microscopic-sized food to it. The gut is straight. Water is brought down into the shell and also is excreted through a hole at the top of the shell.

This is a fossil scaphopod of the Eocene Dentalium stramineum The specimen is 7 cm long and almost complete. It is from Simi Valley, southern California. Fossil scaphopods are not common. 

This is a modern specimen of Dentalium. If it were not for modern specimens, scientists would probably never have determined that these simple tubes were made by mollusks.

This is another modern specimen of Dentalium.

Molds and casts

This picture shows a specimen of the Eocene Turritella andersoni (height 7.5 cm) from southern California (please see one of my earlier posts if you want more information about this species). If the specimen is removed (i.e., is weathered away or simply fell out), it leaves behind an external mold = an impression of the external surface. This external mold is a negative surface, that is to say, it could literally "hold water." 

This next picture shows the external mold of the original specimen and, to the right, an external cast of this mold. Most collectors commonly do not bother to collect external molds. At some localities, however, that is all you can find, and if you need to identify the genus and species, it will help if you create a latex external cast of the external mold. That way, you can make a "positive" out of a "negative." All you need is some liquid latex, like the kind you can put on the back of a rug to keep it from sliding around on a floor. Carefully pour the liquid latex into the external mold (try not to create any bubbles), and let the latex dry. Removal of the latex cast is easy; just pull it off. A latex external cast is shown above just to the right of the external mold. In some, cases nature creates external casts by filling in the external mold with some foreign substance. In the above, because I used latex to make the external cast, therefore, it is called an artificial-external cast.

The above picture shows an internal cast, which shows the interior of a high-spired gastropod shell (most likely, a Turritella). The 6-cm high shell was hollow, and silt and mud filled the shell after the death of the gastropod. Later, the sediment converted into solid rock, the shell was destroyed, and all that is left is the internal cast. Equivalent terms for an internal cast are endocast, internal "mold," or, my personal favorite, steinkern (= a German word meaning a "rock center or core"). Steinkerns are not very useful for determining genus or species. Some early workers, unfortunately, used them for naming new species. Doing so has caused serious taxonomic (classification) problems for subsequent workers, who commonly refer to such a species as a nomen dubium = a name representing a species that is not identifiable from the original specimen (type) used to describe it.  

This final picture shows the external mold (6 cm wide) of the bivalve Laevicardium californiense. The fossil is of Plio-Pleistocene age and from the Santa Barbara Formation, Santa Barbara, southern California.

fossil pine cones

a fossil pine cone, height 10.5 cm

I have always been intrigued by fossil pine cones, especially the ones that were transported by river currents to a ocean shoreline and ended up being deposited in the shallow-marine environment alongside Turritella shells (see some of my earlier posts) and shark teeth. 

The rare specimen shown above is from float material (weathered out and laying loose on the surface) from the Pico Formation near Newhall, southern California. The formation in this area was deposited in a marine-delta environment. There had to have been pine trees growing in the adjacent, ancient San Gabriel Mountains east of the delta. This pine cone floated down a braided river (full of coarse debris consisting of pebbles and cobbles) and was deposited in fine-grained sandstone near the ancient shoreline. 

another fossil pine cone, height 8.5 cm

The rare specimen shown above is from float material found near Topanga State Beach, southern California. Much less is known about its provenance (origin) than the Pico Formation specimen.

Identification as to the family or genus of pine tree for both of the illustrated specimens is needed. The identification process is not an easy task. Just the presence of pine cones in a sedimentary rock deposit is most helpful, nevertheless, because they indicate that a mountainous area was adjacent to the burial site.

An exceptional trilobite-trace fossil

Trilobites (see the link  http://en.wikipedia.org/wiki/Trilobite ) were Paleozoic arthropods that crawled along the shallow-sea floor. When a trilobite stopped to rest, it made a shallow burrow in the mud or silt. These burrows were commonly filled with sediment and later fossilized as "resting" trace fossils called by the Latin name, Rusophycus. Remember from one of my earlier blogs, a trace fossil shows behavior of a fossil organism.

The above picture is the bottom of a Cambrian Rusophycus from the Inyo Mountains, central California. The burrow is 10 cm in length. Scratch marks made by the trilobites legs are visible on the bottom of the burrow.

These two slabs (both about 11 cm, widest dimension) of slightly metamorphosed Cambrian siltstones from eastern California contain a cluster of Rusophycus. The burrows were probably aligned parallel to an ancient current that brought food to the trilobite. Rusophycus commonly has a bilateral symmetry that was caused by its two rows of legs that moved back and forth in its burrow. This leg action was necessary because its gills were attached to its legs, thus it had to move its legs (even when stationary in its burrows), in order to obtain oxygen from the shallow-sea water.

Ahh, finally we get to a truly exceptional specimen of Rusophycus. It is undoubtedly the best specimen I have ever seen. It is preserved three-dimensionally (8.5 cm long, 3.5 inches) and is of Cambrian age from the Salt Spring Hills, eastern California. The above picture is the bottom view, which shows the scratches made by the legs of the trilobite. The specimen is a plaster replica of the actual specimen, which is now in a museum collection. I painted the plaster replica so as to make it look more like the actual specimen.

This is a side view of the same specimen shown above. The bottom of the burrow is at the top of the picture. I painted the antennae red, so you can readily see them. The fact that the antennae are visible indicates that the remains of the trilobite that made this resting trace are within the burrow, thereby proving the trilobite made this burrow. In situ (in place) specimens of trilobites in their burrows are very rare.