An Eocene crab from southern California

If a collector is fortunate, some localities yield a few fossil crabs. This post is about a species of crab, Orbitoplax weaveri (Rathbun, 1926) which can be found in Eocene (approximately 50 million year old) rocks on the west coast of the United States, from southwestern Washington to southern California.

Orbitoplax weaveri is the updated genus name of this crab. It has been erroneously placed in other genera, thus in the older literature this species was called Plagiolophus weaveri and then Glyphithyreus weaveri.

The main part (carapace) of this recently collected specimen is 22 mm (nearly an inch) wide. There are also parts of three of its legs on the left side of the specimen. The collector who found and photographed the specimen kindly gave it to me.

This is another picture of the carapace of the same specimen, after it was removed from the rock. This picture was taken under low-angle lighting to show the sculptural details.

This is another and larger (main part 25 mm) specimen of O. weaveri, which I collected many years ago from the same locality as the one shown above. Its right pincher is intact. Also, a impression of the claw part of the left pincher is present. The total width (from left to right) of the specimen is 55 mm. 

Complete or nearly complete crabs like the ones shown above are typically found in silty sands, which were deposited under relatively quiet-water conditions. Just down section a few meters, there are storm beds with numerous Turritella gastropods, whose shells display preferred orientation caused by current action.

A 50-million year old chambered nautiloid shell

The living "pearly nautilus," also called the "chambered nautilus," is a favorite seashell of many collectors. Today, the biodiversity (number of species) of these animals is very low, and they are confined to tropical waters in the equatorial region of the western Pacific. As certain times in the geologic past, however, when warm oceans extended north and south of where they are now, chambered shells similar to the "pearly nautilus" had high biodiversity, and their distribution was widespread (cosmopolitan). These chambered shells are commonly referred to as coiled nautiloids.

On August, 2016, I created a post about the "pearly nautilus," and two of my pictures are shown again here for comparative purposes. I encourage you to use the "search box" at the top right-hand side of this blog page to find this post and read it again. I also give some interesting details about the life habits of this animal.

Exterior of a modern-day "pearly nautilus."

            Maximum diameter is 14 cm

Interior of same specimen shown above 

This present post concerns one of these ancient widespread groups of coiled nautiloids, namely, an extinct genus belonging to genus Aturia Bronn, 1838.  It was widespread (cosmopolitan) and its geologic time range was Paleocene to Miocene (approximately 40 million years long).

In particular, this post is about Aturia myrlae Hanna, 1927, an early to middle Eocene species of genus Aturia. The ancient geographic distribution of this species covered an area now referred to as central California, southern California (including Ventura, Los Angeles, and San Diego counties), and Baja California Sur, Mexico. Aturia lived in subtropical to tropical ancient environments. So, if you are lucky enough to find one of these fossils, you can be certain that it represents a warm-water ancient environment. Specimens are not that common because, like other coiled nautiloids, Aturia was a predator, thus, their numbers were few.

The next three pictures show a partial specimen of Aturia myrlae from Simi Valley, southern California. The widest dimension (diameter) of this incomplete specimen is 14 cm. 

Side view showing the complex outlines (septal pattern or suture pattern) of the chamber walls (septa). The suture pattern of Aturia is a very distinctive character of this genus and is in sharp contrast to the simple-curved suture pattern of the "pearly nautilus," which belongs to genus Nautilus.

Back side view of the same specimen. 

Front view of the same specimen. Notice the presence of the siphuncle (see a reference picture at the beginning of this post), which was a hollow tube that connected all the empty chambers and allowed for nitrogen gas to be dispersed to all the chambers. In so doing, the shell achieved buoyancy when the gas was pumped in, and the shell sank when the gas was pumped out. The shell could, therefore, move up and down in the water column.

Colors of scallops

If you have been reading my latest posts, you will know that I have been focusing on colors in certain minerals. In this present post, I shall discuss colors of certain clams called scallops, which belong to the family Pectinidae. The family is comprised of many genera.  Those belonging to genus Pecten are called pectens (not: pectins, which refers a group of colloidal substances used to jell various foods or cosmetics). 

Pectinids are especially abundant as fossils, if found in the right environment. Their geologic time range is Triassic to modern day. Along the west coast of the USA, fossil pectinids are  common in sediments of Miocene and Pliocene age (20 million to about 3 million years ago). Today, the strong muscle that holds the two valves together is a popular seafood delicacy. 

The two valves of a pectinid are referred to as the left and right valves. 

In addition to the primary (radial) ribs, important morphologic parts of a pectinid are the auricles ("ears") along the hinge line. As shown above, on the right valve, the anterior auricle is the most elongate. On some pectinids a byssal notch is located directly below the elongate anterior auricle. This notch is where the byssus (a bundle of hairlike material used for attachment) exits the shell.


Today, there are approximately 250 species of pectinids. Some have shells which are bright orange, yellow, red, blue, purple, brown, white, or combinations thereof. This color variation comes mainly from heredity although environment can play a role (e.g., some muddy-bottom pectinids have darker color than sandy-bottom pectinids.

The four colored specimens (each one is about 5 cm width) shown above are of species of the pectinid Chlamys, and they show some of the variation in the color. Some people think that they are hand painted, but the colors of these specimens occur naturally. Chlamys lives by having its byssus attached to a rock, shell, or other hard surface.

These are the right (= the specimen on the left side of this image) and left valves of a modern specimen of Chlamys hastata (2.7 cm wide), dredged from 210 feet depth, at the shelf break just south of Long Beach, southern California.

Nodipecten subnodosus, 15 cm wide, exterior and interior of a right valve. This species is most commonly found in modern waters in the Gulf of California, Mexico.

Nodipecten has been observed anchored by a byssus to hard substrates, but they have been observed also to have the ability to swim for short distances. At least temporarily, therefore, they are not anchored by a byssus. Thus, its byssal notch is not well developed. Nodipecten swims by pulsatory clapping together of their valves. This swimming action is exhausting and cannot be sustained for very long; it is normally used only to escape from predators (e.g., sea stars).

Patinopecten caurinus (Gould, 1850), 16.5 cm wide, exterior of a large right valve modern-day specimen from Skagway, Alaska. Patinopecten has a well developed byssal notch, therefore, it lived by attaching to hard surfaces. 

Patinopecten caurinus, 16 cm wide, exterior of a large right valve fossil specimen from a late Pliocene (3 million years old) bed found between Goleta and Santa Barbara, California. If you look carefully, you can see the minute daily growth lines of this shell.

Barnacles make interesting fossils

Barnacles are classified as cirriped crustaceans and belong to phylum Arthropoda (a large group that also includes trilobites, crabs, insects, etc.). The name "cirriped" (cirri, curl; ped, foot) refers to the six pairs of delicate appendages which are used for filter feeding.

Barnacles are not very different from other arthropods in that they hatch as an egg, have a short existence as free-swimming larval forms, and molt (shed) to grow larger. In adult life, however, barnacles do not resemble other arthropods at all. With a shelly (calcareous) covering of many plates enclosing their shrimp-like body, most barnacles grow attached to hard substrate. Genus Megabalanus Hoek, 1913, one of the so-called “acorn" barnacles, is common in the fossil and modern-day record of the eastern Pacific. 

Few people know that Charles Darwin, yes, that Charles Darwin, was an expert on barnacles. He derived some of his concepts about evolution based on his detailed studies of them.

bivalve shell is 3.4 cm in diameter

A bivalve shell (the "jingle" shell Anomia) with a large Megabalanus base (circular) at the top of the shell and 15 Megabalanus shells elsewhere on the shell. These fossils are of late Pleistocene age from the vicinity of Long Beach, Los Angeles County, southern California.

large barnacle is 1.5 cm high 

A Megabalanus shell encrusting a shallow-marine gastropod shell, and a small Balanus shell encrusting the larger Megabalanus shell. All of these fossils are of late Pleistocene age from the same locality as those shown in the previous image.

acorn-barnacle operculum: smaller parts are 4.5 mm length;
 larger parts are approximately 5.5 mm length

Covering the top of an acorn-barnacle shell is a calcareous structure consisting of four interlocking plates. These two pairs of plates close together, just like a tight-fitting “lid,” used when the barnacle is not feeding or when it is disturbed. The opercular plates, which show continuous growth records (like tree rings) and are not shed during molting. Upon death of the animal, these plates eventually fall apart, thus they are mostly missing on fossil barnacles. The pair of plates shown (above) on the left are called terga, and those on the right are called scuta.

cluster is 2.2 cm wide

 The image shown above is a small cluster of modern-day barnacles with their opercular plates in life position. These are specimens of Balanus amphitrite saltonensis Rogers, 1949 from the Salton "Sea," an inland lake with very salty waters in southeastern California. This lake was created by accident in the early 1900's, when an aqueduct overflowed. This subspecies of barnacle, which is closely allied to the globally widespread B. a. amphitrite Darwin, 1854, was  introduced into the lake. This introduction was most likely via  migratory water birds.

cluster is 21 cm in maximum dimension

A cluster of five large-sized Tamiosoma gregarious from Pliocene rocks in the central San Joaquin Valley of central California.

each component is 2.8 cm length

These four images above show the exterior (above) and the interior (below) of two scuta of Tamiosoma gregarious found in Pliocene beds in the central San Joaquin Valley of central California. These are unusual finds.

2.6 cm height

This is the side view of a modern-day "gooseneck" barnacle (Pollicipes polymers) from a rocky intertidal zone in the vicinity of Goleta, Santa Barbara County, California. Notice the leathery stalk (pedicle) which is used by the barnacle to attach to rock. The stalk does not get preserved, thus, in the fossil record all one can find are the individual calcareous plates that make up the upper part of the animal.

The geologic time range of barnacles is Paleozoic (Silurian) to Holocene [= modern day]. Gooseneck barnacles evolved first. "Acorn" barnacles did not appear until the early Cenozoic.

3.5 cm maximum diameter

Lastly, I show an image of the barnacle genus Chelonibia, which attaches to the carapaces of sea turtles. This modern-day specimen is from Baja California Sur, Mexico.

Colors of fluorite

The mineral fluorite consists of calcium fluoride (CaF2). It can have variation in color, largely due to impurities in its crystal structure. Flourite can be found throughout the world. It is common in hydrothermal deposits, where it can be associated with quartz, calcite, baryte, and galena (lead sulfide). 

Uses of fluorite include jewelry making, as well as a flux for smelting. 

The largest crystal (green color one) is 4 cm long.

Flourite can crystallize in several forms, including the octahedral form, which is my favorite.

Octathedral crystals: the clear one is 3 cm tall.

Flourite has a hardness of four on the Mohs Scale. Optically clear crystals, like the one shown above, has low aberration, thereby making them valuable in the construction of microscopes and telescopes.

Copper and Molybdenite

COPPER (Cu) is a naturally occurring pure element with a reddish-orange color.

This is the element copper in its pure natural form. The specimen, which is 5 cm long, shows the dendritic habit of copper. The gray material is gangue (wall rock) material consisting of calcite.

Copper has with very high electrical and thermal conductivity, and the main uses for this soft element are for electrical wire, plumbing parts, and industrial machinery. In order to make copper harder, it is purposely combined (alloyed) with other metals:

brass = an alloy of copper and zinc

bronze = an alloy of copper and  tin

cupronickel = alloy of copper and nickel (used in making coins, like the U.S. nickel = 75% copper and 25% nickel).

Copper is one of the few metals that occurs naturally as a directly usable form. This led to its being used by early humans, as far back as 9000–8000 BC. The discoveries of making alloys out of copper happened later (e.g., the Bronze Age about 3700–100 BC).

Like aluminum, copper is readily recyclable without any loss in quality. It has been estimated that 80% of all copper ever mined is still in use today.

MOLYBDENITE (MoS2) is the mineral molybdenum sulfide, the principal source of the metallic element molybdenum. This high-temperature hydrothermal mineral is silvery/gray in appearance (similar to graphite = pencil "lead"), is greasy to the touch, and peels apart (in one direction) in somewhat heavy but flexible sheets.

This specimen of molybdenite is 4 cm long (from upper left to lower right).

Molybdenite can withstand extreme temperatures without significantly expanding or softening. The element molybdenum  readily combines with other elements and forms hard, stable alloy materials, which are used for making high-strength steel. In particular, these alloys are used in military armor, aircraft parts, electrical contacts, industrial motors, etc. 

This specimen came from a quartz veinlet in granite at a commercial mine in Climax, Colorado.

Some varieties of the mineral gypsum

I decided to do a post on the mineral gypsum just after my previous post on calcite because the two minerals can resemble each other. 

Gypsum is a very common mineral and is the most common sulphate mineral. It forms as an evaporite mineral and is commonly found in dry-lake beds. Gypsum consists of calcium sulphate dehydrate (CaSO4•2H20). If it becomes dehydrated it forms plaster of paris. 

Gypsum is very soft and can be easily scratched by a fingernail. On the Mohs Scale of Hardness, gypsum has a hardness of 2. Diamond has a hardness of 10 on this same scale. By the way, it does not fizz in acid.

This mineral is used in making fertilizers, plaster, wallboard (drywall), blackboard chalk, and some cements.

There are several varieties of gypsum, and some are shown below.
The scale in each of the three images is the same: increments of centimeters.

This is the clear (transparent) variety called selenite. It contains no significant amount of the element selenium; rather "selenite" refers to the ancient Greek word for the Moon.

This is the tabular, massive (fibrous/silky) variety called satin spar.

These five crystals show "fishtail" twins or "swallowtail" twins of gypsum. I discussed the topic of twinning in crystals in one of recent posts.

Crystals of gypsum can be extremely large in size and are the largest crystals (39 ft. = 12 m long) of any mineral on Earth. An image of these largest crystals is shown above. Note the human for scale. This image is from Wikipedia (accessed Sept. 2017) and shows the "Cave of the Crystals in Naica, Mexico" ["Cristales Cueva de Naica, México], where these enormous crystals are found.

White Sands National Monument in southern New Mexico (USA) consists of a 270 sq. mile expanse of white gypsum sand/dunes. The gypsum was eroded by way from nearby gypsum beds and deposited in the adjacent valley. As a teenager, I visited White Sands and enjoyed sliding down the dunes. The experience is much better than sliding down normal sand dunes made of quartz sand because the gypsum crystals are soft and not abasive. This image is from Wikipedia (accessed Sept. 2017).

Calcite and some of its interesting properties

The mineral calcite consists of calcium carbonate CaCO3. It is a very common mineral that makes up limestone. Beginning students in geology labs quickly learn that calcite can be easily identified because it fizzes in weak hydrochloric acid.

Also, if the crystal of calcite is clear (transparent), any writing underneath of it is "doubled" when viewed. This effect is shown in the first two images below. The first image is of  a small, clear crystal, about 3 cm across. It is sitting on a white piece of paper, with the letter "X" written on it. Light is refracted through the crystal and produces the "doubling" effect (called double refraction). Single perfect crystals like this one, have what is known as the rhombohedron shape (a six-sided prism).  

The second image is of a longer (7.5 cm) clear rhombohedron crystal, sitting on a white piece of paper with the word "calcite" written on it. 

There are also other colored varieties of calcite, as shown in the following "color wheel." For scale, the green crystal is 4 cm length.

These varieties, which I had available, show a range in color of calcite, from orange, red-orange, red, yellow-brown, clear, light yellow, blue, green, and (in the center) honey color. There are other intermediate colors. Impurities in the crystals cause the different colors.

The "black looking" crystal at the bottom of this "color wheel" is actually a clear crystal, and the black background used in making the photo shows through this clear crystal.

A last picture, shown below, is of a stalactite of calcite from the ceiling of a cave. The calcite crystals formed by dissolving out of underground water and dripping into an open space (a cave). The structure is 5 cm in height.

Eocene oyster Cubitostrea sellaeformis

If you have been a reading my posts for the last three years, you will know that I have a strong interest in fossil oysters. I return to them with this post, which concerns the middle Eocene (about 45 million years old) oyster Cubitostrea sellaeformis (Conrad, 1832), known from Alabama, Georgia, Louisiana, North Carolina, South Carolina, Texas, and Mexico. This oyster has a strongly arched, large-size shell, and, if you look at the digital image below, you can see the U-shaped "line" that separates the two valves shown in side view.

I collected this complete specimen in 1989, when I visited southern Alabama. The specimen is from the upper part of the Lisbon Formation. The lower valve (left valve), which is thick and heavy,  sat on the bottom of the shallow ocean. The upper valve (right valve), which is lid-like is smaller and comparatively lighter.

This is the exterior of the lower valve, which is 14 cm long.

This is the exterior of the upper valve, which is 13 cm long. The "ears" (auricles) are part of the other valve.

These are the interiors of both valves, with the lower valve on the left side of the picture, and the right valve on the right side of the picture. If you look closely, you can see the muscle scar, especially on the upper middle part of the lower valve.

If you are wondering what caused the peculiar shape of this species, "join the crowd." No one has determined the answer with any degree of certainty. Its large size allowed it to live in shallow-marine waters in front of barrier beaches (unlike brackish-water, lagoon-living smaller oysters). It is possible that individuals crowded together and took on unusual shapes so as to withstand agitated-water conditions. The "ears" possibly served as stabilizing "anchors" for the lower valve.

Two Late Cretaceous species of the bivalve Pterotrigonia

In some cases, recognition of two species of the same genus can be problematic. In the examples shown here, however, the recognition  is straightforward. The species are of a shallow-marine bivalve (clam) of Late Cretaceous age from the west coast of North America. These species lived as burrowers in subtidal shelfal depths.

Pterotrigonia klamathonia, 4.5 cm length, Santa Ana Mountains,
Turonian age, Orange County, southern California.

Pterotrigonia klamathonia (Anderson, 1958), which is of Turonian age, is characterized by its closely spaced radial ribs, 20 to 25 in number.

Pterotrigonia evansana, 4.5 cm length, Campanian age,
Simi Hills, Ventura County, southern California.

Pterotrigonia evansana (Meek, 1858) is geologically younger and is of Coniacian through Campanian age (see time table below). This species is characterized by its widely spaced radial ribs, commonly 10 or so in number.

Genus Pterotrigonia, which is the state fossil of Tennessee, is extinct. This genus belongs to the family Trigoniidae, whose geologic time range is latest Jurassic through the end of the Cretaceous. The family was very widespread in the world during the Cretaceous.