Monday, March 12, 2018

Andalusite, a mineral with an unusual feature

Andalusite is a metamorphic rock mineral (aluminum-neosilicate mineral = Al2Si05). Because its formation involves contact metamorphism (i.e., heating of rocks near the intrusion of igneous magma), andalusite is resistant to high heat and can be used in making spark plugs, furnaces, and kilns.

Cross-section of a crystal of andalusite (variety chiastolite);
 33 mm width
The variety chiastolite contains dark inclusions of graphite (carbon), which form a very distinctive and sharply delineated cruciform pattern, when shown in cross section. The graphite is pushed aside by crystal growth during metamorphism. Specimens can be of gemstone quality. Chiastolite is also used to make amulets and charms.

Oblique view of same specimen as above; 30 mm height.

Notice how the black band of graphite on the side of the crystal is coincident with one of the rays of the cruciform pattern. Each  of the rays is coincident with a black band. This crystal occurs in a mica schist, which is the most common occurrence of andalusite. 

Sunday, February 25, 2018

The marine gastropod Fusitriton oregonensis: An interesting species today and in the past

Fusitriton oregonensis (Redfield, 1848) belongs to family Ranellidae (the so-called "tritons"). This species lives today most commonly in cool and relatively deep waters from the Bering Sea to northern California. It has been reported in Japan, and it has been reported (mostly as a fossil) in southern California. It is the state seashell of Oregon. The floating larvae of F. oregonensis can last for an extraordinarily long time (up to 4.5 years), and this would explain why it can be found in Japan today. 

This species has a medium-size shell (4 to 5 inches in length) with an overall fusiform shape. Its six convex whorls have 16 to 18 axial ribs nodulated by the crossing of weaker spiral ribs. There is a single parietal tooth near the top of the aperture.

The picture above is an apertural view of a modern specimen 87.7 mm height (3.5 inches) from beach drift at Friday Harbor, San Juan Islands, Washington. If you have ever visited the area, you will known that the ocean water there is cold enough to discourage a normal person, without a thick wetsuit, from swimming in it. The holes you see in the shell are the result of exposure to erosion while the shell was on the beach. At Friday Harbor, this species is intertidal. Southward, it lives in deeper waters (up to several hundred meters).
Abapertural view of same modern specimen.

The fossil record of F. oregonensis is from approximately middle Pliocene (approximately four million years ago) to recent.
The two pictures shown below are of a fossil specimen 55.5 mm height (2.2 inches) of late Pleistocene age (30,000 to 50,000 years old) from a marine terrace at a beach cliff near Santa Barbara. The shells in this marine-terrace deposit lived during the Wisconsin Glacial Stage, which was the fourth and last stage of the great Pleistocene Ice Age. Based on a comparison with modern bathymetric, temperature, and geographic ranges, the shells indicate a maximum water depth of 10 m and a temperature range from 11 to 20 degrees Celsius (cool temperate). This would have been cooler than the sea temperature off Santa Barbara today but similar to that off the northern California coast today. 

Apertural view of a fossil specimen missing its upper part.

Abapertural view of fossil specimen.

Sunday, February 11, 2018

Mystery sand spheres

A fellow geologist recently sent me the following pictures and information about "sand spheres" he found in the general area of the shoreline region of the late Pleistocene Lake Manix, in the Mojave Desert, between Barstow and Baker, southern California. Lake Manix formed by overflow of the Mojave River between 500,000 and 25,000 years ago.

The spheres are all of small size and range from 0.51 to 1.5 cm in diameter (e.g., they are about the size of a U.S. dime). They consist of friable (= fragments come loose from the spheres when rubbed) and angular, coarse-grained material called grus, which results from the granular disintegration (weathering) of granite in an arid climate. The material making up the spheres is slightly cemented by calcium carbonate. 

The above picture is a cross-section of a sliced sphere. Compared to armored mud balls, the "sand spheres" do not have a mud core and are too uniformly of small size. Armored mud balls form when a clump of gooey mud begins to roll around under a flow of water and fragments of rock adhere to the mud surface.

The "sand spheres" shown above, in the background, might have formed in place. The "loose" ones in the foreground are derived from this more concentrated mass of them.

If you "lean in" on this far-away shot, you can make out how numerous the small spheres are (thousands and thousands) among the much larger fragments of angular rock. The "sand spheres" litter the ground sort of like rabbit or deer droppings. 

The "sand spheres" are at an elevation of 563 m, which is slightly above the Pleistocene shoreline, thus they were most likely not formed by a shoreline process. There has not been any lake water at this elevation in 18,000 years. The "sand spheres" look fairly fresh. They are on the surface and although they are not very delicate, thus it is doubtful that they have been around since the Pleistocene. My colleague believes that they might be a product of a local (recent?) downpour. 

Although the nonmarine "sand spheres" of Lake Manix resemble "sand balls" created by small "bubbler" crabs on modern beaches in the tropical ocean waters of the Indo-Pacific, they cannot share the same origin. By the way, if you have the interest you will be amazed and amused by a BBC Blue Planet video (online) that shows how these crabs form the "sand balls."

In closing, we do not know the origin of these Lake Manix spheres and could not find anything in the literature about them. Determining how they formed would be a worthy project. 

Saturday, January 27, 2018

Bursa californica: a fossil and modern-day gastropod

Bursa californica (Hinds, 1843) is a modern-day, shallow-marine gastropod (ocean snail), whose fossil record goes back to the early Pliocene (about 3 million years ago).

Genus Bursa Röding, 1798 belongs to the small family of large sea shells called Bursidae. Their common name is "frog shells" because the intersection of spiral and transverse ribs can result in a strong nodulose pattern of many knobs, producing a "warty" or "frog skin" appearance.

Bursa californica, also known as the "Californian Frog Shell," is characterized by having mostly two protruding ridges (varices) along the left and right margins of the shell. Nevertheless, the warty appearance is not evident on this particular species. 

Bursa californica is found from Monterey, along the central coast of California, to the Gulf of California, mainly in offshore waters. The animal lives mostly on silty-sand bottoms in depths of about 60 to 350 feet. They are active predators and feed on bristle worms (polychaetes), which they anesthetize with acidic saliva. After a storm, some of the shells can wash up on an adjacent beach. The shell is tan-cream in color with a whitish aperture (opening).

Apertural view (front) of a modern specimen
(9 cm height) from Mexico.
Abapertural view (back) of same specimen
Apertural view of a fossil specimen
(8 cm height) from Palos Verdes Sand,
late Pleistocene, Playa del Rey,
 southern California.
Abapertural view of same specimen.

Top view of spire of same specimen.

A growth series (juvenile to adult) of fossi specimens
of Bursa californica from Palos Verdes Peninsula, southern California.
Specimens 2.7, 5 cm, and 8 cm high.

Friday, January 12, 2018

California's Official State Dinosaur

In 2017, the State of California declared its official state dinosaur to be Augustynolophus morrisi Prieto-Márquez et al. (2014), a duck bill of Late Cretaceous age. Duck bills are classified as hadrosaurs.
The generic name, Augustynolophus Prieto-Márquez et al., 2014, is a combination of the Augustyn family, who helped support the Los Angeles County Museum of Natural History, and the suffix “lophus,” in reference to its similarity to Saurolophus, another duckbill, 30 to 40 feet in length, bipedal, and with a battery of flat teeth used for chewing coarse vegetation.  

Augustynolophus, like Saurolophus, has a bony, spikelike crest that projects up and back from the top of the skull. This spike is an extension of the nasal bones.

Augustynolophus morrisi is of late Maastrichtian age, about 68 million years old. The geologic time diagram below depicts the recognized intervals of time that constitute the Late Cretaceous. The Maastrichtian was the last stage of the Late Cretaceous. The red star indicates the approximate occurrence of this dinosaur.

The bones of A. morrisi have been found in the Moreno Formation along the west side of the San Joaquin Valley, Fresno County, central California.

In terms of vertebrate (back-boned) fossils, the Moreno Formation is mainly known for its marine-reptiles (plesiosaurs and mosasaurs). It also contains marine-mollusk fossils (oysters, other bivalves, and gastropods). Augustynolophus morrisi lived in a  marshy, coastal area.

Augustynolophus morrisi is endemic (only found) in California.
For excellent copyrighted pictures of this dinosaur, also known as "Auggie," just "Google" its scientific binomial (two-part) name.

The species name, morrisi, is in honor of the late William J. Morris (1923-2000), a highly respected vertebrate paleontologist who had many dinosaur and other vertebrate-fossil discoveries on the west coast of North America, especially in Baja California, Mexico. His final teaching position was at Occidental College in the Eagle Rock neighborhood of Los Angeles, California. He was also a Research Associate in Vertebrate Paleontology at the Natural History Museum of Los Angeles County.

If you want to see  the entire list of official California names of other geologic entities, go to <>, and click on the associated thumbnail images in order to see larger images. 

Thursday, December 28, 2017

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.

Monday, December 11, 2017

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.