A Dolomite Ventifact

A ventifact is a stone that has been shaped by the erosive action of windblown sand (rarely ice crystals), bouncing along (saltating) only a few feet above the ground. The abrading grains of sand “file” (pit, etch, striate, groove, rill, and/or polish) away at the rock surface, which acts as a windbreak, and as result a sharp edge (facet) forms. Ventifacts form in arid environments (deserts) where there is a strong wind and a steady (but not overpowering) supply of sand or ice crystals. Windblow processes are referred also to as aeolian processes.  

Ventifact rock type (lithology) is commonly a very hard material like quartzite, chert, obsidian, or some basalts. Less common is dolomite, whose chemical formula is CaMg(C0₃)_2. Dolomite reacts only weakly with cold, dilute hydrochloric acid (HCl). Limestone (CaCO₃), which is another carbonate rock, is much softer than dolomite and, unlike dolomite, reacts vigorously with cold, dilute hydrochloric acid.

The ventifact shown below is a stone of dolomite (22 cm width and weighing about 2.5 pounds). Where it was found is unknown, but it was probably found in a desert in southern California.

The first view shown below is the top view of the stone. Most of the grooves are on the left side, but, if you look closely, you can see a smaller set of grooves on the lower right-hand area. Although this smaller set is in parallel alignment with the other set, the smaller set is at a sharp angle relative to the larger set. The only way to explain this phenomenon is that there had to been a change, over time, of 180° in wind direction. The facet (a sharp edge of demarcation between the two different-angled sides of the upper surface of the rock) is readily visible. 

For your information, the planet Mars has been modified by aeolian processes more than another other solid planet in our Solar System. One half of the rocks found at the Mars Path Finder Vallis Landing site in 1996 are ventifacts, and they were formed by northeast to southwest winds. For more information about that site, see the following article (free for reading):

Bridges, N.T., et al. 1999. Ventifacts at the Pathfinder landing site. Journal of Geophysical Research v. 104, no. E4, pp. 8595–8615.

Terrestrial Pulmonate Gastropods Found At A Locality In Southern California 

Terrestrial-pulmonate gastropods have the ability to breathe air. They represent an informal taxonomic group that includes terrestrial (land) and freshwater families, and even a few marine families.

Over many years, I have found several different kinds of terrestrial pulmonates (those with shells and some without) in my yard, in northern Los Angeles County, southern California. Even though these kinds of gastropods are not in my field of expertise, I tried to identify them, with considerable help from my friends at the Malacology Department of the Natural History Museum of Los Angeles County. 

The result of my efforts, after many years of “collecting” right in own yard, is that they are four families present: four genera, and four species. Also, I was surprised to learn that these taxa originated in Europe and were later introduced into the USA. Also, they were all named between 1758 and 1822.

Shelled Pulmonates

Rumina decollata (Linnaeus, 1758)

Common name: decollate snail

Family: Achtinidae

Distribution: native to Europe and the Mediterranean region; introduced to the USA (California, Arizona, Florida, Georgia, No. Carolina, So. Carolina, and Texas) and to Cuba, Mexico, and Bermuda.

Remarks: They remain hidden in debris unless it rains, then they crawl about over sidewalks and driveways. They are voracious predators and eat any other snails they encounter, especially Cornu aspersum.

Images: (in order of appearance): Juvenile specimen (one view) with its non-truncated shell (9.5 mm height). Adult specimen (four views) with its truncated shell 31.5 mm height; front view, back view, tilted side view, and top view.

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Cornu apserum (O.F. Müller, 1774)

Common name: brown garden snail

Family: Helicidae

Distribution: native to Europe, North Africa; introduced to many parts of the USA  (including California) and to South America.

Remarks: These snails are vegetarians and can be very destructive if they find their way into your garden. This species is the most common terrestrial snail in Los Angeles County, southern California. They were common in my garden, until Rumina decollata showed up; now, Cornu aspersum is very rare there. 

Images: Adult specimen, 27.9 mm height (3 views): front, back, and right side, showing outer lip edge.

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Oxychilus sp. 

Common name: glass snail

Family: Oxychilidae

Distribution: native to Europe, North Africa, and parts of the Middle East; introduced to many parts of the USA (including California). 

Remarks: The common name refers to being able to see “through” their somewhat transparent shell. These snails prefer humid places under leaves and stones. This snail is one of two similar species; it is probably O. draparnaldi (Beck, 1837). Positive identification would need anatomical studies.

Images: Adult specimen, 4.2 mm height, 11.5 mm width, 3 views: front, top, and bottom.

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Slug Pulmonate

Ambioglolimax valentianus (Férussac, 1822) or Ambiogloimax nyctelin (Bourguignat, 1861)

Common name: Three-band garden snail 

Family: Limacidae

Distribution: native to Europe (Spain and Portugal); introduced to China, Japan, and USA (including California, Oregon, and Washington).

Remarks: One of two very similar species. Positive identification requires internal anatomical studies. This genus has an internal shell (small and flat).

Image: Largest individual approximately 35 mm length.

ADDITIONAL REMARKS:

Europe is the origin for the majority of the many of the non-native terrestrial gastropods found in the world. Their distributions are constantly changing and basically becoming more widespread. If you are interested in the details, please see the following (free) pdf:

Gladstone et al. (2020). Spatiotemporal patterns of non-native terrestrial gastropods in the contiguous United States. NeoBiota 57:133–152.

In my study, I found the following reference useful:

Cōcke, J. 1995. Common land snails of Los Angeles County. Privately published. 

ANORTHOSITE: A “tell-tale” rock with an early history in our Solar System

Anorthosite is a plutonic igneous rock (formed at high temperature and pressure deeply underground) and is characterized by its mostly calcium-rich plagioclase feldspar composition (90 to 100%), with a minimal mafic (iron and magnesium) component. The plagioclase feldspar is a gradational solid-solution series, and its composition ranges from anorthite (calcium rich) to albite (sodium rich), with each member forming at a particular temperature and pressure. Early mineralogical authorities (e.g., Dana) considered that each part of this series represents individual minerals. The consensus today is that only the end members (anorthite and albite) are distinct minerals. 

If the preceding paragraph makes little or no sense to you, please see my Aug. 21, 2020 blog post entitled Bowen’s Reaction Series. In that post, you will see my poster on the two mineralogical sequences of several minerals. On the upper right-hand side of this poster is the sequence beginning with the mineral anorthite, which is the calcium-rich end member of the plagioclase solid-solution series. The sample of anorthite (35 mm wide) used on my poster is shown here.

The mineral anorthite can be the main component of the rock anorthosite (a plagioclase feldspar). Although anorthosite can be essentially mono-mineralic, its plagioclase-feldspar composition can be different from region to region. Two hand specimens of anorthosite from the San Gabriel Mountains, northern Los Angeles 

County are shown below. 

I do not know the exact composition of this first hand specimen (119 mm wide), but its bluish-gray color is indicative of it being more toward a high percentage of calcium.

This second hand specimen (135 mm high) is a bimineralic rock. It has a similar grayish appearance indicative of a high-calcium content as present in the previous hand specimen but also contains a significant content of mafic (dark) minerals. This second specimen is more appropriately referred to as an anorthosite-gabbro rock. Some geologists might call it by its equivalent name of norite.

Anorthosite is not that common on Earth but can be locally extensive mainly in batholiths (large bodies, many kilometers across). Every continent has a fairly large area consisting of Precambrian-age exposures of this rock, mainly near the ancestral cores (shields) of the continents. Examples are in the Bushveld Complex in Africa, the Grenville Province of eastern Canada, the Lake Superior region in Minnesota, as well as in Nambia/Angola, India, Australia, eastern Europe, etc.). The anorthosite found in southern California is unusual because 1) it occurs near the edge of a continent occurrence and 2) its small extent.

Anorthosite has been found also on the Moon, in certain rare varieties of chondritic meteorites, in the comet Wild 2, and has been detected on Mars. In these occurrences, as well as on Earth, anorthosite represents apparently a primordial phase of planetary and other astronomical rocky material.

As evident in the image above (source Wikipedia.org), the Moon’s crust consists and two sharply contrasting materials. There is the mostly bright material, which is mostly anorthosite. It is confined to the lunar highlands. There is also the dark material, which is basalt (lava). It is confined to the maria regions. After the Moon formed about 4.5 billion years ago, it had a huge magma ocean over a solid interior. As the magma coolled, iron and magnesium silicates crystallized and sank to the bottom. Plagioclase-feldspar crystallized and floated up to form the anorthostie lunar crust. Later, about 4 billion years ago, magma rose and infiltrated the lunar crust, where reactions occurred and basalt was formed. Afterward, the surfaced was intensely bombarded by meteors, thus causing the cratered surface of the lunar highlands. When bombardment ceased, lava flowed from the inside of the moon through volcanoes and cracks in the crust. This lava filled the mare and cooled to form basalts (3.7 to 2.6 billion years old). Since then, some more meteors have hit the surface. The present-day regolith (soil) consists of fine dust.

  

Blister Pearls

A pearl is a calcareous object produced within the soft tissue (mantle) of an oyster (family Pteriidae). The sheen or luster (iridescence) of a pearl, referred to as nacre or “mother-of-pearl,” is caused by the intergrowth of microscopic crystals of aragonite (calcium carbonate) and conchiolin (a protein). Both are formed by the mantle tissue (soft part) of an oyster. As I have mentioned on some of my previous posts, nautiloids and some gastropods can make nacre. Nacre is relatively soft and it ranges from 2.5 to 4.5 on the Mohs scale. A copper penny has a hardness of 3.5 and a knife blade has a hardness of about 5.5. 

As a “side note”: aragonite can form two ways: inorganically or organically. If it is inorganically formed, then it is a mineral. If it is organically formed, then it is a biomineral. Only biogenic aragonite has the nacreous sheen.

The purpose of this blog post is to focus on the blister pearls or “half pearls,” which are the result of part of the growth of the pearl against the inside of the shell, rather than totally within mantle tissue. Blister pearls can be as big as 10 mm diameter. Blister pearls are not valuable, but they can still have beautiful iridescence.

This image shows several blister pearls (each one about 7 mm diameter) on a cut-out portion (6 cm diameter) of a pearl-oyster shell (genus, species, and provenance unknown). 

Pearls can be formed by freshwater (river) oysters or by marine oysters. In some cases, other types of bivalves, both fossil and modern-day, are known to produce pearls. These fossil pearls are usually not preserved very well and have been recrystallized to calcite, with a loss of the sheen or luster.

All pearls, whether they occur in oysters living in shallow-marine waters or in freshwater rivers, are formed in response to an irritant (e.g., a grain of sand becomes embedded, by chance, into the mantle). The mantle then secretes nacre around the irritant. Pearls that are cultured (note: they are not simulants) in aqua-labs have a tiny piece (machine rounded) of another oyster shell artificially introduced (“seeded”) into the mantle of the host oyster. Several species of the saltwater oyster Pinctada and, to a much lesser extent Pteria species, are the main pearl-producing oysters.

Two views (exterior and interior) of a right-valve (7 cm height) of the saltwater oyster Pinctada sterna (provenance unknown), with an unusually large (2 cm diameter) blister pearl. This species ranges from southern California to Peru and can be as much as 10 cm in height. The posterior wing on at the top of the left side of the first image is missing.

"Hammer Oysters"

Today, in tropical oceans, there are approximately eight species of unusually shaped bivalves (clams) called “hammer oysters.” That name is because their shells have a “T” shape and can somewhat resemble oysters. “Hammer oysters” live predominantly in coarse sands on reef flats or in crevices of coral rocks in warm shallow-marine waters. They belong to the family Malleidae Lamarck, 1818, which has a geologic range from Jurassic to Recent. In this post, I am focusing on the malleids that belong to genus Malleus Lamarck, 1799, which is known only from the recent record in the Indo Pacific, Australia, and the Caribbean. The name Malleus is the Latin word for hammer.

Malleus albus Lamarck, 1819: right-valve exterior of a late-juvenile specimen [height 162 mm, width 27 mm] (an adult specimen is twice this size). Shallow water on rock flats; common; Indo-Pacific waters.

The attached small shell about half-way down on the valve is a cemented oyster.

This is the left valve of the same specimen.

Same specimen but showing the interior of each valve (left valve on the left side of image, right valve on the right side of image).

The next series of images shows Malleus malleus Lamarck, 1819: adult specimen [height 95 mm, width 175 mm]. Indo-Pacific region. Shallow water; common; Indo Pacific waters.

Right-valve exterior.

Left-valve exterior.

Interior of both valves (right on bottom, left on top)

Internally, the shell of Malleus has a prominent, circular to shortly oblique (V-shaped) depression (ligamental pit), near the top center of the hinge. This depression is where a leather-like, strong ligament helps to hold the two valves together.

The image above is from the same right valve shown immediately above.

The hinge line of Malleus has also a byssal notch just anterior to the ligamental pit. This notch is where byssal threads emanate from the hinge, and these strong threads are used for attachment of the shell to the substrate.

There is a wide and prominent nacreous ("mother of pearl) area below the hinge line, but this area does not continue too far. Along the posterior side of this nacreous area is the adductor muscle scar (elliptical shape). This strong muscle does much to keep the valves together during life.

The characteristic narrow projections (“wings”) along the hinge of Malleus serve as stabilizers, allowing the bivalves to stay in place in the sands, and not be easily dislodged by the swirling currents. Some species of Malleus can lay unattached (free) on the sand, and others can be wedged in crevices. Malleus is unlike true oysters, which are attached (cemented) to the substrate.

The largest snake of all time

Most likely, the snake that comes to mind when you read this title is the living anaconda. Actually, the world’s largest snake is a fossil. It is the middle Paleocene (58 to 60 million years) Titanoboa cerrejonensis Head et al., 2009, from Cerrejón, northern Colombia, South America. Titanoboa belongs to family Boidae, which includes boas and anacondas. 

This sketch shows a scaled comparison between the size of a 6-foot tall human and a reconstructed 42-foot (2,500 pound) gartantuan Titanoboa cerrejonensis. 

Based on an exceptionally large vertebra, some fragments of a jawbone, and other bones, Titanoboa cerrejonensis has been calculated, on the basis of mathematical modeling, to have been up to 42 feet (12.8 m) long and weight more than one ton (about 2,500 pounds). The very interesting details of how the remains of this snake were originally discovered were written up in an article (“How Titanoboa, the 40-foot-long snake was found”) in the Smithsonian Magazine (April, 2012). This article is available online for free.

Titanoboa cerrejonensis lived in a swamp area within a Paleocene tropical-forest environment. Although this snake resembled a modern-day, tree-dwelling boa constrictor (which can be up to 14 feet long and 100 pounds), T. cerrejonensis behaved like the South American semi-aquatic green anaconda (Eunectes murinus), which is up to 15 feet long and about 500 pounds. Based on the remains of the jawbone, researchers determined that the head of T. cerrejonensis was about two feet long and had many more teeth than anacondas, which, as mentioned above, are classified also as boas. Titanoboa cerrejonensis probably ate crocodiles, turtles, and fish. 

While on the topic of snakes, I thought that you might want to know when the earliest true (no legs) snakes appeared in the fossil record. The answer is about 100 million years ago during the Cretaceous Period (late Albian to Cenomanian time). These earliest snakes probably evolved from burrowing lizards.

Lastly, you might find it interesting to know that the earliest venomous snakes (e.g., cobras and sea snakes) appeared 25 million years ago (late Oligocene/early Miocene). 

The Importance of Morphological Variation

 This post is about a concept that is a very important in the study of recent and/or fossil shells. It is "how much variation in morphology (size, shape, etc.) does a species have?" One would expect differences in size between juvenile and adult shells of a species, as well as differences in color (which would not be a concern for fossils). The over-riding question becomes, however, how much variation occurs in a population of individuals of a species. The best way to attempt to answer this question is to collect as many specimens as possible, in order to establish a baseline for what is the normal range of morphology for a particular species.

The scale is in centimeters.

In the case of the mangrove gastropod Tympanotonos fuscatus from Casamaance in Senegal, West Africa, it would be necessary to collect at least six specimens in order to establish its morphological variation. It extremely important to know that they all lived together at a single locality. The largest specimen shown here is 49 mm in height; the smallest specimen is 36 mm in height.

The above series of specimens helps to establish that the morphology of this species shows an insensible gradation from beaded to spiny forms. Otherwise one might mistakenly believe that these specimens represent six different species, which would give erroneous information about the biodiversity of gastropods at this one locality.

The degree of morphological variation in mollusks that live in shoreline environments (for example, mangrove swamps and tidal flats) are subject to much variation in salinity, turbulence, water depth, water temperature, and so on. These kinds of environments create micro-environments, which, in turn promote morphologic variation. This kind of information, if available for a particular family or genus, should always be used in paleontological studies, so as to avoid the over naming of species.