"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.

Stalactites and Stalagmites

My previous post concerns the topics of groundwater and water table, both of which are crucial in understanding the present topic.

In limestone terrains, when rain water, which has become acidic by interacting with decaying plant matter, percolates into the ground and goes into the water table, water-filled caves are created in the limestone. If the water table drops during a drought, then these caves become air-filled areas with dripping water that forms stalactites on the ceilings of the caves and stalagmites on the opposite floor directly below.

The image above is from Wikipedia.org. No location or a scale were given.

If a stalactite and its corresponding stalagmite join together (as shown in the image above), then a stalagnate is formed. 

Stalactites and stalagmites consist of layers (rings) of polycrystalline aggregates of calcium carbonate, which is the material that makes up limestone.

Cross section of a stalactite or a stalagmite (7 cm diameter). Most of the surface is encrusted, but underneath the encrustation, the growth layers are visible. Locale unknown.

The next three views show the side views of the same hand specimen:

Left side (7 cm across). You can see the "popcorn" structure of the calcium-carbonate deposits,

Next side (5.5 cm) across. The "popcorn" structure dominates this surface.

The "back side" (7 cm wide) of the specimen, showing annulations but no "popcorn.'

Cave "popcorn,' also called coralloids, consists of small nodes of calcite or aragonite that form when water fills the pores in these minerals and then air flows over the surface. Caves with "popcorn" are big, wet, and breezy (e.g., South Dakota's Black Hills region).

If you go back and look at the images, you will see that the internal structure of this hand specimen has no "popcorn." The"popcorn" occurs only on one side although the beginnings of it begin on the reverse side. I conclude, therefore, that this stalactite or stalagmite had a relatively dry beginning, followed by a time when the cave-environment became wetter and also experienced breezy conditions that were unidirectional (causing the "popcorn" to form on the down-wind side). The annulations on the other side show that the wind affected that side also, but the water was blown along the sides of the column and accumulated on only one side, where the calcite nodules formed.

Stalactites and stalagmites grow extremely slowly, usually less than 10 cm (4 in.) every thousand years. The largest known stalagmite is 8.2 m (27 ft.) long. It is in "The White Chamber" of the Jeita Grotto in Lebanon.

A RETROSPECTIVE

I started my blog in May 2014. This past week, after six and a half years, the number of people reading my blog posts reached 100,000.

I taught geology/paleontology for over 40 years, and during that interval, I taught only a total of  approximately 12,000 students. My blog has reached eight times more people. 

Currently, I have 212 blog posts available for viewing. The topics range from ammonites to the gastropod genus Xenophora. Please use the Google Search engine link at the top of each of my posts in order to see if I have written a post about anything that interests you in the world of paleontology/geology.

Based on the records kept by Google, my all-time most popular post (it has three parts) concerns the geology/paleontology of Mt. Everest. I wrote them up back in late March, 2016. To access them, type in Mt Everest geology or Mt Everest fossils in the above-mentioned search box.

Some other of my most popular posts are:

Megalodon (largest shark teeth of all time)

Dimetrodon (an early mammal-like reptile)

Desmostylus (an extinct shallow-marine, hippo-like mammal)

St. Francis Dam Disaster in California (two parts)

Mystery Sand Spheres (original + two follow-up parts)

Eohippus (the first horse)

Sodalite vs. lapis (beautiful royal-blue minerals)

Campanile (largest snail ever, you have to see it to believe it)

I shall continue to write up my educational blogs and bring you up-to-date, straight-forward, reliable, and readable information.

Thank you all for your interest and support.

Groundwater

Groundwater is rain and snow melt that percolates into the ground and accumulates to a depth that depends on the amount of rainfall and/or snowmelt. This depth is known as the water table, which can rise and fall with the changing of the seasons.

Groundwater moves downhill, and where erosion cuts below the water-table level, there will be leakage of groundwater into a stream area (or, if the flow is low, there will be springs and seeps).

Groundwater moves very slowly; if pollutants are poured into the ground and reach the water table, it might take centuries for the pollutants to be flushed from the local area.

As depicted in the diagram above, each water well that takes out groundwater lowers the water table in the immediate vicinity of the well; if too many water wells are pumping out groundwater, then the water table will drop an the wells will have to be deepened = an expensive undertaking.

The topic of my next post concerns what happens to groundwater in limestone terrains. 

Hippochrenes amplus: an elegant gastropod

One of the more unusual looking but memorable middle Eocene gastropods (snails) is Hippochrenes amplus (Solander, in Brander, 1776). It is unusual because of its large and very wide “wing.” This genus belongs to superfamily Stromboidea and, depending on which researcher's classification system you choose to use, the genus belongs to either family Rostellariidae, the latter of which includes also the genus Tibia (see previous post), or belongs to family Hippochrenidae. 

The first picture shown here is a published sketch, made by Solander (in Brander, 1766), of a complete specimen of Hippochrenes amplus from England. 

The original name of this gastropod was Strombus amplus Solander. Solander (1733–1782) was a pupil of C. Linnaeus, the Swedish naturalist and originator of the Linnaean (1758) system of nomenclature of lifeforms. Solander's paper describing Strombus amplus might be the first time Linnaean nomenclature (i.e., a binomial name) was used to name a fossil. 

Solander's species was was later transferred to the genus Hippochrenes Montfort, 1810 (v. 2). This genus name is derived from the early 17th century, via Latin and from Greek, and refers to "hippocrene," literally "a fountain of the horse" (from hippos [horse] + krene [fountain]). The latter refers to a "fountain' on Mount Helicon,  sacred to the Muses. According to legend, the fountain was produced by a stroke of Pegasus' hoof (www.lexico.com). 

This next picture is of a specimen of H. amplus that I collected from an upper middle Eocene bed in southeastern England. It is 152 mm in height, but its fragile "wing" (along the right side of the shell) is mostly missing.

The next two pictures are also of an actual specimen (height 125 mm, incomplete) of Hippochrenes amplus collected by my colleagues from the same, above-mentioned fossil bed containing shallow-marine gastropods and bivalves in southeastern England, but, unfortunately, the specimen is not complete. Nevertheless, you can see at least half of its "wing," as well as  the whorls (revolutions) of the shell, except the anterior end of the shell.

Hippochrenes amplus is best known from upper middle Eocene (Bartonian Stage) strata in southeastern England. Specimens are known also from Eocene strata elsewhere in Western Europe.