Whale Shark

The modern-day whale shark Rhinocodon typus (family Rhinocodontidae) is dark gray in color with white spots and stripes. Its body has a very wide and squarish head (up to 5 feet wide) with many (about 300), rather small, single-cusped teeth. The whale shark is commonly referred to as a whale because of its large size, but Rhinocodon typus is a shark, not a whale. A whale is a mammal, and the whale shark is a giant filter- feeding elasmobranch (shark = a fish). The whale shark is the largest known shark today, thus it is the  largest known living fish. It can up to about 40 feet in length. The first three images show the left side, top, and front (mouth) views of a model of R. typus.

The whale shark is a slow-moving fish that is found globally today, except in the Mediterranean Sea. It likes tropical waters, and is known to seasonally congregate in the following areas: California to Chile, especially in the Gulf of California (=”Sea of Cortez”) in Baja California (Mexico); Yucutan (Mexico); Belize; Western Australia (especially lagoons and coral atolls) to Japan; Maldives Island (Indian Ocean); and New York to central Brazil.

Whale sharks feed on microscopic zooplankton (copepods) and, to a lesser degree, on small fish, fish eggs, and small squids. They are filter feeders: they have modified pads (gill rakers) along their gills and can screen the water for food as it moves through the gills. For their larger prey, they use suction feeding (sucking in large quantities of water containing the food). Whale sharks are harmless to humans. Their teeth are small.  

 

As shown in the diagram above, the earliest known “whale” sharks lived approximately 56 million years old (late Paleocene or early Eocene in age) and belong to the genus Palaeorhinodon. Their teeth, which are small and single cusped, have been found in Africa, central Asia, Europe, and eastern North America (e.g., South Carolina and Virginia). Their geographic distribution coincides with the ancient Tethys Sea, which stretched from Java and India and westward through southern Europe. These warm waters affected, as well, the east coast of the United States. To my knowledge, they did not live along the west coast of the United States during this time even though the climate was warm there. 

Also, as shown in the diagram above, the first known whale shark belonging to genus Rhinocodon appeared during the middle Oligocene, about 28 million years ago. Their teeth, which are also small and single cusped, have been found along the east coast of North America, where warm warms lingered. They did not occur along the west coast of the United States because of continued climate cooling in this area. The fossil record of Rhinocodon ranges from the middle Oligocene to modern times.

In the modern oceans, in addition to whale sharks, there are two other similar looking, giant filter-feeding elasomobranchs: megamouth sharks, and basking sharks. Their geologic time ranges are younger than that for whale sharks (see diagram below).

Megamouth (family Megachasmidae, genus/species Megachasma pelagios) is another large shark that eats plankton. This shark ranges from the late Oligocene/early Miocene, about 23 million years ago, to today. Recent individuals can reach 12–15 feet long. This fish was

unknown to science until 1976, when a specimen was found entangled in a parachute anchor of a U.S. oceanographic vessel off of Oahu, Hawaii. This shark is named for its enormously distensible mouth and protrusible jaws containing overly 100 rows of small hooked teeth. It has a poorly calcified skeleton, therefore, it is a weak swimmer. A drawing of it is shown below:

Furthermore, “Megachasma pelagios was discovered in 1976 off the coast of the Hawaiian Islands. The specimen shown below is the second megamouth ever found. It was caught by commercial fishermen in 1984 near Catalina Island, southern California. it is a 14.7-foot make, weighing approximately 1,540 pounds. In 1990, a live megamouth shark was caught off the coast of Dana Point, southern California. It was fitted with sonic transmitters and released. The transmitters revealed that the shark spends most of its time well below the surface, spending the night 50 feet below the surface, diving to 490 feet at dawn, and returing to shallower waters at dusk. Many marine animals display this vertical migration pattern as they follow plankton in the water.” (From signs displayed at the megamouth exhibit at the Los Angeles County Museum of Natural History, 2021). A head shot of this actual specimen is shown below:

The basking shark genus/species Cetorhinus maximus) is shown below. It is the world’s second largest fish (36 feet long). Like the whale shark and megamouth, the basking shark is a surface-feeder that eats plankton. It prefers temperate waters and shuns polar and equatorial waters. The geologic range of C. maximus is middle Miocene (15 million years ago) to today.

 

 

BIOMINERAL VS. MINERAL

 According to mineralogy textbooks, a mineral is a solid INORGANIC substance of natural occurrence. That definition rules out human-made simulants (e.g., cubic zirconia and nearly all mossanite --- see one of my recent previous posts). That definition also rules out organic substances secreted by organisms, even though these organic compounds have the same (or nearly same) chemical composition as “true” minerals. What to do? The answer is to use the terms biomineral, organic mineral, or biogenic mineral when dealing with organically secreted shells, teeth, bones, kidney stones, etc.

All five kingdoms and 55 phyla of organisms contain members that create biominerals. At last count, there are at least 60 known biominerals, but new ones are being discovered all the time. Most (80%) are crystalline, and some (20%) are amorphous. Many of these biominerals can be in the following groups (with examples given below): carbonates (calcite, aragonite), phosphates (dahllite, francolite), sulfates (gypsum, barite), silica (opal), iron-oxides (magnetite, goethite, ferrihydrite), iron sulfides (pyrite), halides (fluorite), and oxalates (weddelite).

Three comparisons of a biomineral versus its mineral counterpart are given below: 

FIRST COMPARISON: The biomineral that makes up vertebrate (including humans) bones and teeth is called dahllite [= hydroxylapatite (or also spelled hydroxyapatite)] consisting of Ca5(PO4)3(OH), which is = 45% calcium-rich phosphate, 33% organic matrix material (mainly collagen), and 22% water.

The following image is of an Eocene mammal tooth (5.3 cm length) consisting of generally well preserved dahllite:

The mineral apatite Ca5(PO4)3(F, Cl, OH) is an inorganic substance consisting of calcium phosphate combined, in varying amounts, with fluorine, chlorine, and hydroxyl ions. The mineral apatite, furthermore, is an end member of the complex apatite group, or series, of minerals with varying amounts of F, Cl, and OH. The mineral apatite has been long been known as a calcium-phosphate series of minerals found in igneous rocks (especially in hydrothermal veins) and in phosphate-rich sedimentary rocks. In summary, the biomineral dahllite [= hydroxylapatite] and the mineral apatite are chemically quite similar, and the difference is based on the content of the elements fluorine and/or chlorine, which are both subject to variation depending on the surrounding environment.

The following image is of a crystal (2 cm height) of inorganic apatite, with a hardness of 5 on the “Moh’s Scale of Hardness” (0 to 10, with 10 being the hardest = diamond).

 

SECOND COMPARSION: The biomineral aragonite (calcium carbonate [CaCO3 + organic matrix material]) versus the mineral aragonite (calcium carbonate [CaCO3]). Biomineral aragonite, common in some invertebrates, especially certain gastropods, bivalves, and, cephalopods (nautiloids and ammonites) can have iridescent “mother-of-pearl” luster, which I have mentioned in several of my previous posts. This luster indicates that there has been no alteration of the original composition of the shell material. The mineral aragonite never has iridescent luster, as is obvious in the image below of inorganic aragonite.

The following image is of the inside of a modern-day abalone shell (18.7 cm long) showing its “mother-of-pearl” biomineral aragonite luster. See my previous post July 28, 2020 for more details.

The next image is of the interior of a modern-day Nautilus shell (16 cm wide) showing its “mother-of-pearl” biomineral aragonite luster. See my previous post August 1, 2016 for more details.

The next image is of radiating crystal clusters of two specimens of aragonite of inorganic origin. Small cluster 2 cm wide, larger cluster 4 cm long. Impurities cause the reddish color.

THIRD COMPARISON: The biomineral opal (SiO2-xH2O) versus the mineral opal (SiO2-xH2O). Biomineral opal (transparent) is present in the test [microscopic shell] of single-cell, micro-organisms like diatoms [plant kingdom] and radiolarians [animal kingdom], as well as in glass sponges. The mineral opal, which can have an amazing array of colors but not transparency, is present in some sedimentary deposits.

The following two images are of the biomineral (opal), which forms the tests of a modern-day, microscopic radiolarians from deep-ocean sediment.

                                          single radiolarian

                             six more examples of radiolarians

The following image is of the biomineral (opal), which forms the exoskeleton (21 cm long) of Euplectella, a modern-day glass sponge. See my March 15, 2020 post for more details.

The last image is of a fragment (1 cm) of the mineral (opal) of precious opal. See my post Dec. 14, 2018 for more details.