Some representative Late Cretaceous Gastropods from the Pacific Coast of North America

There are about 200 reported genera of Late Cretaceous (about 100 to 65 million years ago) shallow-marine gastropods known from the Pacific coast of North America. This post focuses on a few examples of them.

Tessarolax is one of the most "eye-catching" examples. It is an extinct genus characterized by having four long digitations, used to support its shell on soft ocean-floor sediments. The above images are of three different species at oblique views. The specimens range from 70 to 120 mm (max. dimensions). Tessarolax belongs to family Aporrhaidae (pronounced "a-poor-rye-id-dee"). Tessarolax, which is only of Cretaceous age, was widespread in the world.

These two images of Tessarolax are of two different species showing how their shells relate to the ocean floor (substrate). Both specimens are approximately 50 mm, from left to right. Very careful collecting and cleaning are required in order to obtain such high-quality specimens. For the latest information on Tessarolax and also on the details about the species represented by the above-illustrated specimens, see the reference: 

Saul, L.R. and R.L. Squires. 2015. Pacific slope of North America record of the Cretaceous aporrhaid gastropod Tessarolax: Evolutionary trends, mode of life, and paleobiogeography of the genus. Contributions in Science v. 523, pp. 37–65. (pdf available for free---just "Google" the title of the paper.

Anchura is another example of an aporrhaid gastropod having an extension of its shell in order to provide stability on the ocean floor.
For more information on Anchura, see the reference:

Elder, W.P. and L.R. Saul. 1996. Taxonomy and biostratigraphy of Coniacian thorugh Maastrichtian Anchura (Gastropoda: Aporrhaidae) of the North American Pacific slope. Journal of Paleontology 70(3):381–399. (pdf available for free---just "Google" the title of the article).

These two specimens of the gastropod Xenophora look like two small stacks of pebbles. This genus, which is extant, is known as colloquially as the "carrier shell" because it collects and carries pebbles and/or other shells (by cementing them onto its own shell), in order to provide protection/camouflage for itself. In essence, it was the first fossil collector. For more information on this fossil Xenophora, see the reference:

Squires, R.L. and L.R. Saul. 2001. New Late Cretaceous gastropod from the Pacific slope of North America. Journal of Paleontology 75(1):46–65. (pdf available for free---just "Google" the title of the article).

This specimen (height 50 mm) belongs to genus Paosia, formerly known by its junior synonym name of Trajanella. Paosia has a long geologic time range of 75 million years. It is mainly known from Europe, but a lineage became established along the Pacific coast of North America for 25 million years. For more information, see the reference: 

Squires, R.L. and L.R. Saul. 2004. The pseudomelaniid gastropod Paosia from the marine Cretaceous of the Pacific slope of North America and a review of the age and paleobiogeography of the genus. Journal of Paleontology 78(3):484–500.

Trochactaeon (T.) packardi (see my Post 39 of Feb. 3, 2015): An unusual Late Cretaceous gastropod from California. The specimen on the left is 6 cm height and shows the front view. The specimen on the right is 4.6 cm height and shows the back view. This genus, which was restricted to the Cretaceous, was widespread, including California, in warm waters during the Late Cretaceous.

Alamirifica corona genus and species Saul and Squires, 2003; two views, front and back (height 37 mm). Late Cretaceous (Turonian), southern California. This specimen is exceptional because it shows all the exquisite details of its aperture. For more information about this species, see the reference: 

Saul, L.R. and R.L. Squires, 2003, New Cretaceous cerithiform gastropods from the Pacific slope of North America. Journal of Paleontology 77(1):442–453.

Pentzia hilgardi (White, 1889); two views, front and back (height about 10 mm). Late Cretaceous (Campanian to early Maastrichtian). Genus Pentzia consists of only a single species, which is found only in Washington, California, and northern Baja California (Mexico). This species has an usually long geologic time range, which is approximately 13 million years. Most gastropod species last for much shorter time intervals. For more information about this genus and species, see the reference:

Squires, R.L and L.R. Saul, 2003, New Late Cretaceous (Campanian and Maastrichtian) marine gastropods from California. Journal of Paleontology 77(1):50–63.

Igneous Rocks

Igneous rocks are derived from magma = "red-hot," molten matter emanating from inside the Earth.

The above diagram shows the basic classification of igneous rocks. A substitute rock for rhyolite is any glassy textured rock, like pumice, obsidian, or pele's hair. Another name for "dunite" is peridotite.

This cross section of the Earth' crust shows where the major types of igneous rocks form. Note: basalt = flood basalt (on the continents) and pillow basalt (on the floor of the oceans).

This diorama, which is a more graphic way of showing the table presented at the beginning of this post, shows representative pictures of the major types of igneous rocks. These specimens are mostly hand-size or smaller.

The rocks above the centimeter scale in the diorama are volcanic (extrusives) that cooled quickly from magma extruded onto the Earth's surface, whereas those below the scale are plutonic (intrusives) that cooled slowly from magma bodies inside the Earth.

For each extrusive rock, there is a counterpart intrusive rock. For example, pumice (as well as rhyolite), and their underground (intrusive) counterpart granite, generally have similar chemical compositions (i.e., they both have minerals rich in quartz and potassium feldspar). Andesite and its underground (intrusive) counterpart diorite have their own similar chemical composition (e.g., they both have minerals low in quartz and intermediately high in potassium feldspar). Basalt and its underground (intrusive) counterpart gabbro have their own similar chemical composition (e.g., they both have minerals without quartz and those high in potassium feldspar).

specimen 9 cm height

 Pumice, from the 1980 eruption of Mt. Saint Helens, Washington.

Side view of the same specimen, showing the layers of pumice.

specimen 8.5 cm height

A different type of pumice (locality unknown), showing "air holes." Pumice is so "light" in density, it can float on water.

specimen 5 cm height

Obsidian is volcanic glass with a low amount of iron, which produces the black color. Obsidian forms by rapid cooling. This kind of rock, which breaks into very sharp pieces, is used commonly for making arrowheads.

specimen 8.5 cm height

"Snowflake obsidian," a natural volcanic glass containing the mineral cristobalite (= the white "snowflake" crystals consisting of radially clustered crystals). The cristobalite formed due to the partial crystallization of the glass. This specimen is from Millard County, Utah.

specimen 7 cm long; it comes apart live a stand of hair

Pele's "hair" is a volcanic glass spun by blowing-out during quiet fountaining of fluid lava. It is named for the Hawaiian goddess of volcanoes, Pele.

specimen 22.5 cm height

This large cobble of rhyolite shows "flow banding" caused by flowage of crystals before the magma solidified.

specimen 9.5 cm height

Crystals in this piece of granite are visible because the magma cooled very slowly (thousands of years) underground. The large pink crystals are potassium feldspar. The black mineral is biotite, and the other minerals are quartz (gray) and sodium-rich plagioclase (white).

specimen 9.5 cm height

Andesite (a common rock in the Andes Mountains of South America-hence the name "andesite"). The crystals are not visible because they cooled quickly and did not grow to a larger size.

specimen 6 cm height

Diorite is the intrusive-rock equivalent of andesite. Hornblende is a common constituent, as is intermediate form of plagioclase (a mixture of both sodium-bearing and calcium-bearing plagioclases).

specimen 11 cm height

Basalt is the most common rock on the ocean floor; in fact, its makes up the ocean floor if you subtract the mud and sand that settled on top of it. The iron content of basalt makes it black in color. Basalt is also found on the Moon = the dark patches (the mare).

specimen 7m height

This is a basalt stalactite (a dripping from the top of a lava tube in New Mexico). 

specimen 13.5 cm height

This basalt-lava volcanic "bomb," while still molten, twisted in the air before hitting the ground.

specimen 4.5 cm width

Basalt can contain inclusions, called xenoliths (="stranger rocks"), which consists of a fragment of a plutonic rock. The xenolith was picked up by the basaltic magma as it moved upward toward the surface of the Earth.

specimen 3.5 cm height

Gabbro has visible crystals because it cooled slowly underground. This rock, which is very low in quartz, has abundant hornblende and calcium-rich plagioclase. Gabbro is found also on the Moon = the "white" patches.

specimen 9 cm height

This rock, called dunite, consists almost entirely of olivine.