Two Late Cretaceous species of the bivalve Pterotrigonia

In some cases, recognition of two species of the same genus can be problematic. In the examples shown here, however, the recognition  is straightforward. The species are of a shallow-marine bivalve (clam) of Late Cretaceous age from the west coast of North America. These species lived as burrowers in subtidal shelfal depths.

Pterotrigonia klamathonia, 4.5 cm length, Santa Ana Mountains,
Turonian age, Orange County, southern California.

Pterotrigonia klamathonia (Anderson, 1958), which is of Turonian age, is characterized by its closely spaced radial ribs, 20 to 25 in number.

Pterotrigonia evansana, 4.5 cm length, Campanian age,
Simi Hills, Ventura County, southern California.

Pterotrigonia evansana (Meek, 1858) is geologically younger and is of Coniacian through Campanian age (see time table below). This species is characterized by its widely spaced radial ribs, commonly 10 or so in number.

Genus Pterotrigonia, which is the state fossil of Tennessee, is extinct. This genus belongs to the family Trigoniidae, whose geologic time range is latest Jurassic through the end of the Cretaceous. The family was very widespread in the world during the Cretaceous.

Trigonarca californica

This post concerns a common Late Cretaceous bivalve (clam) that lived in California approximately 92 million years ago (Turonian time). It is Trigonarca californica Packard, 1922, which is known from northern California (Siskiyou County) to southern California.

The specimens shown below are from the Santa Ana Mountains of Orange County, and they were collected from the Baker Canyon Member of the Ladd Formation. As this locale, where specimens can be abundant, this species lived in sandy, warm, shallow-marine waters. A collector recently kindly donated these specimens.

Right-hand valve of Trigonarca californcia Packard. Length 4.4 cm.

          

This unusual specimen shows the somewhat separated valves of a formerly closed-valved specimen

of Trigonarca californica Packard. The hinge with its distinctive teeth are nicely preserved. Length  of the left-hand valve (at the front of the photograph) is 4.3 cm.

The sturdy shell of this species has the shape of a rounded triangle. Its teeth (dentition) are distinctive and consist of numerous, relatively heavy, short, straight teeth along its hinge.

Genus Trigonarca, which belongs to family Glycymerididae, was widespread, with occurrences in North America, Europe, South Africa, and India. Trigonaraca is of Late Cretaceous age.

A Late Cretaceous stalked crinoid stem

Crinoid remains are extremely rare in the Late Cretaceous fossil record of California. A friend recently donated a stalked crinoid-stem fossil collected from Upper Cretaceous rocks in the Santa Ana Mountains, Orange County, Southern California. I have seen many fossils from these rocks but never a crinoid. Its geologic age is Turonian (about 90 million years old). The genus of this fossil is unknown.

This specimen is 8 cm long and 3 mm wide. I also put a modern-day crinoid "stem" (from Cuba) alongside, for comparison; it is 6.5 cm long and nearly 3 mm wide. You can definitely see that the fossil is, indeed, a crinoid.

Crinoids are echinoderms. Some other examples are sea stars (starfish), brittle stars, sea urchins, and sand dollars. Crinoids were very common in Paleozoic faunas, and their remains have contributed substantially to Paleozoic limestones. Crinoids today are less abundant than they once were, but at the present time there are approximately 25 stalked genera (all attached to the ocean floor and restricted to depths greater than 100 m). There are also about 90 or so unstalked genera, and these are able to swim about when they are adults.

This drawing shows the main morphologic parts of a stalked crinoid (i.e., having a column or "stem"). The "stem" was originally somewhat flexible during life and could sway slightly with the prevailing water currents.

Both the fossil and modern-day columns shown above in the photo are missing their calyx (where the stomach was located) and their arms. 

A Cretaceous bivalve that looks like a feather

Inoperna bellarugosa Popenoe, 1937 (length 7.3 cm = 73 mm).

The bivalve genus Inoperna, which belongs to the mytilid family (includes mussels), lived during the Triassic (latest part), and the Jurassic to Cretaceous (a total geologic time range of just over 60 million years). It was globally widespread in warm seas, and it had its widest distribution during the Jurassic. It showed up on California during the Cretaceous.

The genus is characterized by a narrow elongate shell, with nearly parallel dorsal and ventral margins. Its surface is divided by a diagonal ridge, with numerous strong ribs above the ridge and smooth shell beneath the ridge. The shape and ribbing of this genus resembles that of a feather and is very memorable. Individuals of this genus apparently lived on the sea floor and were stationary.

The species shown above is from Turonian strata of Late Cretaceous age in the Santa Ana Mountains, Orange County. As you might remember from my last post, the Turonian (93.5–93 million years ago) was the warmest time of the Cretaceous Period.

The etymology (name derivation) of I. bellarugosa is from bella, Latin for beautiful, and rugosa, Latin for wrinkles. I think that you will agree that it is aptly named.