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.