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 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.