Cucullaeid bivalves of California

Cucullaeid bivalves (clams) are locally common fossils in Jurassic through Eocene shallow-marine rocks throughout the world. They are most abundant in Cretaceous rocks. Today, they are represented by a single species, which lives in warm waters from northern India to Japan and New South Wales, Australia. Malacologists (scientists who study recent mollusks) use the name Cucullaea labiata (Lightfoot, 1847) to refer this modern species. Cucullaea labiata is, furthermore, the type species [= the official reference species] that defines genus Cucullaea Lamarck, 1801. The following two images are of this species.

Cucullaea labiata (Lightfoot, 1786), Recent. The normal size of this species is approximately 10 cm (3.9 in.) in length (left side to right side). The upper image shows both valves. The left valve is in the foreground. The interior of the right valve (in the background) shows the long and straight hinge line, which bears a row of very tiny essentially vertical teeth along most of the hinge, as well as  two much larger, horizontal teeth at both ends of the hinge (see the enlarged lower image). Image source: en.wikipedia.org

Although the name Cucullaea is used today for modern species of cucullaeids, at least 20 other names of genera have been used to refer to the fossil species of cucullaeids. The hinges of the fossils are somewhat similar to, but distinct from, the hinges of the recent  cucullaeids; thus the name Cucullaea should not be used for the fossils. Many paleontologists have chosen, therefore, to refer to the fossil-cucullaeid species as Cucullaea (Idonearca), which employs the use of a genus and a subgenus name. Future detailed work might show, however, that the name Idonearca Conrad, 1862, is a separate and distinctive genus. 

The following images are of some of the C. (Idonearca) species found in the fossil record of California.

Cucullaea (Idonearca) gravida Gabb, 1864. Left valve, length 53 mm. Baker Canyon Sandstone Member, Santa Ana Mountains. Late Cretaceous TURONIAN age (about 93 million years old).

Same specimen but showing the hinge, which had to be cleaned. The rock is very hard and cleaning was difficult.

Cucullaea (Idonearca) cordiformis Packard, 1922. Right valve, length 70 mm. Chatsworth Formation, Simi Hills. LATE MIDDLE

 CAMPANIAN age (about 76 million years old).

Same specimen but showing the hinge. Again, the rock is very hard, and cleaning the hinge was slow and difficult. 

The next three images are of Cucullaea (Idonearca) mathewsonii (Gabb, 1864) of PALEOCENE age (about 60 million years old).

Santa Susana Formation, Santa Ynez Canyon, Santa Monica Mtns. Right valve, length 65 mm. 

Santa Susana Formation, Santa Monica Mtns. Right valve, length 77 mm. Like some other species of Cucullaea (Idonearca), this species has a considerable range in shape.

Same specimen as the immediately preceding one but showing both valves and the wide, large depression (gap) situated where the soft-material ligament was once located.

The following chart shows the geologic time divisions that correspond to the above-mentioned Idonearca species: 

C. (Idonearca) mathewsonii




C. (Idonearca) cordiformis






C. (Idonearca) gravida