A clam for the ages

Venericardia is a widely distributed genus of shallow-marine bivalves (clams) in the family Carditidae. This genus was abundant during the Paleocene and Eocene epochs (a cumulative range of 66 to 34 million years ago). The highest biodiversity of Venericardia was in the Eocene. The genus needs detailed classification studies of all its various subgenera found throughout the world. Detailed studies are much needed to evaluate the likely possibly of over naming of species/subspecies found on the west coast of North America. The genus is now extinct, with the last survivors of this genus dying out apparently in the early Miocene.

Lamarck (1801) first described the genus Venericardia based on well preserved shells of this bivalve in the Paris Basin, France. The aragonite shells of this genus are commonly large and very sturdy, with wide radial ribs crossed by concentric growth lines on the exterior. The interior of the shells are characterized by long posterior teeth, much shorter anterior teeth, and two prominent elliptical-shaped muscle scars connected ventrally by a continuous line. Some species have prominent nodes along the inner margins of the shell.

These two images show the plaster replicas of the exterior and interior surfaces of a left-hand valve (6.5 cm high and 7 cm wide) of Venericardia planicosta Lamarck, 1801 from an Eocene shell bed at Grignon, Paris Basin, France. 

Venericardia (Pacificor) lutmani Turner, 1938, lower Eocene (Ypresian Stage), southwestern Oregon; plaster replicas. Left image is the exterior of a left valve, and the right image is the exterior of the right valve of a single specimen (9.25 cm high and 10 cm wide) which, upon burial, became separated from one another. 

These two images are the corresponding interior views of the same plaster replica shown immediately above.

Venericardia (Pacificor) hornii calafia Stewart, 1930, middle Eocene (Lutetian Stage), southern California. Exteriors of left-hand and corresponding right-hand valves of the same specimen (whose valves [9 cm high and 9 cm wide] are closed very tightly).

Dorsal (hinge) view of same specimen of V. (P.) h. calafia shown immediately above. Left valve is on the left side of image, and right valve is on the right side of image

Venericardia was an infaunal (burrowing) suspension feeder that lived buried just beneath the surface-water interface. Its optimum habitat was in relatively deep, shallow-marine (shelfal) environments, but its shells are commonly found as transported remains in coastal-storm beds.

Stingray stinger and teeth

Stingrays belong to the cartilaginous fishes [Class Chondrichthyes], which includes sharks, stingrays, electric stingrays, skates, and sawfish. Cartilage is soft, unlike true bone, and is very difficult to preserve. Fossilized remains of cartilaginous fishes consist mainly of their teeth, spines, dermal scales, and, in some cases, vertebrae.

Stingrays evolved from sharks, when the shark's dorsal fin developed into a whip-like tail. Sharks have a long geologic history, extending back to the Devonian (about 400 million years ), whereas stingrays extend back only to the Early Cretaceous (about 140 million years or so). 

Bat rays, named for their wing-shaped pectoral fins, are a variety of stingrays. Bat rays are euryhaline, which means that they can tolerate a wide rang of salinities, hence they are found in estuaries, bays, and rocky-bottom shorelines (including kelp beds).
  
Bat rays feed on mollusks, crustaceans, and small fish. They use their wing-shaped fins to move sand and mud, thereby exposing their prey. Bat rays are common along the eastern Pacific coast between Oregon and the Gulf of California. 

The maximum size of a bat ray is about 200 pounds, with a wingspan of about 6 feet (see my artwork below).

All stingrays have a venomous stinger (spine) near the base of their tail. The two views (lateral and side) shown above are of a complete stinger, 8.3 cm long and 4 mm wide, are from a modern-day, full-sized adult specimen of the bat ray Myliobatis californicus from Ventura, southern California.  

As shown below, stingray teeth are flat and form tightly-packed rows. Unlike sharks, which have sharp and piercing teeth for eating fish, stingray teeth are used only for crushing and grinding shells (clams, crabs, etc.). The crushed shell is inedible and is ejected, whereas the soft parts of these shelled animals are then swallowed. 

Articulated stingray teeth in both jaws of a full-sized, modern-day adult Myliobatis californicus from Ventura, southern California. 

Stingray teeth fall out and are replaced continuously in living specimens. Fossil stingray teeth are, therefore, moderately common locally. They occur mainly as fragments of partial sets of teeth.

Miocene Round Mountain Silt (14 million years old); largest set of teeth 24 mm length. 

Middle Eocene Llajas Formation (47 million years old), Simi Valley; 45 mm length.

Middle Eocene Gosport Sand (42 million years old), Little Stave Creek, Alabama; longest set of teeth 15 mm. 

Fossil freshwater stingrays, some of which are nearly complete and articulated, are known from deposits as old as Paleocene age (about 60 million years old) in Italy and Germany. Other well preserved freshwater specimens are known from Eocene deposits in Wyoming and Miocene deposits in Indonesia.

A middle Eocene heart urchin

Heart urchins, also called spatangoids, are echinoderms (sand dollars, sea stars, etc.), which are generally characterized by having 5-rayed (pentameral) symmetry. This post focuses on a middle Eocene heart urchin known as Schizaster diabloensis Kew, 1920. It was named for its occurrence in sedimentary layers near Mount Diablo, just east of San Francisco.

A hand specimen of siltstone rock from the Llajas Formation has three specimens of
S. diabloensis on the same bedding plane. The hand specimen is 5 cm (2 in.) wide.

This species of heart urchin was common in northern and southern California during the middle Eocene (approx. 47 million years ago). The specimens shown here are from the Llajas Formation in Simi Valley, California. This formation was deposited in shallow-marine, warm-water conditions. The entire geologic time range for this species is late Paleocene through middle Eocene.

Five specimens of S. diabloensis from the Llajas Formation. The largest specimens are
  2 cm (0.8 in.) wide. All are top-side up.

Echinoderms, past and present, are strongly gregarious and can occur in great numbers on the ocean floor. Spatangoids have a fossil record extending back to the Cretaceous. They are burrowers and living below the surface provides protection against predators. During the Cretaceous, many new forms of predators evolved, which, which gave the force for some echinoderms (like spatangoids) to adapt to these adverse conditions by becoming infaunal (i.e., burrowers), mainly in fine-grained deposits, like siltstone.

You can readily see the five-rayed symmetry of the feeding grooves on the dorsal (top) surface of each specimen. The central groove, called ambulacrum III, is the longest and is sunken on most spatangoids, whereas the two posterior grooves are smaller. 

A late Paleocene Turritella

This is my second blog in a series of eight that deal with the same subject (plus a ninth one that summarizes the eight). It shows a late Paleocene Turritella species, which is morphologically different from the Late Cretaceous species that I previously showed.

height 65 mm

height 43.5 mm

                                                                                   

Turritella infragranulata Gabb, 1864

AGE RANGE: Cenozoic (late Paleocene = about 58 million years ago).

GEOGRAPHIC RANGE: Northern to southern California.

REMARKS: This large species is very distinctive because of the nodes on some of its spiral ribs.

William More Gabb, who was the first paleontologist of the Geological Survey of California, described and named this species in 1864. If you wish to read an interesting article that I wrote a few years ago about his profound contributions to the paleontology of California, visit the California Geology website (see below)--

In their search box, for Title of Article, type %William More Gabb% and be sure to put a % symbol before and after the name. In the second line, type 1999; and in the third line, type paleontology.

for the link, click HERE and then click "Go to link"

The link should take you to vol. 52, issue 04, and p. 11. Also, while you visit this website, you will see many other useful articles on California Geology. Although you can read their articles online, it seems that you cannot download a pdf for your own use.

NEXT POST:  AN  EOCENE TURRITELLA