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.

Freshwater Bivalves

Freshwater bivalves (clams), which are commonly referred to as freshwater “mussels,” are found in rivers, streams, and lakes throughout the world. Historically, they have been especially diverse in eastern North America (e.g., Tennessee, Alabama, Georgia, North Carolina, and Viriginia). In the 1800’s, their shells were used for making “pearly buttons” because the interiors of their shells are irisdescent (having rainbow-like colors).

This post highlights three examples of extant freshwater clams. They belong to the bivalve family Unionidae, which has a geologic record extending into the Mesozoic. Shells belonging to this family (the unionids) are characterized by having a thin, brown to dark green external shell layer (periostracum) and a much thicker, beautifully iridescent interior shell layer (nacreous layer). The nacreous layer is commonly not preserved, however, in geologically older shells. The outer layer helps protect the shell from acidic waters. Their hinges, like the overall shell, is solid, heavy, and bearing prominent dentition (teeth).

Plagiola lineolata, eastern North America. This species ranges from the Pleistocene to Recent. These first two views are right-valve exterior and interior of a specimen with length 5.3 cm and height 4.1 cm. Note the iridescence of the interior of the valve.

This view is of the right-valve interior of another specimen of P. lineolata: specimen length 8.3 cm, height 6.2 cm. The hinge of the right valve of this species has a single cardinal tooth and a single lamellar tooth on the right valve.

Quadrula quadrula (Rafinesque, 1820), eastern North America. This is the exterior of a right valve: length 7.6 mm, height 6.6 mm. This species ranges from Pleistocene to Recent. Notice the pustules on the surface. Many unionid bivalves have pustules.  

Tritogonia verrucosa Say, 1814, eastern North America. This is the exterior of a right valve:  length 12.2 mm, height 6.6 mm.

Anodonta grandis, Santa Margarita Lake, San Luis Obispo County, California. Right valve, somewhat weathered, exterior view and interior views, length 10.2 cm, height 6.5 cm. This genus is widespread in the fossil record and ranges from the Late Cretaceous to Recent. The shell of Anodonta is very thin, and the damage seen here on the exterior surface of this shell also affected the growth of the interior of the shell, in the form of small "bumps." The hinge of this species of Anodonta is much weaker than on the unionid species shown above, most likely because this Anodonta lived in quieter waters.

Freshwater bivalves can have considerable morphologic variation in their shells because of ecologic modifications and because of male and female differences.

It is also interesting to note that freshwater bivalves have a larval stage, which is parasitic in fish gills. These bivalves, therefore, can achieve rapid dispersal in river systems because of the distribution of the fish.

Freshwater bivalves are essential are the environment because they continuously pump water through their siphons, thereby continuously filtering and cleaning the water they live in. Some adults can live up to 100 years!


In recent decades, freshwater bivalves have undergone drastic declines because of water pollution, habitat desctruction, dam construction, and exotic species introduction.