Pycnodonte (Gryphaea) convexa (Say)
|Description: The bottom (left) valve is larger than the flatter upper (right) valve. The bottom valve is strongly convexed and only has concentric growth ridges. The upper valve is strongly concaved and has concentric growth ridges but may also have shallow radiating grooves. The inner surfaces of both are smooth except for the large muscle scar. In large individuals, the bottom valve can be quite thick and may weigh up to a pound. The hinge is straight along the surface of the upper valve. The bottom valve may be attached to other objects, more than likely other Exogyra or Pycnodonte shells, but to a lesser extent then the modern type of attachment seen in modern oyster populations. Both valves may have drill holes from or be encrusted by the boring sponge, Cliona cretacica.
Commonality: This oyster occurs very commonly in the oyster beds of the Navesink formation at Big Brook.
Similar fossils: Both valves are distinctive and are hard to confuse with any other oyster except Exogyra. Pycnodonte differs from Exogyra by its non-spiral nature and lack of ornamentation.
Size: Pycnodonte can be between 2 to 10 cm (about .75 to 4 in) wide and long.
Notes: Pycnodonte, along with Exogyra, formed vast oyster beds in a more or less continuous belt parallel to the coastline of that time. This belt starts at New Jersey, runs down to Georgia, over to Texas and up to Kansas.
Some paleontologists think that Pycnodonte, along with other oysters that have highly convexed shells such as Exogyra, might have become extinct at the end of the Cretaceous because of their inability to effectively flap their upper valve to remove the accumulation of sand in the bottom valve. If this were so, then the genius would not have lasted as long as they did and would not be as prolific.
Interestingly, this oyster lasts through to the end of the Cretaceous and is fairly prolific in the early Tertiary. Exogyra apparently becomes extinct at the end of the Cretaceous. They may be found articulated when found in situ.
Some older sources label the genera of this oyster as Gryphaea. This is an older assignment and can be disregarded.
|Outside view of
Lower and upper valve
|Inside view of
Lower and upper valve