Exogyra costata Say
|Description: The bottom (left) valve is larger than the flattened upper (right) valve. The bottom valve is also spirally coiled and is strongly convexed. The surface of the shell contains numerous ridges or costae along with concentric growth ridges. The upper valve is flat and spiral shaped with numerous sharp angled ridges on the outer surface. This valve can be slightly concaved to slightly convexed. 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 rounded along the curved surface of the upper valve. The bottom valve may be attached to other objects in its youth (more than likely other Exogyra or Pycnodonte shells). Both valves may have drill holes from or be encrusted by the boring sponge, Cliona cretacica.
Commonality: This oyster very common in the oyster beds of the Navesink formation at Big Brook. They are some lenses of Exogyra that occur uncommonly from the Red Bank and infrequently as molds in the Mt. Laurel.
Similar fossils: Both valves are distinctive and are hard to confuse with any other oyster except Pycnodonte. Exogyra differs from Pycnodonte by its spiral nature and ornamentation of costae.
Size: Exogyra can be between 2 to 10 cm (about .75 to 4 in) wide and long.
Notes: Exogyra, along with Pycnodonte, formed vast oyster beds in a continuous belt parallel to the coastline of the Cretaceous. This belt started at New Jersey, runs down to Georgia, over to Texas and up into Kansas.
Some paleontologists think that Exogyra, along with other oysters that have highly convexed shells such as Pycnodonte, might have become extinct at the end of the Cretaceous or shortly after. This was due to their inability to effectively flap their upper valve to remove the accumulation of sand in the bottom valve. It could also be argued though that the genius would not have lasted for 25 million years as a genus. It would also not be as prolific as a species if it were in fact inefficient at the most basic and important biological activity that this group of pelecypods performs. It is more likely that Exogyra may have become extinct at the end of the Cretaceous from whatever caused over 75% of the species known to exist at that time to go extinct. Interestingly, the Pycnodonts do not go extinct at the end of the Cretaceous and are prominent members of the Paleocene fauna.
Exogyra costata is one of the index fossils for the Monmouth Group (Mt. Laurel, Navesink, Red Bank formations in New Jersey). They may be occasionally found articulated that is both valves together, when found in situ.
Bottom Valve and Top Valve
Bottom Valve and Top Valve