| Comments |
Pycnodonte convexa is an oyster
that makes up a large portion of those fossils that are found
in the layers of the Navesink Formation. This oyster belongs
to a group called the Gryphaea, which dominate the beds
of sedimen. In fact they are found over a range of several feet
within the formation. The name Gryphaea comes from the
fact that this group of oysters is "gryph-shaped."
Some of the features common to the members
of this group are (1) the very small attachment area at the tip
of the left umbo, (2) left valves with growth patterns that are
spirally enrolled, (3) spirogyral left valves, which are large
and also highly convex, as well as opercula-form right valves
that are either small, flat, or concave, (4) growth squamae which
are closely appressed to the left valve's contour, (5) umbonal
part of left valve is lacking in chamber formation, (6) no umbonal
cavity under the hinge area on the left valve, and finally (7)
the left valves are divided by the presence of radial sulci.
The gryph-shaped oysters evolved in response
to a special set of circumstances that occured when the type
of sediment on which they lived changed from a more rocky substrate
to one comprised mostly of muddy sediments. This change caused
those whose shells were flatter to die out, as the more curved
shells thrived because they were able to lift themselves off
of the substrate and prevent mud particles from entering the
shell. Features which distinguish the Gryphaea from other oyster
species are their variably coiled shells and the fact that they
evolved more extreme shapes due to environmental pressures. The
Gryphaea lived in an environment that they were specially tailored
to, but eventually they went extinct, either due to the disappearance
of their main habitat or being out-competed by more adaptable
species of animals. The latter is the more probable of the two
because of the lesser likelyhood that an environment such as
that in which they lived was completely gone, especially for
such a widespread group.
|