Look familiar? They are Lyrodesma Hinge Teeth!
(Attn: New Photos of Lyrodesma dentations resembling Mystery Fossil Now Available!)
Put forth as a possible explanation to the mystery fossil by Roger Cuffey of Penn State,
"The interlocking teeth on the hinge of pelecypod ("bivalve") shells. Several of your 3- & 4-plate specimens look a lot like some of those, such as on trigoniids & heterodonts. If so, these would likely be molds/impressions/casts (of those structures), because most (though not all) pelecypods' shells are aragonite, and so would have dissolved away or inverted to calcite long ago."
I began looking for specimens of pelecypods from the type-Cincinnatian that have sufficient shell preservation to show details of their hinge teeth. I found some specimens of Lyrodesma (species undetermined) from the Kope Formation, Southgate member of Carrollton, Kentucky that had shell preservation. Among them were two specimens that showed the hinge teeth. On one specimen you can see that there are ridges between the teeth. These ridges tell me that Lyrodesma teeth could have formed these mystery plates as molds between them.
This specimen has matrix down in there between the teeth, so I'm still looking for a specimen that can tell me if the ridges are curved in the same direction as is evident on the multi-plate specimens of the mystery fossil.
Here are some images of the Lyrodesma from the "Southgate" site in Carrollton Kentucky.
Look familiar? They are Lyrodesma Hinge Teeth!
The specimen is a single valve filled with matrix consisting of mostly crinoid
material. The hinge teeth as seen at the top of the picture.
Here is a closeup of the hinge teeth showing the ridges on the sides of the
teeth. Note the orientation of the teeth.
Each tooth is convex and terminates at relatively sharp edges. The mystery plates, on the other hand, terminate with a blunt, smooth surface. The above hand drawing illustrates the edges of the hinge teeth.
If the mystery plates were fillings between the teeth, they should be concave,
which we do find. The mystery plates have what appears to be a rough,
"open" edge (
see this illustration
), and a smooth, "closed" edge as well as a generally wedged shape.
This may be consistent with pelecypod teeth external molds if the smooth,
"closed" edge of the molds were facing the back of the pelecypod valve
(making contact with the valve surface) and the rough "open" edge were
facing the open end of the valve. This is mostly conjecture, as I need to find a
more complete specimen of Lyrodesma that shows the hinge teeth all the
way back to the valve surface with no matrix obscuring the view.
The
multi-plate specimens
of the mystery fossil resemble the Lyrodesma teeth in that they are
oriented in radial patterns in a similar way to the pelecypod teeth. Most of the
multi-plate mystery fossils form a 90 degree angle with four plates. Some,
however, form as little as a 45 degree angle with four plates.
Here is another view of the Lyrodesma hinge teeth, this time looking
down from the umbo and with greater contrast to show the ridges on the sides of
the teeth.
Have another look at the images of the multi-plate specimens and the descriptions of the mystery fossils. There are many similarities to the Lyrodesma teeth. However, more analysis and better samples of Lyrodesma are needed. The analysis I have given on this page only serves to justify the claim that the "pelecypod hinge teeth hypothesis" may be a viable possibility for the identity of the mystery fossil.
Also, check out the evidence I have found that ridged impressions were common in this Cyclora Fossil Hash and that the "nodules" found hanging out of the sides of the mystery fossils have been found to be both gastropod steinkerns and crinoid columnals. That page is titled Secondary Phosphatization .
Bill Heimbrock
1621 Westwood Ave
Cincinnati, OH 45214
(513) 921-2335
billheim@cinci.rr.com
