Cretaceous Ammonite

So what's unusual about an ammonite?  After all they seem to pop up everywhere and this doesn't even look like an ammonite!

This is (at least to me)  an unusual ammonite find from the Shanklin - Sandown area of the Isle of Wight. It was found on the high tide mark among the pebbles on the beach in early July 2014. Naively I thought it was some kind of dinosaur bone at first but this was dismissed with the kind assistance of the folk at the Sandown Museum.

 

In fact it is the cast of an reasonably large ammonite chamber. The creature died between 116 and 133 mya (Lower Cretaceous - Aptian to Barremian)

In life it would have swum about like a modern day nautilus and from the outside would have  looked like a larger version of the ammonites that will appear on other posts shortly.

Once it had died it would have fallen to the sea floor where the contents of its shell would have rotted as the remains of the creature lay on its side. Sand would have  covered the shell and would have seeped into the chambers within. The  ammonite would have been eroded by the sea  on its upper surface, which presumably explains some of the asymmetry of the fossil, until it is fully covered by sand. After millions of years the shell would probbaly remain in place until it fell out of the cliffs or was exposed by wave action on the foreshore. Once this happens the shell would disintegrate and the hardened inner chamber infills then separate. One of these is what I found on the shoreline between Shanklin and Sandown.

The  rock apparantly comes from the Ferruginous Sands that make up the tall cliffs that run along Sandown Bay to Shanklin and on which at one point the Isle of Wight Railway runs, it seems from the sound  of the passing train as it echoes at the foot of the cliffs below, perilously close to the edge near Lake.

What I find particularly interesting about it are the various notches in it that were presumably for ligaments or vessels of some kind. There are quite a few on what I take to be  the inner surface and fewer on the outer, including a single central one in the middle of the outer surface.

I owe the background and explanation of this fossil to Trevor Price , the Community Learning Officer of the Dinosaur isle Museum and I hope that he won't mind me mentioning him here.