Up here in Western New York we have some large Devonian coiled nautiloids (Nephriticeras magister) that get to be more than 30 cm diameter, and in several specimens I've encountered the innermost whorls are not well preserved if at all.  Often the inner ones when they are preserved are geodized - crystals growing inward from the chamber walls but still leaving an open cavity.  Makes it really hard to work out a good specimen from the concretion they are found in.  In the Cretaceous ammonites I've worked with it seems either the siphuncle or breaks (from borers, etc) must have provided conduits for fine sediments to enter the shell - especially in the finer deposits of chalky sediments - and even then the smallest innermost whorls may be absent.
As for the ammonite specimen, probably not a placenticeratid - sutures are way too simple and the sutural amplitude index (ratio of suture pattern height to length, a measure of how deep the critters could live) is way too small in placenticeratids compared to this specimen.  Placenticeras stuck to shallow bottom areas (usually close to shore) - the complex sutures of that ammo were not related to depth but rather to prevent the shell from breaking too much should a mosasaur decide it wanted some calamari for dinner but not quite succeed.
- Rick Batt
near Buffalo, NY

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From: Conchologists List on behalf of John Timmerman
Sent: Mon 11/26/2007 1:22 PM
To: [log in to unmask]
Subject: Re: [CONCH-L] Fosssil, and a question about such fossils ...

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