>At 08:54 AM 8/20/2004, Andrew Grebneff wrote:
>>Both are indeed Gonioceras sp(p).
>>Gonioceras is a flattened orthocone... in cross-section the shall
>>has a flattened venter; the dorsum is convex centrally and the
>>edges are tapered to sharp edges, with concave sulci running along
>>either side.
>>These beasties have been called "nautiloids", but have incredibly
>>complex siphuncles, unlike the simple siphuncles of true nautiloids.
>>I collected Gonioceras in western Newfoundland in 1990.
>
>Thank you for your help!
>
>So in both cases shown on my web page, we have external casts
>of the "top" of the nautiloid...   was it a full flattened oval
>orthocone?  I haven't found many pictures of Gonioceras on the
>web, so it's hard to visualize what the other "bottom" side would
>look like.  I think I was misled by the way each chamber's
>impression seems to come to a point - but that's because
>the cone was rounded and left the bedding plane at that point.
>
>- John

>John,
>You may have an internal view of the upper portion of the shell, with the
>rest of the tube broken away.  Possibly the lower half of the shell was
>much thinner and more easily lost.
>
>Allen Aigen  NYC


I attach jpeg images of Gonioceras scanned from Treatise K . One is
an outline in anterior view, with position of siphuncle marked. One
is a photo of a fragment in ventral view, showing the sinuous septa.
The other is a drawing of a complete shell, with a group of septa and
section of siphuncle included.

The venter is slightly convex but fairly close to flat, and the shell
may well have rested on the sea bottom. The dorsal surface is
compressed, with a thicker section in the center, where the head of
the animal was located. At full adulthood thew aperture became
constricted.

Your specimen, like the illustated one here, is a steinkern or
internal cast, with the septal walls preserved; the camerae
(chambers) are infilled with matrix; the shell walls are broken away,
probably still adhering to the external mold (which is presumably
still attached to the countryside somewhere).

Calling this an orthocone ("straight cone") is strictly correct but
most orthocones are cylindrical in section. Gonioceras is very
compressed dorsoventrally.
--
Andrew Grebneff
Dunedin
New Zealand
Fossil preparator
<[log in to unmask]>
Seashell, Macintosh, VW/Toyota van nut

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