Weird Trepospira sp.
Gastropod
Middle Pennsylvanian - Desmoinesian Stage
Marmaton Group - Altamont Formation
Lake Neosho Shale Member
St. Louis, Missouri
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The Trepospira shown in the attached file has
the ornamentation
all around the periphery of the shell instead of stopping half way
around like the usual T. depressa do. Have you seen anything
like this? It is obviously a different species, but I have been unable
to find anything remotely like it in the usual references, including
the Treatise, nor on the Internet. Maybe one of your online experts
knows what it could be.
References would be appreciated.
Richard Gottfried
Discussion
Peripheral nodes on the early whorls of a Trepospira
is normal. Later
whorls are smooth, but are sometimes broken off. Most Trepo
specimens are found mature, but this one is either a complete juvenile
or an incomplete mature specimen.
John
Had another look at my Trepos - nodes are normal on
the inside
of the whorl, even through maturity. On this specimen, the outer
edge of the last whorl is broken off, leaving a row of nodes that
appear to be on the outside of the whorl. Nodes are best developed
in the juvenile shell, making them more likely to be preserved if the
shell is broken. I have several specimens like the one pictured.
John
Richard asked for suggestions on the identification
of the specimen
and a few suggestions were given, although the final resolution of
the issue was not given. Apart from the matter of species identification,
this specimen illustrates important aspects of fossil identification: how
to recognize when a specimen is incomplete or damaged. When I saw
the photos the specimen seemed to have something wrong about it,
which turns out to be the situation. I just looked at some samples
of Trepospira and am satisfied that John is right about the
ornamentation and that the specimen illustrated is different in its
manner of preservation, not in characters that would indicate
recognizing a separate species.
The specimen is incomplete and has suffered selective
breakage. In
the 3rd photo on the web page, the band of nodes is seen to
overhang the edge of the whorl like a shelf, which is an unnatural
condition for this genus. Therefore, one can conclude that an outer
whorl has been broken away and the band of nodes seen on the
edge is actually a remnant of the missing outer whorl, not a band of
ornamentation on the periphery of the shell. The nodes on this genus
develop on the inner edge of each whorl and partly overlap the
periphery of the previous whorl and nodes do not develop on the
periphery. The overlap area is a zone of thicker shell and it tends
to remain behind when most of the outer whorl is broken off. Although
we commonly we see nodes on the outer edge of Trepospira specimens,
this is a result of shell breakage and poor preservation. Apparently
most of the shells Richard is working on are preserved in this
manner, so the mental image of the species is an image of a broken
shell, not a complete shell.
This is a problem all of us face when trying to sort
species and make
identifications. Most fossil shells are incomplete in some way, so we
have to interpret the missing portions and fill in the gaps. The
value of our identifications depends on how well we can make
corrections for missing information and place each specimen in a
reasonable category. The only good way to do this properly is by
experience, but that is where the hobbyist can often do as well as
the pro. All it takes is the extra hours needed to learn about a
species or group and become as informed about it as anyone else.
The reason for selective shell breakage in a gastropod
like
Trepospira is easy to understand. The shell was composed of
aragonite, a mineral that is unstable over long intervals of geologic
time and the shell tends to dissolve and/or convert to calcite. Walls
of the thinner outer shell whorls tend to dissolve more completely,
leaving the thicker inner portions with some shell remaining. Where
dissolution has been extensive it is common to find only internal
molds (steinkerns) of these shells in some deposits.
Tom Y.
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