Cephalopods from the
Lusitanoceras granosum Zone in the Mississippian (Lower
Carboniferous) Chainman Formation, West Central Utah
Cephalopods from the Lusitanoceras granosum zone were first
described from rocks in Utah by Miller, Youngquist and Nielsen in
They are found
in the Camp Canyon Member of the Chainman Formation in West Central
For some reason ammonoids in this zone had stronger longitudinal
than those that came before and after.
Lusitanoceras granosum (shown
left and left) is a globular, almost spherical, only slightly
compressed ammonoid with longitudinal lirae, and a rather small
umbilicus. This species was refered previously to Goniatites
subcircularis, formerly Neoglyphioceras subcircularis is
another longitudinally lirate ammonoid, the difference being the
sinuous constrictions and the compressed form.
The specimen on the
left is 14mm
dia. and the one on the right is 22mm dia.
utahensis (Miller Youngquist & Nielsen) 1952
Described as Lyrogoniatites
utahensis in 1952. These are fairly common wherever the Camp
Member of the Chainman Formation is exposed. The left specimen
44mm dia. and the ones in the right photo range from 25mm to 50mm.
ornatissimum (Miller and Youngquist) 1948
ornatissimum is intermediate between Girtyoceras
Both Girtyoceras and Eumorphoceras have an
oxyconic shell at maturity, the former having constrictions
the juvenile shell and the latter having ribs. Sulcogirtyoceras,
found in rocks younger than those containing Girtyoceras and
older than those containing Eumorphoceras has both ribs and
constrictions on the juvenile shell. These two photos show the
dimorphic character of the dimorphoceratids, the left photo is of the
juvenile shell, it is actually 11mm diameter.
The right photo is a latex cast of an older individual, still
some of the juvenile characters, the ventro-lateral sulcus is forming,
and the ribs and constrictions are starting to disappear, it has not
reached the oxyconic stage of maturity, it is 25mm diameter.
Back in the '40s almost all
late mississippian goniatitic ammonoids found in the U.S. with an
narrowly rounded or sharp first lateral saddle were refered to Goniatites
choctawensis, and the ones with a rounded first lateral saddle
refered to Cravenoceras (see Miller and Furnish 1940).
just shows how things change with each generation of paleontologists.
These two specimens are the only fossils I have found that can be
refered to C. choctawensis. The
top specimen is an internal
mold with part of the shell preserved.
This is a crushed shell of a
rather large adult ?C. choctawensis? that
shows the crenulate pattern of the sculpture
and growth lines.
Refered to Dimorphoceras
humphreyi Youngquist 1949, by Miller, Youngquist and Nielsen in
11mm diameter, the ventral
lobe of the suture is divided into four smaller lobes, which are each
orthoconic nautiloid, is quite common in this part of the Chainman
usually found only in sections.
This photo is about 10x natural
to show the fine tranverse lirae. Notice the shell repair at
the midpoint of this specimen.
references you can go to the alphabetic list
of Utah Fossil Cephalopods and then to the reference