Septa (singular septum) are thin walls or
partitions between the internal chambers (camerae)
of the shell of a cephalopod, namely nautiloids
or ammonoids.
As the creature grows, its body moves forward
in the shell to a new living chamber, secreting
septa behind it.
This adds new chambers to the shell, which
can be clearly seen in cross-sections of the
shell of the living nautilus, or in ammonoid
and nautiloid fossils.
The septa are attached to the inside wall
of the shell, thus dividing the phragmocone
into camerae.
Where the septum meets the shell a suture
line forms; in some ammonoids these lines
became extremely complex and elaborate, providing
strength without the necessity of added weight.
Elaborate sutures allowed for thinner shells,
and hence less time needed for shell growth
and less time spent in the vulnerable juvenile
stage.
The nature and structure of the septa, as
with the camerae, and siphuncle, and the presence
or absence of deposits, are important in classification
of nautiloids.
In some nautiloids, such as the Orthoceratidae,
the septa tend to be widely spaced, resulting
in large, long camarae.
In others such as the Ellesmerocerida, Oncocerida
and Discosorida the septa are crowded closely
together.
In some straight-shelled forms like Actinoceras,
calcium carbonate deposits extend from the
camera (mural deposits) to the septa (episeptal
deposits).
It is possible to calculate the strength of
cephalopod septa on the basis of their thickness
and curvature, and from this the shell's implosion
depth can be estimated.
This has in turn been used to estimate maximum
depth ranges for many living and extinct cephalopod
groups, on the assumption that these animals
would not normally venture deeper than two-thirds
of their shell's implosion depth.
Ordered by increasing depth, these estimated
maximum depth ranges are: Discosorida (<100
m); Oncocerida and Tarphycerida (<150 m);
Actinoceroidea (50–150 m); Ellesmerocerida
(50–200 m); Belemnitida (50–200 m, exceptionally
to 350 m); Bactritoidea (c. 400 m); Endoceroidea
(100–450 m); Orthocerida (150–500 m);
Nautilida (200–600 m); Aulacocerida (200–900
m); and Sepiida (200–1000 m
