Like other related oncocerids, the shell of
Trimeroceras is straight and of equal width and height up to the septum of truncation and bearing horizontal markings, with a later curved apical part. It is often found lacking its apical part past the septum of truncation.
Mobility Like other brevicones,
Trimeroceras had lost the ability to swim effectively but had retained the hyponome as part of the respiratory apparatus. However, as the hyponomic sinus was located close to the anterior end, any sudden use of jet propulsion would only cause the animal to swing forwards, followed by a backwards swing when breathing again. While still useful for slow movement, it would thus not be a useful way to escape predators, with a more likely possibility being for
Trimeroceras to retract its protruding soft parts in its shell.
Size dimorphism T. cylindricum and
T. bulbosum both exhibit size dimorphism which may be
linked with sex. This is largely reflected in the shells, with some specimens (macroconchs) being distinctly larger in size than the others (microconchs). In the case of
T. cylindricum, microconch shells range between 18–21 mm of height and width, while those of macroconchs are found to be in a 22–26 mm range. Though it isn't known with certainty to which sex the larger shell size belonged, it has been hypothesized that the extra space in the shell may have been taken up by an organ analogous to the
spadix and antispadix of
Nautilus. This would make the macroconchs representative of males. ==References==