Platysomiform

Platysomiform refers to a group of extinct ray-finned fish that existed from the Devonian to the Triassic periods. The term itself is not a formal taxonomic rank but rather a descriptive term for a body shape. Platysomiform fish are characterized by their extremely deep and laterally compressed bodies, which gives them a disc-like appearance. This body shape is a significant departure from the more elongated forms seen in many other ray-finned fish. Their fins were often relatively small and positioned in a way that supported their flattened morphology.

The evolutionary significance of the platysomiform body plan is thought to be related to their habitat. It is hypothesized that their deep, flattened bodies were an adaptation for living in environments with restricted movement, such as dense aquatic vegetation or crowded shoals. This shape could have provided advantages in maneuvering through tight spaces and potentially evading predators by presenting a difficult target to grasp. Fossil evidence suggests that platysomiforms inhabited both freshwater and marine environments.

While the exact phylogenetic relationships of platysomiforms are complex and have been subject to revision, they are generally considered to be a diverse assemblage within the actinopterygian lineage. Their unique morphology makes them easily identifiable in the fossil record, and their study offers insights into the adaptive strategies of ancient fish. The decline of platysomiforms is noted to have occurred by the Triassic period, with their ecological niches potentially being filled by other fish groups that evolved different body forms.