[1] They were small to middle-sized fish, generally with laterally-compressed deep bodies, some with almost circular outlines,[2] adapted for manuverability in reef-like environments, though the group was morphologically diverse.

[3] Most, but not all members of the groups had jaws with round and flattened teeth,[4] well adapted to crush food items (durophagy), such as echinoderms, crustaceans and molluscs.

At the beginning of the Late Cretaceous they reached their apex of morphological and species diversity (much of this due to fossils found in the Sannine Formation of Lebanon, such as Gebrayelichthyidae and Ichthyoceros), after which they began to gradually decline, with a more sudden decline at the end of the Cretaceous due to the collapse of reef ecosystems, finally becoming extinct during the Eocene.

Pycnodonts from the Late Triassic Zorzino Limestone in Italy had short and stout jaws with big crushing teeth for eating hard-shelled prey, while other actinopterygians like saurichthyids and birgeriids mainly occupied top predator piscivorous niches.

The difficulty of placing them on a phylogenetic tree arises from the fact that they are a clade defined by a high number of autapomorphies (characteristics shared by a single taxon), which makes them easy to identify, but also makes the study of their relations with other actinopterygians difficult, since characteristics shared by other groups might be obfuscated by the immense amount of features and diversity of pycnodonts.

[16] Polypterus Acipenser Pycnodontiformes Caturus Ionoscopus Amia Calamopleurus Macrosemius Semionotus Obaichthys Dentilepisosteus Masillosteus Cuneatus Lepisosteus Atractosteus Arapaima Pholidophorus Elops As a means to avoid potential competition, the families of Pycnodontiforms evolved different body and jaw shapes, resulting in a highly diverse group.

[17] Paramesturus Palaeobalistum Micropycnodon Mesturus Gyrodus Arduafrons Piranhamesodon Apomesodon Brembodus Eomesodon Gibbodon Macromesodon Stenamara Turbomesodon Turboscinetes Thiollierepycnodus Neoproscinetes Proscinetes Stemmatodus Acromystax Ocloedus Lemanja Tepexichthys Scalacurvichthys Polazzodus Sylvienodus Coelodus Oropycnodus Pycnodus Paranursallia Nursallia Abdobalistum Capasso, Luigi (2023).