Why is the classification of Paracanthopterygii considered provisional?

The classification of the superorder Paracanthopterygii is considered provisional because the interrelationships among the groups assigned to it are not yet well established. While there is general agreement that trout-perches (Percopsiformes) and cods (Gadoidei) are closely related, the inclusion of other fish groups remains a subject of ongoing scientific study.1

The composition of the superorder has shifted over time as research has progressed. For example, groups such as clingfishes and eelpouts were previously classified as paracanthopterygians but are now categorized as perciform fishes. Similarly, the order Polymixiiformes was once included in the superorder but is now identified as the closest living relative of the Acanthomorpha, a clade that includes both the Paracanthopterygii and its sister group, the Acanthopterygii. Furthermore, debate persists regarding whether paracanthopterygian fishes are more closely related to atherinomorph fishes or if they represent more primitive members of the acanthomorphs.1