Palaeonisciformes

Palaeoniscoids (also spelled paleoniscoid, or alternatively paleoniscids) are an extinct grouping of primitive ray-finned fish (Actinopterygii), spanning from the Silurian/Devonian to the Cretaceous. Though often assigned to their own order, the Palaeonisciformes (or superorder Palaeoniscimorpha), they are generally considered paraphyletic, but their exact relationships to living ray-finned fish are uncertain. While some and perhaps most palaeoniscoids likely belong to the stem-group of Actinopteryii (less closely related to modern ray-finned fish groups than all modern ray-finned fish are to each other) it has been suggested that some may belong to the crown group, with some of these possibly related to Cladistia (containing bichirs) and/or Chondrostei (which contains sturgeons and paddlefish). Many palaeoniscoids share a conservative body shape and a similar arrangement of skull bones, though paleoniscoids as a whole exhibit considerable diversity in body shape.

== Historic background == 200px|thumb|left|Acropholis stensioei (fossil at the Geological Museum in Copenhagen) is a taxon referred to Palaeonisciformes based on superficial resemblance with Palaeoniscum The systematics of fossil and extant fishes has puzzled ichthyologists since the time of Louis Agassiz, who first grouped all Palaeozoic ray-finned fishes together with Chondrostei (sturgeons, paddlefishes), gars, lungfishes, and acanthodians in his Ganoidei. Carl Hermann Müller later proposed to divide actinopterygians into three groups: Chondrostei, Holostei, and Teleostei. Later, Edward Drinker Cope included these three groups within Actinopteri. The same classification is also used today, though the definitions of these groups have changed significantly over the years. The sister group to Actinopteri are the Cladistia, which include Polypterus (bichirs), Erpetoichthys and their fossil relatives. All together are grouped as Actinopterygii.