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
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. A few additional classification schemes were proposed over the years. Lev Berg erected the superorder Palaeonisci, in which he included early actinopterygians that belonged to neither Chondrostei nor Polypteri (Cladistia). Mostly following Berg, Jean-Pierre Lehman grouped the Actinopterygii into 26 orders, among others the Palaeonisciformes with the two suborders Palaeoniscoidei and Platysomoidei. Numerous genera of early actinopterygians have been referred to either Palaeonisciformes or to one of its suborders based on superficial resemblance with either Palaeoniscum (Palaeoniscoidei) or Platysomus (Platysomoidei), especially during the early and middle parts of the 20th century. Palaeonisciformes, Palaeoniscoidei, and Platysomoidei have therefore become wastebasket taxa. They are not natural groups, but instead paraphyletic assemblages of the early members of several ray-finned fish lineages. Palaeoniscoidei have traditionally encompassed most Paleozoic actinopterygians, except those that exhibit atypical body forms (such as the deep-bodied Platysomoidei, or those assigned securely to any of the living groups of ray-finned fishes. The same can also be said about the family Palaeoniscidae sensu lato, to which several genera not closely related to Palaeoniscum have been referred in the past. The grouping of "palaeonisciforms" was based largely on shared plesiomorphic features, such as the forward position of the eye, the large gape or the presence of rhombic scales. However, such symplesiomorphies are not informative with regard to phylogeny, but rather an indication of common ancestry. In modern biology, taxonomists group taxa based on shared apomorphies (synapomorphies) in order to detect monophyletic groups (natural groups). They use computer software (e.g., PAUP) to determine the most likely evolutionary relationships between taxa, thereby putting previous hypotheses of such relationships to the test. As a consequence, many genera have been subsequently removed from Palaeonisciformes and referred to distinct orders (e.g., Saurichthyiformes). The term Palaeonisciformes has mostly disappeared from the modern literature or is nowadays only used to refer to the "primitive" morphology of a taxon (e.g., "palaeonisciform skull shape" or "palaeoniscoid body shape"). In order to make the Paleonisciformes, Palaeoniscoidei or Palaeoniscidae monophyletic, these terms should only be used in a strict sense, i.e., when referring to the clade of actinopterygians that includes Palaeoniscum and the taxa closely related to it. A monophyletic clade including several taxa classically referred to the Palaeonisciformes (e.g., Aesopichthys, Birgeria, Boreosomus, Canobius, Pteronisculus, Rhadinichthys) was recovered in the cladistic analysis by Lund et al. This clade, coined Palaeoniscimorpha, is also used in subsequent publications. Recent cladistic analyses also recovered clades containing several genera that have historically been grouped within Palaeonisciformes, while excluding others. Due to the delicate nature of fossils of ray-finned fishes and the incomplete knowledge of several taxa (especially with regard to the internal cranial anatomy), there is still no consensus about the evolutionary relationships of several early actinopterygians previously grouped within Palaeonisciformes.
Classification The following list includes species that have been referred to Palaeonisciformes (or Palaeoniscidae, respectively), usually because of superficial resemblance with Palaeoniscum freieslebeni. Many of these species are poorly known and have never been included in any cladistic analysis. Their inclusion in Palaeonisciformes (or Palaeoniscidae) is in most cases doubtful and requires confirmation by cladistic studies. Which taxa should be included in Palaeonisciformes sensu stricto (or Palaeoniscidae sensu stricto) and which ones moved to other orders or families, respectively, is a matter of ongoing research.
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