Shadow of the Palaeoniscoids

Palaeoniscum freieslebeni, copyright James St. John.

Depending how you cut it, the ray-finned fishes (Actinopterygii) are arguably the most diverse group of vertebrates in the modern fauna. They are the dominant vertebrates in all aquatic environments, they encompass an enormous array of species, and they have evolved a bewildering assemblage of morphologies. But despite their current pre-eminence, the early evolution of actinopterygians remains rather understudied. The earliest actinopterygians appear in the fossil record in the Late Silurian/Early Devonian but, until fairly recently, the majority of Palaeozoic ray-finned fishes have often been lumped into a catch-all holding tank, the 'Palaeonisciformes'. This was a vague assemblage of fishes united by plesiomorphic features such as ganoid scales (heavy, bony scales with an outer layer of enamel, also found in modern gars and sturgeons), a single dorsal fin and a heterocercal tail (with the upper arm of the tail fin longer than the lower). The key genus of the group, the Permian Palaeoniscum, had a fusiform (or torpedo-shaped) body; at first glance, it would not have looked dissimilar to a modern herring. However, it lacked the mobile jaw structure of modern teleost fishes, with the maxilla and preopercular bones being fixed together. As such, it would have lacked the modern fish's capacity for suction feeding (Lauder 1980). Prey capture by Palaeoniscum would have been a simple smash-and-grab affair. Palaeoniscoid fishes remained a component of both marine and freshwater faunas until the end of the Cretaceous before being entirely supplanted by modern teleost radiations such as the ostariophysans and percomorphs.

Reconstruction of Acrolepis gigas, copyright DiBgd.

The core concept of 'Palaeonisciformes' has united fishes with a fusiform body shape like Palaeoniscum; depending on the author, more divergent contemporary fishes such as the deep-body platysomoids might be combined in the same order or treated separately. By modern standards, former 'Palaeonisciformes' probably combine stem-actinopterygians, stem-chondrosteans, stem-holosteans and possibly even stem-teleosts. As such, the term Palaeonisciformes has tended to fall out of favour, though the less formal 'palaeoniscoid' remains a useful descriptor. Nevertheless, the exact phylogenetic position of many palaeoniscoid taxa remains unestablished. Part of this is due to a lack of observable detail: though those heavy ganoid scales preserve well, they effectively cover up internal skeletal features. Many palaeoniscoids are preserved as compression fossils, effectively not much more than intriguing silhouettes. However, part of the problem is simple neglect. Palaeoniscoids are not rare fossils; in some formations, they may be the dominant part of the fauna by a large margin. They certainly deserve a closer look.

REFERENCE

Lauder, G. V., Jr. 1980. Evolution of the feeding mechanism in primitive actinopterygian fishes: a functional anatomical analysis of Polypterus, Lepisosteus, and Amia. Journal of Morphology 163: 283–317.