The Teleost Fuse

A while back, I discussed the group of fish known as the Holostei, the gars and bowfin. The Holostei constitute one branch of the clade Neopterygii which includes the majority of living ray-finned fishes. However, their success in the modern environment pales in comparison to that of their sister group, the Teleostei.

Siemensichthys macrocephalus, an early teleost of uncertain affinities, copyright Ghedoghedo.

Teleosts are such a major component of ray-finned fishes that it is simpler to list those members of the modern fauna that do not belong to this clade: the aforementioned gars and bowfin, sturgeons and paddlefish, and the bichirs of Africa. Everything else belongs to the great teleost radiation, representing about 96% of all modern fishes. The earliest fishes generally recognised as teleosts come from marine deposits of the Late Triassic in the form of the Pholidophoridae of Europe. The earliest known members of the crown group are from the Late Jurassic (Nelson et al. 2016). Teleosts have been recognised as an apomorphy-defined clade; the crown clade has been dubbed the Teleocephala. Among the features that have been used to define the Teleostei are the presence of a mobile premaxilla. In my previous post, I explained how the mobile maxilla of neopterygians including bowfins improved feeding by creating suction when the mouth was opened. Having both the maxilla and premaxilla mobile enhances this process further. In some of the most advanced teleosts, such as dories and ponyfish, the connection between the jaws and the cranium is entirely comprised of soft, flexible tissue, allowing the jaw apparatus as a whole to be catapulted towards unwary prey. Other features that have been highlighted include a strongly ossified caudal skeleton with long uroneural spines derived from the neural arches of the vertebrae, and the lower lobe of the caudal fin supported by two plate-like hypural bones articulating with a single vertebral centrum (Bond 1996).

Leptolepis coryphaenoides, one of the earliest teleosts with cycloid scales, copyright Daderot.

Of course, not all these features necessarily appeared in lock with each other. A phylogenetic analysis of basal teleosts by Arratia (2013) identified the aforementioned features of the caudal skeleton as absent in some of the basalmost teleosts. The condition of the premaxilla is ambiguous in Prohalecites, the earliest stem-group teleost from the Middle-Late Triassic boundary. It appears to be absent in the Aspidorhynchiformes and Pachycormiformes, Mesozoic orders that are currently regarded as on the teleost stem but not part of the Teleostei. However, as was found with the mobile maxilla in gars, one can't help wondering whether this character has been affected by the uniquely derived upper jaw morphologies in these orders. Other features identified by Arratia (2013) as supporting the Teleostei clade include the presence of two supramaxillary bones, a suborbital bone between the posterior margin of the posterodorsal infraorbitals and the anterior margin of the opercular apparatus (subsequently lost in the teleost crown group), and accessory suborbital bones ventrolateral to the postorbital region of the skull roof.

The earliest teleosts in the Pholidophoridae and other basal lineages retained the heavy ganoid scales of thick bone that may still be seen in modern Teleostei. Lighter, thinner cycloid scales first appear with the Early Jurassic Leptolepis coryphaenoides (Arratia 2013) and are the basal scale type for the teleost crown group (in some derived subgroups, the scales would become further modified or even lost). The greater mobility permitted by these lighter scales may have been another significant factor in the teleost explosion. By the Cretaceous period, stem-teleosts had radiated into a variety of specialised forms such as the gigantic predatory Ichthyodectiformes (of which Xiphactinus grew up to four metres in length) and the deep-finned Araripichthys. The three major subgroups of the crown Teleostei—the Elopomorpha, Osteoglossomorpha and Clupeocephala—had diverged from each other by the end of the Jurassic. The stem-teleosts would disappear with the end of the Mesozoic; the crown teleosts would dominates the world's waters from that time on.

REFERENCES

Arratia, G. 2013. Morphology, taxonomy, and phylogeny of Triassic pholidophorid fishes (Actinopterygii, Teleostei). Journal of Vertebrate Paleontology 33 (6 Suppl.): 1–138.

Nelson, J. S., T. C. Grande & M. V. H. Wilson. 2016. Fishes of the World 5^th ed. Wiley.