Advanced jawed fishes I: teleostomes (Osteichthyes)
The first bony fishes are represented by fragments and microfossils from the Late Ordovician. From these ancestors, three distinct classes arose: the acanthodians, sarcopterygians, and actinopterygians. The three together make up the grade Teleostomi, whereas sarcopterygians and actinopterygians together constitute the subgrade Euteleostomi (euteleostomes have historically been referred to as Osteichthyes, literally “bony fishes”, but that taxon lacks official rank although it remains a sentimental favorite and is easier to remember than “euteleostomes”).
Teleostomes are grouped together because they share cranial, scale, and fin similarities, but especially because both acanthodians and actinopterygians possess three otoliths (sarcopterygian lungfishes have two otoliths and coelacanths have only one). Acanthodians diversified in the Silurian and Devonian and lasted through the Permian. The euteleostome Actinopterygii (“ray-fins”) are known first from scales in Late Silurian deposits, whereas Sarcopterygii (“fleshy or lobe fins”) appear in the Early Devonian.
Euteleostomes share numerous characteristics, including the bone series in the opercular and pectoral girdles, the pattern of their lateral line canals, fins supported by dermal bony rays, a heterocercal tail with an epichordal (upper) lobe, replaceable dentition, and a swim bladder that developed as an outpocket of the esophagus. Sarcopterygians diversified into extinct and modern coelacanths, lungfishes, and tetrapodomorphs – the latter group including rhizodontimorphs, osteolepidimorphs, and the elpistostegalians that gave rise to tetrapods. Actinopterygians underwent tremendous multiple radiations, producing the cladistian bichirs, the chondrosteans (many fossil groups plus modern sturgeons and paddlefishes), and neopterygians, including gars and related fossil groups, Bowfin and related fossil groups, and ancient and modern teleosts.
Although we tend to view the more advanced fishes as improvements over the primitive taxa, in part because the former are represented today, placoderms and acanthodians existed literally side by side with the “more advanced” forms for more than 100 million years. At some point, for climatic or biological reasons that are unclear, the innovations of the more derived gnathostomes, or the evolutionary constraints placed upon the more primitive groups, led to a replacement of one group by the other. The result was an incredible series of explosions of species belonging to four or five very different lineages, derived forms of which are still alive today.