The Chondrostei are first represented in the Lower Devonian by the solitary Palaeoniscid genus Cheirolepis, a contemporary of the earliest Crossopterygii. They occur throughout the Mesozoic period, except in the Cretaceous, and also in the Eocene, and while steadily diminishing in number and variety they gradually approximate to their degenerate and in some respects highly specialised descendants, the Sturgeons and Paddle-Fishes of the existing Fish fauna. Of the seven families included in the group the Palaeoniscidae are the oldest and the most generalised. The Platysomidae are a specialised offshoot from the Palaeoniscidae, and, if they are rightly to be considered as Chondrostei, perhaps the same may be said of the problematic Belonorhynchidae. On the other hand, there are certain features in the Catopteridae which indicate an approach to Fishes of an altogether more modern type. Finally, the Chondrosteidae represent a stage in a career of degeneration, the climax of which is reached by the modern Polyodontidae and Acipenseridae.
Fig. 283.—Palaeoniscus macropomus. Restoration, nearly one-half nat. size. (From Traquair.)
Fig. 284.—Outline restoration of the skull and secondary pectoral girdle of Palaeoniscus macropomus. an, Angular; br.r, branchio-stegal rays; cl, clavicle (cleithrum); d, dentary; d.ect, dermal lateral ethmoid; f, frontal; i.cl, infra-clavicle; i.op, suboperculum; mx, maxilla; n, nostril; op, operculum; or, orbit; p, parietal; p.f, pectoral fin; p.mx, premaxilla; p.op, preoperculum; p.t, post-temporal; s.cl, supra-clavicle; s.o, circumorbitals; sq, squamosal; the single median bone overlying the short rostrum is probably a dermal mesethmoid, and the one intercalated between the squamosal and post-temporal a supra-temporal. The dotted lines indicate sensory canals. (From Traquair.)
Fam. 1. Palaeoniscidae.[[579]]—Fishes with fusiform bodies, short dorsal and anal fins, and usually with a complete investment of articulating rhombic, rarely cycloid, ganoid scales (Fig. 283). Fulcra generally present at the bases of the median fins, and especially along the dorsal border of the upper caudal lobe. Ribs are not known to be present. Skull invested by a very complete series of paired dermal bones, which in number and disposition conform to the normal Teleostome type (Fig. 284). The secondary upper jaw includes both premaxillae and large maxillae; and, as a rule, both the dentary and splenial bones of the lower jaw are dentigerous. Except for the absence of an interoperculum, the opercular series of bones is complete, including numerous branchiostegal rays. There is a single small median jugular plate.
The Palaeoniscidae are remarkable both for their individual and specific abundance and for their extensive range in time. Represented only by Cheirolepis in the Middle Old Red Sandstone and Devonian, the family attained its maximum development in the later Palaeozoic rocks (Carboniferous and Lower Permian), became rare in the Mesozoic, finally dwindling away at the close of the Jurassic period. Their geographical distribution in the past is hardly less remarkable. In various geological formations they have been found in Great Britain and Ireland, in widely remote parts of continental Europe, and in North America, South Africa, and Australia. Cheirolepis, Amblypterus, Canobius, Phanerosteon, Elonichthys, Cryphiolepis, Palaeoniscus, and Trissolepis are Palaeozoic genera. Gyrolepis, Urolepis, Coccolepis, Oxygnathus, and Centrolepis are characteristic Mesozoic forms.
Fig. 285.—Restoration of Eurynotus crenatus. in.cl, Infra-clavicle; l.l, lateral line; orb, orbit; other reference letters as in Fig. 284. (From Traquair.)
Fam. 2. Platysomidae.[[580]]—More or less deep-bodied Fishes, with elongated dorsal and anal fins, a high head, short jaws, usually armed with bluntly conical tritoral teeth, and a complete investment of high, narrow, rhombic scales. They agree with the Palaeoniscidae in their osteology and in most other essential features, and they flourished in large numbers during the Carboniferous and Permian periods. Platysomus ranges from the Lower Carboniferous to the Upper Permian in Great Britain and continental Europe, and also occurs in the Carboniferous of North America. Eurynotus (Fig. 285), and the singularly deep-bodied Cheirodus (Fig. 286), in which pelvic fins are unknown, are British Carboniferous genera.