The Fishes here included in the Holostei constitute the Protospondyli and Aetheospondyli of Smith Woodward.[[587]] In the former group vertebral centra are either entirely absent, or, if present, their components in the form of alternating hypo- and pleuro-centra invariably remain distinct in the tail. The latter group has been instituted for the provisional reception of two highly specialised families of uncertain relationships, which differ from the Protospondyli in their higher grade of vertebral structure, the centra always being complete without any indication of distinct hypo- and pleuro-centra.
The Holostei first appear in the Permian, where they are represented by a single genus (Acentrophorus). During the Mesozoic period they were abundant in the Trias, reaching their maximum development and becoming the dominant Fishes of the period in the Jurassic. In the Cretaceous they began to decline, and in the Tertiaries became reduced to the two families which at the present day are the sole survivors of the group.
Of the six families of Protospondyli the Semionotidae are the oldest and most generalised, and the Macrosemiidae a closely allied group. The Pycnodontidae are a highly specialised and terminal offshoot. The Eugnathidae obviously lead to the Amiidae, and from the same stock it is probable that the Pachycormidae have been derived. The relations of the Aspidorhynchidae and Lepidosteidae (Aetheospondyli) are extremely doubtful. That the two families are allied seems probable, but beyond the possibility of a remote connection with the Protospondyli there is no clue to their ancestry.
Fig. 292.—Restoration of Lepidotus minor. Upper Jurassic, Dorset. × ⅕. (After Smith Woodward.)
Fam. 1. Semionotidae.—Small-mouthed, fusiform or deep-bodied Holosteans with rhombic scales, rarely, as in Aetheolepis, cycloid in the caudal region. All the fins possess fulcra. Teeth more or less conical, with a tendency to become tritoral in certain genera. Jugular plate present or absent. Acentrophorus (Upper Permian); Semionotus (Trias of England, Germany, S. Africa, and N. America); Lepidotus (Fig. 292) (Trias of Germany, Jurassic of Europe and India, Cretaceous of Brazil); the deep-bodied Dapedius (Lias of Dorset, Fig. 293), and Aetheolepis (Jurassic of New South Wales) are characteristic genera.
Fig. 293.—Restoration of Dapedius politus. Lower Jurassic, Dorset. × ¼. (After Smith Woodward.)
Fam. 2. Macrosemiidae.—Closely related to the Semionotidae, but with a more extended dorsal fin. Macrosemius (Upper Jurassic of England, Germany, France); Notagogus (Upper Jurassic of Naples, Bavaria, France); Petalopteryx (Upper Cretaceous of Syria).
Fam. 3. Pycnodontidae.—Highly specialised deep-bodied Fishes, with a small mouth and characteristic grinding or tritoral teeth. Scales rhombic. Fulcra absent. Dorsal and anal fins long. There is no jugular plate. The family ranges from the Lower Lias to the Lower Eocene, inclusive. Mesodon, Mesturus, Gyrodus, and Microdon are Jurassic genera. Coccodus and Xenopholis occur in the Upper Cretaceous of Syria (Mount Lebanon), and Pycnodus in various European Eocene formations.