The Early Bony Fishes

The ray-finned bony fishes, Actinopterygii, arose at least as early as the Silurian, and forms such as Cheirolepis radiated in the Devonian (see pp. 66-7). The clade was traditionally subdivided into three, the chon-drosteans, holosteans and teleosts, which form a rough time sequence of origins and radiations. The terms 'chondrostean' and 'holostean', however, refer to para-phyletic groups that include successive outgroups of the teleosts, and they are used here only in that grade-group sense. The bony fishes underwent three major phases of radiation:

1 basal actinopterygian ('chondrostean') radiation, Carboniferous-Triassic;

opercular preopercular subopercular

opercular preopercular subopercular supratemporal

10 mm dermosphenotic nasal

supratemporal

10 mm dermosphenotic nasal

Supratemporal Canal

premaxilla maxilla branchiostegal rays dentary 5 rTim

Fig. 7.5 Characters ofMimia,abasal actinopterygian from the Late Devonian: (a) cross-section of a scale, showing ganoin, a tissue that characterizes actinopterygians; (b) and (c),body and skull in lateral view. (Modified from Gardiner, 1984.)

premaxilla maxilla branchiostegal rays dentary 5 rTim

Fig. 7.5 Characters ofMimia,abasal actinopterygian from the Late Devonian: (a) cross-section of a scale, showing ganoin, a tissue that characterizes actinopterygians; (b) and (c),body and skull in lateral view. (Modified from Gardiner, 1984.)

2 basal neopterygian ('holostean') radiation, Triassic-Jurassic;

3 teleost radiation, Jurassic-present.

7.3.1 The first actinopterygians

Basal actinopterygians are characterized by specialized ganoid scales,which are thick bony elements composed of spongy bone on the inside, covered with dentine, and then ganoin on the outside, a layered shiny enameloid material (Figure 7.5(a)). Actinopterygian (and sar-copterygian) scales also have a characteristic rhomboid shape and peg-and-socket articulations that lock them together (Figure 3.19(b)). Scales of this type have been found in the Upper Silurian of China and Europe and the Lower Devonian of Canada and Australia.

The first complete specimens of actinopterygians are known from the Devonian, fishes such as the cheirolepidid Cheirolepis (Figure 3.19), a heavily built fish covered with tiny bony scales. Also from the Upper Devonian, Mimia from Australia (Figure 7.5(b, c)) is more derived than Cheirolepis in several features (Gardiner, 1984; Gardiner and Schaeffer, 1989). Its teeth are capped with acrodin, a dense variety of dentine, and it has a distinctive postcleithrum,a dermal element in the shoulder girdle region. The skull of Mimia (Figure 7.5(c)) shows a number of actinopterygian characters. The lower jaw has a large dentary bone that bears teeth and encloses a sensory canal. Teeth in the upper jaw are present on the maxilla and premaxilla, as well as on a midline braincase element in the palate, the parasphenoid and on many other small bones in the palate. The maxilla is locked into the cheek and it is a strong hatchet-shaped element.

The bones of the skull roof are highly variable in actinopterygians. The nasal bone lies at the front and it contacts the dermosphenotic above the orbit, which in turn meets the supratemporal behind (Figure 7.5(c)). In later actinopterygians, a dermopterotic element appears behind the dermosphenotic, produced by fusion of the supratemporal and intertemporal, and supraorbitals appear between the nasal and the der-mosphenotic. There is no postorbital and no squamos-al, bones primitively present in sarcopterygians (see pages 68-73). Note also the large eye socket — actinopterygians are visual predators that rely on good eyesight.

Further back in the skull, an array of thin dermal bones, the opercular series, covers the gill region. The preopercular lies above the maxilla, and behind it are the opercular and subopercular.Below the subopercu-lar, and sweeping round beneath the dentary, is a series of overlapping bony plates, the branchiostegal rays, numbering typically 12-13 in basal actinopterygians.

7.3.2 The basal actinopterygian radiation

The basal actinopterygians form a series of outgroups to the derived neopterygians, a group that includes the majority of post-Palaeozoic actinopterygians. At one time, virtually all the Carboniferous to Triassic actinopterygians were called 'palaeoniscids', a waste-basket term.The most basal actinopterygians included

Preopercular Canal

Fig. 7.6 Basal actinopterygians from the Carboniferous (c),Permian,Triassic (a,b,d-g) and extant (h—j): (a) the ptycholepid Ptycholepis; (b) the redfieldiid Redfieldius; (c) the amphicentrid Amphicentrum; (d) the pholidopleuriform Pholidopleurus; (e) the saurichthyid Saurichthys; (f) the perleidiform Perleidus; (g) the perleidiform Cleithrolepis; (h) the bichir Polypterus; (i) the sturgeon Acipenser; (j) the paddlefish Polyodon. [Figure (a) after Schaeffer et al., 1975; (b) after Schaeffer and McDonald, 1978; (c, h,i) after Nicholson and Lydekker, 1889; (d) after Bürgin, 1992; (e) after Rieppel, 1985; (f,g) after Lehman, 1966; (j) after Stahl, 1974.]

Fig. 7.6 Basal actinopterygians from the Carboniferous (c),Permian,Triassic (a,b,d-g) and extant (h—j): (a) the ptycholepid Ptycholepis; (b) the redfieldiid Redfieldius; (c) the amphicentrid Amphicentrum; (d) the pholidopleuriform Pholidopleurus; (e) the saurichthyid Saurichthys; (f) the perleidiform Perleidus; (g) the perleidiform Cleithrolepis; (h) the bichir Polypterus; (i) the sturgeon Acipenser; (j) the paddlefish Polyodon. [Figure (a) after Schaeffer et al., 1975; (b) after Schaeffer and McDonald, 1978; (c, h,i) after Nicholson and Lydekker, 1889; (d) after Bürgin, 1992; (e) after Rieppel, 1985; (f,g) after Lehman, 1966; (j) after Stahl, 1974.]

Cheirolepis and Mimia, and these were followed by numerous non-neopterygians, traditionally lumped together as 'chondrosteans', known especially from the Carboniferous, Permian and Triassic. One basal group, the guildayichthyids, known from two genera from the Bear Gulch Limestone (see Box 7.1), were small deep-bodied forms (Lund,2000).

The stegotrachelids, such as Moythomasia from the Upper Devonian of Europe and Australia (Gardiner, 1984), show some additional derived features of the skull. The ptycholepids, such as Ptycholepis from the Triassic and Lower Jurassic of North America and Europe (Figure 7.6(a)),have a reduced dermosphenot-ic element that no longer contacts the nasal and numer-

Cladogram Fish Amphibians

A ACTINOPTERYGI

Fig. 7.7 Cladogram showing relationships of the basal ray-finned bony fishes (Actinopterygii), based on the work of Gardiner and Schaeffer (1989), Grande and Bemis (1996) and Lund (2000). Extant families are indicated in bold. See Box 3.6 for context of Actinopterygii and Box 7.6 for neopterygian relationships. Synapomorphies are: A ACTINOPTERYGII,dermosphenotic T-shaped and contacts nasal, postorbital absent, squamosal absent, dentary with enclosed mandibular canal, one or two pairs of extrascapulars, single dorsal fin, scales and dermal bones with ganoin, rhomboidal scales with peg-and-socket articulation; B, distinct acrodin crown on all teeth, postcleithrum differentiated from anterior body scales; C CLADISTIA,round body form, postrostrals median and paired, maxilla posterior end narrow, maxilla mandibular fossa absent, median gular absent, lateral gular extended, clavicles rudimentary, postspiraculars present, caudal outline rounded, caudal fin rays webbed; D ACTINOPTERI,accessory vomerine tooth plate, branching rays in all fins; E, intertemporal bone meets nasal bone, supra-angular element in mandible; F,prismatic ganoin in teeth and scales, suborbital bone, antopercular bone; G,dermopterotic element; H,reduction in number of branchiostegal rays; I, snout blunt and rounded, preopercular reduced, suborbitals absent, marginal teeth reduced or absent, crushing tooth plates, body deep and laterally compressed; J, marginal teeth peg-like or absent, crushing tooth plates present, two sets of radials in median fins, body deep and laterally compressed; K,keystone-shaped dermosphenotic, series of supraorbital bones; L CHONDROSTEI,operculum reduced, elongate posterior extension of the parasphenoid, body scalation reduced to tiny elements or absent; M ACIPENSERIFORMES, palatoquadrates with anterior symphysis, triradiate quadratojugal, gill-arch dentition confined to first two hypobranchials and upper part of first arch, suboperculum has anterior process, preopercular canal in a series of ossicles and mandibular canal short or absent, infraorbital canal in a series of ossicles, premaxilla and maxilla absent; N,operculum absent, fewer than four branchiostegal rays, endocranium with extensive rostrum, dorsal and ventral rostral bones, ventral process of post-temporal bone; O,numerous irregular supraorbital bones between nasal and dermosphenotic; P,cerebellar corpus undivided, arches over fourth ventricle and produces a median anteriorly-directed portion; Q,premaxilla and antorbital bones present, hyomandibular nearly vertical; R,premaxilla with rudimentary nasal process, equal number of radials in dorsal and anal fins, upper caudal fin elongated beyond end of body.

ous suborbital bones behind the eye socket. The amblypterids, from the Carboniferous and Permian, have a dermopterotic element that abuts or overlaps the dermosphenotic.

A number of Carboniferous, Permian and Triassic families appear to form a clade (Figure 7.7), character ized by reduced numbers of branchiostegal rays in the gill region: some forms have only one, instead of the more typical 12-13. The redfieldiids, such as Redfieldius from the Upper Triassic and Lower Jurassic of North America (Figure 7.6(b)), are characterized by modifications to the snout and orbit region. These fishes are known worldwide in the Triassic and especially in the freshwater lakes of the Newark Supergroup (Upper Triassic to Lower Jurassic) of eastern North America (Schaeffer and McDonald, 1978). Amphicentrum, an amphicentrid from the Carboniferous (Figure 7.6(c)), is deep-bodied and compressed from side to side. The dorsal and anal fins are very long and the paired fins tiny. Amphicentrum has flattened teeth consolidated into a beak and tooth plates that presumably were used for crushing hard-shelled prey. Other possible members of this clade, such as the Permian dorypterids, were also deep-bodied, and had elongate dorsal fins.

Further basal actinopterygian clades radiated in the Carboniferous to Jurassic interval. The palaeoni-sciforms existed throughout this time. They show additional supraorbitals and a keystone-shaped der-mosphenotic, characters shared with more derived actinopterygians. The birgeriids are known from the Triassic and Early Jurassic and they were large, up to 2 m long and equipped with massive jaws and teeth. The pholidopleuriforms, such as Pholidopleurus from the Mid-Triassic of Switzerland (Figure 7.6(d)), were slender fishes with large rectangular flank scales (Bürgin, 1992). The saurichthyiforms such as Saurichthys (Figure 7.6(e)) were elongate fishes, up to 1m long, with the dorsal and anal fins set well back and close to the tail (Rieppel, 1985). The jaws are narrow and long, and it is likely that Saurichthys was an ambush predator adapted for rapid bursts of swimming when it leapt at its prey. The perleidiforms, also best known from the Triassic, include mainly small fishes, some such as Perleidus (Figure 7.6(f)) with slender bodies, and others such as Cleithrolepis (Figure 7.6(g)) with deep bodies. These fishes, as well as contemporary peltopleuriforms and pholidopleuriforms, had nearly symmetrical tail fins, although the internal skeleton, the backbone, still bent upwards in the primitive heterocercal style. The perlei-diforms and some of the other Triassic relatives, such as the peltopleuriforms, appear to fall on the evolutionary line to more advanced actinopterygians, as suggested by some modifications to the jaws and the increasing symmetry ofthe tail (Figure 7.7).

Remnants of the basal actinopterygian radiation have survived to the present, but with only eight genera in three families. The bichirs, Polypteridae, are heavily armoured fishes (Figure 7.6(h)) that live in the streams and lakes of tropical Africa. Their dorsal fin runs the whole length of the body and is divided into finlets, each with a spine at the front. The sturgeons, Acipenseridae, are large fishes, 1-6 m long, that live in northern waters and are the source of commercial caviar, their eggs. Sturgeons have very poorly ossified endoskeletons and the scales are reduced to five rows of large bony plates (Figure 7.6(i)). The paddlefishes, Polyodontidae (Grande et al., 2002), have long flat snouts that are about one-third of the total length of up to 3 m and they feed by straining plankton out of the water (Figure 7.6(j)). Sturgeons and paddlefishes appear to be related and to form a clade including the Jurassic chondroste-ids, which may be termed Chondrostei in a restricted sense (Figure 7.7).

The phylogeny of this series of mainly Carboniferous to Triassic fishes has proved hard to establish, but cladistic analyses, concentrating on skull and fin characters (Gardiner and Schaeffer, 1989; Grande and Bemis, 1996), have produced a cladogram (Figure 7.7) that consists of a series of outgroups to the Neopterygii. The modern basal actinopterygians, the bichir and the sturgeons and paddlefish, are located at different positions in the cladogram, the bichir (Polypteridae) low in the sequence and the sturgeons (Acipenseridae) and paddlefish (Polyodontidae) in the middle. These relationships have been confirmed by a study of mitochondrial DNA sequences (Inoue et al., 2003), although the morphological tree is unstable and further character evidence is required.

7.3.3 The basal neopterygian radiation

A major new actinopterygian clade, Neopterygii, arose in the Late Permian and radiated through the Mesozoic and Cenozoic. Neopterygians are characterized by major modifications to the feeding apparatus (see Box 7.4), braincase and tail. In the tail, the terminal portion of the backbone is reduced and the tail-fin rays are symmetrical above and below, more fully than even in the perleidiforms. The scales covering the body became thinner and flexible in many lines and changed from a rhomboid to a circular, or cycloid, shape. There are four main groups of neopterygians, the gars, the semionotids and other extinct forms,

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