Box Jaws And Feeding In Bony Fishes

Skull Bony Fish

Amia illustrates an intermediate kind of jaw apparatus between that of the palaeonisciforms and the teleosts. The skull of Amia (illustration I(a)) shows how the jaws are relatively shorter than in the basal actinopterygians (cf. Figure 3.19(d, e)). The maxilla is highly mobile and a new element, the supramaxilla, is attached to it. This mobile maxilla hinges at the front and can swing out some way to the side. This is associated with changes to the main jaw joint between the lower jaw and an internal unit composed of the hyomandibular, symplectic (another new element) and palatoquadrate, termed the jaw suspensorium (illustration I(b)). When the jaws of a neoptery-gian open, the cheek region of the skull expands sideways, which gives a sucking effect, efficient at drawing in small particles of food or prey animals.

I The jaws of non-teleost neopterygians: (a) skull of Amia showing the major jaw elements; (b) detailed view of the jaw joint elements in the early neopterygian Watsonulus, reconstructed with the outer skull elements removed. [Figure (a) after Patterson, 1973; (b) after Olsen, 1984.]

I The jaws of non-teleost neopterygians: (a) skull of Amia showing the major jaw elements; (b) detailed view of the jaw joint elements in the early neopterygian Watsonulus, reconstructed with the outer skull elements removed. [Figure (a) after Patterson, 1973; (b) after Olsen, 1984.]

Fish Teeth Types

The heads of bony fishes of 'chondrostean', 'holostean' and teleost grades show three rather different sets of jaw opening adaptations (illustration II). Palaeonisciforms opened their jaws in a wide 'grin', suitable for grabbing large prey, whereas most neopterygians protrude their jaws forwards and the open mouth is roughly circular (Schaeffer and Rosen, 1961). This protrusion is most marked in higher teleosts, where the sudden opening of the mouth produces a marked suction effect. The jaw-closing action is equally important. When the tube-like teleost mouth is closed by pulling the lower jaw and maxilla back, the food is retained, whereas simple closure by raising the lower jaw could blow some of the food out again.

Advanced teleosts, the Neoteleostei (see pp. 182-4), show a further modification of the jaw apparatus (Alexander, 1967). The maxilla loses its role as the main tooth-bearing element in the upper jaw and the enlarged premaxilla takes over, whereas the maxilla acts as a lever, pushing the premaxilla forwards as the jaws open (illustration III(a)). The maxilla is attached to the lower jaw and to the suspensorium. As the mouth opens, an anterior adductor muscle (illustration III(b)) pulls the top of the maxilla back and the lower jaw is pushed forward. The maxilla also rotates slightly about its long axis and a process on the top of the maxilla, which interlocks with one on the premaxilla, causes the premaxilla to be protruded.

Fish Premaxilla

III The jaw action and musculature of acanthomorph teleosts: (a) lateral diagrammatic view of the major jaw elements with the mouth closed (left) and open (right), showing the relative movements and rotations of the bones; (b) jaw musculature of Epinephelus, showing the muscles and bones indicated in (a). [Figure (a) after Alexander, 1975, courtesy of Cambridge University Press; (b) after Schaeffer and Rosen, 1961.]

the bowfins and the teleosts (Gardiner et al., 1996).

The gars, Lepisosteidae, consist of two genera that live today in North and Central America and Cuba. Lepisosteus (Figure 7.8(a)),a 1-2mpredatory fish,lives in warm-temperate fresh and brackish waters of North America. It has long jaws and captures its prey by lunging and grasping with its long needle-like teeth. The genus Lepisosteus has been traced back to the Cretaceous, and is a good example of a living fossil, an apparently slowly evolving lineage that has generally remained at low diversity. Gars were formerly more widely distributed, occurring in the Cretaceous and Lower Tertiary of North and South America, Europe, Africa and India, but their distribution has since shrunk.

Further basal neopterygians arose in the Late Permian and Triassic and radiated in the Jurassic in particular, but only one lineage has survived to the present. The Semionotidae include about 25 genera of small, actively swimming fishes, such as Semionotus (Figure 7.8(b)), that have nearly symmetrical tails and large dorsal and ventral fins. The tooth-bearing elements, the maxilla and dentary, project well forwards and they are lined with small sharp teeth. Semionotids occur in great diversity in some areas, such as the Newark Group (Upper Triassic and Lower Jurassic) lakes of the eastern seaboard of North America, where they appear to have formed species flocks (see Box 7.5). The dapediids, possible relatives of the semionotids, were deep-bodied fishes of the Triassic and Jurassic.

Box Jaws And Feeding Bony Fishes

Fig. 7.8 The diversity of basal neopterygians, dating from the Triassic (b), Jurassic (c, d) and recent (a, e): (a) the gar Lepisosteus; (b) the semionotid Semionotus; (c) the macrosemiid Macrosemius; (d) the pycnodont Proscinetes; (e) the bowfin Amia. [Figures (a, e) after Goode and Bean, 1895; (b) after Schaeffer and Dunkle, 1950; (c) after Bartram, 1977; (d) after Woodward, 1916.]

Fig. 7.8 The diversity of basal neopterygians, dating from the Triassic (b), Jurassic (c, d) and recent (a, e): (a) the gar Lepisosteus; (b) the semionotid Semionotus; (c) the macrosemiid Macrosemius; (d) the pycnodont Proscinetes; (e) the bowfin Amia. [Figures (a, e) after Goode and Bean, 1895; (b) after Schaeffer and Dunkle, 1950; (c) after Bartram, 1977; (d) after Woodward, 1916.]

The macrosemiids of the Triassic to Cretaceous were small fishes (Figure 7.8(c)) often with a long high dorsal fin. They have some unusual bones in the skull, a series of seven rolled little bones beneath the orbit (the infraorbitals) and two tubular infraorbitals behind it. The pycnodontiforms, also from the Triassic to Cretaceous, are mostly deep-bodied forms with long dorsal and anal fins and a symmetrical (homocercal) tail fin. Proscinetes (Figure 7.8(d)) has an elongated snout and a pavement of crushing teeth on the upper and lower jaws that were used to crush molluscs or echinoderms, as indicated by their gut contents (Kriwet, 2001).

The Halecomorphi, a group of basal neopterygians that arose in the Triassic and survives today, is most closely related to the teleosts, and together the two form the Division Halecostomi (see Box 7.6). The haleco-morphs are characterized by a specialized jaw joint involving an additional ventral element, the symplec-tic, as well as the quadrate (see Box 7.4, illustration I(b)). The earliest halecomorphs, the parasemionotids of the Triassic, were small fishes with large eyes and neopterygian jaw patterns. The modern bowfin, Amia

(Figure 7.8(e) and Box 7.4, illustration I(a)), lives in fresh waters of North America, where it is an active predator on a wide variety of organisms ranging in size up to its own body length of 0.5-1 m. Amiids were formerly diverse and have shown slow evolution (Grande and Bemis, 1998).

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