Illustration Of A Cephalaspis

The classic theory for the origin of jaws is that they formed from modified anterior gill arches (Figure 3.11(a)). In jawless fishes, the gill slits are separated by bony or cartilaginous arches. A hypothetical ancestral basal vertebrate with eight gill slits and nine gill arches evolves into an early gnathostome by the loss of four gill slits, and the fusion and modification of the anterior three gill arches. The most anterior may form parts of the floor of the braincase. The second gill arch might have been modified to form the palatoquadrate, the main part of the upper jaw, and Meckel's cartilage, the core of the lower jaw (mandible). The third gill arch was then supposedly modified in part to provide a skull bone and a mandible bone that formed part of the jaw joint, the hyomandibular in the skull and the cerato-hyal in the lower jaw.

Certain of the cephalaspid osteostracans, such as Hemicyclaspis (Figure 3.10(a)) and Cephalaspis, are extremely well preserved, and it has been possible to extract a great deal of anatomical and biological information from the specimens.

The upper surface of the head shield (illustration (a)) shows two oval openings for the eyes, the orbits and a narrow keyhole-like slit in front of them in the midline, the nasohypophysial opening. Behind it, and still in the midline, is a tiny pineal opening, associated with the pineal gland in the brain that might have been light-sensitive.

There are three specialized areas on the head shield marked by small scales set in slight depressions, the dorsal field in the midline behind the orbits, and the two lateral fields (illustration (a)). They might have had an additional sensory function. These areas are connected to the auditory region of the brain by large canals in the bone that may have transmitted nerves or contained fluid. The fields may have functioned in detecting movements in the water nearby, either by physical disturbance of the water, or by weak electrical fields.

The curved notches on either side at the back of the head shield are occupied by the pectoral fins (illustration (a)), and pointed cornua run back on either side. The underside of the cephalaspid head shield (illustration (b)) shows a large mouth at the front with a broad area of small ventral scales behind. Around the edges of this scale field are eight to ten gill openings on each side.

Cephalaspis Stensio

Cephalaspid anatomy and function: (a) head shield of Cephalaspis in dorsal view, showing sensory fields; (b) head shield of Hemicyclaspis in ventral view; (c) internal structure of the head shield of Kiaeraspis, showing the brain and related sense organs and nerves; (d) locomotion of Escuminaspis in dorsal (left) and lateral (right) views. Abbreviations in (d): A, anterior component of force produced by tail; L, lateral component; R, resultant of A and L; c.g., centre of gravity. [Figures (a-c) after Zittel, 1932; (d) after Belles-Isles, 1987.]

Cephalaspid anatomy and function: (a) head shield of Cephalaspis in dorsal view, showing sensory fields; (b) head shield of Hemicyclaspis in ventral view; (c) internal structure of the head shield of Kiaeraspis, showing the brain and related sense organs and nerves; (d) locomotion of Escuminaspis in dorsal (left) and lateral (right) views. Abbreviations in (d): A, anterior component of force produced by tail; L, lateral component; R, resultant of A and L; c.g., centre of gravity. [Figures (a-c) after Zittel, 1932; (d) after Belles-Isles, 1987.]

The most notable features of the cephalaspid head shield are to be seen inside. The bony parts enclosed much of the brain and sensory organs, as well as parts of the blood circulatory system and digestive system. The brain and its associated cranial nerves, the major nerves that serve the various parts of the head region, have been reconstructed by the Swedish palaeontologist Erik Stensio (1927) with a fair degree of confidence because of the extensive bony envelope (Illustration (c)). The large orbits and inner ear regions are quite clear. Even the semicircular canals of the inner ear, the organs of balance, can be seen. The brain stem itself is located in the midline, and it was made from the three main portions seen in primitive living fishes, the medulla at the back which leads into the spinal cord, the pons in the middle, and the telencephalon (forebrain) in front with an elongate hypophysial sac running forwards from it. The cranial nerves III (eye movement), V2 (mouth and lip region), VII (facial), IX (tongue and pharynx) and X (gill slits and anterior body) have been identified by comparison with living vertebrates. The five broad canals running from the lateral sensory fields to the vestibule of the inner ear show clearly (illustration (c)).

An analysis of the locomotion of cephalaspids (Belles-Isles, 1987) has shown that they were capable of sustained swimming, short bursts of fast locomotion, and fairly delicate manoeuvring, rather like sharks that live on or just above the sea-bed. The shape of the head in side view is an aerofoil, so that forwards movement would have tended to produce lift. When the tail beat from side to side, it produced a resultant force that drove the fish forwards and slightly downwards (illustration (d)). The downwards component was produced by the presence of the long upper lobe on the tail, but it was counteracted by lift at the head end, and possibly also by the pectoral fins.

Cephalaspid fossils have been found in freshwater sediments from streams, lakes, and deltas, and in marine sediments. They may have foraged for detrital matter on the bottoms of lakes, moving by pulling their bodies along with the muscular pectoral fins. They could apparently also swim for long distances, however, in search of new feeding grounds, or rapidly to escape predators.

Anatomical evidence, however, suggests that the gill-arch theory may not be so simple in reality. The gill lamellae in lampreys develop medially to the supporting skeleton, whereas the gills of gnathostomes develop laterally to the skeleton, so there must have been a transition from internal to external gill arches before the jaws evolved. Mallatt (1999) has argued that jaws evolved first for breathing: the mandibular branchial arch in the pre-gnathostome enlarged first to improve the intake of oxygenated water. Only later, with the evolution of suction feeding, did the jaws take on a feeding function.

Developmental studies suggest that the classic theory is probably incorrect (Kuratani et al., 2001). In gnathostomes, the jaws develop from cells that arose initially from the neural crest (see section 1.4.4), and similar cells are seen early in development of the lamprey. In the lamprey, however, these cells go on to develop into the upper lip and velum, whereas in gnathostomes they become the precursor of the jaw. There is evidence for a major repatterning process that happens during development, and this implies that it is wrong to expect to see precursors of jaws in the adults of jawless vertebrates.

Developmental genetic studies of mice (Depew et al., 2002) show that the first branchial arch has mandibular and maxillary bulges, precursors of the Meckel's cartilage and the palatoquadrate respectively. The homeobox genes Dlx5 and Dlx6 code for identity and anteroposterior orientation of the upper and lower jaws. Gnathostomes show nested Dlx gene action: they possess three pairs of Dlx homeobox genes that come into play sequentially. Lampreys show Dlx expression in their branchial arches, but the action is not nested, so indicating perhaps the condition in pre-gnathostomes.

3.4.2 Jaw attachments and gnathostome relationships

The palatoquadrate in gnathostomes is generally attached to the neurocranium, the main portion of the skull that enclosed the brain and sensory organs, in various ways. In early sharks, such as Cobelodus from the Upper Carboniferous of North America (Figure 3.11(b)), there is a double attachment with links fore and aft, the amphistylic condition.

Gnathostomata Origin Jaws Attachment

Fig. 3.11 The evolution of jaws and jaw suspension: (a) the 'classic'theory for the evolution of jaws from the anterior two or three gill arches of a jawless form (left) to the fully equipped gnathostome (right); gill openings in black; H, hyomandibular; S, spiracular gill opening; (b) braincase, jaws, and gill supports of the Carboniferous shark Cobelodus,to show the amphistylic system of jaw attachment to the neurocranium; (c) braincase and jaws of the modern shark Carcharhinus,with the jaws closed (top) and open (bottom), to show the hyostylic system of jaw support and the highly mobile palatoquadrate; (d) head of a chimaerid chondrichthyan, to show the autostylic, or fused, system of jaw attachment. [Figure (a) based on Romer, 1933; (b) after Zangerl and Williams, 1975; (c) based on Wilga et al., 2000 and other sources; (d) based on various sources.]

Fig. 3.11 The evolution of jaws and jaw suspension: (a) the 'classic'theory for the evolution of jaws from the anterior two or three gill arches of a jawless form (left) to the fully equipped gnathostome (right); gill openings in black; H, hyomandibular; S, spiracular gill opening; (b) braincase, jaws, and gill supports of the Carboniferous shark Cobelodus,to show the amphistylic system of jaw attachment to the neurocranium; (c) braincase and jaws of the modern shark Carcharhinus,with the jaws closed (top) and open (bottom), to show the hyostylic system of jaw support and the highly mobile palatoquadrate; (d) head of a chimaerid chondrichthyan, to show the autostylic, or fused, system of jaw attachment. [Figure (a) based on Romer, 1933; (b) after Zangerl and Williams, 1975; (c) based on Wilga et al., 2000 and other sources; (d) based on various sources.]

The amphistylic pattern has been modified in two main ways. In most modern fishes, the palatoquadrate contacts the neurocranium at the front only, and the jaw joint is entirely braced by the hyomandibular. On opening the jaw, the palatoquadrate can slide forwards, which increases the gape (Wilga et al., 2000). This is the hyostylic jaw suspension condition (Figure 3.11(c)). The second modification has been to exclude the hyomandibular from support of the jaw, and to fuse the palatoquadrate firmly to the neurocranium, the autostylic condition. This is typical of certain fish groups, the chimaeras (Figure 3.11(d)) and lungfishes, as well as the tetrapods.

Living gnathostomes are grouped in the clades Chondrichthyes and Osteichthyes (bony fishes and tetrapods), and two extinct clades are the Acanthodii

Gnathostomata

Fig. 3.12 Cladogram showing the relationships of the main groups of jawed fishes, based on Maisey (1986), Janvier (1996), Donoghue etal. (2000), Coates and Sequeira (2001a), and Goujet (2001). See Box 3.1 for context of Gnathostomata; see Box 3.6 for phylogeny of Sarcopterygii. Synapomorphies: A GNATHOSTOMATA,jaws composed of a primary upper (palatoquadrate) and lower (Meckel's cartilage) jaw component, supporting hyoid arch (not in placoderms), separate endoskeletal pectoral and pelvic girdles and fin skeletons, basals and radials supporting dorsal and anal fins, horizontal semicircular canal; B,teeth erupt from a dental lamina, fusion of nasal capsule to the rest of the chondrocranium, postorbital connection between palatoquadrate and braincase, internal rectus eye muscle inserts in a posterior position in the orbits, superior and inferior oblique eye muscle with an anterior insertion in the orbit; C,ventral cranial fissure, glossopharyngeal nerve foramen exits dorsally posterior to otic capsule, palatobasal process, interhyal, lateral line passes through scales, paired fin radials barely extend beyond level of body wall.

of the Ordovician to Permian, and the Placodermi of the Silurian and Devonian. Placoderms are generally ranked as the basal gnathostomes, then chon-drichthyans, and acanthodians and osteichthyans are paired by a n umber of synapomorphies (Figure 3.12).

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