Box Chondrichthyan Relationships

Living chondrichthyans are readily classified as either sharks and rays (elasmobranchs) or chimaeras (holocephalans), and most fossil taxa can be assigned to one or other branch of chondrichthyan evolution. There has been a great deal of debate about the placement of major taxa, whether for example the holocephalans are part of the elasmobranch clade, with symmori-dans and cladoselachids as their outgroups, or whether there was a clear division between the clades Elasmobranchii and Subterbranchialia (Gaudin, 1991), the view accepted here (see cladogram).

The hybodonts, ctenacanths and xenacanths form successive outgroups to the Neoselachii, but below that, the classification of elasmobranchs is uncertain and one possible solution, based on the work of Gaudin (1991) and Coates and Sequeira (2001a), is shown here. The relationships of the extant neoselachians are debated, but the pattern indicated here is supported by several analyses (de Carvalho, 1996; Shirai, 1996).

Molecular analyses of chondrichthyan phylogeny so far do not support the morphological tree. Douady and Douzery (2003) find that Galeomorphii are paraphyletic to other sharks and that batoids are not a part of Squalea and Hypnosqualea, but are a basal group to all the sharks, quite separate, as was the traditional, pre-cladistic view. Winchill etal. (2004) confirm the molecular evidence against Hypnosqualea, but they find some evidence for monophyly of Galeomorphii.

Cladogram showing postulated relationships of cartilaginous fishes, based on Gaudin (1991), de Carvalho (1996), Shirai (1996) and Coates and Sequeira (2001a). Synapomorphies: ACHONDRICHTHYES, prismatically calcified cartilage, second or single dorsal fin situated at pelvic level, a metapterygium articulating with 5+ radials and with an anteriorly directed proximal facet and a posteriorly directed axial radial series, myxopterygial claspers, elongate hyoid rays and various braincase characters (Coates and Sequeira, 2001a, p. 253); B SUBTERBRANCHIALIA, pelvic metapterygium that spans the entire fin base; C ELASMOBRANCHII, hypochordal (lower) lobe of caudal fin large; D, hyomandibular crescentic; E SYMMORIIDA, posterior dorsal fin with delta-shaped cartilage; F, dorsal spine concave forwards and with large opening behind; G EUSELACHII, braincase with elongated otic region, anal fin, tribasal pectoral fin (metapterygium, mesopterygium, propterygium); H, two dorsal fin spines, fin spines with pectinate ornament, deeply inserted fin spines; I, palatoquadrates fused at the symphysis, calcified ribs, pelvic metapterygium articulates with all or all but first radials; J NEOSELACHII, extrabranchial cartilages on hyomandibular (epihyal) only, right and left coracoids fused; K SQUALEA, ectethmoid process present, orbital articulation present, suborbital shelf absent, basitrabecular process present, notochordal constriction reduced, complete haemal arches in precaudal tail region; L, ectethmoid process absent, notochord constricted along entire vertebral column, enlarged supraneurals preceding second dorsal fin, precaudal haemal processes as elongate as lower caudal skeleton, spiracle valve present, longitudinal precaudal keel present; M HYPNOSQUALEA, anterior expansion of pectoral fin.

E SYMMORIIDA

A CHONDRICHTHYES

M HYPNOSQUALEA

Ikan Chondrichthyes

Fig. 7.3 Early subterbranchialians (chimaeras and relatives) from (a-e) the Carboniferous and (f) the Jurassic: (a) the iniopterygian Sibyrhynchus; (b) the iniopterygian Iniopteryxin ventral view; (c) the chondrenchelyiform Chondrenchelys; (d) the holocephalan Helodus; (e) upper and lower dentition ofthe holocephalan Deltoptychius; (f) the holocephalan Ischyodus. [Figures (a,b) after Zangerl and Case, 1973; (c, d) after Moy-Thomas and Miles, 1971; (e) after Patterson, 1965; (f) after Schaeffer and Williams, 1977.]

Fig. 7.3 Early subterbranchialians (chimaeras and relatives) from (a-e) the Carboniferous and (f) the Jurassic: (a) the iniopterygian Sibyrhynchus; (b) the iniopterygian Iniopteryxin ventral view; (c) the chondrenchelyiform Chondrenchelys; (d) the holocephalan Helodus; (e) upper and lower dentition ofthe holocephalan Deltoptychius; (f) the holocephalan Ischyodus. [Figures (a,b) after Zangerl and Case, 1973; (c, d) after Moy-Thomas and Miles, 1971; (e) after Patterson, 1965; (f) after Schaeffer and Williams, 1977.]

sharks, this jaw apparatus, combined with large numbers of serrated teeth, is extremely effective at gouging flesh from large prey. The serrated teeth of neoselachi-ans contrast with the cladodont teeth of earlier groups such as the hybodonts, which had three, five or more major points, and were good for capturing prey and holding it, but not for gouging and butchering. The neoselachian jaw system works well for those sharks that feed on smaller prey: the jaws open rapidly and wide and they produce powerful suction to draw in swimming crustaceans and small fishes.

Neoselachian senses are also enhanced. Neoselachi-ans have larger brains than most other fishes, larger even than amphibians and reptiles of the same body weight, and the sense of smell is improved over earlier sharks (at least to judge by the size of the nasal capsules).

The swimming abilities of neoselachians are evidently better than those of earlier sharks. The noto-chord is enclosed in, and constricted by, calcified cartilage vertebrae, whereas the primitive chon-drichthyans had a simple notochordal sheath. This strengthening of the backbone helps neoselachians resist compressional forces during fast swimming. The limb girdles are strengthened by fusion or firm connection in the midline, which allows more powerful muscle activity. The basal elements (the radials) in the paired fins are reduced and most of the fin is supported

Fig. 7.4 Modern sharks and rays: (a) the jaws of the giant Tertiary galeomorph shark Carcharocles,reconstructed from isolated teeth and probably too large; (b) restoration of the giant fossil Carcharocles and compararison of its size with the living great white shark Carcharodon (in box); (c) the modern squalomorph shark Squalus; (d) the modern ray Raja. [Figure (a) based on Pough etal., 2002; (b) courtesy of Mike Gottfried; (c) after Schaeffer and Williams, 1977; (d) after Young, 1981.]

Fig. 7.4 Modern sharks and rays: (a) the jaws of the giant Tertiary galeomorph shark Carcharocles,reconstructed from isolated teeth and probably too large; (b) restoration of the giant fossil Carcharocles and compararison of its size with the living great white shark Carcharodon (in box); (c) the modern squalomorph shark Squalus; (d) the modern ray Raja. [Figure (a) based on Pough etal., 2002; (b) courtesy of Mike Gottfried; (c) after Schaeffer and Williams, 1977; (d) after Young, 1981.]

by flexible collagenous rods called ceratotrichia or actinotrichia (Figure 7.4(c)).

The modern neoselachians fall into five main clades (de Carvalho, 1996; Shirai, 1996; see Box 7.2). 1 The galeomorphs, the largest group of some 250 species, are divided into the orders Heterodontiformes (the bullhead sharks, 8 species), Orectolobiformes (the carpet sharks, including the whale shark, 30 species), Lamniformes (the mackerel sharks, including the great white shark, 15 species) and Carchariniformes (the requiem and hammerhead sharks, 200 species). Galeomorphs mainly inhabit shallow tropical and warm temperate seas and they feed on crustaceans and molluscs, fishes and, on occasion, humans (see Box 7.3). The basking and whale sharks, up to 17 m long, are the largest living sharks, but they feed on krill, small floating crustaceans that they strain from the water. An even larger fossil shark has been reported. Carcharocles, a relative of the living great white shark, is known only from triangular teeth up to 168 mm long which are found in sediments dating from the Palaeocene to Pleistocene, but especially in the Miocene and Pliocene. Early reconstructions of its jaws, based on these large teeth (Figure 7.4(a)), gave it a 3-m gape and a total body length of 18-30 m. A comparative study of its teeth (Gottfried et al., 1996), however, has suggested that

BOX 7.3 CRETACEOUS JAWS!

BOX 7.3 CRETACEOUS JAWS!

Stories of shark attacks on humans and other large animals are common. In Cretaceous times, sharks attacked dinosaurs and other large reptiles of land and sea, as shown in two recent studies of lamniform sharks. Shimada (1997) documents predatory behaviour by the ginsu shark Cretoxyrhina from the Upper Cretaceous Niobrara Chalk of Kansas. In one specimen, a complete 5-m-long Cretoxyrhina skeleton is closely associated with bones of the large teleost Xiphactinus (see Figure 7.9(f)), and other sharks contain smaller teleost fishes in their stomach areas. Vertebrae of mosasaurs (see p. 243) show series of bite marks and some even have Cretoxyrhina teeth embedded in the bone. Xiphactinus and mosasaurs were themselves active predators, so Cretoxyrhina was evidently the top predator, or 'superpredator', in Niobrara Chalk seas, something like the great white shark today. There are three lines of evidence that Cretoxyrhina was attacking live prey: (1) some bitten bones show evidence of healing (Martin and Rothschild, 1989); (2) whole large fishes in the stomach area were presumably attacked and swallowed; (3) the tooth shape is the 'tearing type', with long slender cusps and gaps between teeth.

While Cretoxyrhina was probably an active predator, Schwimmer et al. (1997) argue that the Late Cretaceous lamniform Squalicorax was a scavenger, feeding on carcasses of mosasaurs, plesiosaurs, marine turtles and even dinosaurs (hadrosaurs and an ankylosaur). Squalicorax teeth have been found embedded deeply in mosasaur, turtle and dinosaur bones, and there is no sign of healing. This implies that the shark was scavenging the carcass of a dead animal that was either floating at the surface, or lying on the sea-bed. Further evidence of scavenging is that other tetrapod bones from marine Upper Cretaceous rocks show scratch marks that match precisely the pattern of serrations on Squalicorax teeth, and some large vertebrate carcasses are surrounded by shed Squalicorax teeth.

Read more at http://www.elasmo-research.org/education/evolution/cretoxyrhina.htm,

http://www.elasmo-research.org/education/evolution/squalicorax.htm, http://www.oceansofkansas.com/sharks.html and http://www.oceansofkansas.com/bite.html.

Shark attack in the Late Cretaceous: (a) right metatarsal of a young hadrosaur showing an embedded Squalicorax tooth; (b) a rib of the mosasaur Platecarpusshowing scratch marks produced by Squalicorax. (Photographs by Jon Haney; courtesy of David Schwimmer.)

Carcharocles was a mere 10-20 m long, with females significantly larger than males. The teeth are very like those of the living (but much smaller) Carcharodon. Nonetheless, this was a terrifying giant marine predator (Figure 7.4(b)).

2 The hexanchiforms, the frilled and cow sharks, are a small group of mostly benthic, deep-water sharks that are found worldwide. They eat crustaceans, bony fishes and other sharks, and bear live young. Hexanchiforms have a single dorsal fin and six or seven long gill slits, whereas other sharks have two dorsal fins and five gill slits.

3 The squaliforms, three families containing over 70 species, include forms such as Squalus (Figure 7.4(c)), the spiny dogfish. Squaliforms generally live in deep cold waters and they retain spines in front of the dorsal fins.

4 The squatiniforms are a small group containing one family, known from the Late Jurassic to the present day. These sharks, represented today by 13 species of Squatina, the angel shark and monkfish, have changed little since the Mesozoic. They have flattened bodies, broad pectoral fins projecting at the side and a long slender tail. At times, the squatinomorphs have been classified as rays (batoids), sharing with them features of the skull, vertebrae, fins and musculature (Shirai, 1996).

5 The batoids include more than 500 species of skates and rays. They are specialized mainly for life on the sea-floor, and have flattened bodies with broad flap-like pectoral fins at the sides and many have long whip-like tails. The eyes have shifted to the top of the head and the mouth and gill slits are underneath. The batoids swim (Figure 7.4(d)) by undulating the pectoral fins. The teeth are usually flattened, arrayed in pavements and are adapted for crushing hard-shelled molluscs.

7.2.2 Changes in hunting style andthe neoselachian radiation

The neoselachian sharks underwent a dramatic radiation in the Jurassic and Cretaceous, when they lived side-by-side with the hybodonts, which disappeared at the end of the Cretaceous. Most of the earlier shark groups had died out in the Carboniferous and Permian, but the xenacanths and ctenacanths survived well into the Triassic. There is no evidence that the new shark groups were competitively replacing their forebears: indeed, an observer in the Late Triassic might have had some trouble finding any chon-drichthyan fishes other than hybodonts. It is odd also that the dramatic radiation of neoselachians corresponded with the radiations of other marine predators, the ichthyosaurs and plesiosaurs (see

Chapter 8), some of which at least must have competed for the same food.

Thies and Reif (1985) suggested that the neoselachi-an radiation was an opportunistic response to the sudden appearance of abundant new sources of food in the radiation of the actinopterygian bony fishes, particularly the semionotids and other basal neopterygians in the Late Triassic and the teleosts from the Early Jurassic onwards. Here were new fish groups, present in vast shoals throughout the world, fast-moving, thin-scaled fishes. The early neoselachians, perhaps originating from Triassic ctenacanths, had capabilities of speed, manoeuvrability, a flexible jaw system and enhanced sensory systems, all essential for hunting the fast-moving bony fishes.

The early neoselachians were all apparently near-shore hunters that probably radiated in response to the evolution of teleost fishes and squid. Many modern sharks still specialize in this activity. A new feeding mode, fast offshore hunting, arose in the mid-Cretaceous, probably in response to increases in size and speed of teleost fishes and squid, and a move by them offshore. Marine reptiles, such as ichthyosaurs and long-necked plesiosaurs, may have been fast enough to compete with the new sharks, and indeed to eat smaller species. The Late Cretaceous mosasaurs (see p. 243), however, may have been too slow to compete with the sharks and may themselves have been eaten by larger shark species.

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