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Elasmobranchii - Accessscience from Mcgraw-Hill Education Page 1 of 4 Elasmobranchii - AccessScience from McGraw-Hill Education Page 1 of 4 Elasmobranchii Article by: Schaeffer, Bobb American Museum of Natural History, New York, New York. Publication year: 2014 DOI: http://dx.doi.org/10.1036/1097-8542.214800 (http://dx.doi.org/10.1036/1097-8542.214800) Content • Evolutionary development • Modern features • Bibliography • Additional Readings The subclass within the Chondrichthyes (cartilaginous fishes) that includes the sharks (Euselachii) and the skates and rays (Batoidei). The other subclass within Chondrichthyes, according to traditional classifications, is the Holocephali (chimaera, or ratfishes). It is probable that both groups arose independently during the Silurian or Early Devonian from a group of extinct armored fishes, the Placodermi. The elasmobranchs are distinguished by separate gill openings, amphistylic or hyostylic jaw suspension, and sensory ampullae (of Lorenzini) in the head region. Characters shared with the holocephalans include a variably calcified cartilaginous endoskeleton, placoid scales, urea-retention mechanism, clasper organs in the male for internal fertilization, and the absence of an air (swim) bladder. See also: Chimaeriformes (/content/chimaeriformes/129900); Chondrichthyes (/content/chondrichthyes/133100); Placodermi (/content/placodermi/520900); Scale (zoology) (/content/scale-zoology/604100); Swim bladder (/content/swim-bladder/672500) Evolutionary development The history of the elasmobranchii can best be understood in terms of three successive evolutionary levels: cladodont, hybodont, and modern. Because of their incomplete fossil record, it is impossible to work out the detailed phylogenetic relationships of the extinct and living groups. Cladodonts The cladodont sharks first appeared in the Devonian; except for the specialized pleuracanths (Fig. 1a), which persisted into the Triassic, they had disappeared by the end of the Pennsylvanian. The cladodonts all possess amphistylic jaw suspension. Their teeth have a pointed central cusp, two or more lateral cusps, and a flattened disklike base. The notochord was persistent and was not replaced by centra. http://www.accessscience.com/content/elasmobranchii/214800 7/23/2015 Elasmobranchii - AccessScience from McGraw-Hill Education Page 2 of 4 Fig. 1 Lateral views of fossil sharks. (a) Pleurocanthus, a specialized cladodont. (b) Cladoselache, a cladodont. (c) Hybodus, a hybodont. The basal part of the pectoral fin skeleton is variable in design but never tribasal. The radial cartilages of the pectoral and pelvic fins are not divided and extend more or less to the fin margin. The pelvic plates are separated. Claspers are usually present. The dorsal fins are preceded by spines. The caudal fin is heterocercal, nearly equilobate, and the radials of the hypochordal lobe are unsegmented. The body scales are usually multilobed, each lobe having a separate pulp cavity. Some cladodont sharks possess relatively large semicircular canals; in others these are the same size as in modern sharks. See also: Copulatory organ (/content/copulatory-organ/161360); Ear (vertebrate) (/content/ear-vertebrate/208600) Cladodonts were primarily pelagic predators, capable of seizing and tearing prey. The relatively stiff fins suggest that their maneuverability was more limited than in modern sharks. Representative cladodonts include the Devonian genera Cladodus and Cladoselache (Fig. 1b). See also: Devonian (/content/devonian/189500) Hybodonts The hybodonts first appeared in the Mississippian Period. Although their skeleton resembles that of the cladodonts, they were more advanced in having only a tribasal pectoral fin skeleton, divided and shortened paired fin radials, clearly differentiated anal fin, and ribs and hemal arches along the entire length of the unreduced notochord. The acquisition of the tribasal pectoral fin, present in all later sharks, and a more flexible hypochordal lobe in the caudal fin apparently enabled these sharks to maintain their equilibrium more effectively than the cladodonts. http://www.accessscience.com/content/elasmobranchii/214800 7/23/2015 Elasmobranchii - AccessScience from McGraw-Hill Education Page 3 of 4 Some hybodonts had cladodont-like teeth and presumably fed on swimming prey, but much of the adaptive radiation at this level involved the development of crushing or grinding dentitions related mainly to bottom feeding. The late Palaeozoic genus Ctenacanthus is a good example of a conservative hybodont; the Mesozoic Hybodus (Fig. 1c) has a more advanced fin structure and a crushing dentition. Ctenacanthus lived from the Mississippian into the Permian, and Hybodus from the Triassic through the Cretaceous. Modern elasmobranchs Modern sharks arose during the Jurassic Period long before the hybodonts had become extinct in the Late Cretaceous. The transition from the hybodont to the modern level is poorly documented by fossils. However, there are three groups of aberrant (part hybodont, part modern) living sharks. Heterodontidae (Port Jackson sharks), Chlamydoselachidae (frilled sharks), and Hexanchidae (six-gill and seven-gill sharks). All except the Port Jackson sharks are rare deep-water forms, and each group has a somewhat different combination of hybodont and modern shark characters. These archaic sharks suggest the kind of evolutionary experimentation that took place during the rise of the modern sharks. See also: Jurassic (/content/jurassic/361100); Selachii (/content/selachii/613500) The skates and rays (batoids) are mostly bottom-dwelling elasmobranchs that possess many features distinct from the sharks. Present evidence suggests that the batoids arose during the Jurassic from some benthic hybodont group. Modern features Morphologically the modern shark level may be defined by the following features: hyostylic jaw suspension, shortened jaws, jaw protrusion mechanism, complete replacement of the notochord by calcified centra, fusion of the pelvic plates, and presence of only single-lobed placoid scales. Although the rostrum was short and blunt in the cladodonts, it became elongated in some hybodonts and, with a few exceptions, it projects well beyond the mouth in all modern sharks (Fig. 2). Fig. 2 Mackeral shark (Isurus). (After H. B. Bigelow and W. C. Schroeder, Fishes of the Western North Atlantic, pt. 1, 1948) Skates and rays differ from the sharks in having a depressed body, modification in the mode of gill ventilation, freer hyostylic jaw suspension, elaboration of the pavement dentition for crushing or grinding, enlargement of the pectoral fins, and disappearance of the anal fin. With the pectoral fins assuming the main role in locomotion, the posterior part of the body, including the caudal fin, became reduced to whiplike proportions (Fig. 3). http://www.accessscience.com/content/elasmobranchii/214800 7/23/2015 Elasmobranchii - AccessScience from McGraw-Hill Education Page 4 of 4 Fig. 3 Skate (Raja). (After H. B. Bigelow and W. C. Schroeder, Fishes of the Western North Atlantic, pt. 2, 1953) Bobb Schaeffer Bibliography H. B. Bigelow and W. C. Schroeder, Fishes of the Western North Atlantic, pt. 1, 1948, pt. 2, 1953 C. E. Bond, Biology of Fishes, 2d ed., 1997 E. S. Herald, Living Fishes of the World, 1961 F. H. Pough et al., Vertebrate Life, 5th ed., 1998 B. Schaeffer, Comments on elasmobranch evolution, in P. W. Gilbert et al. (eds.), Sharks, Skates and Rays, 1967 Additional Readings J. C. Carrier, J. A. Musick, and M. R. Heithaus (eds.), Biology of Sharks and Their Relatives, 2d ed., CRC Press, Boca Raton, FL, 2012 G. S. Helfman et al., The Diversity of Fishes: Biology, Evolution, and Ecology, 2d ed., John Wiley & Sons, Chichester, West Sussex, UK, 2009 J. T. Springer and D. Holley, An Introduction to Zoology: Investigating the Animal World, Jones & Bartlett Learning, Burlington, MA, 2013 http://www.accessscience.com/content/elasmobranchii/214800 7/23/2015 .
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