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Home , Cladodont, Cladoselache, Elasmobranchii, Placodermi

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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 (cartilaginous ) that includes the () and the skates and rays (Batoidei). The other subclass within Chondrichthyes, according to traditional classifications, is the (, or ratfishes). It is probable that both groups arose independently during the or Early from a group of extinct armored fishes, the . The elasmobranchs are distinguished by separate gill openings, amphistylic or hyostylic suspension, and sensory ampullae (of Lorenzini) in the 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); (zoology) (/content/scale-zoology/604100); (/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 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 , 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 was persistent and was not replaced by centra.

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Fig. 1 Lateral views of fossil sharks. (a) Pleurocanthus, a specialized cladodont. (b) Cladoselache, a cladodont. (c) Hybodus, a hybodont.

The 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 () (/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 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.

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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 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 , and Hybodus from the Triassic through the .

Modern elasmobranchs

Modern sharks arose during the 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 . 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 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 , 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 (). (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).

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Fig. 3 (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 World, Jones & Bartlett Learning, Burlington, MA, 2013

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