Chondrichthyes Sharks Rays Chimaeras

The class Chondrichthyes, or cartilaginous fishes, comprises the sharks, rays (including skates), and chimaeras, and it is one of the major groups of aquatic, gill-breathing jawed vertebrates. Approximately 1,050 species of chondrichthyans are living today, but their extensive fossil record dates back to the Late Ordovician (some 450 million years ago); the number of extinct species is not precisely known. Most chondrichthyans inhabit marine continental shelf areas, but some are deep-water and about 4 percent are freshwater.

Chondrichthyan fishes share several unique evolutionary specializations indicative of their common ancestry, such as a cartilaginous skeleton reinforced by superficial deposits of prismatic calcification, internal fertilization through claspers (male intromittent organs), and a special mode of tooth attachment and replacement (teeth occur in numerous rows and are constantly shed and replaced or fused into tooth-plates). Sharks and rays are fur

Sharks Intromittent Organ
Mouth of a Caribbean reef shark (Jeffrey L. Rotman/CORBIS)

ther united in the subclass Elasmobranchii ("plate-gill," referring to the individual gill clefts of their branchial arches), while the forty or so species of chimaeras form the subclass Holocephali (with the upper jaws fused to the braincase). A few Paleozoic taxa may fall outside of these subclasses (for example, the Devonian Pucapampella and Cladoselache), but their classification is uncertain.

Living chondrichthyans share specialized sensory and reproductive systems. Their fine-tuned sensory receptors, perhaps partly responsible for their evolutionary longevity, include the inner ear for sound, lateral line for perturbations in the water, and ampullae of Lorenzini for electrical field detection, along with advanced olfactory and visual units. Their reproductive strategies parallel those of mammals; some ground sharks nourish developing fetuses in the oviduct through maternal-fetal connections, and stingrays produce "uterine milk." Even intrauterine cannibalism has been documented (mackerel sharks). Gestation strategies vary within certain orders, but about 40 percent of all species (including catsharks, skates, chimaeras) lay egg cases, while the rest give birth to live young directly. More than one egg is fertilized at a time, and there is no rearing of the pups after birth. Chondrichthyans sexually mature at advanced ages in comparison to bony fishes (more than thirty years for some species), produce relatively few young per gestation, and may have prolonged gestation periods (more than two years for the spiny dogfish).

Fossil History

Fossil chondrichthyans are known primarily from isolated teeth, denticles, and fragmentary skeletal remains; complete fossilized skeletons are rare. The earliest chondrichthyan fossils, from the Late Ordovician of Colorado, are small external teethlike structures called pla-coid scales, similar to those coating modern sharks. The earliest fossil teeth are from Early Devonian deposits of Spain (Leonodus, 400 million years ago), but teeth become more common in the fossil record only during the Carboniferous. The earliest skeletal fragments are neurocrania (cartilaginous skull encompassing the brain and cephalic sensory organs) from the Middle Devonian (380 million years ago) of Bolivia and South Africa (Pucapam-pella) and Antarctica (Antarctilamna).

Relatively little is known from the earliest phases of chondrichthyan evolution, but many different lineages became established by the Late Devonian and Early Carboniferous, even though some of these were short-lived. Early elasmobranchs were morphologically very diverse, including the eel-like and mostly freshwater xenacanths (with conspicuous bifurcated teeth—for example, Triodus and Ortha-canthus), stethacanthids (with "spine-brush" complexes), cladoselachians (perhaps the best known early sharks), and ctenacanthids (with more modern pectoral fin endoskeletons). Some two dozen new families flourished in the Carboniferous, including the distinctive edes-tids and eugeneodontids (for example, Edestus and Helicoprion, with vertical tooth-whorls), and morphologically bizarre relatives of modern holocephalans (for example, Harpagofututor and Belantsea, from the Bear Gulch limestones of Montana, 325 million years ago). Some chondrichthyan groups persisted into the Tri-assic (xenacanths, edestids), but many families vanished in the Permian.

Elasmobranchs with the features of modern sharks, such as calcified vertebrae, appeared during the Jurassic (for example, Palaeospinax and Hopleacanthus), along with the first rays

(for example, Spathobatis and Asterodermus, similar to modern guitarfishes). One prominent group of Mesozoic sharks was the hybodonts, which originated in the Paleozoic (Pennsyl-vanian), diversified in the Mesozoic, but did not survive beyond the Cretaceous, going extinct when many living shark families became established. Some modern lineages can be found in the Late Jurassic Solnhofen limestones of Bavaria (150 million years ago— for example, angelsharks, hornsharks, and chi-maeras), but the earliest record of a fossil shark belonging to a living family (Hexanchidae) is from the Early Jurassic, some 200 million years ago. Most modern families, many known only from isolated teeth, became established during the Cretaceous and Paleocene. Significant Cretaceous fossil sites include Cenomanian (97 million years ago) and Santonian stages (87 million years ago) from Lebanon, with sawfishes, guitarfishes, and skates, along with sharks, all modern in appearance. More complete fossil remains are also known from Eocene deposits (52 million years ago) of Monte Bolca, Italy, (marine rays, sharks) and Green River, Wyoming, (freshwater stingrays).

Phylogenetic Relationships and Modern Diversity

Recent phylogenetic studies unite certain Meso-zoic elasmobranchs along with living sharks and rays in the group Neoselachii ("new sharks"), while Paleozoic elasmobranchs are more distantly related to neoselachians. Studies of evolutionary relationships among modern elasmobranchs indicate that rays are mono-phyletic (with a single origin and containing all lineages descended from the original ancestor) and are descended from sharklike ancestors sometime during the Early Mesozoic. In this scenario, "sharks" are not a natural group, as some squalomorphs (including cow, bramble, squaloid, angel, and sawsharks) are more closely

Figure 1A, B

Evolutionary Relationships among Living Holocephalan Families and Living Elasmobranch Orders

A Holocephalan Families

Recent Holocephali (c h i ma era s)

CALLORHYNCHIDAE

RHINOCHIMAERIDAE

(longnose chimaeras;7 sp p.)

CHIMAERIDAE

(shortnose chimaeras;25 sp p.)

B Elasmobranch Orders

Recent * Elasmobranch i i

Galeomorphii

Squalomorphii

• HETERODONTIFORMES

• LAMNIFORMES

(mackerel sharks; 16 spp)

■ CARCHARHINIFORMES

(ground sharks; 220 spp )

■HEXANCHIFORMES

■ ECHINORHINIFORMES jf

- SQUALIFORMES

(dogfish sharl<s;96 spp.)

' SQUATINIFORMES (

(angelsharks;15spp.) ^

■PRISTIOPHORI FORMES

• PRISTIFORMES

■ "RHINOBATIFORMES"

■ TORPEDINIFORMES

(electric rays,58 spp )

■ RAJIFORMES

•MYLIOBATIFORMES 3>

Sources: Modified from Nelson, J. S. 1994. Fishes of the World, 3d ed. New York: John Wiley; and Carvalho, Marcelo R. de. 1996. "Higher-level Elasmobranch Phylogeny: Basal Squaleans and Paraphyly." In Interrelationships of Fishes, edited by M. L. J. Stiassny, G. D. Johnson, and L. R. Parenti; Numbers of species are compiled from various sources.

Note: Living families and orders are in upper case, with common names and current number of species in parentheses. The relationships among rays are still uncertain (unresolved in the cladogram); Rhinobatiformes are in quotation marks because their monophyly is highly doubtful at present.

related to rays than to galeomorphs (including bullhead, carpet, requiem, tiger, mako, and white sharks). The morphological transition between shark and ray is evident in one fossil elasmobranch (the Jurassic Protospinax from

Solnhofen). This evolutionary scheme is at odds with the fossil record, as large gaps remain for some shark groups (for example, squaloids, bramble sharks), even though it is the most corroborated theory to date.

Elasmobranchs constitute about 95 percent of living chondrichthyan diversity; only three out of the approximately sixty families are of chimaeras. Living families of sharks are grouped into eight or nine orders (containing some 400 species), and they usually have elongate bodies, one or two dorsal fins (some with fin spines), a forked or lunate caudal fin for propulsion, paired pectoral and pelvic fins, and five to seven lateral gill slits. Sharks vary in size from whale sharks of 15 m to lantern sharks of 15 cm, and they range from bluish-gray pelagic forms to colorful benthic species. The 580 or so species of rays are usually divided into five orders, most with enlarged pectoral fins continuous with the head forming the disc (except in sawfishes and guitarfishes), and ventral gill slits (water intake is usually done through enlarged, muscular spiracular openings). Rays vary in size from 7 m across the disc (manta rays) to 15 cm in length (some electric rays). They inhabit from the shoreline to abyssal depths and are morphologically diverse (including sawfishes, electric rays, skates, gui-tarfishes, and venomous stingrays). South American potamotrygonid stingrays are exclusively freshwater, and some marine stingrays are pelagic. Both sharks and rays are carnivorous, but some of the largest species filter plankton for food.

Conservation

Consumer demand for shark-related products has increased over the past decades, and consequently many populations have declined by an estimated 70 to 90 percent because of overfishing (for example, dusty and sandbar sharks off the northeastern United States). Most countries do not manage their shark fisheries, and predatory fishing practices that kill indiscriminately are commonly employed (for example, long-lining and finning). Some species are already depleted (for example, sawfishes in the

Gulf of Mexico), and others face extinction in the near future. Elasmobranchs are increasingly important in the growing ecotourism industry, contrary to their maligned, popular Jaws image. Chondrichthyans are ecologically important apex-predators, and their reproductive peculiarities (internal fertilization, slow sexual maturation, long gestation, and production of few young) render them especially vulnerable to population decline.

—Marcelo Carvalho

See also: Bony Fishes; Evolutionary Biodiversity; Geological Time Scale; Phylogeny; Systematics

Bibliography

Cappetta, Henri, Chris Duffin, and Jiri Zidek. 1993. "Chondrichthyes." In The Fossil Record 2, edited by Michael J. Benton, pp. 593-609. London: Chapman and Hall; Carvalho, Marcelo R. de. 1996. "Higher-level Elasmobranch Phylogeny: Basal Squaleans and Para-phyly." In Interrelationships of Fishes, edited by M. L.J. Stiassny, G. D. Johnson, and L. R. Parenti, pp. 35-62. San Diego: Academic; Compagno, Leonard J. V. 1999. "Systematics and Body Form." In Sharks, Skates and Rays, edited by William C. Hamlett, pp. 1-42. Baltimore: Johns Hopkins University Press; Janvier, Philippe. 1996. Early Vertebrates. London: Oxford University Press; Nelson, J. S. 1994. Fishes of the World, 3d ed. New York: John Wiley; Springer, Victor G., and Joy P. Gold. 1989. Sharks in Question. Washington DC: Smithsonian Institution Press.

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