Temporal range: Wenlock–Recent
|Great white shark
Elasmobranchii // is a subclass of Chondrichthyes or cartilaginous fish, that includes the sharks (Selachii) dogfish sharks (Squalus acanthias) and the rays or skates (Batoidea). "They resemble the true fishes in external form, but differ from them so widely in structure that they are placed in a class by themselves."
"The elasmobranchs exhibit a number of structural advances over the cyclostomes; there are paired fins, a lower jaw, gill arches, and placoid scales. Among the peculiarities which separate the elasmobranchs from the true fishes (Pisces) are the absence of membrane bones, of an air bladder, and of true scales, and the presence of skeletal characteristics which are not found in true fishes. Two subclasses of living elasmobranchs are recognized: the Selachi or sharks and rays, and Holocephali or chimaeras." 
Members of the elasmobranchii subclass have no swim bladders, five to seven pairs of gill clefts opening individually to the exterior, rigid dorsal fins, and small placoid scales. The teeth are in several series; the upper jaw is not fused to the cranium, and the lower jaw is articulated with the upper.
In contrast, hyostyly involves an ethmoid articulation between the upper jaw and the cranium, while the hyoid most likely provides vastly more jaw support compared to the anterior ligaments. Finally, in euhyostyly, also known as true hyostyly, the mandibular cartilages lack a ligamentous connection to the cranium. Instead, the hyomandibular cartilages provide the only means of jaw support, while the ceratohyal and basihyal elements articulate with the lower jaw, but are disconnected from the rest of the hyoid. The eyes have a tapetum lucidum. The inner margin of each pelvic fin in the male fish is grooved to constitute a clasper for the transmission of sperm. These fish are widely distributed in tropical and temperate waters.
Many fish maintain buoyancy with swim bladders. However elasmobranchs lack swim bladders, and maintain buoyancy instead with large livers that are full of oil. This stored oil may also function as a nutrient when food is scarce.  Deep sea sharks are usually targeted for their oil, because the livers of these species can weigh up to 20% of their total weight.
The brain is more highly developed than that of the cyclostomes. It possesses two remarkably large olfactory lobes, a cerebrum of two hemispheres, a pair of optic lobes, and a cerebellum which projects backward over the medulla oblongate. There are ten pairs of cranial nerves. The spinal cord is a dorsoventrally flattened tube with a narrow central canal; it is protected by the vertebral column. Spinal nerves arise from its sides in pairs.
The surface (skin) is covered with placoid scales or dermal denticles which form shagreen. They represent a primitive exoskeletal structure and have been the starting point for the development of the scales and bony plates of the true fishes. A placoid scale consiste of a bony plate with a spine in the center compound of dentine and covered with enamel. Over the jaws they are modified as "teeth" with their points directed backward, and are used for holding and tearing prey.
Fossilised shark teeth are known from the early Devonian, around 400 million years ago. During the following Carboniferous period, the sharks underwent a period of diversification, with many new forms evolving. Many of these became extinct during the Permian, but the remaining sharks underwent a second burst of adaptive radiation during the Jurassic, around which time the skates and rays first appeared. Many surviving orders of elasmobranch date back to the Cretaceous, or earlier.
Nelson's 2006 Fishes of the World arranges the class as follows:
- Subclass Elasmobranchii
- †Order Squatinactiformes
- †Order Protacrodontiformes
- †Infraclass Cladoselachimorpha
- †Order Cladoselachiformes
- †Infraclass Xenacanthimorpha
- †Order Xenacanthiformes
- Infraclass Euselachii (sharks and rays)
- †Order Ctenacanthiformes
- †Division Hybodonta
- †Order Hybodontiformes
- Division Neoselachii
- Subdivision Selachii (Selachimorpha) (modern sharks)
- Superorder Galeomorphi
- Superorder Squalomorphi
- Subdivision Batoidea (rays)
- Subdivision Selachii (Selachimorpha) (modern sharks)
Recent molecular studies suggest the Batoidea are not derived selachians as previously thought. Instead, skates and rays are a monophyletic superorder within Elasmobranchii that shares a common ancestor with the selachians.
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- Skaphandrus.com Elasmobranchii