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Unlike traditional scales, sharks have placid scales, known as denticles. Denticles are V-shaped and are made of layers of dentine and a surface of enamel. [24] Riblets are sockets in the shark's skin which hold the denticles. [22] These denticles on the skin allow for the shark to move quietly, swiftly, and almost effortlessly.
Rhincodontidae is a shark family which includes the whale shark, the sole extant member and the largest living fish. A single extinct genus, Palaeorhincodon , is known from the Paleocene as well. [ 1 ]
The whale shark (Rhincodon typus) is a slow-moving, filter-feeding carpet shark and the largest known extant fish species. The largest confirmed individual had a length of 18.8 m (61.7 ft). [8] The whale shark holds many records for size in the animal kingdom, most notably being by far the most massive living non-cetacean animal.
Physical characteristics of sharks – shark skeleton, respiration and skin Dermal denticle – small outgrowths which cover the skin of sharks; Ampullae of Lorenzini – sensing organ that helps sharks and fish to sense electric fields; Electroreception – the biological ability to perceive electrical impulses (see also Ampullae of Lorenzini)
Spiracle of a shark (bighead spurdog, Squalus bucephalus) The spiracle is a small hole behind each eye that opens to the mouth in some fish. In the jawless fish, the first gill opening immediately behind the mouth is essentially similar to the other gill openings.
Shark skeletons are very different from those of bony fish and terrestrial vertebrates. Sharks and other cartilaginous fish (skates and rays) have skeletons made of cartilage and connective tissue. Cartilage is flexible and durable, yet is about half the normal density of bone. This reduces the skeleton's weight, saving energy. [29]
A spiracle is a small hole found behind each eye. These can be tiny and circular, such as found on the nurse shark (Ginglymostoma cirratum), to extended and slit-like, such as found on the wobbegongs (Orectolobidae). Many larger, pelagic species, such as the mackerel sharks (Lamnidae) and the thresher sharks (Alopiidae), no longer possess them.
The most ancient types of shark-like fish date back to 450 million years ago, during the Late Ordovician period, and are mostly known by their fossilized teeth and dermal denticles. [5] However, the most commonly found fossil shark teeth are from the Cenozoic era (the last 66 million years).