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Squalomorphi
Squalomorphi is a superorder of sharks, generally characterized by lacking traits such as an anal fin, nictitating membrane, or suborbital shelves in the cranium. Squalomorphs are also called squalea, or squaleans. There are about 163 living species in 11 families. Squalean sharks are divided into six orders: Echinorhiniformes, Hexanchiformes, Squaliformes, Squatiniformes, Pristiophoriformes and the extinct Protospinaciformes. Extant orders Order Hexanchiformes The order Hexanchiformes is a very small and primitive order consisting of cow sharks and frilled sharks. These sharks have only 1 dorsal fin, 6-7 gill slits, and no nictitating membrane on the eyes. Cow sharks are fairly large and stocky, while frilled sharks are smaller and have eel-like bodies. The oldest fossils from this order can be dated back to the mid-Jurassic. There are six living species in 4 genera, and 2 families. * Family Hexanchidae (Cow sharks) * Family Chlamydoselachidae (Frilled sharks) Order Sq ... [...More Info...] [...Related Items...] OR: [Wikipedia] [Google] [Baidu] |
Shark
Sharks are a group of elasmobranch cartilaginous fish characterized by a ribless endoskeleton, dermal denticles, five to seven gill slits on each side, and pectoral fins that are not fused to the head. Modern sharks are classified within the Division (taxonomy), division Selachii and are the sister group to the Batoidea, Batomorphi (Batoidea, rays and skate (fish), skates). Some sources extend the term "shark" as an informal category including Extinction, extinct members of Chondrichthyes (cartilaginous fish) with a shark-like morphology, such as hybodonts. Shark-like chondrichthyans such as ''Cladoselache'' and ''Doliodus'' first appeared in the Devonian Period (419–359 million years), though some fossilized chondrichthyan-like scales are as old as the Ordovician, Late Ordovician (458–444 million years ago). The earliest confirmed modern sharks (Selachii) are known from the Early Jurassic around , with the oldest known member being ''Agaleus'', though records of true shar ... [...More Info...] [...Related Items...] OR: [Wikipedia] [Google] [Baidu] |
