Concept

Cladoselache

Résumé
Cladoselache is an extinct genus of shark-like chondrichthyan (cartilaginous fish) from the Late Devonian (Famennian) of North America. It was similar in body shape to modern lamnid sharks (such as mako sharks and the great white shark), but was not closely related to lamnids or to any other modern (selachian) shark. As an early chondrichthyan, it had yet to evolve traits of modern sharks such as accelerated tooth replacement, a loose jaw suspension, enameloid teeth, and possibly claspers. Some 20th century studies considered Cladoselache to be a basal (early-diverging) member of Elasmbobranchii, the fork of cartilaginous fish which leads to modern sharks and rays. More recent studies have identified distinctive traits of the chondrocranium (cartilaginous braincase), dorsal fin spines, and pectoral fin bases. These newly identified features support a close relationship to symmoriiforms, a small group of bizarre chondrichthyans such as the bristle-spined Stethacanthus. Cladoselache and symmoriiforms may be more closely related to chimaeras (a modern group of unusual deep-sea fish) than to true sharks and rays. Growing to several meters in length, Cladoselache is considered to have been a fast-moving and fairly agile marine predator due to its streamlined body and deeply forked tail. From both an anatomical and historical perspective, is one of the best known of the early chondrichthyans in part due to an abundance of well-preserved fossils, discovered in the Cleveland Shale on the south shore of Lake Erie. In addition to the cartilaginous skeleton, the fossils were so well preserved that they included traces of skin, muscle fibers, and internal organs, such as the kidneys. The anatomy of Cladoselache shows a mixture of derived and ancestral characteristics. The skeleton is composed of tessellated cartilage, a complex tissue unique to chondrichthyans. Tessellated cartilage combines flexible cartilage fibers with a loose mosaic of irregular calcified plates, known as tesserae.
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