Taxonomy and Phylogeny
The frilled shark was first scientifically recognized by German ichthyologist Ludwig Döderlein, who visited Japan between 1879 and 1881 and brought two specimens to Vienna. However, his manuscript describing the species was lost, and so the first description of the frill shark became authored by American zoologist Samuel Garman, working from a 1.5 m (4.9 ft) long female caught from Sagami Bay in Japan. His account, entitled "An Extraordinary Shark", was published in an 1884 volume of Proceedings of the Essex Institute. Garman placed the new species in its own genus and family, and gave it the name Chlamydoselachus anguineus from the Greek chlamy ("frill") and selachus ("shark"), and the Latin anguineus for "snake-like". Other common names for this species include frill shark, lizard shark, scaffold shark, and silk shark.
Several early authors believed the frilled shark to be a living representative of otherwise long-extinct groups of elasmobranchs (sharks, rays, and their ancestors), based on its multi-pointed teeth, the articulation of its upper jaw directly to the cranium behind the eyes (called "amphistyly"), and its "notochord-like" spinal column with indistinct vertebrae. Garman proposed that it was allied with the "cladodonts", a now-obsolete taxonomic grouping containing forms that thrived during the Palaeozoic era, such as Cladoselache from the Devonian period (416–359 Ma). His contemporaries Theodore Gill and Edward Drinker Cope suggested it was instead related to the hybodonts, which were the dominant sharks during the Mesozoic era. Cope went as far as to assign this species to the fossil genus Didymodus.
More recent investigations have found that the similarities between the frilled shark and extinct groups may have been overstated or misinterpreted, and that this shark exhibits a number of skeletal and muscular traits that firmly place it with the neoselachians (modern sharks and rays), and more specifically with the cow sharks in the order Hexanchiformes (though systematist Shigeru Shirai has proposed that it be placed in its own order, Chlamydoselachiformes). Nevertheless, the frilled shark belongs to one of the oldest still-extant shark lineages, dating back to at least the Late Cretaceous (c. 95 Ma) and possibly to the Late Jurassic (c. 150 Ma). Because of its ancient ancestry and "primitive" characteristics, it has been described as a "living fossil".
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