Defining membership within Squaliformes based on an unshared morphological character is not possible, and thus crafting a membership card for this order of fishes may have to await the results of molecular studies. Amongst living elasmobranches, Squaliformes is usually accepted as a sister group to an evolutionary branch consisting of angel sharks (Squatiniformes), saw sharks (Pristiophoriformes), and rays. Fossils interpreted as representing Squaliformes have been laid down in deposits at least 150 million years old, and future discoveries will surely push this trail marker back deeper into the past. In this chapter, we will consider the order Squaliformes to comprise a more traditional and inclusive group of sharks, which includes the bramble sharks (Echinorhinidae), the dogfish sharks (Squalidae), the gulper sharks (Centrophoridae), the lantern sharks (Etmopteridae), the sleeper sharks (Somniosidae), the rough sharks (Oxynotidae), and the kitefin sharks (Dalatiidae). ![]() Some do not include the bramble sharks, Echinorhinidae, and others exclude additional groups, justifying their decisions on various fine morphological details. However, higher-level systematics is not pure science, and hence it should not be surprising that experts may define the order Squaliformes somewhat differently. ![]() The order Squaliformes contains 22 genera, 98 formally described species, and at least 17 known but undescribed species.
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