Sharks and chimaeras diversified hugely in the Carboniferous, and their diversity declined during the Permian and Triassic. Some of the Carboniferous groups survived into the Mesozoic, notably the hy-bodont sharks, some ctenacanths and the holocepha-lans, also evolved from Palaeozoic ancestors. Modern sharks, the neoselachians, diversified in the later Meso-zoic and Cenozoic seas.
7.2.1 Neolselachii:the modern sharks
The neoselachians, including all modern sharks and rays, arose in the late Palaeozoic and radiated particularly during the Jurassic and Cretaceous to the modern diversity of 42 families. Their early history is hard to track because the sole remains are often teeth (cartilaginous skeletons do not preserve well) and teeth are hard to classify (Cuny and Benton, 1999). The neoselachians are characterized by numerous derived characters (de Carvalho, 1996; Shirai, 1996), such as the possession of calcified centra. Many other derived characters relate to a more adaptable feeding system and capabilities of faster swimming than in precursor shark groups.
The jaws of neoselachians open more widely than in earlier forms because of greater mobility about the jaw joint and a highly kinetic palatoquadrate and hyomandibular (see p. 54, Figure 3.11(c)). This allows most neoselachian sharks to have a wide gape, as the palatoquadrate moves forwards relative to the brain-case and the hyomandibular rotates as the lower jaw opens (Wilga et al., 2000). The snout is usually longer than the lower jaw and this means that the mouth opens beneath the head rather than at the front. In larger
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