Chondrichthyes The First Sharks

The first chondrichthyans ('cartilaginous fishes'), distant ancestors of modern sharks and rays, may be indicated by isolated scales and teeth from the Upper Ordovician and Silurian (Sansom et al., 2001). The first definitive remains containing prismatic calcified cartilage (see below) date from the Early Devonian. An early shark, Cladoselache from the Upper Devonian of Ohio (Figure 3.15), reached a length of 2m. The skin does not seem to have borne scales, although small multicusped tooth-like scales have been found on the edges of the fins, in the mouth cavity, and around the eye.

Externally the tail fin is nearly symmetrical, but internally the notochord bends upwards into the dorsal lobe only (the heterocercal tail condition). There are two dorsal fins, one behind the head, and the other halfway down the body, and the anterior dorsal fin has a spine in front. There are two sets of paired fins, the pectoral and pelvic fins, each set approximately beneath one of the dorsal fins, and each associated with girdle elements of the skeleton. Cladoselache was probably a fast swimmer, using sideways sweeps of its broad tail as the source of power, and its pectoral fins for steering and stabilization. As in modern sharks, the skeleton of Cladoselache is made from calcified cartilage, in other words, cartilage invested with some calcium phosphate, but not true bone. Calcified cartilage is

Photos Chondrichthyes Cartilage

Fig. 3.14 Diverse placoderms: (a) the rhenanid Gemuendinain dorsal view; (b,c) the antiarchs Pterichthyodes (b) and Bothriolepis (c) in lateral view; (d) the petalichthyid Lunaspisin dorsal view; (e) the ptyctodont Ctenurellain lateral view. [Figures (a,c-e) after Moy-Thomasand Miles, 1971; (b) after Hemmings, 1978.]

Fig. 3.14 Diverse placoderms: (a) the rhenanid Gemuendinain dorsal view; (b,c) the antiarchs Pterichthyodes (b) and Bothriolepis (c) in lateral view; (d) the petalichthyid Lunaspisin dorsal view; (e) the ptyctodont Ctenurellain lateral view. [Figures (a,c-e) after Moy-Thomasand Miles, 1971; (b) after Hemmings, 1978.]

known also in placoderms and several agnathans, but chondrichthyans appear to be unique in having prismatic calcified cartilage, arranged as small platelets or prisms.

Cladoselache is usually assigned to the order Cla-doselachida, a group restricted to the Late Devonian. Other shark remains are known from rocks of this age (Zangerl, 1981), but none as well as Cladoselache. The major chondrichthyan lineages arose in the Devonian, but radiated in the subsequent Carboniferous Period, so this later history is discussed in more detail in Chapter 7.

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