A spiny defence
Primitive sharks like Asteracanthus and Hybodus (see lower picture), have hard spines on the leading edge of their dorsal fins (along the top of the body). These were to provide protection from predators; if attacked, the shark would stick up these spines, making it very difficult for the attacking animal to swallow its prey. The Horn Shark, found along the California coastline, does exactly the same thing today.
