222 Evolution? The Fossils Say YES!
FIGURE 9.11. The evolution of the sharks. (Drawing by Carl Buell)
AcanthodianCladoselacheStethacanthusFalcatusXenacanthusCtenacanthusHybodontModern sharkModern raySilurian Devonian Miss. Penn. Permian Mesozoic Cenozoicteeth and feed on plankton by filtering them through their gills. Other sharks are specialized
mollusk eaters with crushing teeth, especially the flat-bodied skates and rays. Because shark
skeletons are made of cartilage and do not fossilize well, we know them primarily from their
teeth (which are made of bone and enamel) and often from the bony spines that many sharks
had in their bodies and fins. Although the basic shark design has been successful for over 400
million years, sharks show considerable evolutionary change through time, contrary to the
creationist books and websites (fig. 9.11). The earliest sharks, the cladodonts from the Devo-
nian, had a large, very primitive “skull” (actually a cartilaginous precursor of the skull called
a chondrocranium), very broad-based stiff pectoral fins, thick spines in front of their dorsal