Historical Geology Understanding Our Planet\'s Past

Sharks breathe by drawing water in through the mouth, passing it over the

gills, and expelling it through distinctive slits behind the head. The body of the

shark is heavier than water, requiring it to swim constantly or else sink to the bot-

tom.Instead of skeletons composed of bone as with most fish, shark skeletons are

made of cartilage, a much more elastic and lighter material. However, cartilage

does not fossilize well. About the only common remains of ancient sharks are

teeth, found in marine rocks of Devonian age onward (Fig. 95).

MARINE INVERTEBRATES

The Devonian marine invertebrates were similar to those that evolved in the

Ordovician and include prolific brachiopods, corals, crinoids, trilobites, and

Figure 95Excavation for
shark teeth at Shark’s
Tooth Hill, Kern County,
California.
(Photo by R.W. Pack,
courtesy USGS)

Historical Geology