Fish Tales 221Jaws: The Evolutionary Story
The term “fish” is of value on restaurant menus, to anglers and aquarists, to stratig-
raphers and in theological discussions of biblical symbolism. Many systematists
use it advisedly and with caution. Fishes are gnathostomes that lack tetrapod char-
acteristics. We can conceptualize fishes with relative ease because of the great evo-
lutionary gaps between them and their closest living relatives, but that does not
mean they comprise a natural group. The only way to make fishes monophyletic
would be to include tetrapods, and to regard the latter merely as a kind of fish.
Even then, the term “fish” would be a redundant colloquial equivalent of “gna-
thostome” (or “craniate,” depending upon how far down the phylogenetic ladder
one wished to go).
—John Maisey, “Gnathostomes”One of the great evolutionary breakthroughs in vertebrate history was the origin of jaws.
Before jaws appeared, vertebrates were severely limited in what they could eat (mostly filter
feeding or deposit feeding or living as parasites like lampreys and hagfish), and thus in their
lifestyles and body size. Jaws allowed vertebrates to grab and break up a food item, which in
turn meant they could eat a wide variety of foods, from other fish to plants to mollusks and
so on. This then allowed vertebrates to evolve into a great many different ecological niches
and body sizes, including superpredators that ate all other kinds of marine life. Eventu-
ally, vertebrates used their jaws and teeth for many other things besides eating, including
manipulating objects, digging holes, carrying material to build nests, carrying their young
around, and making sound or speech.
Once jaws appeared in the Silurian, there was a tremendous evolutionary radiation
(fig. 9.1) of different kinds of jawed vertebrates, or more properly, gnathostomes (which means
“jawed mouth” in Greek). As the quote from John Maisey points out, we have been accus-
tomed to using the word “fish” to describe most of these vertebrates, but that word has no
meaning in systematics. Fish are simply vertebrates or gnathostomes that live in the water
and are not land animals, or tetrapods. The group is ecological and paraphyletic and not a
natural taxon whatsoever. However, for the purposes of this book, we will keep the terminol-
ogy simple, recognizing that fish is a label of convenience and not some real biological entity.
The radiation of gnathostomes began with the dominant group in the Devonian, the
extinct placoderms (fig. 9.1). These creatures had thick bony plates covering their head and
shoulders, but the rest of the skeleton was made of cartilage. Some of them had sharp bit-
ing plates on the edge of their mouth shields and reached up to 10 meters (30 feet) long,
the largest predator the world had seen up to that time. Others were weighted down with
armor over the entire front half of their body (including jointed armor on their pectoral fins
that resembled crab legs) and apparently ate small slow prey on the sea bottom. Still others
developed flat bodies like rays and skates. All of these different body shapes evolved rapidly
in the Devonian and then vanished at the end of that period.
The next group to branch off the family tree of gnathostomes (figs. 9.1 and 9.11) were the
sharks, or chondrichthyans (“cartilaginous fish”). We think of the terrors of the movie Jaws
or the documentaries that feature sharks attacking divers, but sharks are actually much more
complex and interesting than that. Most are highly effective predators with rows of razor-sharp
teeth, but the largest sharks (whale sharks, megamouth sharks, and basking sharks) have tiny