Fish Tales 223fins, and a rigid tail that was almost symmetrical. In the Mesozoic, the hybodont sharks are
considerably more advanced, with a reduced, more flexible chondrocranium, pectoral fins
with narrower bases that allowed more maneuverability, more specialized teeth, and a more
flexible tail for powerful swimming. These trends are all continued in the living sharks, or the
Neoselachii, which have a chondrocranium that is highly reduced, so they can protrude their
upper and lower jaws very far; highly maneuverable pectoral fins; a wide variety of tooth
types; and an even more flexible tail. Sharks may have been around for a long time, but they
show considerable evolution during this history, and any creationist who claims otherwise
has no familiarity with the real fossil record of sharks.
After the sharks and rays split off the family tree (fig. 9.1), the next groups up the clado-
gram are all known as osteichthyans, the “bony fish.” They break into two main groups: the
“lobe-finned fish,” which include lungfish, coelacanths, and, of course, the tetrapods, to be dis-
cussed in the next chapter; and the “ray-finned fish,” so called because they support their fins
with many long bony rays. Ray-finned fish make up about 98 percent of the species of living
fish; only the hagfish, lamprey, lungfish, coelacanth, and chondrichthyans are not members
of this group. Like the other main groups of fishes, ray-finned fish appeared in the Devonian,
but they have been evolving rapidly ever since then, with hundreds of genera and thousands
of species known from both the fossil record and the living world (figs. 9.12 and 9.13). As with
sharks, the lineage has been around for 400 million years, but they show remarkable changes
over that long history and major changes in the ways in which they feed and swim.
The earliest (mostly Paleozoic) ray-finned fishes (known by the paraphyletic waste-
basket name “chondrosteans”) have heavy bone surrounding the head region and simple
“snap-trap” jaws with limited flexibility and limited room for muscles that close them shut.
Although they have a lot of bone in their skeletons, large parts are also made of cartilage,
FIGURE 9.12. The evolutionary radiation of bony fishes. (From Kardong 1995; reproduced with permission of
the McGraw-Hill Companies)