224 Evolution? The Fossils Say YES!
another primitive sharklike feature. Their bodies are also covered with heavy rhombohe-
dral scales, and their tails are very sharklike in having the upper lobe much larger than the
lower lobe. By the Mesozoic, these primitive groups had mostly died out with the exception
of a few living fossils, such as the sturgeon and the paddlefish. In their place came another
great radiation of more advanced ray-finned fish (known by the paraphyletic wastebasket
name “holosteans”), which can easily be distinguished from their primitive relatives. Their
skulls are still made of fairly solid bone, but the upper jawbones (premaxillary and maxillary
bones) are hinged at the front of the skull, allowing them to open their mouths wider and
grab larger prey. The back part of the skull is also less solidly bony and more open, so they
have enlarged jaw muscles for a stronger bite. Unlike chondrosteans, the holosteans have
almost no cartilage in their skeletons but are completely bony. Their scales are thinner and
smaller, so they are not as heavily armored as primitive ray-finned fish. The tail is nearly
FIGURE 9.13. The transition from (A) primitive bony fish (“palaeoniscoids”) with simple robust “snap-trap”
jaws and heavy bones throughout their skulls to (B) more advanced “holostean” fish (such as the bowfin Amia,
shown here), with more protrusible jaws and less ossified skulls, to (C) the modern teleost fish, with a very
lightly ossified skull and a protrusible jaw for sucking down prey. (From Schaeffer and Rosen 1961; used by
permission)
(A)(B)(C)