Page 451 - Introduction to Paleobiology and The Fossil Record

438 INTRODUCTION TO PALEOBIOLOGY AND THE FOSSIL RECORD

trunk shield from the Devonian of many parts of the world
head shield
(Box 16.5).
After the Devonian, the actinopterygians
seem to have radiated three times. The fi rst
radiation (Devonian-Permian) consisted of
100 mm the palaeonisciforms (Fig. 16.10a), a para-
anal fin pelvic fin pectoral fin

(a) phyletic group of bony fishes with large bony
scales and heavy skull bones. The second
dorsal fin spines
radiation of bony fi shes, an assemblage termed
the “holosteans”, occurred in the Late Trias-
sic and Jurassic. Semionotus (Fig. 16.10b), a
10 mm
small form that has been found in vast shoals,
anal pelvic intermediate pectoral fin had more delicate scales than the palaeonisci-
fin spine spines spine forms, and a jaw apparatus that could be
spine partly protruded, hence providing a wider
(b)
gape.
The third and largest radiation of actinop-
terygian fishes, occurred in the Late Jurassic

and Cretaceous (Fig. 16.10c), with the diver-
sification of the teleosts. Teleosts are the most

diverse and abundant fishes today, including

25 mm
(c) 23,000 living species, such as eel, herring,
salmon, carp, cod, anglerfi sh, fl ying fi sh, fl at-

fish, seahorse and tuna. The huge success of
this radiation may be the result of their
remarkable jaws. Palaeonisciforms opened
their jaws like a simple trapdoor, holosteans
10 mm
(d) could enlarge their gape a little, but teleosts
can project the whole jaw apparatus like an
upper lobe dorsal fins lateral line canal extendable tube (Fig. 16.10d). This came
about because of great loosening of the ele-
ments of the skull: as the lower jaw drops, the
tooth-bearing bones of the upper jaw (the
lower maxilla and premaxilla) move up and for-
lobe
anal fin pelvic fin 10 mm pectoral fin wards. Rapid projection of a tube-like mouth
(e) allows many teleosts to suck in their prey,
while others use the system to vacuum up
Figure 16.8 Jawed fi shes of the Devonian: (a) food particles from the seafloor, or to snip

the placoderm Coccosteus; (b) the acanthodian precisely at fl esh or coral.
Climatius; (c) the actinopterygian bony fi sh
Cheirolepis; (d) the lungfi sh Dipterus; and (e) the
lobefi n Osteolepis. (Based on Moy-Thomas & The evolution of sharks:
Miles 1971.) an arms race with their prey?
During the Carboniferous, numerous extraor-
dinary shark-like fishes arose, and these were

The lungfi sh Dipterus (Fig. 16.8d) was a long, clearly important marine predators. A second

slender fish that hunted invertebrates and shark radiation took place in the Triassic and
fishes, and crushed them with broad grinding Jurassic. Hybodus (Fig. 16.11a) was a fast-

tooth plates. The “rhipidistian” Osteolepis swimming fish, capable of accurate steering
(Fig. 16.8e) was also long and slender, and using its large pectoral (front) fi ns. The
was an active predator. These lobefi ns had hybodontiforms had a range of tooth types,
muscular front fi ns, and could have used these from triangular pointed fl esh-tearing teeth to
to haul themselves over mud from pond to broad button-shaped crushers, adapted for
pond. Specimens of these fi shes are known dealing with mollusks. It is rare to fi nd whole