The Osteichthyes includes the true bony fishes, which are the most
highly developed and abundant of all fishes. They possess an internal
bony skeleton, well-developed jaws, an air-bladder, and, typically, an
external covering of overlapping scales.
Included in this class are a primitive group of fishes called
_crossopterygians_. These were very abundant in the Devonian and are
believed to be the ancestors of the amphibians. The modern lungfishes
also belong to the class Osteichthyes, and these primitive fishes, which
are now found only in Australia, South America, and Africa, breathe by
means of gills and lungs which have been developed from the air-bladder.
Although not abundant as fossils, the remains of these specialized
fishes have added much to present knowledge concerning the development
of certain of the higher vertebrates.
The remains of bony fishes have been collected at many localities in
Texas, and fossils of this type have been found primarily in rocks of
Cretaceous age but have been reported from other rocks as well. Fish
fossils are more commonly found in the form of teeth (Pl. 37),
vertebrae, scales, and an occasional well-preserved skeleton.
[Illustration: Plate 37]
[Illustration: SHARK TEETH × 1]
[Illustration: CONODONTS
(GREATLY ENLARGED)]
[Illustration: PRIMITIVE ARMORED FISH
PLACODERM (DEVONIAN)
× ½]
_Conodonts_ (Pl. 37) are small, amber-colored, tooth-like fossils which
are believed to represent the teeth of some type of extinct fish.
Although geologists do not know a great deal about the origin of these
strange fossils they are of value in micropaleontology. Conodonts have
been reported from several Paleozoic formations in Texas and are useful
guide fossils in some areas.