Texas Fossils: An Amateur Collector's Handbook

by William Henry Matthews

CLASS AGNATHA.—

Fishes belonging to this class are primitive, jawless, and represented by the living lampreys and hagfishes. The first agnathans appeared in the [Ordovician] and were armored by a bony covering on the front part of their bodies. These primitive fishes, called ostracoderms, are the earliest recorded fishes and, in addition, appear to be the first known [vertebrate] animals. The ostracoderms first appeared in late Ordovician time, increased in numbers in the [Silurian], and were extinct by the end of the [Devonian].

CLASS PLACODERMI.—

These are primitive jaw-bearing fishes, the majority of which were heavily armored ([Pl. 37]). The placoderms were shark-like in appearance, and some of them grew to be as much as 30 feet in length. Members of this class appeared first in the [Devonian] and lasted into the [Permian], at which time they became extinct. Placoderms are rare in Texas, but the fragmentary remains of these primitive fishes have been found in Devonian rocks in central Texas.

CLASS CHONDRICHTHYES.—

This class includes such modern forms as the sharks, rays, and skates. They are characterized by skeletons which are composed of cartilage and are very abundant in the marine waters of today. The earliest known representatives of this class are reported from rocks of [Devonian] age, and they have been relatively common up to the present time.

Shark teeth ([Pl. 37]) can be found in Texas in [Pennsylvanian], [Permian], [Cretaceous], [Paleocene], [Eocene], and [Miocene] rocks. These are probably the most common [vertebrate] fossils to be found in Texas and are usually found in thin-bedded marine limestones or clays.

CLASS OSTEICHTHYES.—

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.