Fig. 24. Graptolites. (a) Diplograptus (×2). (b) Dendrograptus (×3). (c) Phyllograptus (×2).
The [chitinous] [graptolite] [exoskeleton] is commonly preserved as a flattened carbon residue; their remains may be locally abundant along the bedding planes of certain black or dark gray shales.
Graptolites are known from rocks that range from [Cambrian] to [Mississippian] in age, and they are among the most important guide fossils for [Ordovician] and [Silurian] rocks.
Graptolites have been reported from [Cambrian] rocks in central Texas and from the [Ordovician] of west Texas ([fig. 24]). The most abundant of these occur in certain Ordovician rocks in the Trans-Pecos area where they are common fossils in certain formations.
Subphylum Vertebrata
The vertebrates are the most advanced of all chordates. They are characterized by a skull and a bony or cartilaginous internal skeleton, with a vertebral column of bone or cartilage. This subphylum is commonly divided into two superclasses, the Pisces (the fishes and their relatives) and the Tetrapoda (the four-footed animals).
As mentioned earlier, most amateur collectors collect very few [vertebrate] remains, and for this reason this group is not discussed in detail. However, the more important vertebrate classes are briefly reviewed to enable the reader to have some understanding of this important group of animals. This part of the handbook will also serve as an introduction to some of the interesting and unusual, but now extinct, animals that have inhabited Texas in the geologic past. Among these animals are giant fishes, primitive amphibians, and many different types of dinosaurs. Included also are such unusual mammals as the giant ground sloths, saber-tooth cats, mammoths, and mastodons, all of which are now extinct. The remains of these, and many other interesting extinct vertebrates, may be seen in the geological collections of the Texas Memorial Museum at Austin. Many of these displays are accompanied by drawings which depict the scientific restoration of the animal’s soft parts and show how the animal may have appeared in life.
Superclass Pisces
The members of this superclass are commonly called fishes and are the simplest and most numerous of all vertebrates. They are aquatic, free-moving, cold-blooded (their blood maintains the temperature of the surrounding water), and breathe primarily by means of gills. However, some forms (the lungfishes) breathe by means of a lung developed from the air-bladder.
The most recent fish classification recognizes four major classes, the Agnatha (primitive jawless fishes), the Placodermi (armored fishes with primitive jaws), the Chondrichthyes (sharks and related forms with cartilaginous internal skeletons), and the Osteichthyes (true bony fishes).