The graptolites are a group of extinct colonial animals which were very
abundant during early Paleozoic time. They are characterized by a
chitinous exoskeleton consisting of rows of cups or tubes which housed
the living animal. These cups are attached to single or branching stalks
(fig. 24) which in some forms were attached to sea weeds, rocks, or
other foreign objects where they led a fixed existence. The stalks of
the unattached graptolites grew on floats (fig. 24a) and these floating
forms attained wide geographic distribution. It is also possible that
some of the attached forms were fixed to floating objects, such as sea
weed, and thus were distributed in this manner.
Previous classifications have recognized the graptolites as members of
the phylum Coelenterata. As coelenterates they were assigned, at various
times, to the classes Hydrozoa, Scyphozoa, and Graptozoa. In addition,
they were also classified as bryozoans by certain of the early
paleontologists. This publication, in keeping with recent changes in
taxonomy, considers graptolites to be an extinct group of hemichordates.
This classification is based upon research in which uncompressed
graptolites were etched out of chert and studied in great detail.
Information derived from these relatively undistorted specimens
indicates a much higher degree of body organization than was previously
suspected, and as a result of these studies most paleontologists now
consider graptolites to be some form of primitive chordate.
[Illustration: 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.