Holothuroid body impressions have been reported from the Middle [Cambrian], and sclerites from rocks as old as [Mississippian].
[Phylum] Chordata
The chordates are the most advanced of all animals and are characterized by the presence of a well-developed nervous [system] and a body supported by a bony or cartilaginous notochord and/or spinal column. In the higher chordates (the vertebrates) the notochord is normally replaced by bone, but in the lower chordates (for example, the graptolites) it remains in a cartilaginous condition.
The [phylum] Chordata contains only two subphyla of paleontological significance. These are the subphylum Hemichordata, composed of primitive chordates (including the graptolites which are important fossils), and the Vertebrata, which includes all animals with backbones.
Subphylum Hemichordata
The hemichordates are characterized by a well-defined notochord which runs the length of the body, but they do not possess a true backbone. Only one class, the Graptolithina, is of paleontological importance.
CLASS GRAPTOLITHINA.—
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. 24]a) 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.
