THE MAIN KINDS OF FOSSILS 41 



The record of fossil sea-urchins in Ohio consists of abundant spines in the Pennsylvanian 

 limestones. No complete specimen has yet been found. 



Phylum Chordata 



The most advanced animals are those with a backbone and a spinal cord; they are placed 

 in the phylum Chordata which also includes other, more primitive animals without a backbone 

 but with a nerve cord (notochord) running down the length of the body from head to tail. Only the 

 most important of these lower chordates from the standpoint of the fossil record (Class Grapto- 

 zoa, subphylum Hemichordata) are mentioned in this manual. The chordates with a backbone are 

 subdivided into two superclasses, Pisces (fishes and their relatives) and Tetrapoda (four-footed 

 animals). The geologic range of the phylum as a whole is Cambrian to present. 



SUBPHYLUM HEMICHORDATA . 



General . The living representatives of this subphylum are the tongue worms which com- 

 bine chordate and nonchordate characteristics. They are included here because the graptolites, 

 formerly considered as coelenterates, are now placed in this subphylum. 



Class Graptozoa. This is an extinct group (Cambrian to Lower Mississip- 

 pian) of colonial animals abundant in some black shales, scarcer in other kinds of 

 sediments. Superficially, they look like pencil marks or tiny sawblades (fig. 46); 

 actually, they are colonies of individual animals arranged in rows along a common 

 axis. 



Few graptolites have been recorded from the Paleozoic rocks of Ohio, partly 

 because black shales are not abundant in our Ordovician and Silurian, partly for Fig. 46 



lack of collecting. Our most abundant graptolites are Silurian in age. 



SUPERCLASS PISCES. 



General^ The animals generally called fishes include many classes which are as different 

 from each other as birds are different from mammals. They are related in that they are cold- 

 blooded, breathe primarily by means of gills, and have paired side fins and a tail fin. The four 

 classes of Pisces are described separately. 



Class Agnatha . These are the jawless fishes, of which a modern representative is the 

 lamprey. The early members of the class had bony armor which covered the front part of the 

 body. The jawless fishes are doubtfully represented in the Cambrian and are certainly known 

 from the Ordovician to the present. 



Agnatha abounded in the Devonian and Mississippian seas of Ohio, along with fishes of 

 other classes. Complete skeletons are sometimes found. The most abundant remains are those 

 found in the "bone beds" of the Devonian but these are all fragments of teeth, spines, and armor. 



Class Placodermi. This is an extinct group of 

 jawed fishes which first appeared in the Silurian and per- 

 sisted into the Permian, when they became extinct. Some 

 of them bore extensive bony armor (fig. 47); in others 

 there were bony plates only on the head. The class 

 includes the so-called spiny sharks and the arthrodires or 

 jointed-necked fishes which sometimes attained giant size. 



Ohio has long been famous for its fossil fishes. The Fi „ 47 



Devonian and Mississippian rocks of the state have yielded 



