38 



OHIO FOSSILS 



siphuncle 



septa 



Fig. 41 



suture 



Fig. 42 



CLASS CEPHALOPODA. The squids, devilfish, 

 octopus, and their allies are not numerous in present- 

 day seas. In the Paleozoic and Mesozoic seas they 

 were much more numerous and varied. One living form, 

 the pearly Nautilus, has a coiled shell which is exter- 

 nally like a snail shell but internally is divided into 

 compartments by walls (septa) which the gastropods 

 do not possess. The septa are pierced by round holes 

 through which a tube-like organ, the siphuncle (fig. 41), 

 passes. The cephalopods are exclusively marine. 

 Their geologic range is Cambrian to present. The 

 three subclasses are discussed separately. 



Subclass Nautiloidea . In this group the animal 

 lives inside the shell which is straight or coiled, and 

 divided into compartments whose edges (sutures) are 

 simple (fig. 42). The nautiloids are found from the Cambrian to the 

 present. They are not abundant in any of the Paleozoic rocks of Ohio 

 but a variety of genera and species, some of them spectacularly 

 large, are found in all marine formations. 



Subclass Ammonoidea. The ammonoids also have a straight 

 or coiled external shell which is divided internally into compartments 

 but in this group the sutures are complex, that is the septa are crin- 

 kled and their edges (sutures) are wavy. This is an extinct group 

 which first appeared in the Devonian and died out before the end of 

 the Cretaceous. 



Ammonoids are scarce in the Devonian of Ohio; they are found 

 sparingly in the Mississippian and Pennsylvanian of the state. 



Subclass Coleoidea . In the squid and its allies, which consti- 

 tute this subclass, the shell is inside the body and may be shaped like a cigar or a shoe horn. 

 Coleoids are found from the Mississippian to the present but were specially abundant in the 

 Jurassic. They have not been recorded from the Paleozoic rocks of Ohio. 



Fig. 43 



Phylum Annelida 



The segmented worms have left little fossil record but it begins with 

 rocks as old as Cambrian. It includes such indefinite evidence as trails, 

 tracks, castings, and burrows, as well as scolecodonts, the jaws and 

 denticles (teeth) of some worms. Some segmented worms secrete a cal- 

 careous tube, which may be straight or coiled, and fossil tubes of this 

 nature, attached to brachiopods or other objects (fig. 43) are abundant in 

 some rocks. The Annelida are marine, freshwater, and terrestrial. The 

 earthworm is the best known example. Calcareous annelid tubes have been 

 recorded from the Paleozoic rocks of Ohio. 



Phylum Onychophora 



This group of caterpillar -like creatures would scarcely deserve mention here but for the 

 fact that two fossil genera may belong to this phylum. If the fossil representatives are correctly 

 placed in this phylum, its geologic range is Cambrian to present. There is no record of fossil 

 onychophores in Ohio. 



