THE MAIN KINDS OF FOSSILS 



39 



Phylum Arthropoda 



GENERAL . In all of these, the skeleton is external, jointed, and consists of chitin, some- 

 times strengthened by calcium carbonate. The most familiar are the insects, spiders, lobsters, 

 and crabs; less familiar are the scorpions, barnacles, and extinct groups such as the trilobites 

 and eurypterids. The phylum contains many classes of which only those most important as fos- 

 sils are discussed. 



CLASS CRUSTACEA . These are the lobsters, crabs, crayfish, and their relatives; most 

 of them are marine but the crayfish lives in fresh water and some of the crabs can live on land. 

 The geologic range of the class as a whole is Cambrian to present. Fossil crustaceans are not 

 abundant in the Paleozoic rocks of Ohio. Fossil crayfish may be expected in the freshwater 

 Pleistocene beds. 



Subclass Ostracoda. The ostracodes are among the oddest of the crustaceans. They are 

 all small and the best way to examine the living ones is under a low-power binocular microscope. 

 At first the ostracode looks like a tiny clam with its valves shut; if it is left alone, it will open 

 its valves and poke out its jointed legs - proof that it is an arthropod. Fossil ostracodes are 

 abundant in some formations and are among the best index fossils for subsurface beds. Some 

 Ohio Paleozoic beds contain abundant ostracodes. 



CLASS ARACHNOIDEA. This group includes scorpions and spiders, which have a scanty 

 geologic record, the horseshoe crabs, and the eurypterids, an extinct group (Ordovician to 

 Permian) most abundant during Silurian and Devonian times. Eurypterids have been recorded 

 from Ohio rocks but they are never abundant. 



CLASS TRILOBITA . This is another extinct group (Cambrian to Permian) which can be 

 recognized by the two grooves running from head to tail which divide the body into three distinct 

 areas (fig. 44). The threefold division may be obscure 

 on the head and tail of some trilobites but it is always 



clear in the thoracic (mid-section) segments. The glabella 



trilobites were exclusively marine; their remains are 

 fairly common in some formations. 



The Paleozoic formations of Ohio have yielded 

 many and varied kinds of trilobites. The Ordovician 

 beds at Cincinnati are famous for the abundance of 

 trilobites in some of them and the Silica formation 

 (Devonian) is equally famous for good specimens. 

 Some of our Mississippian and Pennsylvanian trilo- 

 bites, though very small, are highly ornamented with 

 spines and beautifully perfect specimens are some- 

 times found. 



CLASS INSECTA. The insects are a very 

 ancient group, distinguished from the other arthropods 

 by the fact that they have a distinct head, thorax, and 

 abdomen, not more than three pairs of legs, and in 

 the adult of most groups, one or two pairs of wings. 



Some of the most spectacular insects are those 

 of the Pennsylvanian; during this period cockroaches 

 up to 4 inches long and dragonflies with a wingspread 

 of 30 inches lived in the coal forests. Many kinds of 

 fossil insects have been recorded from Ohio, mostly 

 from the Pennsylvanian. 



Fig. 44 



