PRE-CAMBRIAN AND PALEOZOIC ERAS 165 



great evolutionary changes others continue virtually unchanged for mil- 

 lions of years. Dynasties of animals wax and wane, but the cockroach 

 goes on forever. 



Large size characterized many Pennsylvanian insects. Thus one of a 

 group of insects closely resembling modern dragonflies had a wingspread 

 of about 34 inches, making it the largest insect ever recorded. 



Aside from cockroaches the most numerous insects were of a group now 

 extinct, the Paleodictyoptera (Fig. 8.21). These insects are of interest as 

 the ancestors of all other winged insects. 



FIG. 8.21. Pennsylvanian insects. Paleodictyoptera (upper, 

 and lower right). Cockroach (lower left). (Reprinted by per- 

 mission from Textbook of Geology, Part II, Historical 

 Geology, by Louis V. Pirsson and Charles Schuchert, pub- 

 lished by John Wiley & Sons, inc., 1 91 5, p. 752.) 



We recall that amphibians first appeared during the Devonian period, 

 apparently having evolved from crossopterygian fishes (pp. 159-163). We 

 have noted that these early amphibians were of a group known as laby- 

 rinthodonts (sometimes called Stegocephalia). The Pennsylvanian period 

 saw expansion of this group, some eighty-eight species being known. Some- 

 what lizardlike in appearance, the labyrinthodonts had relatively large 

 heads, which were frequently broad and flat (Fig. 8.19). A third or pineal 

 eye commonly occurred between, and slightly behind, the more usual pair. 

 This was evidently another inheritance from crossopterygian ancestry. 

 Limbs were short, extending laterally from the body (Fig. 8.19) and form- 



