steps 



Bdrth. especlall 



appli) 



(modifio 



Dunbor 1949). 



Cosmic era : 3-5 billion years ago; tidal disruption of an ancestral sun and origin of 



earth. 

 Azoic era : formation of a stable cold exterior shell to the earth; origin of oceanic de- 

 pressions and continental platforms; first formation of water and a thin atmosphere. 

 Archeozoic era : first plant life - bacteria, marine algae, (elevation of Laurentian Up- 

 lands and peneplation of continents). 

 Proterozoic era : marine algae abundant; animal life chiefly sponges and segmented 



marine worms, (peneplanation of continents). 

 Paleozoic era : 500 million years ago. 



Cambr ian period: marine invertebrates only. 



Ordovician period : marine invertebrates continue predominance; rise of armored 



fishes. 

 Silurian period : first invasion of land by plants; rise of air-breathing scorpions 



and millipedes and of fresh-water fishes. 

 Devonian period : first forests and extensive land floras; diversification of fresh- 

 water fishes, rise of labyrinthodont amphibians, and increase in land fauna, 

 especially spiders, mites, and wingless insects. 

 Mississippian period : increase of amphibians. 



Pennsylvanian period : luxuriant swamp floras cosmopolitan in distribution, mostly 

 of spore-bearing types; fresh-water clams and amphibians abundant and on 

 land, giant insects, spiders, centipedes, snails, and first reptiles. 

 Permian period : (elevation of Appalachian and Ouachita Mountains); decline of 

 ancient flora and rise of conifers; modern insects and advanced types of 

 amphibians and reptiles appear. 

 Mesozoic era : 200 million years ago . 



Triassic period : (desert climates); plants mostly rushes, ferns, cycads, conifers; 



stegocephalian amphibians and dinosaurs numerous, archaic mammals appear. 

 Jurassic period : reptiles evolve higher and more diversified forms, first toothed 



birds and frogs appear. 

 Lower Cretaceous period : woody Angiosperras spreading over world. 

 Upper Cretaceous period : (great inland seas, warm climate world-wide); modern 



genera of deciduous hardwood trees predominant, sedges and grasses appear- 

 ing; clams and snails common in fresh-water, culmination of dinosaurs, 

 toothed birds, archaic mammals (elevation of Rocky Mountains). 

 Cenozoic era : 60-70 million years ago. 



Paleocene and Eocene epochs : (inland seas recede, Appalachian region pene- 



plained (Schooley) but later again uplifted, climate warm and humid) hardwood 

 forests predominant, palms abundant; modern mammals and birds replace 

 archaic forms. 

 Oligocene epoch : 40 million years ago (continent peneplained); turtles, alligators, croco- 

 dile at maximum. 

 Miocene epoch: 29 million years ago (western mountains becoming elevated, climate 



turning drier and colder); grasses disperse over open plains; insects reach full de- 

 velopment and mammal fauna expands ■ 

 Pliocene epoch : 12 million years ago (continued elevation of western mountains, espe- 

 cially Sierra Nevadas; lower Great Basin becomes arid); grasslands become exten- 

 sive and desert vegetation develops in southwest; mammals at maximum and man- 

 ape changing into man. 

 Pleistocene epoch : 1 million years ago (continental glaciation); great mammals disappear. 

 Psychozuic era : 26-30 thousand years ago (glaciers recede). 



Recent epoch: man becomes predominant, rise of civilization. 

 Note: The epochs, Paleocene to Pliocene inclusive, are often grouped and designated the Tertiary 

 Period, and the Pleistocene and Recent epochs the Quaternary Period. 



to continue the succession into the indefinite future. 

 As higher types evolved in each taxonomic group, 

 primitive forms mostly died out. The first primitive 

 wingless insects appeared in the Devonian ; in the 

 Pennsylvanian there were giant forms of primitive 

 dragonflies, cockroaches, and grasshoppers. One 

 cockroach had a wingspread of nearly 12 cm. By the 

 Permian, these giant forms disappeared and were re- 

 placed by many modern orders. During early Meso- 



zoic, most modern families of insects were estab- 

 lished, and by the Upper Cretaceous many modern 

 genera are recognizable. 



Of the two great groups of warm-blooded ani- 

 mals, the earliest mammals had originated by the 

 Triassic. Modern orders did not become well diflfer- 

 entiated until very late Cretaceous or early Paleocene, 

 modern families by the Oligocene, and modern genera 

 by the Pliocene (Simpson 1953). 



The biotic community 25 



