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Times of origin of vertebrate classes are known to the epoch for the higher fishes and for amphibians, and to within about a period for reptiles, birds, and mammals. Origin of the phylum in the Ordovician rather than earlier is doubtful because of the sparsity of early Paleozoic fresh-water deposits. An interval of one or two periods elapses between the first appearance and maximum diversification of a vertebrate class. Times of rapid expansion of successive classes and subordinate groups are distributed rather evenly throughout geologic time since the Ordovician and do not cluster around era boundaries or coincide with local orogenic episodes.

Local conditions in northwestern Europe following the Caledonian orogeny may have favored the evolution of land-dwelling vertebrates. No relationship of this type can be traced for the origin of other vertebrate classes. Reptiles, more fully adapted to land life than amphibians, appear in Pennsylvanian coal swamps, though the unique occurrence of the advanced reptile Petrolacosaurus associated with a Permian type flora in late Missourian (upper Pennsylvanian) deposits at Garnett, Kansas, suggests a possible upland origin of reptiles.

The origin of mammals is known from a rather imperfect evolutionary sequence from the Permo-Triassic Karroo deposits of South Africa. Similar faunas inhabited Asia. Although local environmental effects of the Cape orogeny may have influenced evolution in the direction of mammals, the trend toward mammalian structure was initiated early in the Permian, well before conditions became favorable for reptilian life in South Africa, and continued until the end of the Triassic. Close approximation in time of the pre-Molteno compressive movements and technical origin of mammals is purely fortuitous. Nor can late Paleozoic glaciation of the southern hemisphere have been a critical factor, for it preceded marked development toward the mammalian condition by a full Period.

If birds, as is probable, originated from arboreal reptiles, widespread forest conditions must have been the chief environmental factor favoring their development. Forests flourish in both quiescent and tectonically active times, so no correlation should be expected between orogeny and the origin of birds.

Scarcity of deposits containing fossil vertebrates makes the localization of areas of origin of classes or orders almost impossible.

Evidence concerning the temporal duration of land connections between continents is afforded by the distribution of terrestrial and fresh-water vertebrates. The Bering land bridge is documented by successive faunal interchanges between North America and Eurasia during the Cenozoic. More spectacular is the profound modification of the South American Tertiary fauna when connection with North America in the Pliocene permitted intermigration. Precise location of land bridges must be established on evidence (mainly geologic) other than the distribution of fossil vertebrates, and faunal distributions must be interpreted in the light of geologic data.

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