, Volume 24, Issue 7, pp 433-509

Evolution of the madro-tertiary geoflora

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Summary

1. The Madro-Tertiary Geoflora comprises semiarid live oak-conifer woodland, chaparral, arid subtropic scrub, plains (desert) grassland, and subdesert to desert vegetation.

2. The taxonomic and adaptive relations of its plants suggest that it was derived chiefly from subtropical to warm temperate groups that evolved in response to the expansion of a new adaptive zone—dry climate.

3. The geoflora seems to have had an origin in southwestern North America because (a) types that appear to be ancestral to Madro-Tertiary lineages are represented in the Cretaceous and Paleocene floras of that region, (b) they are not known from the temperate Arcto-Tertiary Geoflora to northward, and (c) they are not recorded in the humid phases of the Neotropical-Tertiary Geoflora.

4. Geologic and paleoclimatic data suggest that southwestern North America was generally a lowland region, characterized by tropic savanna climate during the Cretaceous and Early Tertiary.

5. During this interval, ancestral Madro-Tertiary phylads probably were evolving chiefly in sites away from the moist lowland floodplains, in drier areas provided by local edaphic and climatic conditions.

6. With a strong linear component to selection imposed by a gradual trend toward increased aridity, these scattered, isolated phylads of subtropic to warm temperate affinities may have been undergoing megaevolution during the Cretaceous and Paleocene, giving rise to numerous, highly specialized taxonomic and adaptive types.

7. When first recorded in moderate numbers in the Eocene and early Oligocene, they are closely similar to modern species.

8. As dry climates continued to expand in area and to increase in severity, the Madro-Tertiary Geoflora migrated widely over southwestern North America. The stage during which it attained dominance in each province was governed chiefly by geographic position and time.

9. In response to the evolution of new dry climates in the later Cenozoic, each of the generalized Madro-Tertiary communities was differentiated into a number of modern associations that are adapted now to more localized dry environments.

This review is a summary of research which has been generously supported by the Carnegie Institution of Washington (1934–46), the National Research Council (1939–41), the Geological Society of America (1948–50), the John Simon Guggenheim Memorial Foundation (1952–3), the Committee on Research, University of California, Los Angeles (1946–48, 1950–52), and the National Science Foundation (1954–57).