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A Theory of Angiosperm Evolution

Daniel I. Axelrod
Evolution
Vol. 6, No. 1 (Mar., 1952), pp. 29-60
DOI: 10.2307/2405502
Stable URL: http://www.jstor.org/stable/2405502
Page Count: 32
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A Theory of Angiosperm Evolution
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Abstract

The following theory of angiosperm evolution may explain the history of the phylum following its initial appearance from a still undiscovered proangiosperm stock (or stocks). Permo-Triassic. Geological and botanical data are adduced which support strongly the inferences that (1) angiosperms probably were in existence by Permo-Triassic time, that (2) they probably occupied upland regions remote from lowland sites of deposition, that (3) they were distributed in the diverse environments of the tropical zone, that (4) at least some of them were ancestral to modern primitive types of monocotyledons and dicotyledons which are now represented in tropical regions, and that (5) they were then undergoing quantum evolution from proangiosperms which occupied upland regions. While most groups were confined initially to the tropical zone, during the quantum phase some family types probably started to exploit the more extreme marginal environments of the uplands which were prominently developed during this stage. Some tended to differentiate from the tropics both north and south, others evolved within the tropics as well as in marginal environments to southward, whilst some probably exploited the tropics as well as adjacent regions lying to northward. None of these early types has yet been recorded. Late Triassic-Jurassic. With a return to more equable climates during this stage, some of the types which had developed earlier in the uplands of the more extreme tropical and marginal tropical environments probably became extinct as their areas tended to disappear. This created in random manner numerous discontinuities between the earlier more closely related groups. Expansion of humid environments now enabled surviving types in upland areas to exploit the broad regional climates, and most groups probably were now evolving in the mode of phyletic evolution. Although many angiosperm families probably had developed in the uplands, only a few have been recorded in lowland deposits of this age. Nonetheless, both monocotyledons and dicotyledons of present day primitive family types appear to be recognizable.

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