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The Bases of Angiosperm Phylogeny: Paleobotany

1975; Missouri Botanical Garden; Volume: 62; Issue: 3 Linguagem: Inglês

10.2307/2395274

2162-4372

Jack A. Wolfe, James A. Doyle, Virginia M. Page,

Plant Taxonomy and Phylogenetics

Analysis of certain fossils that have been used as evidence for a pre-Cretaceous origin and diversification of the angiospenns indicates that these fossils are non-angiospermous.A review of the sequential development of morphological features exhibited by Early Cretaceous pollen and leaves indicates that the initial radiation of the angiospenns was occurring at that time.The monocotyledons and dicotyledons diverged early before either class attained a moderate degree of diversity.Subclass Magnoliidae has retained the morphological grade exhibited by the earliest dicotyledonous pollen and leaves.The next youngest grade of Early Cretaceous pollen is found in putatively primitive members of Hamamelididae and Ranunculidae.Stratigraphic series of Early Cretaceous foliage indicate a transition from the morphological grade of Magnoliales to leaf morphology characteristic of Rosidae.Late Cretaceous wood assemblages are characterized by a preponderance of anatomical features that have been previously postulated as primitive and typically lack features thought to be advanced.Twenty years ago, the inclusion of paleobotany in a symposium of this nature would have been little more than a token gesture.The then prevalent concept was that by the Cretaceous -the time of their first entry in significant numbers in the fossil record -the angiospenns were well diversified into extant orders and families, and even genera (Axelrod, 1970).The diversification having taken place in some area remote from basins of deposition and fossilization (e.g., tropical uplands or Malesia), paleobotany would provide almost no evidence bearing on the phyletic relationships between major angiosperm groupings.During the last twenty years, however, a growing body of evidence has indicated that the angiospenns were undergoing their major evolutionary radiation during the Cretaceous ( Fig. 1 ) .The lack of any definite angiosperm pollen in a great array of pre-Cretaceous samples led Scott et al. (1960) andHughes (1961) to question the supposed great antiquity of angiosperm diversification.More significantly, palynological work on Cretaceous sequences has uniformly shown that, from the level of the first known and morphologically simple angiosperm pollen types, successively younger horizons yield successively more complex and 1 Dr.