A SURVEY OF THE FOSSIL RECORD FOR DICOTYLEDONOUS WOOD AND ITS SIGNIFICANCE FOR EVOLUTIONARY AND ECOLOGICAL WOOD ANATOMY

Data on fossil dicotyledonous wood were assembled in order to 1) test the Baileyan model for trends of specialisation in dicotyledonous wood anatomy by addressing the question - were 'primitive' wood anatomical features (as defined by the Baileyan model) more common in the geologic past than at present?, 2) infer, on a broad geographic scale, past climatic regimes, and long term climatic change, and 3) assess the extent of knowledge of fossil dicotyledonous woods. The resulting database has information on 91 anatomical features for over 1200 fossil dicotyledonous woods. The incidence of selected anatomical features was plotted through time (by geologic epoch) for the world and for two regional groupings (roughly corresponding to the Laurasian and Gondwanan supercontinents). For comparison to the fossil wood record, the incidence of wood anatomical features in the Recent flora was obtained from the 5260 record OPCN database for extant dicotyledonous woods. The fossil record supports the Baileyan model for xylem evolution, as the incidence of 'primitive' features (e.g., scalariform perforation plates) is much higher in the Cretaceous than in the Tertiary, while the incidence of 'advanced' features (e.g., short vessel elements and simple perforation plates) is lower. From the Cretaceous to the Tertiary, there are pronounced changes in the incidence of features believed associated with conductive efficiency. The tempo and timing of changes in ray composition, storied structure, and elaborate axial parenchyma distribution patterns are different from those for tracheary elements as the former increase gradually throughout the Tertiary. Incidences of features, such as ring porosity and elaborate vessel groupings, that are associated with markedly seasonal climates in the Recent flora do not approach modern levels until the Neogene. Correlations of wood anatomical features with ecology do not appear to have been constant throughout time, as in the Cretaceous different features provide conflicting interpretations of the climate. Throughout the Tertiary, there are differences between the 'Laurasian' and 'Gondwanan' regions in the incidences of wood anatomical features, and these differences are consistent with well-established ecological trends in wood anatomy in the extant flora; during the Miocene and Pliocene, wood anatomical characteristics of these two regions are nearly identical to those of the present-day. The Late Tertiary increase in seasonality of the Northern Hemisphere is recorded in the fossil wood record. Our review of the literature indicates there is a considerable need for more information on well-dated woods, particularly for woods of Paleocene age (less than 40 known world-wide), and from continuous sequences across the Cretaceous-Tertiary boundary, and the mid-Eocene to Oligocene, both times critical in the development of modern vegetation.