Surprisingly modern Latest Cretaceous-earliest Paleocene woods of India

Abstract

Background and approach - The Deccan Intertrappean Beds of Central India contain a diverse assemblage of fossil plants, including petrified woods from 15 localities. These beds are dated at c. 67-64 Ma, i.e. latest Cretaceous-earliest Paleocene and span the K-Pg boundary, a significant time in angiosperm history. At this time, the Indian tectonic plate was halfway on its journey from Gondwana to its collision with Asia, and relatively close to the equator. We provide descriptions in IAWA Hardwood List codes for 47 species of Deccan fossil woods, based on our examination of thin sections of these woods, mostly holotypes that are housed at the Birbal Sahni Institute of Palaeobotany, Lucknow, India. An appendix lists all validly published Deccan wood species of which we are aware, including 52 that we were not able to examine. Main results - The Deccan fossil woods described herein include the oldest known occurrences of some orders, families or genera viz. Lamiales (Lamiaceae), Achariaceae (Hydnocarpus-like wood), Anacardiaceae, Simaroubaceae (Ailanthus-like and Simarouba-like woods), subfamily Leeoideae (Vitaceae), subfamily Myrtoideae (Myrtaceae), subfamily Planchoideae (Lecythidaceae), tribe Castilleae (Moraceae), tribes Grewioideae and Sterculioideae (Malvaceae). These first fossil records are discussed with reference to other macrofossil and pollen records of the same or related clades. They complement recent work on the oldest known Olea and Connaraceae also documented by Deccan woods. For the Deccan woods we examined, we could confirm the earlier taxonomic assignment at least down to order or family level for 29 taxa. Ordinal level affinities are ambiguous for eight of the taxa. In two cases, we revised the taxonomic assignment to other families; for another eight, the original assignment was found to be incorrect, but we are unable to suggest alternative affinities. Evolutionary implications - Only 3% of all Deccan woods have scalariform perforations and the incidences of so-called specialized features in the Baileyan sense are high, so these woods have a remarkably "modern" aspect. This is anomalous in comparison with contemporaneous fossil woods from higher paleolatitudes, and seemingly they are more "derived" than the recent flora. In these respects, the Deccan woods constitute a unique assemblage. The low incidence of scalariform perforations suggests xeric conditions, while - in contrast - the low incidence of distinct growth ring boundaries suggests an aseasonal everwet climate. It is speculated that convergent xylem specialization, especially the selection for simple perforations, was enhanced by the climatic conditions found at low paleolatitudes with high temperatures as would characterize the Deccan Intertrappean Beds at the K-Pg boundary.