First Fossil Record of Alphonsea Hk. f. &amp; T. (Annonaceae) from the Late Oligocene Sediments of Assam, India and Comments on Its Phytogeography

Gaurav Srivastava,Rakesh C. Mehrotra,Subho Mozumdar

doi:10.1371/journal.pone.0053177

Abstract

A new fossil leaf impression of Alphonsea Hk. f. & T. of the family Annonaceae is described from the Late Oligocene sediments of Makum Coalfield, Assam, India. This is the first authentic record of the fossil of Alphonsea from the Tertiary rocks of South Asia. The Late Oligocene was the time of the last significant globally warm climate and the fossil locality was at 10°–15°N palaeolatitude. The known palaeoflora and sedimentological studies indicate a fluvio-marine deltaic environment with a mosaic of mangrove, fluvial, mire and lacustrine depositional environments. During the depositional period the suturing between the Indian and Eurasian plates was not complete to facilitate the plant migration. The suturing was over by the end of the Late Oligocene/beginning of Early Miocene resulting in the migration of the genus to Southeast Asia where it is growing profusely at present. The present study is in congruence with the earlier published palaeofloral and molecular phylogenetic data. The study also suggests that the Indian plate was not only a biotic ferry during its northward voyage from Gondwana to Asia but also a place for the origin of several plant taxa.

Highlights

Annonaceae is a typical pantropical family of shrubs, trees and lianas consisting of about 112 genera and 2440 species [1] and is considered as one of the most diverse families of the magnoliid clade
In the present paper we describe a new leaf impression of Alphonsea from the Late Oligocene (Chattian 28–23 Ma) sediments of Makum Coalfield, Assam (Fig. 1), which was located at 10u–15uN palaeolatitude during the depositional period [16] when the suturing of the Indian and Eurasian plates was not complete to facilitate the plant migration [17]
Material for the present study was collected from the Tirap Colliery (27u179200N, 95u469150E) of the Makum Coalfield, Tinsukia District, Assam having an exposure of the Late Oligocene sediments belonging to the Tikak Parbat Formation of Assam

Summary

Introduction

Annonaceae is a typical pantropical family of shrubs, trees and lianas consisting of about 112 genera and 2440 species [1] and is considered as one of the most diverse families of the magnoliid clade. Geological setting Of the several coalfields in northeast India the Makum Coalfield is the most important as it accounts for nearly 90% coal production in this part of the country This field lies between the latitudes 27u159–27u259N and longitudes 95u409–95u559E (Fig. 1) and is situated along the northern flank of the Patkai range. On the southern and south-eastern side of the field are hills, which rise abruptly to heights of 300–500 m from the alluvial plains of the Buri Dihing and Tirap rivers These hill ranges are traversed by the Namdang, Ledopani and Tirap rivers. The Tikak Parbat Formation comprises alternations of sandstone, siltstone, mudstone, shale, carbonaceous shale, clay and coal seams [20] (Fig. 2). The detailed geological account of the Tirap mine section has recently been published [18]

Materials and Methods

Results

Discussion