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

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.


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. Based on the molecular data the family has been placed in Magnoliales, along with Degeneriaceae, Eupomatiaceae, Himantandraceae, Magnoliaceae and Myristicaceae [2]. On the basis of DNA sequences [3,4] and other morphological features [5] the family is strongly considered as monophyletic in origin. It has been considered as an important component of lowland tropical rainforest around the world [6][7][8][9], whose abundance and species richness covary with the temperature and rainfall [9].
The oldest fossil records of Annonaceae are in the form of seeds and pollen from the Maastrichtian of Nigeria and Colombia [10,11] indicating that it is Gondwanic in origin despite the fact that one of the fossil records is from the Coniacean (Late Cretaceous) of Japan [12]. Fossil records of Annonaceae are well documented in the Neogene in contrast to the Palaeogene sediments of India [13][14][15]. 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]. The known palaeoflora and sedimentological studies indicate a fluvio-marine deltaic environment with a mosaic of mangrove, fluvial, mire and lacustrine depositional environments. [18,19]. This is the first fossil record of the genus Alphonsea from the Tertiary rocks of South Asia. An attempt is also made to discuss the evolution and migration of the genus in the region.

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. These river courses expose sections of the coalbearing Tikak Parbat Formation. The Makum Coalfield encompasses Baragolai, Ledo, Namdang, Tikak, Tipong, and Tirap collieries (Fig. 2).
The present study is concerned with the Tikak Parbat Formation assigned to Late Oligocene (Chattian 28-23 Myr) [18,19]. The formation has five coal seams confined within the basal 200 m section [20].
Of the five seams only seam no. 1 and 3 have been exploited throughout the field. The Tikak Parbat Formation comprises alternations of sandstone, siltstone, mudstone, shale, carbonaceous shale, clay and coal seams [20] (Fig. 2). However, the occurrence of plant remains is mainly confined to the grey carbonaceous and sandy shales. The formation is underlain by 300 m of predominantly massive, micaceous or ferruginous sandstones that comprise the Baragolai Formation, which in turn is underlain by 1100-1700 m of thin-bedded fine-grained quartzitic sandstones with thin shale and sandy shale partings that make up the Naogaon Formation [21]. Together the three formations comprise the Barail Group (Fig. 2). In the Barail Group there is an upward trend from predominantly marine to predominantly non-marine palaeoenvironments which represent the infilling of a linear basin on the eastern edge of the Indian plate. The detailed geological account of the Tirap mine section has recently been published [18].

Materials and Methods
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. The specimen was first cleared with the help of a fine chisel and hammer and then photographed in natural low angled light using 10 megapixel digital camera (Canon SX110 and Fuji color 9500).The terminology used in describing the fossil leaf is based on Hickey [22], Dilcher [23] and Ellis et al. [24]. The type specimen is housed in the museum of the Birbal Sahni Institute of Palaeobotany, Lucknow, bearing specimen no. BSIP 40084

Nomenclature
The electronic version of this article in Portable Document Format (PDF) in a work with an ISSN or ISBN will represent a published work according to the International Code of Nomenclature for algae, fungi, and plants, and hence the new names contained in the electronic publication of a PLOS ONE article are effectively published under that Code from the electronic edition alone, so there is no longer any need to provide printed copies. The online version of this work is archived and available from the following digital repositories: PubMed Central, LOCKSS.   Description. Leaf complete, symmetrical, mesophyll, ellipticnarrow elliptic; preserved lamina length 7.6 cm, maximum width 3.2 cm (near the middle portion); apex broken; base symmetrical, seemingly obtuse; margin entire; texture coriaceous; attachment with petiole normal, petiole 0.7 cm long; venation festooned brochidodromous; primary vein moderate in thickness, straight; secondary veins 12 pairs visible, 0.3-1.3 cm apart, not uniform, alternate to sub-opposite, angle of divergence moderate-wide acute (45u-69u), moderate in thickness, joining super-adjacent secondary veins at acute-obtuse angle; intersecondary veins present, composite; tertiary veins random reticulate; marginal ultimate venation looped; areoles present, quadrangular to pentagonal in shape; veinlets branched.

Results
Affinities. The characteristic features of the fossil leaf viz., elliptic-narrow elliptic shape, coriaceous texture, festooned brochidodromous venation, intersecondary veins and random reticulate tertiary veins suggest its close affinity with that of Alphonsea of the family Annonaceae. A number of species of Alphonsea, namely A. lutea Hk. f. & Th., A. madraspertana Bedd., A. sclerocarpa Thw. and A. ventricosa Hk. f. & Th. along with other taxa of the same family examined in the Central National Herbarium, Howrah, India and the Forest Research Institute, Dehradun, India. In A. sclerocarpa the distance between the two secondary veins is greater than that found in the present fossil, while in A. ventricosa the venation is eucamptodromous. A. lutea (Herberium sheet no. CNH 12209) and A. madraspertana (Herbarium sheet no. CNH 14643) are very similar to the fossil leaf ( Fig. 3B; 4B,D). The fossil also shows some resemblance with Annona senegalensis Pers., Uvaria hookeri King and Uvaria zeylanica L. of the same family but the presence of eucamptodromous-brochidodromous venation makes the difference from the present fossil.
Hably [25] reported Alphonsea fossil from the Miocene sediments of Hungary but the modern distribution of the genus being restricted only to South and Southeast Asia (Fig. 5) creates doubt on the Hungarian fossil. Under such circumstances a new species, Alphonsea makumensis Srivastava & Mehrotra, sp. nov., is created, the specific epithet is after the Makum Coalfield. As far as authors are aware, this is the first authentic fossil leaf record of Alphonsea from Southeast Asia.

Modern distribution of Alphonsea
The genus Alphonsea consists of about 30 species distributed in China and Indo-Malayan region [26] (Fig. 5). In China it is mainly found in Hainan and South Yunnan [27], while in Asia it is found in India, Sri Lanka, Myanmar, Thailand, Laos, Vietnam, Cambodia, Malaysia, Indonesia and Papua New Guinea [28]. One of the modern comparable species, Alphonsea lutea, is a tree of Bangladesh, Myanmar, Orissa and Sri Lanka, while the other one, A. madraspertana, is an evergreen tree of the hills of Cuddapah District, Andhra Pradesh and Thiruvannaamalai District, Tamil Nadu [29].

Discussion
On the basis of molecular phylogenetic study within the Annonaceae, two major sister clades were recognized containing the majority of the species, namely the long-branch clade (LBC) and short-branch clade (SBC) [1,30]. The genus Alphonsea belongs to SBC of the tribe Miliuseae which comprises of the following six genera distributed in Asia: Alphonsea, Mezzettia Becc., Miliusa Lesch. ex A.DC., Orophea Bl., Phoenicanthus Alston and Platymitra Boerl. Couvreur et al. [1] suggested that the age of the crown node of SBC is c.33 Ma. Other molecular data suggests that the Alphonsea was separated from its sister genus Platymitra during the Late Oligocene [30]; this is in contrast to Couvreur et al. [1] data which suggests that the separation occurred during the Late Miocene. Our leaf fossil is in favour of Richardson's et al. [30]. Su and Saunders [31] suggested the role of rafting Indian plate in the dispersal of SBC into Southeast Asia i.e. ''Out of India hypothesis''. Their idea gets more support from the fossil record of the family Annonaceae from the Deccan Intertrappean sediments [32][33][34][35][36] suggesting that the family was already present before the collision of Indian and Eurasian plates.
During the Late Oligocene the fossil locality was at 10u-15uN [16] and the suturing between the Indian and Eurasian plates was not complete to facilitate the plant migration [17]. The presence of Alphonsea fossil in northeast India during the Late Oligocene indicates that the genus was originated in India during the Late Oligocene and migrated to Southeast Asia via Myanmar after the complete suturing of Indian and Eurasian plates during the Early Miocene [37]. There are several examples which support that a significant floral exchange happened between India and Southeast Asia during the Neogene. Sterculiaceae is an interesting example as it is represented in the Palaeogene of Southeast Asia by Pterospermum and in India by Sterculia. The former made its first appearance in the Pliocene [38], while the latter is known from the Neogene of Sumatra [39]. Similar is the case with the Mangifera and Semecarpus of the family Anacardiaceae. The oldest fossil records of these taxa are known from the Palaeogene of India. These genera later on migrated to Southeast Asia during the Neogene as evidenced by their fossil records [40][41][42]. The other plant families like Crypteroniaceae [43,44], Melastomataceae [45]  and Lowiaceae [46], along with the palaeotological evidence [47] support the above fact as they were migrated from India to Southeast Asia. The present 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.
For the reconstruction of palaeoclimate of the Makum Coalfield both qualitative and quantitative studies have been made. Qualitative study is based on the floral composition of the families such as Annonaceae, Burseraceae, Calophyllaceae, Combretaceae, Lecythidaceae, Myristicaceae and Rhizophoraceae. The aforesaid families are typical pantropical [48] and their presence in the Makum Coalfield palaeoflora provides evidence that the CMMT (cold month mean temperature) was not less than 18uC. Fabaceae, the most dominant family in the Makum Coalfield [49] whose abundance and richness covary with temperature [9], also indicates a warm climate. The occurrence of Avicenniaceae and Rhizophoraceae is significant in terms of the depositional environment. These families are highly indicative of deltaic, mangrove or lacustrine deposition of sediments in the Makum Coalfield. The presence of palms like Nypa [50] provides further evidence of a coastal plain environment where both temperature and humidity remained high throughout the year [51]. For quantitative study, CLAMP (Climate Leaf Analysis Multivariate Program) analysis was made indicating MAT (mean annual temperature) 28.363.7uC, CMMT (cold month mean temperature) 2365.5uC and a WMMT (warm month mean temperature) of 33.665.2uC. The analysis also indicates a monsoonal climate during the Late Oligocene [19].