Kimin Formation - Springer Link

Palaeobio Palaeoenv (2011) 91:237–255 DOI 10.1007/s12549-011-0059-z

ORIGINAL PAPER

Floral diversity during Plio-Pleistocene Siwalik sedimentation (Kimin Formation) in Arunachal Pradesh, India, and its palaeoclimatic significance Mahasin Ali Khan & Ruby Ghosh & Subir Bera & Robert A. Spicer & T. E. V. Spicer

Received: 31 May 2011 / Revised: 22 August 2011 / Accepted: 25 August 2011 / Published online: 30 September 2011 # Senckenberg Gesellschaft für Naturforschung and Springer 2011

Abstract A morpho-taxonomic study of leaf remains from the upper part of the Siwalik succession of sediments (Kimin Formation; upper Pliocene to lower Pleistocene) of Papumpare district, Arunachal Pradesh, India, revealed 23 species representing 20 genera belonging to 15 angiosperm families. The recovered fossil leaves are comparable to modern Bambusa tulda Roxb. (Poaceae), Mangifera indica Linn., Dracontomelum mangiferum Blume (Anacardiaceae); Chonemorpha macrophylla G. Don (Apocynaceae); Pongamia pinnata (L) Pierre., Millettia pachycarpa Benth., Dalbergia rimosa Roxb., Millettia extensa (Fabaceae); Macaranga denticulata Muell. Arg., Croton caudatus Geisel. (Euphorbiaceae); Combretum decandrum Roxb. M. A. Khan : S. Bera (*) Centre of Advanced Study, Department of Botany, University of Calcutta, 35, Ballygunge Circular Road, Kolkata 700019, India e-mail: [email protected] R. Ghosh Centre of Advanced Study, Department of Botany, 35, Ballygunge Circular Road, Kolkata 700019, India R. Ghosh Birbal Sahni Institute of Palaeobotany, 53, University Road, Lucknow 226007 U.P., India R. A. Spicer Centre for Earth, Planetary, Space and Astronomical Research, The Open University, Milton Keynes MK7 6AA, UK R. A. Spicer : T. E. V. Spicer Institute of Botany, The Chinese Academy of Sciences, 20 NanxincunXiangshan Beijing 100093, China

(Combretaceae); Dysoxylum procerum Hiern. (Meliaceae); Dipterocarpus sp. Gaertn.f. (Dipterocarpaceae); Actinodaphne angustifolia Nees., Actinodaphne obovata Blume., Lindera pulcherrima Benth., Litsea salicifolia Roxb. (Lauraceae); Calophyllum polyanthum Wall. (Clusiaceae); Knema glaucescens Hook.f. (Myristaceae); Canarium bengalense Roxb. (Burseraceae); Quercus lamellosa Smith; Quercus semicarpifolia Smith (Fagaceae); and Berchemia floribunda Wall. (Rhamnaceae). Among these taxa, 11 species are recorded as new to the Neogene flora of India. Analysis of the floral assemblage with respect to the distribution pattern of modern equivalent taxa and the physiognomic characters of the fossil leaves, suggests that a tropical evergreen forest was growing in a warm humid climate in the region at the time of deposition. This is in contrast to modern tropical semi-evergreen forests that occupy the area. Values of mean annual temperature (MAT) of 29.3°C and mean annual precipitation (MAP) of 290 mm have been calculated using leaf-margin characters and fossil leaf size. Keywords Leaf margin analysis (LMA) . Nearest living relative (NLR) . Siwalik . Climate . Upper Pliocene to lower Pleistocene . Arunachal Pradesh

Introduction Arunachal Pradesh, the largest northeastern state of India, lies between 26°42′ and 29°30′N and 90°36′ and 97°30′E. It shares its borders with two Indian states, Assam in the south and Nagaland in the southeast. Myanmar lies to the east, Bhutan to the west, and China to the north. The Arunachal foothills are an important eastern Himalayan Siwalik sector in India. Compared to the northwestern (Jammu, Himachal Pradesh and Uttaranchal) and central

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Himalayan Siwalik sectors (Nepal), few palaeobotanical studies of the eastern Himalayan sectors, especially Arunachal Pradesh, have been carried out, due to the inaccessibility of most of the fossiliferous units in these areas. Very few plant megafossils, especially fossil leaves (e.g. Singh and Prakash 1980; Joshi and Mehrotra 2003, 2007; Joshi et al. 2003; Das et al. 2007; Khan et al. 2007a, b, 2008, 2009), and a solitary animal fossil (Kumar 1997) have been recorded from the Arunachal sub-Himalaya. Until this study, leaf data were insufficient for palaeoclimatic interpretations. Recently, however, diverse types of angiosperm leaves were collected from road cuttings through the upper part of the Siwalik succession (upper Pliocene to lower Pleistocene) from Papumpare district (located between 26°55′ and 28°40′N and 92°40′ and 94°21′ E), and the outcome of the morphotaxonomical study of these taxa is used here to reconstruct the palaeoclimate of the area.

Materials and methods During a field survey, five sections through the upper Siwalik (Kimin Formation) strata with well-preserved leaf remains were found exposed in cuttings along the Itanagar– Bandardewa road (near the 7 km post between Itanagar and Naharlagun) and Chander Nagar–Gohpur road in Papumpare district (Fig. 1). The external morphology of 48 leaves recovered was studied with a hand lens and incident light microscopy. Identifications were made by comparison with modern leaves collected from forests adjacent to the fossil exposures, and with herbarium specimens of modern taxa kept in the Central National Herbarium, Sibpur, Howrah, West Bengal. Photographs of the fossil leaves and their nearest living relatives were taken using an incident light compound microscope (Zeiss Axioskop 40) to show structural details. For the descriptions of leaf impressions and compressions, the terminologies adopted by Hickey (1973) and Dilcher (1974) were used. All the specimens are kept in the repository of the Palaeobotany–Palynology Section, Department of Botany, University of Calcutta.

Geology of the area The geology of the foothills of Arunachal Himalaya has been described by several workers (Agarwal et al. 1991; Karunakaran and Ranga Rao 1979; Kumar 1997; Kumar and Singh 1980, 1982, 1996; Kumar et al. 1983; Prakash and Singh 2000; Ranga Rao 1983; Singh 2007). In Arunachal, sedimentation in the Siwalik foredeep continued from the Miocene to early Pleistocene. The Siwalik Group consists of the Dafla, Subansiri and Kimin formations, in ascending order (Table 1). The sediments of the Siwalik

Palaeobio Palaeoenv (2011) 91:237–255

Group are characterised by soft, coarse-to-medium grained, in places pebbly, salt and pepper sandstones, with intercalations of silt and shale bands, and sporadic streaks and lenses of carbonised wood, lignite and coal. The fossil leaves were collected from late Pliocene to early Pleistocene sediments equivalent to the Kimin Formation. The Kimin Formation includes alternating layers of light-grey to orangecoloured, coarse-grained poorly consolidated sandstone, silty clay and beds and lenses of pebbly conglomerate.

Floral assemblage from the upper Siwalik Group of Arunachal Pradesh, India The recovered fossil leaves are identified as Actinodaphne palaeoangustifolia Antal and Awasthi (Fig. 2), Quercus cf. lamellosa (Fig. 2), Bambusa siwalika Awasthi and Prasad (Fig. 3), Calophyllum suraikholaensis Awasthi and Prasad, 1990 (Figs. 3 and 4), Croton cf. caudatus (Fig. 4), Combretum sahnii Antal and Awasthi (Fig. 5), Actinodaphne cf. obovata (Fig. 5), Litsea cf. salicifolia (Fig. 6), Mangifera someshwarica Lakhanpal and Awasthi (Fig. 6), Dysoxylum raptiensis Prasad and Awasthi (Fig. 7), Knema cf. glaucescens (Fig. 7), Dalbergia cf. rimosa (Fig. 7), Lindera cf. pulcherrima (Fig. 8), Dipterocarpus siwalicus Lakhanpal and Guleria (Fig. 9), Macaranga cf. denticulata (Fig. 9), Berchemia siwalica Tripathi, Pandey and Prasad (Fig. 10), Millettia cf. extensa (Fig. 10), Canarium cf. bengalense (Fig. 11), Dracontomelum cf. mangiferum (Fig. 11), Quercus semicarpifolia Kapoor and Singh, Chonemorpha miocenica Prasad and Awasthi, Millettia siwalika Antal and Awasthi and Pongamia siwalika Awasthi and Lakhanpal, 1990. Diagnostic features of just the newly described fossil leaves are provided as follows. Quercus cf. lamellosa (Fig. 2c, d) Holotype: CUH/PPL/IB7/46 Type locality: Road cuttings along the Itanagar–Bandardewa road in Papumpare district, Arunachal Pradesh. Type stratum: Upper part of the Siwalik succession of sediments (Kimin Formation: upper Pliocene to lower Pleistocene). Diagnosis: Leaf simple, symmetrical, narrow elliptic; preserved lamina length 3.5 cm, maximum width 2.4 cm; base broken; apex acuminate; margin serrate with regular spacing, serration straight-concave, a small spine present at tooth apex; texture chartaceous; venation pinnate, simple, craspedodromous; primary vein single, prominent, stout, almost straight; six pairs of secondary veins visible with angle of divergence broad acute (about 80°), upper veins more acute than those at the base, opposite, 0.3–0.4 cm apart, uniformly curving distally and running parallel to each other, moderately thick reaching the margin, unbranched; tertiary veins fine with angle of origin AO–AR,


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Fig. 1 Location of study area in Papumpare district, Arunachal Pradesh, and a magnified view of the geological setting of the area around Itanagar

pattern percurrent, usually branched, straight or slightly wavy, predominantly alternate, oblique in relation to midvein, close; higher order venation not clearly preserved. Remarks: The diagnostic characters of the fossil leaf, such as narrow elliptic shape, acuminate apex, serrate margin, simple craspedodromous venation, tertiaries oblique in relation to the midvein and percurrent pattern collectively indicate a closer resemblance to Quercus (Fagaceae), especially Quercus lamellosa Smith. Earlier, fossil leaf Quercus semicarpifolia Kapoor and Singh, 1987 has been described from the Paleogene sediments exposed along the Kalka–Kasuli road section, Himachal Pradesh. However, a comparative study of fossil Q. semicarpifolia and the

present fossil specimen indicates that the former differs in being smaller in size, obovate shape, eucamptodromous venation and having different course and arrangement of secondary veins (bifurcating towards the margin). No fossil leaves resembling Quercus lamellosa have been described previously from the Siwalik sediments. Therefore, being different, the present fossil leaf is assigned to a new species, Quercus cf. lamellosa. Croton cf. caudatus (Fig. 4c–g) Holotype: CUH/PPL/IB7/43 Paratypes: CUH/PPL/IB7/43a and CUH/PPL/IB7/43b

240 Table 1 Neogene–Quaternary Stratigraphy of Arunachal Pradesh (Kumar 1997)


PERIOD Q U A T E R N A R Y

SUB HIMALAYA

EPOCH

NAGAPATKOI RANGES

BRAHMAPUTRA PLAINS

Holocene Newer Alluvium

Newer Alluvium

Newer Alluvium

Older Alluvium (Hapoli Fm.)

Older Alluvium

Older Alluvium

Kimin Fm. (upper Siwalik)

Dihing Fm.

Dhekiajuli Fm.

L

Subansiri Fm. (middle Siwalik)

Namsang Fm.

Namsang Fm./Dupi Tila Fm.

U

Dafla Fm. (lower Siwalik)

Girujan Fm.

Girujan Fm.

M

Kimi Fm.

Tipam Fm.

Tipam Fm.

L

Tourmaline Granite

Surma Group?

Surma Group

U Pleistocene

M L

N E O G E N E

U Pliocene

Miocene

Type locality: Road cuttings along the Itanagar–Bandardewa road in Papumpare district, Arunachal Pradesh. Type stratum: Upper part of the Siwalik succession of sediments (Kimin Formation: upper Pliocene to lower Pleistocene). Diagnosis: Leaf simple, asymmetrical, ovate, preserved lamina length 4 cm and maximum width 4.3 cm; apex broken; base peltate; margin serrate with both regular and irregular tooth spacing; texture coriaceous; petiole not preserved; venation pinnate, simple, craspedodromous type; primary vein prominent, straight; three to four pairs of secondary veins visible, first pair of secondaries originating from a single point at the base giving rise to branches in the basal portion of the leaf in a radiating manner and terminating at the margins, opposite, 0.7– 0.9 cm apart; angle of divergence 45–60°, more or less uniform, moderately thick, uniformly curved distally; tertiary veins with angle of origin mostly RR, percurrent, relationship to midvein oblique, opposite; quaternary veins fine and orthogonal. Remarks: The diagnostic features of this fossil leaf are the peltate base, ovate shape, serrate margin, craspedodromous venation, first pair of secondary veins originating from a single point in the basal part producing branches in a radiating manner, RR, percurrent and predominantly opposite tertiaries. These features collectively indicate closest affinity with the modern leaves of Croton caudatus Geisel. (Euphorbiaceae). This is the first record of a Croton fossil leaf from the Cenozoic. Actinodaphne cf. obovata (Fig. 5a, c) Holotype: CUH/PPL/IB7/39

Paratypes: CUH/PPL/IB7/39a and CUH/PPL/IB7/39b Type locality: Road cuttings along the Itanagar–Bandardewa road in Papumpare district, Arunachal Pradesh. Type stratum: Upper part of the Siwalik succession of sediments (Kimin Formation: upper Pliocene to lower Pleistocene). Diagnosis: Leaf simple, symmetrical, broadly elliptical, maximum length 12 cm and maximum width 9 cm; apex missing; base obtuse; margin entire; texture coriaceous; venation eucamptodromous; primary vein (1°) prominent, straight, stout; 3–4 pairs of secondary veins (2°) visible in the available part, 2.8–3 cm apart, alternate, unbranched; angle of divergence 50–60°, apical pairs of secondary veins usually meeting with the superadjacent secondary veins forming a distinct loop; intersecondary veins present, simple to composite; tertiary veins fine, angle of origin mostly RR, in some cases AO, percurrent, sporadically branched, straight to sinuous, rarely curved, mostly alternate, close to distantly placed; quaternary veins fine, angle of origin usually RR, branched, forming orthogonal meshes. Remarks: The diagnostic features of the present fossil leaves (wide elliptical shape, obtuse base, entire margin, eucamptodromous venation, alternate secondaries arising from the primary vein at an angle of divergence of 50–60°, presence of intersecondary veins and RR, percurrent tertiary veins) collectively indicate that the fossil leaf is morphologically most similar to the leaves of extant Actinodaphne obovata Blume (Lauraceae). To date, there are eight records of fossil Actinodaphne from the Cenozoic sediments of India and


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Fig. 3 a Bambusa siwalika Awasthi and Prasad: fossil lamina with obtuse base. Specimen No. CUH/PPL/C3/4iii. b Bambusa siwalika Awasthi and Prasad: part of the fossil lamina with acuminate apex. Specimen No. CUH/PPL/C3/4iii. c, d Calophyllum suraikholaensis Awasthi and Prasad: fossil leaves with parallel secondary veins. Specimen No. CUH/PPL/IB7/42. (Scale 1 cm)

Fig. 2 a Actinodaphne palaeoangustifolia Antal and Awasthi - fossil leaf. Specimen No. CUH/PPL/IB7/40 b Actinodaphne angustifolia Nees. - modern leaf with similar size, shape and venation pattern. c Quercus cf. lamellosa - fossil leaves with serrate margin Specimen No. CUH/PPL/IB7/46. d Quercus lamellosa Smith. - modern leaf with similar size, shape and margin pattern. (Scale 1 cm)

abroad: A. hoelittingensis Ett., A. frangula Ett. and A. dolichophylla Takhtajan from the Tertiary of Hoelting, former U.S.S.R., A. germarii (Heer) Friedrich from the Eocene of Germany (Takhtajan 1969), A. martiniana from the Pliocene of Java (Crié 1888), A. nipponica from the Cenozoic of northern Honshu, Japan (Tanai and Suzuki 1963) and A. palaeoangustifolia from the Siwalik sediments of the Darjeeling District, West Bengal (Antal and Awasthi 1993) and lower Siwalik sediments of Western Nepal (Prasad and Pandey 2008), respectively. The present fossil leaf differs from the already established fossil species of Actinodaphne in size, shape, and course and arrangement of the

secondary and tertiary veins, but shows closer similarities with modern A. obovata, hence, a new specific name Actinodaphne cf. obovata is suggested. Litsea cf. salicifolia (Fig. 6a–d) Holotype: CUH/PPL/IB7/36 Paratypes: CUH/PPL/IB7/36a and CUH/PPL/IB7/36b Type locality: Road cuttings along the Itanagar–Bandardewa road in Papumpare district, Arunachal Pradesh. Type stratum: Upper part of the Siwalik succession of sediments (Kimin Formation: upper Pliocene to lower Pleistocene). Diagnosis: Leaf lamina symmetrical; narrow elliptic; size 8×3.5 cm; apex and base broken; margin entire, seemingly undulated; texture chartaceous; venation pinnate, craspedodromous type; primary vein prominent, stout, almost straight; 5–6 pairs secondary veins visible, less than 0.9 cm apart; angle of divergence acute (about 60°), moderate, uniformly curved distally, usually alternate, unbranched,

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Fig. 4 a Calophyllum suraikholaensis Awasthi and Prasad: part of the fossil leaf magnified to show details of venation. Specimen No. CUH/PPL/IB7/42. b Calophyllum polyanthum Wall.: part of the modern leaf magnified to show details of venation. c Croton cf. caudatus: fossil leaf showing shape, size, base and venation pattern. Specimen No. CUH/PPL/IB7/ 43. d Croton caudatus Geisel.: modern leaf showing similar shape, size, base and venation pattern. e Croton cf. caudatus: part of the modern leaf magnified to show details of venation. f Croton cf. caudatus: part of the fossil leaf magnified to show peltate base. g Croton cf. caudatus: part of the fossil leaf magnified to show details of venation. (Scale 1 cm)

bifurcating towards the margin; intersecondary veins absent, simple; tertiary veins fine, with angle of origin mostly RR, percurrent, straight to sinuous, oblique in relation to midvein, predominantly alternate, close to distant. Remarks: The diagonostic features of this fossil leaf such as symmetrical lamina, narrow elliptic shape, chartaceous texture and nature of secondary veins and tertiary veins indicate its close affinity with the extant Litsea salicifolia Roxb. (Lauraceae). Among the fossil leaves of Litsea from the Indian subcontinent, Litsea polyantha Juss described from the middle Siwalik of Darjeeling foothills (Pathak 1969) is an incomplete leaf with seemingly obovate shape, which is distinct from the narrow elliptic shape of the present species. Litsea bhatiaaii (Mathur 1978) known from the upper Siwalik of Himachal Pradesh is also different from the present species in its larger size and lesser number of secondaries which are relatively widely spaced. Hence, the present species is assigned a new name Litsea cf. salicifolia.

Dalbergia cf. rimosa (Fig. 7e) Holotype: CUH/PPL/IB7/47 Type locality: Road cuttings along the Itanagar–Bandardewa road in Papumpare district, Arunachal Pradesh. Type stratum: Upper part of the Siwalik succession of sediments (Kimin Formation: upper Pliocene to lower Pleistocene). Diagnosis: Leaflet symmetrical, narrow, elliptic; preserved lamina length 6 cm; width 3 cm; apex round; base round; margin entire; texture thick chartaceous; venation pinnate, brochidodromous forming prominent intra-marginal vein; primary vein straight, stout; 6–7 moderately thick secondary veins visible, 0.4–0.5 cm apart, angle of divergence 65– 75°, wide acute to right angle, alternate to opposite, branched, uniformly curved and joined to their superadjacent secondary and forming intramarginal veins, intramarginal veins joined to the margin through cross veins; inter-secondary veins present, simple, common; tertiary


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Fig. 5 a Actinodaphne cf. obovata: fossil leaf. Specimen No. CUH/PPL/IB7/39. b Combretum sahnii Antal and Awasthi: a fossil leaf showing shape, size and venation pattern. Specimen No. CUH/PPL/IB7/ 37. c Actinodaphne obovata Blume.: modern leaf with similar size, shape and venation pattern. d Combretum decandrum Roxb.: a modern leaf showing similar details. (Scale 1 cm)

veins fine, with angle of origin AR-OR, percurrent to reticulate, relation to midvein oblique, close. Remarks: The diagnostic features of this fossil leaflet match those of extant Dalbergia rimosa Roxb. (Fabaceae). Both the fossil and modern leaflet of Dalbergia rimosa possess similar shape, base and venation pattern. This is the first record of fossil leaflets of Dalbergia rimosa from the Cenozoic sediments of India. Knema cf. glaucescens (Fig. 7c, d) Holotype: CUH/PPL/IB7/35 Paratypes: CUH/PPL/IB7/35a; CUH/PPL/IB7/35b and CUH/PPL/IB7/35c Type locality: Road cuttings along the Itanagar–Bandardewa road in Papumpare district, Arunachal Pradesh. Type stratum: Upper part of the Siwalik succession of sediments (Kimin Formation: upper Pliocene to lower Pleistocene).

Diagnosis: Leaf simple, symmetrical, narrow elliptic, preserved length 4 cm, maximum width 2.5 cm; apex broken; base acute; margin entire, texture chartaceous; petiole not preserved; venation pinnate, eucamptodromous; primary vein single, prominent, stout, slightly curved; about 4–5 pairs secondary veins visible, 1–1.5 cm apart, alternate, uniformly curved distally, unbranched, angle of divergence wide acute (70–80°), intersecondary veins present, simple; tertiary veins indistinct. Remarks: The characteristic features of this fossil leaf given above are found in the leaves of extant Knema (Myristicaceae). Leaves of Knema glaucescens Hook. f. are most similar to the fossil. The new fossils and modern leaves of Knema glaucescens possess similar shape, base and venation pattern. However, fossil leaves are different from the modern leaves in their slightly smaller size, greater number of secondaries which are relatively narrow spaced.

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mum width 3.1 cm; apex not preserved; base symmetrical, normal, acute; margin entire; texture chartaceous; petiole not preserved; venation pinnate, camptodromous, brochidodromous, primary vein single, stout, more or less straight; seemingly 4 pairs of secondary veins preserved, widely spaced, 0.9–1.3 cm apart, alternate, basal secondaries more acute than upper secondaries, secondary veins uniformly curved distally and joined together in a series of prominent arches, angle of divergence moderate, 45–65°; tertiary veins fine, with angle of origin usually AO type, reticulate, random to orthogonal, straight, oblique in relation to midvein, predominantly alternate and close. Remarks: The characteristic features of this fossil leaf indicate a close resemblance to modern leaves of Lindera Thunb. (Lauraceae). Comparison with herbarium specimens revealed close similarity to Lindera pulcherrima Benth. As far as the authors are aware, there is no previous record of Lindera Thunb. leaves from the Cenozoic sediments. Macaranga cf. denticulata (Fig. 9d)

Fig. 6 a Litsea cf. salicifolia: fossil leaf. Specimen No. CUH/PPL/ IB7/36. b Litsea salicifolia: modern leaf. c Litsea cf. salicifolia: part of the fossil leaf magnified to show the details of venation pattern. d Litsea salicifolia: part of the modern leaf magnified to show the details of venation pattern. e Mangifera someshwarica Lakhanpal and Awasthi: fossil leaf. Specimen No. CUH/PPL/C3/4i. (Scale 1 cm)

It is worth noting that Knema has not been recorded previously from the Cenozoic sediments of India. Lindera cf. pulcherrima (Fig. 8a–d) Holotype: CUH/PPL/IB7/34 Type locality: Road cuttings along the Itanagar–Bandardewa road and Chander Nagar–Gohpur road in Papumpare district, Arunachal Pradesh. Type stratum: Upper part of the Siwalik succession of sediments (Kimin Formation: upper Pliocene to lower Pleistocene). Diagnosis: Leaf simple, symmetrical, narrow elliptic; preserved maximum lamina length 6.2 cm and maxi-

Holotype: CUH/PPL/IB7/41 Type locality: Road cuttings along the Chander–NagarGohpur road in Papumpare district, Arunachal Pradesh. Type stratum: Upper part of the Siwalik succession of sediments (Kimin Formation: upper Pliocene to lower Pleistocene). Diagnosis: Leaf simple, asymmetrical, ovate, preserved lamina length 4.9 cm and maximum width 5.8 cm; apex missing; base peltate; margin entire; texture coriaceous; petiole not preserved; venation pinnate, simple, craspedodromous type; primary vein prominent, straight; 4 pairs of secondary veins visible, first pair of secondaries originating from a single point at the base giving rise to branches in the basal portion of the leaf in a radiating manner and terminating at margins, opposite, 0.4–0.7 cm apart; angle of divergence 40–60°, more or less uniform, moderately thick, uniformly curved distally; tertiary veins with angle of origin mostly RR, percurrent, relationship to midvein oblique, opposite; quaternary veins fine and orthogonal. Remarks: The characteristic features of this fossil leaf are typical of leaves of extant Macaranga Thouars (Euphorbiaceae). Examination of herbarium specimens indicates greatest similarity to the leaves of Macaranga denticulata Muell. Arg. in shape, size and other features. Moreover, the present fossil leaf shows little similarity to the fossil leaf Macaranga siwalika Antal and Awasthi, 1993 known earlier from the lower-middle Siwalik sediments of the right bank of the Ghish River near Oodlabari, Darjeeling District, West Bengal. Hence a new name Macaranga cf. denticulata is proposed here for the presently described fossil leaf.


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Fig. 7 a Dysoxylum raptiensis Prasad and Awasthi: fossil leaf showing shape, size and venation pattern. b Dysoxylum procerum Hiern.: modern leaf showing similar details. c Knema cf. glaucescens: fossil leaf. Specimen No. CUH/PPL/ IB7/35. d Knema glaucescens Hook. e Dalbergia cf. rimosa: fossil leaf. Specimen No. CUH/ PPL/IB7/47. f Modern leaf. (Scale 1 cm)

Millettia cf. extensa (Fig. 10a–d) Holotype: CUH/PPL/IB7/38 Paratypes: CUH/PPL/IB7/38a and CUH/PPL/IB7/38b Type locality: Road cuttings along the Itanagar-Bandardewa road in Papumpare district, Arunachal Pradesh. Type stratum: Upper part of the Siwalik succession of sediments (Kimin Formation: upper Pliocene to lower Pleistocene). Diagnosis: Leaf simple, obovate; preserved lamina length 19 cm and maximum width 10.3 cm; apex not preserved; base symmetrical, acute; margin entire; texture thick, chartaceous; venation pinnate, simple, eucamptodromous;

primary vein prominent, stout in the basal region, gradually thinning to the apex; slightly curved; 5 pairs of secondary veins visible, alternate, 0.9–2.3 cm apart; angle of divergence wide acute (70–80°), more or less uniform, moderately thick, uniformly curved, curvature is pronounced near the margin; tertiary veins sparse, percurrent. Remarks: The diagnostic features of this fossil leaflet, such as obovate shape, acute base, size, entire margin, eucamptodromous venation, and wide acute angle of divergence of 2° veins that curve towards the margin, suggest its affinity with Millettia W. & A. (Fabaceae). Comparison with herbarium specimens indicates its closest affinity with modern leaflets of Millettia extensa Benth.

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Fig. 8 a Lindera cf. pulcherrima: fossil leaf showing shape, size, apex and venation pattern. Specimen No. CUH/ PPL/IB7/34. b Lindera pulcherrima Benth.: modern leaf showing similar shape, size, apex and venation pattern. c Lindera cf. pulcherrima: part of the fossil leaf magnified to show acuminate apex. d Lindera pulcherrima Benth.: part of the modern leaf magnified to show same type of apex. (Scale 1 cm)

Sixteen fossil leaflets with affinity to Millettia W. & A. are known so far from the Cenozoic sediments (Prasad et al. 1999). Of these, eight are known from the Siwalik sediments from India and Nepal. These are: Millettia siwalica (Prasad 1990a), M. koilabasensis (Prasad 1990b), M. miobrandissiana (Prasad 1994a) and M. imlibasensis (Prasad et al. 1999) from Siwalik sediments of Koilabas, western Nepal; M. palaeoracemosa (Awasthi and Prasad 1990), M. churiensis (Prasad and Awasthi 1996) and M. koilabasensis (Prasad) Prasad and Pandey (2008) from Siwalik sediments of Surai Khola, western Nepal; M. oodlabariensis Antal and Prasad 1996) from the lower Siwalik sediments of Darjeeling District, West Bengal; M. palaeoracemosa (Awasthi and Prasad) Prasad (1994b) and M. kathgodamensis Prasad et al. (2004) from Siwalik sediments of Kathgodam, Uttranchal; M. koilabasensis Prasad from Siwalik sediments of the

Laxmi River beds, Bhutan (Prasad and Tripathi 2000) and M. palaeopachycarpa Agarwal, 2002 from the Neyveli lignite deposits, South India. The studied fossil leaflet differs from already established fossil species of this genus in size, shape, course and arrangement of the secondary and tertiary veins. Dracontomelum cf. mangiferum (Fig. 11e, f) Holotype: CUH/PPL/IB7/45 Paratypes: CUH/PPL/IB7/45a and CUH/PPL/IB7/45b Type locality: Road cuttings along the Itanagar–Bandardewa road in Papumpare district, Arunachal Pradesh. Type stratum: Upper part of the Siwalik succession of sediments (Kimin Formation: upper Pliocene to lower Pleistocene).


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Fig. 9 a, c Dipterocarpus siwalicus Lakhanpal and Guleria: fossil leaves. Specimen Nos. CUH/PPL/C3/4iii; CUH/ PPL/IB7/5. b Dipterocarpus sp.: modern leaf showing similar shape, size, apex and venation pattern. d Macaranga cf. denticulata: fossil leaf. Specimen No. CUH/PPL/IB7/ 41. (Scale 1 cm)

Diagnosis: Leaf simple, asymmetrical, narrow oblong to oblong; preserved lamina length 7.6 cm, maximum width 3.7 cm; apex broken, petiole not preserved; base obtuse; margin entire; texture chartaceous; venation pinnate, simple, eucamptodromous; primary vein (1°) stout, prominent, almost straight, unbranched; secondary veins (2°) 6 pairs visible, angle of divergence wide acute, 65–80°, lowest pairs more acute than above, alternate, 0.7–0.8 cm apart, curving uniformly, unbranched, very few intersecondary veins present, simple; tertiary veins (3°) not preserved. Remarks: The characteristic features of this fossil leaf such as asymmetrical, narrow oblong to oblong shape, obtuse base, entire margin, eucamptodromous venation, wide acute angle of divergence of secondary veins and presence of intersecondary veins reveal close resemblance to the modern leaves of Dracontomelum (Anacardiaceae). After examining herbarium sheets of all the available species of this genus, it was concluded that the leaves of Dracontomelum mangiferum Blume are most similar to the fossil leaf

in shape, size and venation pattern. However, living species are different from the fossil species in its lesser number of secondaries which are relatively narrowly (0.5–0.6 cm) spaced. No fossil leaf resembling Dracontomelum mangiferum from Cenozoic sediments has been recorded previously. The present fossil leaf form appears to be the first record from the Siwalik sediments and is being described here as Dracontomelum cf. mangiferum. Canarium cf. bengalense (Fig. 11a–d) Holotype: CUH/PPL/IB7/44 Type locality: Road cuttings along the Itanagar–Bandardewa road in Papumpare district, Arunachal Pradesh. Type stratum: Upper part of Siwalik succession of sediments (Kimin Formation: upper Pliocene to lower Pleistocene). Diagnosis: Leaf simple, symmetrical, narrow elliptic, preserved length 4 cm, maximum width 2.5 cm; apex broken; base acute; margin entire, texture chartaceous;

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Fig. 10 a Millettia cf. extensa: fossil leaf showing shape, size, base and venation pattern. Specimen No. CUH/PPL/IB7/ 38. b Millettia extensa Benth.: modern leaf showing similar shape, size, base and venation pattern. c Millettia cf. extensa: part of the fossil leaf magnified to show details of venation. d Millettia extensa Benth.: part of the modern leaf magnified to show same venation pattern. f Berchemia floribunda Wall.: modern leaf. g Berchemia siwalica Tripathi, Pandey and Prasad: part of the fossil leaf magnified to show details of venation. h Berchemia floribunda Wall.: part of the modern leaf magnified to show same venation pattern. (Scale 1 cm)

petiole not preserved; venation pinnate, eucamptodromous; primary vein single, prominent, stout, slightly curved; about 6–7 pairs secondary veins visible, 0.7–0.8 cm apart, alternate, uniformly curved up, unbranched, angle of divergence wide acute (65–75°), intersecondary veins present, simple; tertiary veins fine, with angle of origin mostly RR, percurrent, straight to sinuous, oblique in relation to midvein, predominantly alternate, close to distant. Remarks: The distinguishing features of this fossil leaf such as size, shape, margin, apex, base, texture, venation and nature of secondary veins indicate that the fossil leaf is referable to Canarium (Burseraceae). The fossil most closely resembles leaves of Canarium bengalense Roxb. There are no previous records of fossil leaves resembling Canarium from Cenozoic sediments.

Palaeoclimatic significance The fossil leaf assemblage recovered from the Kimin Formation (upper Pliocene to lower Pleistocene) exposed near Papumpare district, Arunachal Pradesh, has yielded 1 monocotyledonous and 22 dicotyledonous taxa. Eleven fossil leaves comparable to modern Dracontomelum mangiferum (Anacardiaceae), Actinodaphne obovata, Litsea salicifolia, Lindera pulcherrima (Lauraceae), Macaranga denticulata, Croton caudatus (Euphorbiaceae), Dalbergia rimosa, Millettia extensa (Fabaceae), Knema glaucescens (Myristicaceae), Quercus lamellosa (Fagaceae) and Canarium bengalense (Burseraceae) are new to the Siwalik flora and also to the Neogene flora of India. The plant megafossils recovered from the Siwalik sediments can be used effectively to interpret palaeoclimate, as the


249

Fig. 11 a Canarium cf. bengalense: fossil leaf showing shape, size, base and venation pattern. Specimen No. CUH/ PPL/IB7/44. b Canarium bengalense Roxb.: modern leaf showing similar shape, size, base and venation pattern. c Canarium cf. bengalense: part of the fossil leaf magnified to show details of venation. d Canarium bengalense Roxb.: part of the modern leaf magnified to show same venation pattern. e Dracontomelum cf. mangiferum: fossil leaf. Specimen No. CUH/PPL/ IB7/45. f Dracontomelum mangiferum Blume: modern leaf. (Scale 1 cm)

nearest living relatives of all the fossil leaf taxa are known. The principal constituents of the modern arboreal vegetation near the fossil localities are moist semievergreen forest elements, i.e., Pongamia pinnata, Duabanga grandiflora, Callistemon lanceolatus, Terminalia catappa, Terminalia miocarpa, Litsea spp., Bauhinia purpurea, Albizia spp., Michelia champaca, Gynocardia odorata, Syzygium spp., Dipterocarpus spp., Gmelina arborea, Ficus spp., Calophyllum polyanthum, Cinnamomum beghalghota, Actinodaphne angustifolia, A. obovata, Alstonia scholaris, Bombax malabaricum, Bombax ceiba, Macaranga denticulata, Knema spp., Bischofia javanica, Canarium strictum, Dalbergia sisso, Anthocephalus chinensis, Elaeocarpus aristatus, E. rugosus, Phoebe goalparensis, Meliosma simplicifolia, Turpinia nepalensis, Lagerstoemia parviflora, Quercus lamellosa, Croton chlorocalyx, etc. Additionally, several climbers and lianas such as Argyreia argentea, Dioscorea alata, Gauania tilaefolia and Buettnaria pilosa, also several species of Combretum and Piper, Mastersia assamica, and Thunbergia coccinea

are present. Amongst the monocots, wild species of Musa occur in isolated pockets. In shady and moist places, ferns and bryophytes occur with moderate abundance. On the outer hills of Arunachal sub-Himalaya, the vegetation also contains some bamboo species. We have followed two different approaches for the palaeoclimatic analysis of leaves: (1) the nearest living relative method (Axelrod and Bailey 1969; Hickey 1977; Tiffeney 1978), which determines palaeoclimate by summarising the climatic tolerances of the nearest extant relative of each taxon in the fossil flora; and (2) the foliar physiognomy method (Dilcher 1973; Dolph and Dilcher 1979; Herman and Spicer 1996, 1997; Jacobs 2002; Kovach and Spicer 1996; Wolfe 1969, 1971, 1979, 1993, 1995; Wolfe and Hopkins 1967; Wolfe and Spicer 1999), which determines palaeoclimate by means of leaf morphology. The distributions of the modern counterparts of the fossil taxa occur in a range of forest types (Table 2). The distributions suggest that the Siwalik florule represents a tropical evergreen forest which experienced heavy rainfall. The assemblage comprises plants whose presumed modern

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Table 2 Present-day distributions of comparable modern taxa recovered from the upper part of the Siwalik succession of sediments of Arunachal Pradesh Fossil taxa Poaceae Bambusa siwalika Awasthi and Prasad 1990 Anacardiaceae Mangifera someshwarica Lakhanpal and Awasthi 1984 Dracontomelum cf. mangiferum. Clusiaceae Calophyllum suraikholaensis Awasthi and Prasad 1990 Dipterocarpaceae Dipterocarpus siwalicus Lakhanpal and Guleria 1987 Rhamnaceae Berchemia siwalica Tripathi et al. 2002 Lauraceae Litsea cf. salicifolia Actinodaphne cf. obovata

Modern comparable species

Forest types

Distribution

Bambusa tulda Roxb.

Moist deciduous

Northeast India, Bangladesh and Myanmar

Mangifera indica Linn.

Evergreen to moist deciduous India and Malaya

Dracontomelum mangiferum Blume

Evergreen

Andamans and Nicobars, Malay Peninsula and Archipelago

C. polyanthum Wall.

Evergreen

Bangladesh, Myanmar and Malaya

Dipterocarpus sp.

Tropical wet evergreen

Indo-Malayan region

Berchemia floribunda Wall.

Evergreen to moist deciduous Sub-Himalayan region and Northeast India.

Litsea salicifolia Roxb. Actinodaphne obovata Blume.

Deciduous Evergreen

Sub-Himalayan region and Northeast India. Sub-Himalayan tract from Sikkim eastwards, Assam, Khasi Hills, Manipur.

Evergreen

Northeast India, Bangladesh and Myanmar

Deciduous

Himalaya from Kumaon eastwards at 4,000–9,000 ft., Khasi hills, Manipur and hills east of Toungoo.

Combretum decandrum Roxb.

Mixed deciduous

Sub-Himalayan region, Bangladesh and central India

Dysoxylum procerum Hiern.

Evergreen

N.E. India and Myanmar

Macaranga denticulata Muell

Evergreen

Sub-Himalayan tract from Sikkim eastwards, Assam, Khasi hills, Chittagong, Upper and Lower Burma

Arg. Croton caudatus Geisel.

Evergreen

Sikkim, Assam, Khasi Hills, Bengal, Chittagong, Myanmar, Malaya Peninsula and Archipelago.

Millettia extensa Benth. Millettia pachycarpa Benth.

Deciduous Evergreen

Pongamia pinnata (L) Pierre.

Evergreen

Myanmar Northeast India, Myanmar and Malaya. India, Sri Lanka and Malaya.

Dalbergia rimosa Roxb.

Deciduous

Sub-Himalaya regions

Knema glaucescens Hook. f.

Evergreen

Andamans, Nicobars, Tenasserium and Malay Penninsula

Canarium bengalense Roxb.

Evergreen

Assam and Silhet

Actinodaphne palaeoangustifolia Actinodaphne angustifolia Nees. Antal and Awasthi 1993 Lindera cf. pulcherrima Lindera pulcherrima Benth.

Combretaceae Combretum sahnii Antal and Awasthi 1993 Meliaceae Dysoxylum raptiensis Prasad and Awasthi 1996 Euphorbiaceae

Macaranga cf. denticulata Croton cf. caudatus

Fabaceae Millettia cf. extensa Millettia siwalika Antal and Awasthi 1993 Pongamia siwalika Awasthi and Lakhanpal 1990 Dalbergia cf. rimosa Myristicaceae Knema cf. glaucescens Burseraceae Canarium cf. bengalense


251

Table 2 (continued) Fossil taxa

Modern comparable species

Forest types

Distribution

Fagaceae

Quercus semicarpifolia Smith.

Evergreen

Kuram valley, Himalaya, East Manipur on the Burma frontier and China.

Quercus lamellosa Smith Chonemorpha macrophylla G. Don

Evergreen Mixed deciduous

Nepal, Sikkim, Bhutan and Manipur. Northeast India, Western Ghats, Tenasserium, Myanmar and Sri Lanka.

Quercus Kapoor and Singh 1987 Quercus cf. lamellosa Apocynaceae Chonemorpha miocenica Prasad and Awasthi 1996

descendants can be broadly classified into evergreen and deciduous forest elements. The evergreen elements constitute 70% of the total assemblage. They are Calophyllum suraikholaensis, Dipterocarpus siwalicus, Actinodaphne cf. obovata, Millettia siwalika, Pongamia siwalika, Actinodaphne palaeoangustifolia, Dysoxylum raptiensis, Macaranga cf. denticulata, Croton cf. caudatus, Dracontomelum cf. mangiferum, Canarium cf. bengalense, Knema cf. glaucescens, Berchemia siwalica, Quercus cf. lamellosa, Quercus semicarpifolia and Mangifera someshwarica. The remaining 30% are deciduous elements and include Bambusa siwalika, Litsea cf. salicifolia, Combretum sahnii, Chonemorpha miocenica, Dalbergia cf. rimosa, Lindera cf. pulcherrima and Millettia cf. extensa. In contrast, the modern forest of this region is of the tropical semievergreen type (Hazra et al. 1996) suggesting that there have been some changes in forest cover vis-à-vis the climate of this region since the Pleistocene. The predominance of evergreen elements in the assemblage indicates the prevalence of a tropical, warm, humid climate in contrast to the relatively dry climate of today. The Foliar Physiognomic method is independent of the systematic relationships of the species and is robust to evolutionary change. Because physiognomy is the result of architectural optimisation in relation to environmental constraints and is grounded on time-stable laws of physics, the potential errors in the interpretation of the palaeoclimate are minimised compared to the nearest living relative method. Eight physiognomic characters have been shown to be correlated with climate such as (1) leaf size distribution (Dilcher 1973; Dolph and Dilcher 1980; Raunkier 1934; Webb 1959); (2) leaf margin type (Bailey and Sinnott 1915, 1916; Baker-Brosh and Peet 1997; Wilf 1997); (3) drip tips (Wolfe 1969); (4) organisation (compound vs. simple; Bailey and Sinnott 1916); (5) major venation pattern (Bailey and Sinnott 1916; Manze 1968); (6) venation density (Manze 1968; Uhl and Mosbrugger 1999; Wolfe 1969, 1971); (7) leaf texture (Wolfe 1969); and (8) leaf base shape (Howard 1969). A detailed physiognomic study of the fossil leaves recovered from

the upper Siwalik sediments of the Papumpare district area in the Arunachal foothills provides considerable data on climatic conditions prevailing at the time of deposition (Table 3). The best indicator of climate appears to be leaf margin form specifically entire versus non-entire. According to Bailey and Sinnott (1916) and Wolfe (1969, 1971, 1979), the woody plants of tropical rainforests tend to possess entire margins whereas in temperate regions they possess toothed margins. In the present study, most of the species of angiosperms are entire-margined (91.3%) indicating a warm tropical climatic condition. Leaf margin analysis (LMA) is a quantitative technique of palaeoclimate reconstruction that applies present-day correlations between the proportions of woody dicot species with entire leaves and mean annual temperature to estimate palaeotemperature from the fossil leaf assemblages. It appears from leaf margin analysis of the whole leaf assemblage that only Quercus cf. lamellosa and Croton cf. caudatus (about 8.7% of total fossil taxa) have non-entire margins. Wolfe (1971) demonstrated a relationship between mean annual temperature (MAT) and the percentage of taxa with entire margined leaves based on 19 modern floras. Later, Wolfe (1979) and Greenwood (1992) derived models from the plots of MAT and percentage of entire margined leaves for eastern Asia and Australia, respectively. They have proposed a regression model converting the plot into an equation as mentioned below: MAT=1.4+0.306p, where p represents the proportion of entire margined leaves in an assemblage (Wing and Greenwood 1993; Wolfe 1979). When this equation is applied to the Arunachal Siwalik foreland basin assemblage, the MAT value comes to 29.3°C. The modern MAT of the Himalayan foothills zone is 24.4°C (mean data of 20 years obtained from Indian Meteorological Department and Champion and Seth 1968), which indicates a mean temperature reduction of 4.9°C since the early Pleistocene. The present estimated MAT (29.3°C) is very significant as it corresponds to the MAT value (29.29°C) obtained for the Siwalik foreland basin during Mio-Pliocene time

252


Table 3 Physiognomic characters of the fossil flora recovered from the upper part of the Siwalik succession of sediments of Arunachal Pradesh Physiognomic characters Fossil taxa

Average leaf size (sq cm)

Leaf margina

Drip tipb

Nature of petiolec

Leaf textured

Leaf basee

Leaf organisationf

Venation patterng

Mangifera someshwarica Dracontomelum cf. mangiferum Calophyllum suraikholaensis Dipterocarpus siwalicus Berchemia siwalica Litsea cf. salicifolia Actinodaphne cf. obovata Actinodaphne palaeoangustifolia Lindera cf. pulcherrima Combretum sahnii

51.05 45.25 16.76 41.34 8.08 18.67 61.9 82.33 12.82 35.04

E E E E E E E E E E

P _ A _ _ _ _ P P P

_ _ _ _ _ _ _ _ _ _

CH CH CO CO CH CO CO CH CH CH

_ _ A _ O _ _ A _ A

S S S S S S S S S S

D D C D C C D D D D

Dysoxylum raptiensis Macaranga cf. denticulata Croton cf. caudatus Millettia cf. extensa Millettia siwalika Pongamia siwalika Dalbergia cf. rimosa Knema cf. glaucescens Canarium cf. bengalense Quercus semicarpifolia Quercus cf. lamellosa Chonemorpha miocenica

39.12 18.95 11.5 83.07 14.7 14.5 12.1 8.6 8.7 17.86 6.6 40.8

E E N E E E E E E E N E

P _ _ _ A P A _ A A P A

_ _ _ N _ _ _ _ _ S _ _

CH CO CH CO CH CH CH CH CH CO CO CO

_ Peltate Peltate A O O O A O O _ _

C S S C C C C S S S S S

C-D C C D D D C C C D C-D D

a

E Entire, N Non-Entire

b

P Present, A Absent, – Indistinct

c

S Swollen, N Normal, – Indistinct

d

CH Chartaceous, CO Coriaceous

e

A Acute, O Obtuse, C Cuneate, At Attenuate, – Indistinct

f

C Compound, S Simple

g

C Close, D Distant

(early–middle Siwalik) from the northwest up to Darjeeling sector of the eastern Himalaya and Nepal in central Himalaya based on other fossil floras (Prasad 2008) (Fig. 12). Leaf size is another important indicator of climate. Leaf size distribution in any forest type is correlated with available moisture. Dilcher (1973) argued that leaf size decreases with decreasing rainfall. Givnish (1976, 1978) also postulated that leaf size should be greatest in the tropics, decreased in the subtropics and increased in the warm temperate forests. When applying the above criteria to the Papumpare fossil flora, we find considerably greater leaf size (microphyll; 2.25–20 cm2 and mesophyll; 20– 182 cm2) which indicates again that a tropical humid climate prevailed in the area during sedimentation.

A strong relationship exists between mean annual precipitation (MAP) and average leaf area (Wilf et al. 1998). To estimate MAP, Wilf et al. (1998) formulated an equation by using the proportion of large-sized leaves in the assemblage of any region as follows: MAP=147.5+6.18p, where p represents the proportion of mesophyll-sized or larger leaves in an assemblage. Following this equation, MAP during the late Pliocene– early Pleistocene in Arunachal sub-Himalaya was calculated as 290 mm. When this MAP value is compared with the present MAP values of different places in the Himalayan foothills [e.g. Jammu (89 mm), Kathgodam (167 mm), Surai Khola and Koilabas (106 mm), Dehradun (180 mm), Oodlabari, Darjeeling (279 mm)], it is seen that their average MAP value (164 mm) is reduced by 125 mm


253

Fig. 12 MAP and MAT values of different Siwalik sectors of the Himalayan Foreland Basin since Mio-Pliocene time

(Fig. 12). This significant difference in the MAP value between present and past might have affected the floral composition of the region. However, if we consider the present day MAP values of northeastern Himalaya, i.e. Assam (274 mm) and Oodlabari, Darjeeling (279 mm), the difference is reduced to 13 mm (Prasad 2008). Prasad (2008) estimated the MAP of the Himalayan foreland basin during Mio-Pliocene time from different Siwalik sectors, i.e. Kathgodam, Uttranchal (258 mm), Koilabas, W. Nepal (303 mm), Surai Khola, West Nepal (288 mm) and Oodlabari, West Bengal (386 mm). Thus, the average MAP of the whole Siwalik foreland basin during Mio-Pliocene time was 308 mm. Hence, the MAP value (290 mm) estimated from the upper Siwalik sediments (upper Pliocene–lower Pleistocene) of Arunachal subHimalaya shows a slight decrease compared to that of Mio-Pliocene times from the northwestern sector of the eastern Himalaya up to Darjeeling, and Nepal in central Himalaya (Fig. 12). Six other physiognomic features (drip tips, organisation, compound vs. simple, major venation pattern, venation density, leaf texture, and leaf base shape), less useful than margin type and leaf size, have also been used to aid determination of past climate. Drip tips (an extended leaf tip) are useful for hastening the runoff of water from the leaf and are also an important physiognomic feature of angiosperm leaves in wet tropical forests (Dorf 1969). In the Papumpare assemblage, due to poor preservation of leaf tips (either found broken or indistinct). the percentage of leaves having drip tips is uncertain. Interestingly, some taxa (such as Quercus cf. lamellosa, Pongamia siwalika, Combretum sahnii, Bambusa siwalika, Lindera cf. pulcherrima, Dysoxylum raptiensis) possess conspicuous drip tips

showing evidence of tropical humid climate during sedimentation in the Siwaliks. The organisation of leaves as simple or compound has been correlated with available moisture/precipitation. Dolph and Dilcher (1979, 1980) suggested that the percentage of simple leaves increases from piedmont to both mountain and coastal regions where precipitation is higher. The majority of leaves from the Siwalik sediments are simple indicating higher precipitation during the Miocene to Pleistocene than at present. Tropical leaves have a pronounced tendency for the lamina to be supplied with many veins; highly branching and free veinlets are less common in temperate plants. Evidence from leaves collected from the Siwalik area is inconclusive on this criterion because both dense and sparse venation patterns occur. The bases of most of the leaves have normal shape; however, a few that have swollen bases are indicative of moist conditions. Considering all the data, it is suggested that the Arunachal sub-Himalaya enjoyed a tropical, warm, humid climate with plenty of rainfall (MAT 29.3°C and MAP 290 mm) during the upper Siwalik sedimentation (late Pliocene to early Pleistocene) in contrast to the present-day relatively drier climate of the area. This trend possibly started after the Pliocene. Acknowledgements The authors are thankful to Dr. Sambhu Chakraborty, Senior Geologist, GSI, Operation Arunachal, Itanagar and Sri Bimalendu De Ex Dy. DG. GSI, Arunachal, for their help during the collection of specimens. Financial assistance by DST, New Delhi, is thankfully acknowledged. Sincere thanks are due to the authorities of The Botanical Survey of India, Itanagar Field Station, Itanagar and Central National Herbarium, Sibpur, Howrah, West Bengal, for providing necessary facilities to consult the herbaria. Special thanks are due to Dr. Madhusadan Mandal, Joint

254 Director of The Botanical Survey of India, Sibpur, Howrah for his encouragement and library facilities during the progress of the work and also to authorities of The Birbal Sahni Institute of Palaeobotany, Lucknow and Geological Survey of India, Calcutta, for providing library and other facilities. We also thank two anonymous reviewers and the editor for their constructive suggestions to improve the quality of the article.

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