1. Introduction

Fabaceae (Leguminosae) is a large and diverse family that has three subfamilies (Caesalpinioideae, Mimosoideae, and Papilionoideae) and envelops about 770 genera and over 19,500 species (Lewis et al., 2005; LPWG, 2013, 2017). Based on recent molecular phylogenetic studies, six subfamilies, namely Caesalpinioideae, Cercidoideae, Detarioideae, Dialioideae, Duparquetioideae and Papilionoideae, have been recognized (Lavin et al., 2005; Bruneau et al., 2008; LPWG, 2017). Fabaceae is cosmopolitan in distribution, representing one of the most spectacular examples of evolutionary diversification in plants. The family is morphologically, physiologically and ecologically exceptionally diverse and is only behind Asteraceae and Orchidaceae. Fabaceae trails Poaceae in agricultural importance, although the spectrum of legume uses is much greater. Legumes are diverse in areas of varied topography with seasonal climates (Axelrod, 1992; LPWG, 2017), extending from lowland rainforests to deciduous and semi-arid forests and savannas (Schrire et al., 2005a, b; Yahara et al., 2013). In India, the family is represented by 179 genera and 1297 species, of which about 23% are strictly confined to India. Moreover, these species are dominantly found in biodiversity hotspots of India such as northeast India, including eastern Himalaya (about 767 species) and the Western Ghats (Sanjappa, 1991, 1995; Chavan et al., 2013).

Legume fossils have been discovered from the Paleocene sediments across the globe (Table 1). The biogeography of legumes is questionable due to ambiguous fossil records (Ma et al., 2017). The Paleocene legume records can provide an opportunity to better understand the ecological niches during its early divergence when the Earth was globally warm and without polar ice caps (Zachos et al., 2001). Attempts have been made to understand the early evolution, diversification and ecology of legumes but due to high species diversity this is still not clear. The fossil records of legumes from the aforesaid period are important in understanding the early ecology of legumes.

Here we report a well-preserved pod and leaflet of legume from the late Paleocene sediments of Tura Formation of Meghalaya, India (Fig. 1). During the deposition of the late Paleocene sediments, the fossil locality was in the southern Hemisphere ~7°S paleolatitude (Scotese, 2001) (www.odsn.de) which is in contrast to the modern latitude (25.44°N).

2. Materials and methods

Fossil fruit and leaflet were collected from the late Paleocene sediments of the Tura Formation, which are well exposed in Damalgiri near Tura, Meghalaya, India (Fig. 1). The exact locality is 16 km southwest of Tura. The fruit and leaflet are preserved as compressions/impressions. The specimens were first cleaned with the help of a fine chisel and hammer and then photographed using a 10-megapixel digital camera (Canon SX110) in low-angled natural light. Details of the fossils were examined under a Leica S8APO stereomicroscope. The terminology used in describing specimens follows Dilcher (1974), Herendeen (1992) and Ellis et al. (2009). To determine the nearest relatives of these fossils, we compared them to specimens at the Central National Herbarium (CNH), Howrah and Forest Research Institute (FRI), Dehradun, India. We also consulted the website of the Royal Botanical Gardens, Kew (apps.kew.org/herbcat/navigator.do; accessed on July 23, September 10, 2020 and August 20, 2021). Systematic relationship of fossils was evaluated through comparison of morphological features such as shape, size, valve striation and venation pattern. The fossils have been deposited in the museum of the Birbal Sahni Institute of Palaeosciences, Lucknow, India.

3. Geological settings

The Tura Formation is well exposed near Tura, which is situated in the Garo Hills district of Meghalaya, India. The formation is divided into lower, middle, and upper members (Biswas, 1962; Chakarborty and Baksi, 1972; Bakshi, 1974; Raja Rao, 1981). The lower member is composed of massive sandstones with pebbly interbeds, while the middle member consists of argillaceous sandstones and lithomargic clays, along with coal seams. The upper member is arenaceous with few pebbly interbeds (Biswas, 1962; Bakshi, 1974; Raja Rao, 1981). A general stratigraphic succession is provided in Table 2. Plant megafossils are found in the bed of the middle member, which contains greyish-white to buff coloured clay stones (Bhattacharyya, 1985) (Fig. 2). Based on the lithostratigraphy and biostratigraphy, the age of the Tura Formation has been assigned as late Paleocene (Sah and Singh, 1974; Saxena et al., 1996; Ambwani and Kar, 2000; Monga et al., 2014). Saxena et al. (1996) have suggested that the palynofloral assemblage of Tura Formation has taxonomic uniformity with the Cherra and Lakadong Sandstone formations of the Khasi Hills (Dutta and Sah, 1970; Sah and Dutta, 1974; Kar and Kumar, 1986), the Therria Formation of Jaintia Hills, Meghalaya (Tripathi and Singh, 1984), and the Mikir Formation of the North Cachar Hills, Assam (Mehrotra, 1981). Because all the aforesaid formations are considered as Paleocene in age, Saxena et al. (1996) have suggested the same age for the Tura Formation. Ambwani and Kar (2000) have suggested a more specific age, i.e., the late Paleocene, for the Tura Formation based on characteristic pollen taxa previously described by Sah and Singh (1974), including Dandotiaspora telonata, D. pseudoreticulata, D. plicata, Polycolpites speciosus, P. cooksoniae, Lycopodiumsporites paleocenicus, Proxapertites microreticulatus, Matanomadhiasulcites maximus, Retitribrevicolporites matanomadhensis, Tricolpites levis, Neocouperipollis rarispinosus, and N. brevispinosus. The late Paleocene age for the Tura Formation has been subsequently supported by Monga et al. (2014). Characteristic plant megafossils reported from the Tura Formation also support the late Paleocene age (Mehrotra et al., 1998; Mehrotra, 2000; Srivastava et al., 2018).

4. Results 4.1. Systematics 4.1.1. Fruit

Order: Fabales Bromhead.

Family: Fabaceae Lindl.

Genus: Leguminocarpum Dotzler.

Species: Leguminocarpum meghalayensis Bhatia, Srivastava et Mehrotra sp. nov. (Fig. 3).

Etymology: The specific epithet “meghalayensis” is named after the fossil locality from where the specimens have been recovered.

Holotype: Specimen no. BSIP 42119.

Repository: Birbal Sahni Institute of Palaeosciences, Lucknow.

Type locality: Damalgiri near Tura, Meghalaya, India.

Horizon: Tura Formation.

Age: Late Paleocene.

Specific diagnosis: Fruit, straight, flattened, broadly linear in shape, wingless; sutures visible, narrow to medium in thickness; valve venation dense, consisting of numerous closely spaced veins, transverse to slightly oblique, anastomosing in the middle, forming a dense reticulum; suture thickened with fascicles of vascular strands and with narrow flange measuring 0.6–1.0 mm in width; fracture marks perpendicular to suture observed.

Description: Fossil well preserved, straight, flattened pod without wings, broadly linear in shape (Fig. 3a and b); the valve texture possibly chartaceous; preserved pod length 6.8 cm and width 1.5 cm; apex and base poorly preserved but seemingly acute and tapered in appearance; margin straight to slightly constricted; seeds or seed chamber not observed; sutures distinct, medium to broad in thickness, with fascicles of vascular strands and thickened into a flange (Fig. 3a–c, h, i) measuring 0.6–1.0 mm wide; difficult to distinguish between placental and non-placental sutures; dense and prominent valve venation present, closely spaced, at some places veins not discernible (Fig. 3b, f–i); transverse to slightly oblique in appearance, veins arising from sutures, dividing several times, at some places bifurcating and generally forming dense reticulate pattern in the middle of the valve; fracture marks present on valves, perpendicular to the suture (Fig. 3b–e).

Affinities: The key characters of the fossil fruit are shape, length, width, apex, base, marginal suture, and striations, which distinctly represent a legume. Morphologically it resembles numerous legume fruits and is difficult to identify to an extant genus. We have compared our fossil fruit with many similar extant and fossil taxa (Appendix A) as shown in Fig. 4. Wang (2012) suggested that fossil fruits earlier assigned to the organ genus Leguminosites Bowebank with uncertain affinities should be assigned to Leguminocarpum Dotzler. Moreover, Leguminocarpon Göppert, Leguminocarpos Göppert, Leguminocarpum Göppert (non Dotzler) and Leguminocarpas Miki used for fossil legume fruits should be abandoned because they are not validly published names (Wang, 2012). Due to insufficient preservation, we follow Wang (2012) and describe the present fossil fruit under the morphogenus Leguminocarpum.

While comparing the present fossil with previously described fruits, we gave particular attention to key characters such as straight to linear, flattened fruit with a vascularized flange present on one of the sutures and transverse to slightly oblique venation. Herrera et al. (2019) described eight fruit morphotypes from the middle to late Paleocene of Colombia. These fruit morphotypes look similar to the present fruit but differ in shape, valve venation and absence of vascularized flange. Leguminocarpon gardneri (Chandler) Herendeen and Crane from the late Paleocene of England (Herendeen and Crane, 1992) and Paleobowdichia lamarensis (Knowlton) Herendeen, D.B.O.S. Cardoso, F. Herrera and Wing and Tobya claibornensis (Herendeen et Dilcher) Herendeen, D.B.O.S. Cardoso, F. Herrera and Wing from latest Paleocene to late early Eocene of United States (Herendeen et al., 2022) also show some similarity with the present fossil but the presence of wings is the major difference. Bhattacharyya (1985) described a few legume fruits from the Paleocene of Meghalaya, India, such as Leguminocarpon albizoides Bhattacharyya, L. derrisoides Bhattacharyya, L. desmodioides Bhattacharyya, L. millettioides Bhattacharyya, and L. pongamioides Bhattacharyya, which bear some similarity with the present fossil fruit but differ in the absence of vascular fascicles, extended flange, pattern of valve venation, as well as shape and size (Appendix A). Given these circumstances, we propose a new fossil species L. meghalayensis Bhatia, Srivastava et Mehrotra sp. nov.

4.1.2. Leaf

Genus: Parvileguminophyllum Herendeen and Dilcher.

Type species: Parvileguminophyllum damalgiriensis Bhatia, Srivastava et Mehrotra sp. nov.

Etymology: The specific epithet “damalgiriensis” is named after the fossil locality from where the specimen was unearthed.

Holotype: Specimen no. BSIP 42120.

Repository: Birbal Sahni Institute of Palaeosciences, Lucknow.

Type locality: Damalgiri near Tura, Meghalaya, India.

Horizon: Tura Formation.

Age: Late Paleocene.

Specific diagnosis: Leaflet shape elliptic, seemingly asymmetrical, with obtuse to reflex asymmetric base; primary venation pinnate; major secondaries eucamptodromous, distance between 2° veins irregular; compound agrophic veins present, 1–2 visible; minor 2° veins present; inter-secondary veins course parallel to secondary veins, terminating into percurrent tertiary veins; intercoastal tertiary veins percurrent with dominantly AR–RO angle of origin; epimedial tertiary veins opposite percurrent.

Description: Leaflet seemingly asymmetrical, elliptic to ovate in shape with medial asymmetry, preserved lamina length 8.2 cm, maximum width 4.8 cm (Fig. 5a and b); lamina un-lobed with entire margin; primary vein moderate in thickness, straight to slightly curved; apex missing; base asymmetrical with obtuse to reflex angle and seemingly round to weakly cordate (Fig. 5a and b); primary venation pinnate; compound agrophic veins, 1–2 visible (Figs. 5 and S1); four major secondary veins visible, eucamptodromous with irregular spacing ranging between 1.2 and 2.3 cm, angle of divergence moderate to wide acute (58°–72°) with excurrent attachment to the primary vein (Fig. 5a and b); minor secondary veins present (Figs. 5 and S1) with angle of divergence narrow to moderate acute (42°–63°); inter-secondary veins present, covering <50% length of the subjacent secondaries, proximal course perpendicular to primary vein, distal course of intersecondary veins parallel to secondary veins and terminating into percurrent tertiaries, frequency 1 to >1 (Fig. 5a and b); intercoastal tertiary veins percurrent, straight to slightly wavy, opposite to alternate, dominantly AR–RO angle of origin (Fig. 5a, b, f); epimedial tertiary veins opposite percurrent with proximal course perpendicular to the primary vein and distal course parallel to intercoastal tertiary veins (Fig. 5a, b, e); exterior tertiary veins not visible; areoles poorly preserved.

Affinities: The characteristic features of the fossil leaf are elliptic to ovate lamina, compound agrophic vein, minor secondaries, eucamptodromous venation of secondary veins and percurrent tertiaries, which shows its near similarity with the genus Rhynchosia Lour (Fig. 6). of the family Fabaceae. A large number of herbarium specimens were studied and compared with the present fossil to determine its nearest living relatives. The present fossil shows similarity to some of the taxa within the Fabaceae, such as Demodium elegans DC., D. latifolium (Roxb.) DC. Dolichos lablab L., Erythrina humeana Spreng., E. addisoniae Hutch. and Dalziel, Mucuna pruriens DC., Pueraria candollei Benth., P. montana (Lour.) Merr., and some from other families such as Alangium chinense (Lour.) Harms of the Alangiaceae, Byttneria aspera Colebr. ex Roxb., Berrya amonilla Roxb. of the Malvaceae and Macaranga pustulata King ex Hook.f., Mallotus macrostachyus (Miq.) Müll. Arg. of the Euphorbiaceae. However, critical comparison with these extant species and with previously described fossil species [e.g., Pueraria miothunbergiana and P. maxima (Wang et al., 2010), Wisteria taoiana (Wang et al., 2006), Swartzia sp. 1 and Ormosia sp. 1 (Herendeen, 1992), Leaf morphotype 6 (Herrera et al., 2019), and P. lamarensis (Herendeen et al., 2022)] indicates that the present fossil differs in a combination of characters (Appendix A). Moreover, the present fossil leaf most resembles the genus Rhynchosia (Figs. 6, 7 and S1) (Appendix A), which are climbing, prostrate or sometimes erect herbs or small shrubs having a pantropical distribution (c a. 140 spp. in Africa–Madagascar; c a. 55 endemic spp. in tropical and subtropical America and c a. 30–35 spp. in warm temperate to tropical Asia to Australia) (Fig. 8; Lewis et al., 2005). Because the present fossil leaf is not satisfactorily preserved, we describe it under an organ genus Parvileguminophyllum Herendeen and Dilcher.

5. Discussion

The origin and evolution of the family Fabaceae, or the legumes, is currently under debate. Phylogenetic studies based on molecular and fossil data have led some researchers to propose that the legumes evolved during the Cretaceous (Wang et al., 2009; Magallón et al., 2015; Koenen et al., 2019). Evidence that supports an early origin of legumes comes in the form of fruits from the late Campanian (about 73.5 Ma) of Mexico (Centeno-Gonzalez et al., 2021), although the affinity of these fossil fruits with legumes has been questioned (Herendeen et al., 2022). In addition, the hypothesized origin of legumes in moist equatorial region of West Gondwanaland during the Late Cretaceous (Raven and Axelrod, 1974; Raven and Polhill, 1981) lacks reliable fossil records (Herrera et al., 2019). Several studies have indicated that the crown node of the family and diversification of multiple major lineages began in the Paleocene (Lavin et al., 2005; Bruneau et al., 2008; Zhao et al., 2021). Abundant fossil records support the early Paleocene origin of legumes (Crepet and Taylor, 1985; Crepet and Herendeen, 1992; Herendeen, 1992; Herendeen et al., 1992; Koenen et al., 2019). Legume fossils have been recorded from the early Paleocene of Colorado, USA in North America and Argentina in South America. Moreover, they have been reported from middle to late Paleocene of Colombia, late Paleocene of England and India and late Paleocene to early Eocene of USA (Table 1). These early Paleogene legume records are important for understanding the family's early divergence and ecology.

Our fossil legumes are consistent with early records of legumes in India, which have previously been discovered in late Paleocene sediments of northeast India (Bhattacharyya, 1985). During the late Paleocene the Indian plate was in the Southern Hemisphere and the fossil locality was near the equator at ~7°S of paleolatitude. This paleolatitude, together with fossil records of legumes from the Paleocene sediments of North America, South America, and Europe suggest that legumes most likely immigrated to India from Africa via the Kohistan-Ladakh Arc during the latest Cretaceous–Paleocene (Chatterjee et al., 2013).

Previous studies have reconstructed climate data from late Paleocene of the Tura Formation (Bhatia et al., 2021). These data suggest that deep time legumes were living in a warm seasonal climate with monsoonal rainfall. In addition, the fossil assemblage of plant taxa at the Tura Formation provides information on the paleoclimate during the late Paleocene. For instance, the presence of Terminalia catappa L. and Nypa fruiticans Wurmb indicate estuarine conditions with large amounts of swampy vegetation (Lakhanpal, 1952; Bhattacharyya, 1983; Mehrotra, 2000). Similarly, the presence of Nelumbo nucifera Gaertn (Bhattacharyya, 1983). and Ipomoea L. (Srivastava et al., 2018) suggest that there were ponds and or other small bodies of water around the fossil locality during the deposition of sediments. Thus, the Tura Formation provides important phytogeographical and climatological information.

6. Conclusions

The early Paleogene was characterized by a warm global climate which redistributed the habitat of marine and terrestrial biota. Understanding the ecology of biota under these warm climates is important for their future conservation. In this communication, we have reported two new fossils of legumes, namely Leguminocarpum meghalayensis Bhatia, Srivastava et Mehrotra sp. nov. and Parvileguminophyllum damalgiriensis Bhatia, Srivastava et. Mehrotra sp. nov. from the late Paleocene sediments of Tura Formation of Meghalaya, northeast India. The Paleocene records of legumes indicate that they might have immigrated to India from Africa via the Ladakh-Kohistan Arc during the latest Cretaceous–Paleocene. Our fossil discoveries, when taken together with previously reconstructed climate data from the Tura Formation of Meghalaya, indicate that legumes were well adapted to a seasonal climate in the late Paleocene.

Acknowledgements

Authors are grateful to the Director of the Birbal Sahni Institute of Palaeosciences, Lucknow for providing necessary facilities and encouragement during the research. They are thankful to the Directors of the Forest Research Institute, Dehradun and the Central National Herbarium, Howrah for providing permission to consult their herbarium. They express their gratitude to the Kew Herbarium, London for providing high resolution images of herbarium sheets. The authors are indebted to Profs Tao Su and Xiangchuan Li and one anonymous reviewer for their valuable suggestions in improving the manuscript.

Author contributions

H.B., G.S., and R.C.M. planned and designed the research. H.B., and G.S., participated in the research. H.B., G.S., R.C.M., wrote the manuscript.

Declaration of competing interest

The authors declare that they have no conflicts of interest.

Appendix A. Supplementary data

Supplementary data to this article can be found online at https://doi.org/10.1016/j.pld.2022.08.001.