The Qinghai–Tibet Plateau (QTP) comprises a platform (sometimes called the Qinghai–Tibet Plateau sensu stricto), the Himalayas, and Hengduan Mountains (Liang et al., 2018; Mao et al., 2021). The latter two parts and adjacent highlands are also called the Pan-Himalaya. Numerous plants are distributed there with many endemic species, probably because of the high diverse landscapes created by continuous geological and climatic activities (Favre et al., 2015; Mao et al., 2021). As the well known biodiversity hotspot of the alpine plants in the world (Sun et al., 2017), many studies have been conducted on evolutionary origin and ecological adaptation of those species occurring in the QTP (e.g., Wen et al., 2014, 2019; Zhang et al., 2019). In the present special issue, we collected 15 related papers on this topic. Among them, two are invited reviews. Mao et al. (2021) provide a comprehensive review of evolutionary origin of species diversity on the QTP. Especially, they outlined major disputes and likely causes in this research topic, including circumscribing and naming the QTP, the QTP uplifts, dating of molecular phylogenetic trees, non-causal correlations between QTP uplifts and species diversification and the unified ice sheet. The authors also summarized genomic advancements related to high-altitude adaptation of both plants and animals. Tong et al. (2021) reviewed the reproductive strategies of animal-pollinated alpine plants on the QTP, involving pollination system, pollen limitation, self-pollination, and sexual system. In this region, 95.4% of animal-pollinated plants are pollinated by insects (i.e., bees, moths, butterflies, and flies) with only 4% by vertebrates (i.e., bats and birds). Self-pollination through self-compatibility shift from outcrossing has become an effective reproductive strategy to overcome pollen limitation in alpine plants. The other 13 research papers aimed to address origin and adaptation of alpine flora involving three major lines of evidence: genomics, ecology, and paleobotany. We hope that the collection of these papers will increase our understanding of the origin, speciation, and adaptation of alpine species on the QTP.

1 Genomics: Phylogeny, Speciation, and High-Altitude Adaptation

Because of the decreasing cost, it is becoming easy to obtain numerous homologous sequences to construct the high-solved and well supported phylogeny for diverse genera. Here, we collect three related case studies. A phylogenomic analysis using nuclear and plastome genes by Zhou et al. (2021) was conducted for the peonies (Paeoniaceae, Paeonia L.). Their results suggest that the Paeoniaceae is a relict and ancient lineage with a divergence from the close relatives in the late Cretaceous. The common ancestor of the extant peonies may have survived in the Pan-Himalaya during the ice ages. The further diversification evolved into two subgenera and seven sections that are widespread in the Northern Hemisphere. In addition, climatic oscillations since the late Pliocene promoted polyploidy speciation including both allotetraploids and autotetraploids. Ye et al. (2021) constructed phylogeny of two species-rich bamboo genera (Poaceae) in the Himalayas and Hengduan Mountains based on simplified genome sequences. They revealed the high discordance of gene topologies and extensive hybridization between identified lineages. They conclude that reticulate evolution seems to be common during species diversification of these two genera. Chen et al. (2021) constructed phylogeny and diversification of the subtribe Gentianinae (Gentianaceae) in the QTP and adjacent regions based on transcriptomes. Similarly, they found the inconsistent phylogenetic relationships of a few identified clades based on nuclear and plastome genes. Both hybridization and incomplete lineage sorting may account for these discordant relationships. Especially, they found many gene duplication events in several phylogenetic nodes. Therefore, hybridization and gene duplication might have together facilitated species diversification of this subtribe in the alpine regions of the QTP in addition to the previously assumed geographic isolation.

The diverse habitats of the QTP provide chances for fast species divergence and adaptive evolution. Similarly, in this issue, we publish three related works. Based on genomic data, Li et al. (2021b) examined the cryptic divergence in an alpine ginger Roscoea tibetica Batalin (Zingiberaceae) in Hengduan Mountains of the QTP. They identified two deeply diverged lineages. However, gene flow occurred because of the second contact of these two lineages after the initial divergence. For two aspens occurring in the high- and low-altitude regions, however, Li et al. (2021a) revealed continuous gene flow since the initial divergence. A clear hybrid zone was recovered in the contacting region of the two species. In addition, for the high-altitude species, many genes related to the high-altitude adaptation were found to experience positive evolution. Therefore, natural selection may also play an important role during the divergence history of these two species. Such a strong selection posted by the arid habitat of the QTP may also exist for the lower plants. Zhang et al. (2021) used transcriptome data to examine the adaptive evolution of one high-altitude algae. Compared with the closely related low-altitude algae, a few genes involved in the antioxidative response, DNA repair, and translational/post-translational modifications were found to show positive evolution in the high-altitude species, possibly because of strong abiotic stresses, for example, extensive UV-B radiation on the QTP. All of these case studies suggest that geographic isolation, natural selection (especially high-altitude stresses), and hybridization may have together promoted the speciation of plants occurring there.

2 Ecological Adaptation

It remains an interesting topic to explore how plants adapt to diverse habitats of the QTP. Here, we collected three research papers related to ecological adaptation in pollination, climate, and functional traits. Harsh environments in the high-altitude region of the QTP, such as low temperature, strong wind, and extensive solar radiation, may change the pollinator spectrum of the outcrossing plants. Pi et al. (2021) compared the animal composition of the effective pollinators of one spring-flowering shrub Elaeagnus umbellata Thunb. (Elaeagnaceae) along three altitudinal gradients in the Hengduan Mountains. They found that the flowers of the high-altitude individuals became smaller with longer floral tubes and more nectar sucrose. Correspondingly, the number of bee pollinators decreased while sunbird pollinators increased. They revealed an obvious association between altitude, change of floral traits, and animal composition of the effective pollinators. In the second paper, He et al. (2021) examined the drought distribution limit of one evergreen oak species (Quercus pannosa Hand.-Mazz. s.l.) in the dry valleys in the SE Himalaya. They measured leaf functional traits of this species in the non-monsoon and monsoon seasons. They found that this oak species decreased growth at the drought stage but increased growth in the monsoon season. Therefore, monsoon duration seems to be a major factor in determining the distribution limit of this species. In the third paper, Zou et al. (2021) explored the relationship between functional traits and phylogenetic relatedness along altitudinal distributions of the Rhododendron species. They found that closely related species had high trait similarity. In addition, trait convergent selection occurred among closely related species with the similar altitudinal distributions. Therefore, both evolutionary history and trait selection may together shape species coexistence along altitudinal gradients.

3 Paleobotany for Ancient Flora of the QTP and Related Paleo-Altitude

Paleontology provides direct evidence for ancient flora of the QTP and further evolution in both floral composition and altitude change with time. In the recent past, many fossils of tropical or subtropical plants have been discovered on the central QTP, which suggested that the ancient climate in some subregions was very warm and humid and the altitude should not have been very high at the age when these fossils formed (Su et al., 2020). In the present issue, we collect four related papers. The extant species of the genus Illigera Blume (Hernandiaceae) are distributed in tropical Africa and Asia. The first fossil record of this genus was reported from western North America. Wang et al. (2021) reported 10 fossil fruits of this genus in the central QTP during the Eocene. These fossils could be placed under Illigera eocenica Manchester & O'Leary, which was first found in western North America in the Eocene. This finding indicates a warm, humid climate and low altitude in the central QTP during this stage and a close floristic link between the QTP and North America. Del Rio et al. (2021) reported Cissampelos L. and Menispermites Lesq. for the family Menispermaceae from the Middle Eocene of the central QTP. This family is now characterized with a pantropical and temperate distribution. Previous fossils for the family have been found in Europe and North America. Therefore, this study collected the ancient floristic connection between the QTP and Europe. In addition, Li et al. (2021c) reported a fossil for the genus Cercis L. (Fabaceae) from the Miocene sediments of the northeastern QTP. This genus currently shows the well known intercontinental disjunct distribution in subtropical and warm temperate regions of the Northern Hemisphere (Li et al., 2019; Wen et al., 2019). The paleo-altitude of this region was further inferred to be less than 2400 m at this stage. In addition, we also include one fossil report from the low-altitude foot of the southern Himalaya (Hazra et al., 2021).

Three of the four fossil reports from the high-altitude QTP suggest that the ancient flora there might comprise many tropical and subtropical genera from the Eocene to Miocene. These genera showed the widespread floristic connections with Northern America and Europe in their historical distributions. These findings and those reported before (Jia et al., 2019; Xu et al., 2019; Tang et al., 2019; Su et al., 2021) suggest that some subregions of the QTP were still at relatively low altitudes at these stages compared with the present high altitudes. Therefore, the total QTP might have never been uplifted together to a high elevation at a certain stage and each subregion was subjected to different uplifts at different stages (Mao et al., 2021).

4 Recommendations

In order to fully understand evolutionary histories and ecological adaptations of alpine plants on the QTP, all lines of evidence exemplified here by 13 research papers and those listed by two reviews (Mao et al., 2021; Tong et al., 2021) are encouraged in the future (also see Anderson & Song, 2020). However, the following related research is especially enforced: (1) species delimitation based on statistical analyses of morphological traits and population genetic data; (2) speciation pattern and species radiation; (3) molecular mechanism and allelic variation for special adaptation; (4) ecological adaptation linking species and community; and (5) continuous evolution from tropical to alpine floras since the Eocene.

The Qinghai–Tibet Plateau (QTP) sensu lato (sl) houses an exceptional species diversity in Asia. To develop a comprehensive understanding of species diversity in this fascinating region, we reviewed recent progress from biogeographic, paleogeographic, paleontological and genomic research of both plants and animals in the QTPsl. Numerous studies have been conducted to examine whether the QTPsl uplift triggered the production of rich species diversity there, whether a Quaternary “unified ice sheet” eliminated plants and animals on the central plateau and how high-altitude species developed the extreme environment adaptations. Major disputes arose about the first issue, mainly from different understanding of the QTP circumscriptions and related uplift, inaccurate dating of molecular phylogenetic trees, and non-causal correlations between uplift and species diversification. The QTPsl uplift is spatially and temporally heterogeneous, and abundant fossils reported recently similarly support such an asynchronous upheaval model across the entire region. Available phylogeographic studies of alpine plants and animals suggested their glacial refugia in the central QTPsl, rejecting a unified ice sheet during the Last Glacial Maximum. Genomic evidence from a limited number of alpine species has identified numerous candidate genes for high-altitude adaptation. In the future, more studies should be focused on speciation and adaptation mechanisms of the alpine species in the QTPsl based on the cutting-edge methods.

Having hundreds of big mountains, the Qinghai–Tibetan Plateau and its southern boundary, the Hengduan Mountains Region, represent one of the world's biodiversity hotspots. In the so-called “Third Pole of the earth”, diverse effects of climate change and habitat heterogeneity could have driven the evolution of plant adaptive strategies. In this review, we collected and compiled recent sources of reproductive biology in animal-pollinated plants, stressing the questions that need further attention, including pollination system, pollen limitation, self-compatibility, and sexual system. We constructed plant–pollinator interaction networks based on the case studies from this region, showing that the majority of pollinators were bees, moths, butterflies, and flies were also important, and bats and birds were rare. As the unpredictable (cold, rainy or windy) weather affects pollinator activities, sexual reproduction seemed to be pollen limited, but available studies show that the exact pattern of pollen limitation is still under debate. Self-compatibility could be either a solution, or a result from pollen limitation. Moreover, plant sexual systems can be regarded as an integrated module that counters pollen limitation. As phylogenetic relationships were constructed, diverse genera experienced rapid radiation were revealed in these mountainous areas. Further studies of evolutionary transitions in reproductive systems of congeners will unveil potential factors shaping the general pattern of adaptive strategies in animal-pollinated plants. Further studies will be beneficial in using advanced new techniques, examining phenotypic selection under common gardens, and considering chemical ecology in interactions between plants and florivores, particularly in pollen fate.

Peonies (the Paeoniaceae, Paeonia L.) are famous garden flowers, medicinal plants, and edible oil crops, but their evolutionary history largely remains unknown. To probe into their phylogenetic relationships, evolutionary history, formation of present distribution pattern, and origins of tetraploids, we sequenced 25 fragments belonging to 20 single copy nuclear genes and 14 chloroplast regions of all species in the genus to reconstruct phylogenetic relationships, date the divergence times of lineages, infer the ancestral biogeographical regions, and document the parents of tetraploids. Our results show that Paeoniaceae separated from the other members in Saxifragales in the Campanian of the late Cretaceous and diverged into two clades, woody and herbaceous clades, in the late Oligocene or early Miocene. They survived and early diverged in the Pan-Himalaya where they migrated eastwards to East Asia and further to NW America, and northwards to Middle Asia, and further to Europe. The woody lineage differentiated into two sublineages with accelerated root or floral disk evolution, while the herbaceous lineage diverged into five sublineages. Multiple glacial and interglacial cycles in Europe in the late Pliocene and early Pleistocene created opportunities for the peony species to meet and hybridize in the Mediterranean refugia, giving rise to eight allotetraploid species and four infraspecific tetraploids. Paeonia daurica Andrews, P. obovata Maxim., and P. tenuifolia L. served as the most important parents. The phylogeny of Paeonia L. implies that a new taxonomic system with two subgenera and seven sections should be proposed.

Reticulate evolution is a common and important driving force in angiosperm evolution. In this study, we analyzed the phylogenetic signals of genomic regions with different inheritance patterns to understand the evolutionary process of organisms using species-rich Himalaya–Hengduan taxa of bamboos (Fargesia Franchet and Yushania Keng). We constructed phylogenetic trees using different sampling strategies and reconstruction methods based on genome skimming and double digest restriction-site-associated DNA sequencing data. We assessed the congruence of topologies generated from different datasets and employed several approaches to reveal the causes of phylogenetic incongruence, including the detection of hybridization and introgression using PhyloNetworks and the D-statistic test (ABBA-BABA test). We found that, in the plastome-based phylogeny, Fargesia bamboos can be clustered into three groups and Yushania was nested within one of them, which contradicts the nuclear–double digest restriction-site-associated DNA sequencing-based phylogeny. Moreover, the genetic variation of chloroplast DNA is significantly correlated with geographical distribution. The strong signal of incomplete lineage sorting, hybridization, introgression, and cytoplasmic gene flow found among genera and species suggests that reticulate evolution is the main cause for the phylogenetic incongruence between nuclear and chloroplast datasets. Our results add evidence that genomes with different inheritance patterns can reveal distinct evolutionary histories of species and suggest that reticulate evolution is prevalent in rapidly diversifying groups.

Gene duplication plays an important role in plants for diversification and adaptation to new habitats. In this study, we aim to reconstruct the genome-scale phylogeny and identify large-scale gene duplication events for the subtribe Gentianinae (Gentianaceae), which is a great symbol of the alpine plants in the Qinghai–Tibet Plateau. We sequenced and assembled 70 transcriptomes from 67 species, representing all six recognized genera in the subtribe Gentianinae plus the closely related outgroups. Using phylogenomic approaches, the backbone relationships of Gentianinae were almost fully resolved with high bootstrap support. Although instances of conflicts were observed between nuclear and plastid phylogenies, six major clades of Gentianinae were consistently recovered in both phylogenies. In addition, we revealed a high occurrence of duplicated genes in our transcriptome assemblies. Using several gene tree reconciliation methods, we collectively identified 10 nodes in the species tree with large concentrations of duplicated genes. Further analysis indicated that many of these duplicated genes likely arose from hybrid polyploidy, which might also account for some of the topological incongruences between nuclear and plastid phylogenies in Gentianinae.

The harsh environments and high biodiversity of the Qinghai–Tibet Plateau (QTP) provide an ideal natural laboratory for studies on adaptive evolution. Low temperature and intense UV-B radiation are the main abiotic stresses to plants at high altitudes. The rapid development of RNA sequencing has enabled us to investigate the molecular adaptations of plants that thrive in the QTP at the genomic level. Despite the ecological importance of Oocystis algae on the plateau, the genetic mechanisms of the adaptations of these algae to this high-altitude environment remain poorly understood. Here, we sequenced and assembled the transcriptomes of Oocystis marina and Oocystis sp. LXD-20, and undertook comparative transcriptomic and evolutionary analyses to reveal their adaptive strategies. Our results identified 348 positively selected genes in Oocystis algae, and functional analyses indicated that many of these positively selected genes were associated with adaptation to abiotic stresses, such as antioxidative response, DNA repair mechanisms, translation, and post-translational modifications. We also identified the cold-responsive and UV-B-responsive genes in O. marina and Oocystis sp. LXD-20, and revealed the transcriptional regulation strategies under stress conditions. Our analyses provided a wealth of sequence data to serve as reference transcriptomes for future studies, and shed light on the adaptive strategies of green algae that live in the QTP.

Natural selection serves as an important agent to drive and maintain interspecific divergence. Populus rotundifolia Griff. is an alpine aspen species that mainly occurs in the Qinghai–Tibet Plateau (QTP) and adjacent highlands, whereas its sister species, P. davidiana Dode, is distributed across southwest and central to northeast China in much lower altitude regions. In this study, we collected genome resequencing data of 53 P. rotundifolia and 42 P. davidiana individuals across their natural distribution regions. Our population genomic data suggest that the two species are well delimitated in the allopatric regions, but with hybrid zones in their adjacent region in the eastern QTP. Coalescent simulations suggest that P. rotundifolia diverged from P. davidiana in the middle Pleistocene with following continuous gene flow since divergence. In addition, we found numerous highly diverged genes with outlier signatures that are likely associated with high-altitude adaptation of these alpine aspens. Our finding indicate that Quaternary climatic changes and natural selection have greatly contributed to the origin and distinction maintenance of P. rotundifolia in the QTP.

Species delimitation is a key foundation for exploring biodiversity. However, the existence of continuous phenotypic variation in widespread species challenges accurate species delimitation based on classical taxonomy. In this study, we investigated the cryptic diversity of a widespread herb (Roscoea tibetica Batalin) in a biodiversity hotspot (the Hengduan Mountains, China) using genotyping by sequencing, examining morphological traits, developing species distribution models, and simulating demographic history. Phylogenomic reconstruction, principal component analysis, and genetic structure inferences indicated that previously reported R. tibetica comprised two monophyletic lineages with a deep divergence. Several morphological diagnostic characteristics were discovered from field and common garden that corresponded to these independent evolutionary lineages. Species distribution models illustrated significant ecological divergence between both lineages. All evidence strongly supported that R. tibetica, as described in previous taxonomy, actually comprises two distinct species. Model test of gene flow and effective population size changes in fastsimcoal2, and a negative Tajima's D-value suggested that recent contact likely occurred between the two lineages. Our results proposed that cryptic diversity in previously reported R. tibetica was possibly associated with phenotypic plasticity in heterogeneous environments and morphological convergence in similar habitats. This study suggests that caution should be exercised when attempting to gain biological insight into species with large-scale morphological variation, and species delimitation should be done in advance.

Congeneric species may coexist at fine spatial scales through niche differentiation, however, the magnitude to which the effects of functional traits and phylogenetic relatedness contribute to their distribution along elevational gradients remains understudied. To test the hypothesis that trait and elevational range overlap can affect local speciesʼ coexistence, we first compared phylogenetic relatedness and trait (including morphological traits and leaf elements) divergence among closely related species of Rhododendron L. on Yulong Mountain, China. We then assessed relationships between the overlap of multiple functional traits and the degree of elevational range overlap among species pairs in a phylogenetic context. We found that phylogeny was a good predictor for most functional traits, where closely related species showed higher trait similarity and occupied different elevational niches at our study site. Species pairs of R. subgen. Hymenanthes (Blume) K. Koch showed low elevational range overlap and some species pairs of R. subgen. Rhododendron showed obvious niche differentiation. Trait divergence is greater for species in R. subgen. Rhododendron, and it plays an important role between species pairs with low elevational range overlap. Trait convergent selection takes place between co-occurring closely related species that have high elevational range overlap, which share more functional trait space due to environmental filtering or ecological adaptation in more extreme habitats. Our results highlight the importance of evolutionary history and trait selection for species coexistence at fine ecological scales along environmental gradients.

Unlike the well-understood cold limit of trees, the causes of the dry trailing edge of trees await explanation. Here we aimed at explaining the drought limit of an evergreen oak species (Quercus pannosa s.l.) in a typical dry valley of the upper Yangtze region, SE Himalaya, where rains (ca. 250 mm/a) are largely confined to the typical monsoon season (July–August) with drought during the remaining 9–10 months. We capitalized on an unintentional year-round irrigation treatment with trees growing along the overflow of a water reservoir serving as moist controls. We measured shoot water potential (Ψ), leaf conductance (g), flushing phenology, leaf mass per area (LMA), foliar and stem δ¹³C, leaf nutrients, and non-structural carbohydrates across the transition from non-monsoon to monsoon season, from April to August 2018. At the dry site, Ψ and g were high during the monsoon but declined to <−3 MPa as drought proceeded in the non-monsoon season. Irrigated oaks retained high values year-round. Oaks experiencing the natural drought flushed at the full strength onset of the monsoon only, that is, 80 days later than irrigated oaks. The annual shoot increment in oaks under natural drought was ca. 10% of that in irrigated oaks. However, mature foliage showed no difference in LMA and δ¹³C between dry and moist sites. We conclude that these oaks drastically reduce their activity in response to drought, with growth strictly confined to the monsoon season, the minimum duration of which, presumably is setting the range limit.

The evolution of floral traits has been thought to be influenced by local, effective pollinators. However, little attention has been paid to the possibility that altitudinal variation in floral traits could be mediated by local pollinator functional groups, particularly a shift from bees to birds. Plant size, floral traits, pollinators and their pollination roles were investigated in the spring-flowering shrub Elaeagnus umbellata (Elaeagnaceae) at three altitudes (1160, 1676, and 2050 m) in Minshan, Sichuan Province, on the northern rim of the Hengduan Mountains, southwest China. Compared to lower altitudes, higher-altitude plants were smaller but the floral tubes were longer, with a larger volume of nectar of lower sugar concentration but with a greater proportion of sucrose. The visitation frequency of bees decreased with altitude, whereas the sunbirds did the opposite. Birds and bees foraged for nectar but not pollen, and birds deposited more pollen grains per visit relative to bees and least were syrphid flies. Excluding birds decreased seed set at high but not at mid- or low altitude. Our study of E. umbellata revealed an association between altitudinal variation in floral traits and a change in the relative abundance of the major pollinators with altitude from majority bees to majority sunbirds. Although abiotic factors also tend to vary with altitude and can affect floral traits, nectar properties of “pro-bird” pollination were observed at high altitude.

Illigera (Hernandiaceae) is a liana genus distributed mainly in the tropical Asia and Africa. Previous fossil records suggested that Illigera was restricted in western North America during the Eocene. Recent paleobotanical investigation has unveiled a Paleogene flora that is totally different from today's vegetation in central Tibet. This provides novel insights into the paleoenvironmental change during the evolution of the Tibetan Plateau (TP). Here, we investigated 10 fruit impressions of Illigera from the early middle Eocene Niubao Formation in the Bangor Basin, central TP. The fossil winged fruits are characterized by their eroded fruit wings, well preserved fusiform locular areas with a median ridge bisecting the fruit, and short veins fanning radially outward. These features allow us to assign these fossils to Illigera eocenica, a species originally discovered in the Eocene of western North America. This is the second fossil occurrence of Illigera worldwide, and the first in Asia. Our finding suggests a warm and humid climate in the central TP during the early middle Eocene, and a close floristic link between Asia and North America during the Paleogene. We also propose a Northern Hemisphere origin and a Paleogene dispersal event from Northern Hemisphere to Africa for Illigera.

Menispermaceae are a pantropical and temperate family with an extensive fossil record during the Paleogene period, especially in North America and Europe, but with much less evidence from Asia. The latest fossil evidence indicates a succession of tropical to subtropical flora on the central Tibetan Plateau during the Paleogene. However, the biogeographic histories of these floras are still unresolved. Here, we report on endocarps and leaves of Menispermaceae from the Middle Eocene of Jianglang village, Bangor County, central Tibetan Plateau. The endocarps belong to two genera: Stephania, which is characterized by a horseshoe-shaped endocarp and with one lateral crest ornamented by spiny to rectangular ribs, and a condyle area; and Cissampelos (s.l.), which has two characteristic lateral ridges and a conspicuous external condyle. Associated leaves belong to the genus Menispermites, and are characterized by actinodromous primary venation, brochidodromous secondary veins, entire margins, and the presence of marginal secondary veins. The biogeographic history of Menispermaceae is complex, but evidence from these new fossils indicates an early diversification of the group in Asia, probably in response to the warming climate during the Eocene. The Jianglang flora appears to be part of a boreotropical flora, connecting Asia with North American and European floras during the Middle Eocene. The modern distribution of menispermaceous taxa found in Jianglang, as well as other families represented in the Jianglang flora, show that a tropical to subtropical climate occurred during the Eocene in central Tibet.

Even though presently indigenous to eastern Himalaya in India, no Engelhardioideae have been reported from the Cenozoic sediments of India till date. Here, we report the first Indian occurrence of a characteristic engelhardioid winged samaroid fruit having a tri-lobed wing (oblong-ovate median lobe and two lateral lobes) and a globose nut from the latest Neogene (Pliocene: Rajdanda Formation) sediments of Chotanagpur Plateau, eastern India. This is the first fossil evidence of relict family Juglandaceae from the Indian Cenozoic. We determine its taxonomic position on the basis of detailed macromorphological comparison with similar extant and fossil specimens and discuss its palaeoclimatic significance in terms of the present-day distribution of modern analogous species. We assign this Pliocene winged fruit specimen to the morphogenus Palaeocarya sect. Monocosta Manchester and describe it as a new species, namely Palaeocarya indica Hazra, Hazra M & Khan sp. nov. Palaeocarya sect. Monocosta has rich fossil records from the Cenozoic sediments of Europe, North America, and eastern Asia (China, Korea), but the modern analog, Engelhardia, is presently native only to India and neighboring Southeast Asia. We discuss the possible causes of disappearance of Engelhardia from the present-day vegetation of Chotanagpur Plateau. Its disappearance may be related to the gradual intensification of monsoonal rainfall seasonality since the Pliocene. Here, we also review in detail the biogeographic history of Palaeocarya sect. Monocosta and suggest its possible migration routes.

The climatic impacts of the Tibetan Plateau since the Neogene and the phytogeographic pattern changes of formerly widely-distributed forest communities on the plateau remain poorly constrained. Today, Cercis L. (Fabaceae) is a well-known arborescent genus typically distributed in subtropical to warm temperate zones of the Northern Hemisphere, and Paleogene fossil occurrences from Eurasia and North America show a long history of the genus in mid-low latitudes of the Northern Hemisphere. Here, we describe a fossil species, Cercis zekuensis sp. nov. based on well-preserved fruits from the early Miocene of the northeastern Tibetan Plateau. Detailed morphological comparison (e.g., ventral margin with a veinless wing) of extant and fossil members of Cercis and other genera confirmed validity of the present taxonomic identity. Based on the comparison with extant relatives and their climate preferences, this unexpected occurrence of thermophilic Cercis in northeastern Tibetan Plateau indicates this area had higher temperature and precipitation in the Miocene than today. Integrated with inferred (paleo-)temperature lapse rates, this indicates a low paleoelevation of less than 2.4 km. In contrast with the present-day alpine climate here (~3.7 km), such a low elevation facilitated a more favorable habitat with comparatively high biodiversity and warm temperate forests at that time, as were evidenced by co-occurring megafossils. Moreover, the present existence of Cercis implies the genus was widespread in interior Asia during the early Neogene and shows its modern disjunction or diversification between eastern and central Asia was possibly shaped by the late Cenozoic regional tectonic uplift and consequential environmental deterioration.