Shu-Feng Li

First Fossil Fruits and Leaves of Burretiodendron s.l. (Malvaceae s.l.) in Southeast Asia: Implications for Taxonomy, Biogeography, and Paleoclimate

Premise of research. Burretiodendron Rehder is a genus of six species that are mainly found on limestone in Southeast Asian mountain forests with a monsoonal climate. Recent molecular evidence supports its placement in Malvaceae s.l. Presently, two species of Burretiodendron have been classified as vulnerable according to the International Union for the Conservation of Nature. Fossil Burretiodendron is also extremely rare in the record, and little is known about its evolutionary history. Methodology. Fossil fruits and leaves were collected from upper Miocene sediments in Yunnan (SW China). Their macro and micro morphology were studied and compared with all extant species of Burretiodendron and with species of fossil and extant Craigia W.W. Sm. & W.E. Evans having somewhat similar fruits. Pivotal results. We recognize fossil remains of Burretiodendron on the basis of fruits described as Burretiodendron parvifructum sp. nov. and associated leaves of Burretiodendron miocenicum sp. nov. Conclusions. These Miocene fossils are the first confirmed occurrence of this genus in the Southeast Asian fossil record. This discovery supports the possibility that southeastern Yunnan was already experiencing frost-free winters and seasonal precipitation with wet summers and relatively dry winters. It also corroborates the hypothesis that this genus might have originated at the Sino-Vietnamese border, where the biodiversity center of the genus is today.

New U-Pb dates show a Paleogene origin for the modern Asian biodiversity hot spots

Yunnan, in southwestern China, straddles two of the world’s most important biodiversity hot spots (i.e., a biogeographic region that is both a reservoir of biodiversity and threatened with destruction) and hosts more than 200 fossiliferous sedimentary basins documenting the evolutionary history of that biodiversity, monsoon development, and regional elevation changes. The fossil biotas appear modern and have been assumed to be mostly Miocene in age. Dating has been by cross-correlation using palynology, magnetostratigraphy, and lithostratigraphy because numerical radiometric ages are lacking. Here we report the first unequivocal early Oligocene age (33–32 Ma) of a section in the Lühe Basin (25.141627°N, 101.373840°E, 1890 m above mean sea level), central Yunnan, based on U-Pb zircon dates of unreworked volcanic ash layers in a predominantly lacustrine succession hosting abundant plant and animal fossils. This section, located in Lühe town, is correlated with an adjacent section in the Lühe coal mine previously assigned to the upper Miocene based on regional lithostratigraphic comparison. Our substantially older age for the Lühe town section calls into question previous estimates for the surface uplift and climate history of the area, and the age of all other correlative basins. The modernization of the biota ~20 m.y. earlier than previously thought overturns existing concepts of vegetation history in southwestern China, and points to Paleogene modernization of the biota in Yunnan and associated Asian biodiversity hot spots.

Occurrence of Christella (Thelypteridaceae) in Southwest China and its indications of the paleoenvironment of the Qinghai-Tibetan Plateau and adjacent areas: Fossils Christella from Central Tibet and SW Yunnan

The uplift of the Qinghai–Tibetan Plateau dramatically changed the regional topography and climate, profoundly impacting the distribution of many plant lineages. Plant responses to environmental changes are particularly prominent in lineages that require ecological factors differentiated from those present before the uplift of the QTP. Two fossil occurrences of Christella H. Lév., Fl. Kouy–Tchéou (Thelypteridaceae), a fern genus now distributed mainly at low elevations of the pantropics with warm and moist habitats, are described based on fossilized Cenozoic leaf fronds recovered from SW China: late Paleocene Christella nervosa (J. R. Tao) C. L. Xu, T. Su & Z. K. Zhou comb. nov. found in Liuqu, southern Tibet and middle Miocene Christella sp. recovered from the Jinggu Basin in western Yunnan. The frond fossils from both sites share key morphological characteristics that diagnose these fossils as Christella. After detailed comparisons, we further clarified Christella papilio (C. Hope) Holttum, a species distributed in warm, humid habitats at altitudes no more than 1300 m, as the nearest living relative of C. nervosa. This finding suggested that southern Tibet had not reached its present elevation during the late Paleocene (ca. 56 Ma). We propose that the uplift, accompanied by severe cooling and aridification after the late Paleocene, caused the disappearance of Christella in southern Tibet, whereas paleoenvironmental conditions enabled the genus to survive in Yunnan. Our study provides the first example of distributional constraints of ferns in SW China in response to paleoenvironmental changes in the Qinghai–Tibetan Plateau and nearby areas.