Sunday, 15 May 2016

Xenoxylon junggarensis: A new Gymnospermous fossil wood from the Late Triassic Huangshanjie Formation of Xinjiang.

Plant fossils are considered to be extremely important in the reconstruction of ancient environments, however, unlike animals, complete plants are extremely rare in the fossil record, with almost all plants known from dissarticulated material, such as wood leaves or pollen, so that these different parts of the plant are usually described as different species. Wood is the toughest material produced by plants (other than wind-blown pollen) and is extremely useful in reconstructing ancient climates, as growth patterns directly related to seasonality are preserved in the wood.

In a paper published in the journal Palaeogeography, Palaeoclimatology, Palaeoecology on 1 January 2016, Mingli Wan of the Department of Palaeobotany and Palynology and the State Key Laboratory of Palaeobiology and Stratigraphy at the Nanjing Institute of Geology and Palaeontology , Weiming Zhou of the School of Earth Sciences and Engineering at Nanjing University, Peng Tang, also of the Department of Palaeobotany and Palynology at the Nanjing Institute of Geology and Palaeontology, and Lujun Liu and Jun Wang, again of the Department of Palaeobotany and Palynology and the State Key Laboratory of Palaeobiology and Stratigraphy at the Nanjing Institute of Geology and Palaeontology, describe a new species of fossil wood from the Late Triassic Huangshanjie Formation of Jimsar County in Xinjiang Province in northwestern China.

The new species is placed in the widespread Mesozoic genus Xenoxylon, and given the specific name junggarensis, meaning 'from Junggar' in reference to the Junggar Basin, where the specimens were found. The species is described from five wood specimens, all of which were fragmented into smaller pieces by weathering. The specimens are interpreted as having come from branches or stems 3-10 cm in diameter, though only the secondary xylem has been preserved, all cortex and peridermal structures having been lost.

Xenoxylon junggarensis from the Norian (Late Triassic) Huangshanjie Formation in Dalongkou Section, Jimsar County, Xinjiang Uygur Autonomous Region, China. (A) Field photo showing the original position, and a small diameter and length (2 cm in diameter and 6 cm in length) of the fossil stem. The hammerhead is ~10 cm long. (B) Field photo showing the original position and a relatively larger size (10 cm × 7 cm) of the fossil stem. The hammer is ~21 cm long. (C) Photomicrograph of a transverse section showing an exceptional wide growth ring (white arrow) and a relatively narrow growth ring (black arrow). (D) Photomicrograph of a transverse section showing ring boundary and the tacheids collapsed into zigzag pattern (white arrow). Note large amount of earlywood and the smaller amount of latewood. Wan et al. (2016).

Members of the genus Xenoxylon have generally been interpreted as having lived in cool wet climates. The climate of the Junggar Basin in the Late Triassic is still the subject of some discussion; the area was located at a latitude of about 60° at the time, equivalent to southern Scandinavia or Siberia today, but the Triassic climate was much warmer, and most climatic interpretations of the Junggar Basin have suggested the area was at least seasonally warm and wet, and never prone to freezing.

The Xenoxylon junggarensis specimens preserve easily observed growth rings, generally taken as an indicator of a seasonal climate, but the pattern of this is somewhat different to that seen in most modern temperate trees, with an abrupt transition from earlywood (spring and summer growth) to latewood (darker autumn growth), unlike that seen in modern trees, where the transition is more gradual as late summer conditions slowly cool and/or dry until photosynthesis becomes impossible over the winter/dry season months. Wan et al. suggest that this indicates that conditions were sufficiently warm and wet for the trees to photosynthesise till the very end of their growth season, suggesting that some other factor may have restricted their growth in the non-growing season. They further suggest that this limiting factor may have been light, with the trees, which were growing at a high latitude having to survive an annual dark season in which they did not receive enough light to carry out photosynthesis.

See also... wood from the northwest of India. Diptocarps, Dipterocarpaceae, are the dominant trees of modern South and Southeast Asian rainforests, and are also found in South America, Africa and Madagascar. The group reach their maximum diversity today on the island of Bornea, where there are over 280 described species of Diptocarp, but the earliest... fossils from the Plio-Pleistocene of northwest India with African affinities.           India separated from Africa about 130 million years ago, and was effectively an island continent until its collision with Eurasia in the Middle Cenozoic. Nevertheless the modern flora and fauna of India show strong affinities with that of Africa, and while there... wood from an Early Eocene kimberlite pipe in northwestern Canada’s Slave Province.                                       Kimberlite pipes are produced by rapid volcanic intrusions carrying magma from the Earth’s mantle rapidly to the surface, often...
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