ブルネイに生える太古の森(A prehistoric forest grows in Brunei)

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ボルネオ島の熱帯雨林で初めて葉の化石を調べ、現在の生態系が古代のものであることが判明 First leaf fossil study of Borneo’s rainforest reveals current ecosystem is ancient

2022-04-26 ペンシルベニア州立大学(PennState)

ペンシルベニア州立大学とブルネイ・ダルサラーム大学の国際研究チームは、ボルネオ島のブルネイで行われた葉の化石に関する初の研究により、現在支配的な樹木群であるフタバガキが、少なくとも400万年前から熱帯雨林を支配していたことを明らかにしました。この研究成果は、学術誌「PeerJ」に掲載され、現在の景観が530万年から260万年前の鮮新世に存在したものと似ていることを示唆しており、多くの絶滅危惧種を支えているこれらの森林の保全にさらなる正当性を与える可能性があるとしています。

<関連情報>

ブルネイ・ダルサラーム島から出土した最初の葉化石は、鮮新世までにボルネオ島でフタバガキが優勢であったことを示す。 First fossil-leaf floras from Brunei Darussalam show dipterocarp dominance in Borneo by the Pliocene

Peter Wilf, Xiaoyu Zou, Michael P. Donovan, László Kocsis, Antonino Briguglio, David Shaw, JW Ferry Slik, Joseph J. Lambiase
PeerJ  Published:March 24, 2022
DOI:10.7717/peerj.12949

Study area.

Abstract

The Malay Archipelago is one of the most biodiverse regions on Earth, but it suffers high extinction risks due to severe anthropogenic pressures. Paleobotanical knowledge provides baselines for the conservation of living analogs and improved understanding of vegetation, biogeography, and paleoenvironments through time. The Malesian bioregion is well studied palynologically, but there have been very few investigations of Cenozoic paleobotany (plant macrofossils) in a century or more. We report the first paleobotanical survey of Brunei Darussalam, a sultanate on the north coast of Borneo that still preserves the majority of its extraordinarily diverse, old-growth tropical rainforests. We discovered abundant compression floras dominated by angiosperm leaves at two sites of probable Pliocene age: Berakas Beach, in the Liang Formation, and Kampong Lugu, in an undescribed stratigraphic unit. Both sites also yielded rich palynofloral assemblages from the macrofossil-bearing beds, indicating lowland fern-dominated swamp (Berakas Beach) and mangrove swamp (Kampong Lugu) depositional environments. Fern spores from at least nine families dominate both palynological assemblages, along with abundant fungal and freshwater algal remains, rare marine microplankton, at least four mangrove genera, and a diverse rainforest tree and liana contribution (at least 19 families) with scarce pollen of Dipterocarpaceae, today’s dominant regional life form. Compressed leaves and rare reproductive material represent influx to the depocenters from the adjacent coastal rainforests. Although only about 40% of specimens preserve informative details, we can distinguish 23 leaf and two reproductive morphotypes among the two sites. Dipterocarps are by far the most abundant group in both compression assemblages, providing rare, localized evidence for dipterocarp-dominated lowland rainforests in the Malay Archipelago before the Pleistocene. The dipterocarp fossils include winged Shorea fruits, at least two species of plicate Dipterocarpus leaves, and very common Dryobalanops leaves. We attribute additional leaf taxa to Rhamnaceae (Ziziphus), Melastomataceae, and Araceae (Rhaphidophora), all rare or new fossil records for the region. The dipterocarp leaf dominance contrasts sharply with the family’s <1% representation in the palynofloras from the same strata. This result directly demonstrates that dipterocarp pollen is prone to strong taphonomic filtering and underscores the importance of macrofossils for quantifying the timing of the dipterocarps’ rise to dominance in the region. Our work shows that complex coastal rainforests dominated by dipterocarps, adjacent to swamps and mangroves and otherwise similar to modern ecosystems, have existed in Borneo for at least 4–5 million years. Our findings add historical impetus for the conservation of these gravely imperiled and extremely biodiverse ecosystems.

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生物環境工学
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