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in Wilf, Zou, Donovan, ... et Lambiase, 2022. DOI: 10.7717/peerj.12949 |
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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| Study area. (A) Inset series showing location of Brunei Darussalam, the capital Bandar Seri Begawan (BSB), and the two fossil sites studied here at Berakas Beach (B), and Kampong Lugu (C). |
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| Kampong Lugu fossil site (see also Fig. 1) and stratigraphic section. A sharp, ca. 30° angular unconformity separates the lower unit, the Miocene Miri Formation, from the horizontal, dark, onlapping beds of the new stratigraphic unit, which is exposed in a horseshoe around the west and north sides of the local hill as shown (marker A; bottom photograph shows the north face of the outcrop). Fossil leaves are abundant throughout the dark claystones. Google Earth Image © 2017 CNES/Airbus. |
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| Dipterocarpus sp. BR01. (A) UBDH F00253b (Kampong Lugu), with well-preserved plications; (B) UBDH F00090a (Berakas Beach), with subtle preservation of plications; (C) UBDH F00156 (Kampong Lugu), preserving base and stout petiole; (D) UBDH F00096 (Berakas Beach), large leaf fragment with good vein preservation; (E) UBDH F00140b (Kampong Lugu), under low-angle unidirectional light to show the stout midvein and strong longitudinal folding of the blade. (F) UBDH F00140a, counterpart of specimen in E, preserving margin and venation details. |
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| Shorea sp. BR04. UBDH F00097b (Berakas Beach). White arrow, shared visual reference point at a wing juncture with the nut body. (A, B) Surface view, showing an ovoid nut with two clasping, obovate, apparently subequal fruit wings (calyx lobes); the wing at left is missing its apex (fragments distal to the wing are dark-stained matrix, not fossil), but the wing at right is relatively complete; (C) Rotational views of CT scans, showing an additional large wing (dark color) embedded in the sediment directly underneath the broken wing at the surface, subequal in size and shape to the more complete wing at the surface. Inset, initial scan that captured fragments of the embedded wing (dark) clasping the obverse surface of the nut well toward the base. |
Conclusions
We report two new late Cenozoic compression assemblages from Brunei Darussalam, a nation with extraordinarily biodiverse and intact tropical rainforests. The new plant fossils are the first from that country and the first Cenozoic compression floras from the Malay Archipelago in the modern era. We also report co-occurring palynofloras, and both the macro- and microfossils are unbiased collections. Our results, most broadly, show that the principal features of northern Borneo’s coastal vegetation (e.g., Thia-Eng, Loke Ming & Sadorra, 1987; Wong & Kamariah, 1999) have changed little for at least 4–5 million years. Dipterocarps overwhelmingly dominate both macrofossil assemblages, showing for the first time from compression floras, which record localized paleoecological information, that the dipterocarp-dominated rainforests that define lowland forest structure throughout Malesia are ancient. At least three genera (Dipterocarpus, Dryobalanops, and Shorea) and four species of dipterocarps are present, and dipterocarps represent 79% of all identifiable macrofossils. All other elements identified are also present in the living Brunei flora and include the first reliable macrofossil occurrences for the region of Melastomataceae, Rhamnaceae (Ziziphus), and Araceae (Rhaphidophora).
Rich palynofloras from the same strata as the leaves detail fern- and mangrove-swamp depositional environments with input from adjacent tropical rainforests and diverse, well-structured communities. The pollen data provide a large number of taxon occurrences that complement the macrofloras, with few overlaps. Dipterocarp pollen is notably rare, at less than 1% abundance. Thus, our work directly tests and supports the idea that the low representation of dipterocarp pollen in many regional assemblages results from significant taphonomic bias, providing a caveat for palynological studies. Macrofossils offer an outstanding opportunity to assess patterns of dipterocarp diversity, abundance, and dominance through time and, more broadly, the evolution of the modern vegetation structure and dominance patterns of Southeast Asia. Our discovery of dipterocarp-dominated coastal rainforests in Borneo from 4–5 million years ago raises the conservation significance of their highly threatened yet still strikingly diverse and ecologically foundational living analogs.
Peter Wilf, Xiaoyu Zou, Michael P. Donovan, László Kocsis, Antonino Briguglio, David Shaw, J.W. Ferry Slik and Joseph J. Lambiase. 2022. First fossil-leaf floras from Brunei Darussalam show Dipterocarp dominance in Borneo by the Pliocene. PeerJ. 10:e12949. DOI: 10.7717/peerj.12949
