Eocene Fagaceae from Patagonia and Gondwanan legacy in Asian rainforests
Eocene Fagaceae from Patagonia and Gondwanan legacy in Asian rainforests
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DOI：
10.1126/science.aaw5139
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发表时间：
2019-06
期刊：
影响因子：
56.9
通讯作者：
P. Wilf;K. Nixon;M. Gandolfo;N. Cúneo
中科院分区：
文献类型：
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作者：
P. Wilf;K. Nixon;M. Gandolfo;N. Cúneo
巴塔哥尼亚的壳斗科化石 橡树科壳斗科被认为其进化起源于北温带森林和东南亚。威尔夫等人。现在报告了来自南半球的 5200 万年前的化石，属于现存的槠属。壳斗科起源的假说仅集中在北半球。祖先栲树可能代表了今天在东南亚雨林中发现的众多古南极植物属之一。科学，本期第 14 页。 eaaw5139 栲树化石表明，祖先山毛榉科树在南半球有重要分布。简介 壳斗科开花植物包括所有橡树、山毛榉、栗树、石橡树以及 10 个属和超过 900 个物种的近缘植物。该科因其非常高的生物量及其在从北温带到热带，特别是东南亚热带地区的森林中的主导地位而脱颖而出。许多壳斗科是定义森林结构的关键物种，通过其著名的营养丰富的果实提供大量食物储备，并具有相当大的经济和文化重要性。到目前为止，在马来群岛以南还没有发现壳斗科的活体或化石，因此，南半球在该科的历史中并未得到认真考虑（南方山毛榉，Nothofagus，属于一个单独的科）。基本原理我们在阿根廷南部丘布特的始新世早期（距今 5200 万年）Laguna del Hunco 植物群中发现了两种壳斗科果序化石，一种成熟，一种未成熟，保存有超过 110 个果实，以及丰富的山毛榉叶子。高度多样化的化石组合代表了冈瓦纳古陆末期的雨林植被；南美洲、南极洲和澳大利亚尚未分离，全球变暖使得这些大陆之间的花卉和动物交流得以实现。随后，澳大利亚向北移动，最终与东南亚相撞，开启了新的生物交流。 Laguna del Hunco 植物群反映了这些地球过程，保存了在澳大利亚和东南亚生存的众多类群，其中一些类群与今天的热带壳斗科有特征相关，并为这一发现提供了丰富的生物地理背景。例子包括桉树（树胶）、Gymnostoma (rhu)、engelhardioid Juglandaceae（核桃科）、Ceratopetalum（山茱萸）、Lauraceae（月桂树）、Ripogonum（supplejack）、Agathis（贝壳杉）、多种罗汉松（黄杉）、Papuacedrus（新几内亚柏树）和 Todea（王树） 蕨类植物）。结果我们将新的果序化石置于壳斗科和现存的亚洲栲属（栗子的近亲）中，因为它们保存了杯托-果实复合体，并且在其穗状果序轴上横向、单独放置； （单个）螺母的完整外壳； (二)不对称阀门；鳞片装饰；浅裂的花被；以及三种带有未展开柱头的线性花柱。化石叶子也与栲属植物一致，并且很可能代表与果序相同的来源植物。两者都与刚刚列出的类群出现在同一地层中，这些类群是现存栲树的当地伙伴，特别是在新几内亚的山地雨林中。这些新化石代表了壳斗科历史范围向南的主要延伸，以及栲属的最古老记录（约 800 万年），该属有约 120 个现存物种，主要分布在从新几内亚到喜马拉雅山和日本的低山海拔地区。结论 化石的诊断特征、始新世早期以及与今天明显相似的花卉群落的出现都表明，栲属是在南半球进化的，很可能来自于更早从北美分散的祖先，并与相关的幸存者类群一起沿着南线到达亚洲。这一发现大大增加了亚洲和马来西亚已知的冈瓦纳遗产，并显示了幸存者谱系的持久性，这些谱系通过时间和空间从冈瓦纳到亚洲追踪了他们喜欢的凉爽潮湿的雨林环境。现代模拟森林通常位于生物多样性丰富的流域地区，现在受到人为变化的威胁，其发生速度比地质过去快几个数量级。丰富的叶子化石、各种昆虫的进食痕迹、大型坚果和相关植物群都表明，这些古老的树木是巴塔哥尼亚始新世早期“橡树月桂”森林的关键物种，就像今天亚洲的栲树一样。随后，随着南极洲分离的最早阶段和区域气候干燥，巴塔哥尼亚的槠属和许多其他雨林类群似乎已经灭绝。发现阿根廷消失的槠雨林。 （上）Laguna del Hunco 的早始新世化石湖床。 （下）从左到右：成熟栲树（大坚果长，17 毫米）和未成熟（长，约 15 厘米）果序节段、岩藻类叶子（长，18.5 厘米）、桉树果序（长，8.2 厘米）和 Papuacedrus prechilensis 叶枝（长，10.2 厘米）的实地发现照片。山毛榉科壳斗科（Fagaceae）在从北温带到热带亚洲和马来西亚的森林中占主导地位，并到达其南端。我们报告了始新世早期栲树的果序，栲树是东南亚的一种丰富多样的壳斗科植物，以及来自阿根廷南部 5200 万年前 Laguna del Hunco 植物群的共生叶子。该化石组合尤其包括许多与今天的槠属相关的植物类群。这一发现揭示了壳斗科冈瓦纳大陆的新历史和东南亚低山雨林的特征树群落。现存的散居群体在新生代气候变化和板块运动中持续存在，因为其组成谱系追踪了后冈瓦纳大陆的中生生物群落数千公里，突显了它们目前对快速气候变化和栖息地丧失的脆弱性。
Fossil Fagaceae from Patagonia The oak family Fagaceae is thought to have its evolutionary origins in northern temperate forests and Southeast Asia. Wilf et al. now report 52-million-year-old fossils from the Southern Hemisphere belonging to the still-living genus Castanopsis. Hypotheses of Fagaceae origins have focused only on the Northern Hemisphere. Ancestral Castanopsis may represent one of numerous paleo-Antarctic plant genera that are found with Castanopsis today in Southeast Asian rainforests. Science, this issue p. eaaw5139 Fossils of Castanopsis suggest an important Southern Hemisphere dispersal of ancestral beech family trees. INTRODUCTION The flowering plant family Fagaceae includes all oaks, beeches, chestnuts, stone oaks, and allies across 10 genera and >900 species. The family stands out for its very high biomass and its domination of forests from the northern temperate zone to the tropics, especially the Southeast (SE) Asian tropics. Numerous Fagaceae are keystone species that define forest structure, supply substantial food reserves through their famously nutritious fruits, and hold considerable economic and cultural importance. Until now, no living or fossil member of Fagaceae had been found south of the Malay Archipelago, and, accordingly, the Southern Hemisphere has not been seriously considered in the family’s history (the southern beech, Nothofagus, belongs to a separate family). RATIONALE We discovered two fossil infructescences of Fagaceae, one mature and one immature with >110 fruits preserved, along with abundant fagaceous leaves in the early Eocene (52-million-year-old) Laguna del Hunco flora of Chubut, southern Argentina. The highly diverse fossil assemblage represents rainforest vegetation from the terminal phase of Gondwana; South America, Antarctica, and Australia had not yet separated, and global warmth allowed floral and faunal interchange among those landmasses. Subsequently, Australia moved northward and eventually collided with SE Asia, initiating new biotic exchanges. The Laguna del Hunco flora reflects these Earth processes in preserving numerous taxa that survive in Australasia and SE Asia, among which several characteristically associate with tropical Fagaceae today and provide rich biogeographic context for the discovery. Examples include Eucalyptus (gum), Gymnostoma (rhu), engelhardioid Juglandaceae (walnut family), Ceratopetalum (coachwood), Lauraceae (laurels), Ripogonum (supplejack), Agathis (kauri), diverse podocarps (yellowwoods), Papuacedrus (a New Guinean cypress), and Todea (king fern). RESULTS We place the new fossil infructescences in Fagaceae and the living Asian genus Castanopsis, a close relative of the chestnuts, because of their preservation of cupule-fruit complexes with lateral, solitary placement on their spikelike infructescence axes; complete enclosure of the (single) nut; (two) asymmetrical valves; scaly ornamentation; lobed perianth; and three linear styles with unexpanded stigmas. The fossil leaves are also consistent with Castanopsis and in all likelihood represent the same source plant as the infructescences; both occur in the same strata with the just-listed taxa that are local associates of living Castanopsis, especially in New Guinea’s montane rainforests. The new fossils represent a major southern extension of the historical range of Fagaceae, as well as the oldest record, by ~8 million years, of the genus Castanopsis, which has ~120 living species and is dominant at lower montane elevations from New Guinea to the Himalaya and Japan. CONCLUSION The fossils’ diagnostic characters, early Eocene age, and occurrence in floral associations markedly similar to today’s all suggest that Castanopsis evolved in the Southern Hemisphere, most likely from an ancestor that had dispersed earlier from North America, and followed the southern route to Asia along with the associated survivor taxa. This discovery substantially increases the known Gondwanan legacy in Asia and Malesia and shows the persistence of the survivor lineages, which tracked their preferred cool-wet rainforest environments through time and space from Gondwana to Asia. The modern analog forests, often located in biodiverse watershed areas, are now threatened by anthropogenic change that is occurring orders of magnitude more rapidly than in the geologic past. The abundant fossil leaves with feeding marks from diverse insects, the large nuts, and the associated flora all indicate that the ancient trees were keystone species in early Eocene “oak-laurel” forests of Patagonia, much like Castanopsis is today in Asia. Subsequently, Castanopsis and many other rainforest taxa appear to have gone extinct in Patagonia with the earliest phases of Antarctic separation and drying regional climates. Discovering Argentina’s lost Castanopsis rainforest. (Top) Early Eocene fossil lake beds at Laguna del Hunco. (Bottom) Left to right: Field-discovery photos of the Castanopsis mature (large nut length, 17 mm) and immature (length, ~15 cm) infructescence segments, a fagaceous leaf (length, 18.5 cm), a Eucalyptus caldericola infructescence (length, 8.2 cm), and a Papuacedrus prechilensis leafy branch (length, 10.2 cm). The beech-oak family Fagaceae dominates forests from the northern temperate zone to tropical Asia and Malesia, where it reaches its southern limit. We report early Eocene infructescences of Castanopsis, a diverse and abundant fagaceous genus of Southeast Asia, and co-occurring leaves from the 52-million-year-old Laguna del Hunco flora of southern Argentina. The fossil assemblage notably includes many plant taxa that associate with Castanopsis today. The discovery reveals novel Gondwanan history in Fagaceae and the characteristic tree communities of Southeast Asian lower-montane rainforests. The living diaspora associations persisted through Cenozoic climate change and plate movements as the constituent lineages tracked post-Gondwanan mesic biomes over thousands of kilometers, underscoring their current vulnerability to rapid climate change and habitat loss.