Research Article

Eocene Fagaceae from Patagonia and Gondwanan legacy in Asian rainforests

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Science  07 Jun 2019:
Vol. 364, Issue 6444, eaaw5139
DOI: 10.1126/science.aaw5139

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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

Structured Abstract

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).

Abstract

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.

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