Résumé
The island of New Caledonia in the South Pacific is a hotspot of plant biodiversity, home to more than 3,000 plant species of which nearly 80% are endemic. These include a number of unique lineages long thought to be relicts from the breakup of Gondwana, although more recent geological work suggests the island was submerged until around 25 million years ago, implying that the current flora was built from more recent dispersal to the island. Understanding the origins of this important flora is complicated by the lack of a fossil record, but a recently discovered assemblage of plant and insect fossils from the western coast of the island may help to shed light on the evolutionary and ecological history of New Caledonian terrestrial communities. The fossils primarily consist of plants preserved as a dense bed of leaf impressions in a granular, clastic-rich limestone layer deposited between marine carbonates that have been previously dated to the Lower Miocene. We interpret the fossil bed as a coastal storm deposit, preserving an allochthonous assemblage derived from nearby plant communities. The assemblage contains nearly 50 unique plant morphotypes despite the small size of the collection (less than two square meters of fossil-rich bedding plane in total). The flora primarily consists of dicotyledonous angiosperm leaves, but also includes ferns, a leafy conifer branch similar to extant Dacrydium, and infructescences similar to extant Gymnostoma (Casuarinaceae). This assemblage therefore provides the first direct fossil evidence for the presence of high diversity floras on New Caledonia, which appear to have been present by at least the Lower Miocene. Preliminary comparisons of fossil leaves with those of living species collected in the area also suggest that precipitation was somewhat higher in the Miocene, based on leaf area and the presence of drip tips in some fossil leaves. Overall, this assemblage highlights the importance of the New Caledonian terrestrial fossil record, and its potential to help understand the evolutionary history of the island's biota.