The geologic context of the newly discovered fossil-bearing outcrops found across New Caledonia (Fig. 1, and Supplementary Figs. 1,4,6,8), their basic features, and their preserved biota are discussed below. As a matter of reference, the New Caledonia archipelago comprises a main island (Grande Terre), its offshore extensions (Isle of Pines and Belep Islands), and a girdle of smaller and much younger islands (Loyaly Islands) that extends parallel to the main island. The Grande Terre is the emerged northernmost part of the ‘microcontinent’ Zealandia24.
Late Cretaceous of Haute-Nessadiou (H-Ness) and Haut-Robinson (H-Rob)
The Upper Cretaceous sedimentary cover of Grande Terre consists of a passive margin megasequence, with coarse detrital terrestrial to marine peri-continental sediments at its base and fine-grained marine transgressive deposits towards the top. We located several fossiliferous outcrops within this sequence in the Haute-Nessadiou area between village of La Foa, Boghen pass, and the village of Moindou. This area contains a NW-SE striking strip of Upper Cretaceous mudstones and thin sandstones, 4 km wide and 16 km long (‘Formation à charbon’, Supplementary Fig. 1) that reflect distal fluvial deposition. Marine bivalves and gastropods indicate deposition in estuarine marine environments while coal seams and other mudstone units contain in situ root horizons, indicating terrestrial deposition and development of soils. Marine invertebrate biostratigraphy (Supplementary Information) suggests a Turonian to late Santonian age (~90-85 MY). Plant fossils occur in millimeter-thick argillite beds in several places and are dominated by conifers, including cones of Araucariaceae and taxodiaceous Cupressaceae (Fig. 2a). Conifer foliage includes small needle-leaved taxa as well as broader leaved forms with a single midvein. One poorly preserved fern specimen and a possible cycad sporophyll with attached seeds were also recovered. At least five taxa of flowering plants (angiosperms) have been found in these deposits, including forms with both entire and toothed margins from moderately sized to large leaves (Fig. 2b-d). Precise systematic placement of these plant fossils is difficult because the cuticles and fine venation are not preserved, and because of the fragmentary nature of the material and thermal alteration of the sediments. No fossil insects were recovered from these sediments, but a number of the angiosperm leaves show marginal feeding traces, galling, and mining made by insects (Fig. 2c,e,f, Supplementary Fig. 3g). Typically, arthropod remains are rare in soft mudstone and non-consolidate sandstone sediments25.
The Haut-Robinson plant outcrop is contained within the same general sequence as that at Haute-Nessadiou. Here the base of the Upper Cretaceous succession is interbedded with volcanic agglomerates, flows and sills 10 to 50 m thick (Supplementary Fig. 1) derived from the trachyte and rhyolite flows that directly overlie the Jurassic basement26. Fluvial sandstones and siltstones in this outcrop contain abundant small woody stems and other fragmentary evidence of vegetation. Leaf fossils are rare and are confined to a few horizons. The material includes at least three taxa of ferns tentatively assigned to Microphyllopteris, Cladophlebis, and Sphenopteris (Supplementary Fig. 3). Similar fossil foliage is generally placed in Gleicheniaceae, Osmundaceae, and Dicksoniaceae, respectively. Small needle-leaved conifers, including the foliage from Elatocladus and Pagiophyllum, are also present. A possible cone scale (Araucarites) also suggests the presence of araucariaceous conifers. Angiosperm leaves are small and rare, but at least two taxa are present. The first has craspedodromous-like venation with the main secondaries terminating in a major tooth at the leaf margin. Similar types of leaves occur in the fossil assemblage from the Cretaceous Winton Formation of Australia27. The second taxon has larger leaves and more steeply/shallowly angled venation. The flora of the Haut-Robinson outcrop is distinct from that of the Haute-Nessadiou Formation, probably due to the more proximal mode of deposition. No fossil insects have been found at the Haut-Robinson plant outcrop, but the fossil leaves show numerous mines and galls.
Miocene of Nepoui Group (Nepoui) and of the Fluvio-Lacustrine Formation
In the absence of unconformably overlying younger sediments, obduction in New Caledonia is not precisely dated; however, there is a consensus that the post-obduction period started in the early Oligocene7-8. The oldest known aerial exposures of weathered ultramafic rocks (ferricretes) have been dated by paleomagnetism from the Late Oligocene28. The earliest terrestrial deposits containing fossils (dating to the early Miocene) are therefore very important for understanding the post-obduction recolonisation process and elucidating the origin of the key extant lineages. Miocene deposits of the Nepoui Formation, as well as possible Miocene deposits of the Fluvio-Lacustrine Formation, both contain diverse and well-preserved plant remains, although with only a few insects or other terrestrial animals (see below). Further field collections are needed to fill this gap.
The Nepoui Group is centered around the Pindai Peninsula on the west coast of Grande Terre. It comprises two formations29: the lowermost part of the lower formation is a ca. 100 m thick sequence of reefal and lagoonal limestones, reflecting the earliest post-obduction sedimentation (Supplementary Fig. 4). This is overlain unconformably by coarse conglomerates, ca. 100 m thick that grade upwards into a finer-grained ‘intermediate unit’ with both marine and terrestrial sediments. The intermediate unit is approximately 15 m thick and is made up of alternating bioclastic and lithoclastic sands, conglomerate lenses, and calcareous mudstone. Numerous imprints and molds of fossil leaves, as well as a few insects, occur in these beds. Silicified and ferruginous fossil wood fragments and silicified infructescences very similar in morphology to extant Gymnostoma (Casuarinaceae; see below) are also common in the conglomerate30 (Supplementary Figs. 5j-k). These beds grade upward into the upper Nepoui Formation, which consists of about 25 m of bioclastic limestones that are rich in coral, algae, and echinoid fragments (Supplementary Information). The ages of both the lower and upper limestones are early Miocene, based on planktonic and benthic foraminifera. Specifically, the Upper Nepoui Formation has been dated to the Aquitanian-Burdigalian (21.4-17 MY), constraining the age of the plant fossils that occur immediately below it. We discovered a fossiliferous plant layer, approximately 10 cm thick within the intermediate unit (Figs 2g, h, Supplementary Fig. 5), that consisted of dense mats of leaves, suggesting an allochthonous accumulation. Preliminary analysis reveals a diverse floral assemblage consisting of more than 40 distinct leaf morphotypes, including around 40 dicot angiosperm leaf morphotypes, one scale-leaved conifer, and a fern, from a very small total collected exposure (ca. two square meters of rock) (Supplementary Information). The variety of forms in a limited exposure, as well as the representation of most morphotypes by only a single specimen, suggests that the Early Miocene source vegetation in this part of New Caledonia was highly diverse (Supplementary Information). The fossil leaves are difficult to assign to extant angiosperm genera or families because of their relatively poor preservation, but two types of woody infructescences recovered are consistent with assignment to Gymnostoma (Casuarinaceae). The larger type, with large prominent bracteoles and large subtending bracts, is broadly similar to infructescences assigned to extant Gymnostoma from the Eocene of Australia and Argentina31. The smaller type is more poorly preserved, but is also consistent with Gymnostoma in its prominent bracteoles that are more widely separated than Casuarina or Allocasuarina. Traces of insect activity occur frequently on the angiosperm leaves (margin feedings, galls; Figs. 2g,h). Compared to plants living in the extant dry, sclerophyll forest of the Pindai Peninsula today, the leaves of Miocene angiosperms were generally larger, suggesting greater precipitation (Supplementary Information). Together with the presence of Gymnostoma, which is abundant on New Caledonia but not in the relatively dry Pindai Peninsula, paleobotanical data are congruent with previous suggestions of higher rainfall in New Caledonia during the early Miocene32.
The Fluvio-Lacustrine Formation (Fig. 1) (Supplementary Information)
This formation comprises the sedimentary infill of depressions mainly located in the Massif du Sud in southern Grande Terre, which includes the Yaté Basin, Plaine des Lacs, Rivière des Pirogues, and Creek Pernod33. This unit, 70 to 80 m thick, formed from the erosion of weathering profiles that developed over peridotites or gabbro cumulates34-35. It also displays evidence of hydromorph (palustrine) pedogenesis with horizons of ferric crust and includes plant roots encrusted with iron oxides, as well as localized layers that are rich in well-preserved fossil plant remains (Figs 3-4). Over most of its area, it is capped by ferruginous cuirasses (Plateau de Gertrude, La Madeleine waterfall, etc.) that probably formed in association with an ancient water table. Fossiliferous layers are distributed throughout the whole series, including in some cuirasses (La Madeleine waterfall). The general pattern of the fossiliferous sites is broadly similar to fossiliferous travertine formations, and a few cases of iron travertine deposits are known in Spain36-37 and in Greece, also on ultramafic formations with ferrihydrite encrusted leaves38.
The precise age of these deposits is difficult to determine. Attempts at paleomagnetic dating of internal duricrusts and lateritic ferricrete on top of the sequence yielded contradictory results ranging from latest Oligocene (25 MY) to Miocene (15 MY). Folcher et al.39 proposed several age hypotheses for the Fluvio-Lacustrine Formation, spanning the latest Oligocene to Holocene (ca. 120,000 years ago). They favour a possible early to middle Miocene age by correlation with tectonic events recorded in Nepoui sediments (coarse conglomerate and erosion sequence positions). We were unsuccessful in our attempt to use electron spin resonance (ESR) to date quartz grains at the two major fossil localities (La Madeleine and the Pont des Japonais), where quartz grains are exceedingly rare.
At the ‘La Madeleine locality’ (Mad), casts of leaves and wood fragments are at the surface of a ferruginous cuirass along the river La Madeleine close to its falls in the Plaine des Lacs. More productive, however, is a small (~10 m long) but rich fossil-bearing layer of ferricrust that occurs in a small hill near the Madeleine waterfall in the Plaine des Lacs (Supplementary Fig. 7). This matrix preserves plant remains in three dimensions and with cellular detail, including leaves, flowers, seeds, and wood (Supplementary Fig. 8). Poorly preserved casts of leaves and wood fragments were also found at the surface of a ferruginous cuirass along the river La Madeleine close to its falls. The fossils from the small hill are preserved as dense mats of plant material that accumulated in great quantity in small sinks, without particular orientation, suggesting deposition in the absence of current. They consist mainly of eudicot leaves (Figs. 3-4) with additional remains of flower petals, inflorescence bracts, and seeds, as well as remains of ferns and gymnosperm branches with attached leaves. Although original organic material is not present in these fossils, some show exceptional preservation of epidermal and anatomical details, including stomata, which should help to improve future identification. Preliminary results indicate the presence of at least 30 plant morphotypes, including likely representatives of the angiosperm families Ericaceae (ex Epacridaceae), Malvaceae-Sterculioideae, Thymelaeaceae, and Myrtaceae. Some fossil representatives are nearly identical to the leaves of extant taxa of Thymelaeaceae (Solmsia) and Ericaceae (Styphelia). Leaf physiognomy and the taxonomic composition of the fossil flora at the La Madeleine site are strongly consistent with present-day vegetation in the area, suggesting that similar plant communities may extend as far back at the late Miocene40. The La Madeleine site also preserves a three-dimension cast of a pair of beetle elytra, which were found in association with a dense leaf mat (Figs. 4a-c). These remains are attributed to an extinct species of scarabaeid beetle, likely related to an extant New Caledonian species found in the rainforest of the same area (Rivière Bleue) (Supplementary Information).
The ‘Pont des Japonais’ (PJap) outcrop includes several fossiliferous layers, along several hundred meters of exposure on a road to the Pont des Japonais. As at ‘Mad’, fossils are preserved in three dimensions in iron oxide and include leaves, seeds, and wood fragments (Figs. 3-4). The density of fossils is lower than at ‘Mad’ and leaves are not typically found as dense mats, but the ‘PJap’ assemblages nonetheless contain a high diversity of leaf morphotypes. The composition of the flora is also different, and includes likely gymnosperms, Calophyllaceae, Ericaceae (ex Epacridaceae), Lauraceae, Malvaceae-Sterculioideae, and Rhamnaceae. The leaf morphology and likely taxonomic affinities of the ’PJap’ fossils, especially the abundant leaf fossils that resemble those of extant New Caledonian Lauraceae, are consistent with a rainforest community similar to that of the current vegetation in the ‘Rivière Bleue’ reserve in the southern Grande Terre41. In general, the ’PJap’ fossil leaves are also larger than those ’Mad’, further consistent with a rainforest community rather than a maquis community. The ‘PJap’ assemblages also contain some evidence of insects; feeding marks are present on few leaves (margin and galls), one layer contains numerous wasp nests (Figs. 4d-f), and another ca. 4 m thick layer contains numerous bee nests built with consolidated mud and very small fragments of iron oxide. These nests, comprising clusters of cells that open to flat surfaces, can be attributed to the ichnogenus Rosellichnus Genise and Bown, 199642. The contemporaneity of such ichnofossils with the rocks that contain them is difficult to establish43, but the absence of any organic remains in them suggests they are fossils. A single fossil orthopteroid forewing has been recovered from the ’PJap’ deposits with an unusual plesiomorphic character (a simple anterior branch of the cubitus posterior CuPa). This feature, which is absent in the extant taxa (Fig. 4g), suggests a very basal position in orthopteroid phylogeny, close to some Permian taxa, and is consistent with the occurrence of early diverging plant and insect lineages throughout the extant biota of New Caledonia.
Comparison of the fossil assemblages
We carried out a correspondence analysis or COA (Supplementary Information) on a dataset that considers the ‘presence vs. absence of the plant families’ encountered in our preliminary analyses of the paleoflora. This analysis suggests, not surprisingly, that Cretaceous fossil floras tend to be different from extant ones. The three Neogene deposits show facies that correspond to different paleoenvironmental and sedimentary conditions, and share more similarities with the modern flora than the Cretaceous ones. The Cretaceous Haut Robinson outcrop (H-Rob) is unique in preserving several families of ferns, not yet found in other New Caledonian deposits, possibly indicating wetter conditions. The Cretaceous Haute-Nessadiou (H-Ness) assemblage is characterized by the presence of Cycadales and Cupressaceae. The contrast between the Cretaceous ‘H-Ness’ and the Miocene Madeleine (‘Mad’) deposits could correspond to an ecological gradient (along axis 2) between a relatively humid environment (‘H-Rob’) and a drier one of a forest/maquis like that represented in the Madeleine deposit in the southern part of Grande Terre, with Nepoui being in an intermediate position. This inference is clearly more valid for the Miocene assemblages only than for a comparison that would include the Cretaceous floras.
The new discoveries show that the Late Cretaceous floras from New Caledonia are generally similar to those from contemporaneous eastern Gondwanan communities but contribute important new data, because such floras are rare. In Australia, the Winton Formation from central Queensland, which spans the Latest Albian or Cenomanian to early Turonian, is one of the few well-known macrofloras in the region27,44. Other Late Cretaceous floras include that from the Waare Formation in the Otway Basin in Central Australia45. New Zealand Late Cretaceous is better represented with floras of Albian to Cenomanian (Clarence Series)46, Cenomanian to Turonian (Tupuangi Formation)47-48, Campanian (Taratu Formation)49 and Latest Cretaceous (Pakawau Group)50. Comparison of the New Caledonia assemblages to these floras confirms the widespread occurrence of taxodiaceous Cupressaceae, which dominate in distal deltaic settings (such as the Pitt Island sequences of the Tupuangi Formation), suggesting their importance in coastal environments. These particular Cupressaceae are absent on New Caledonia today, even though other groups of conifers (Araucariaceae, Podocarpaceae, different Cupressaceae, viz. Callitris and Libocedrus) are diverse. This prominence of conifers during the mid- to Late Cretaceous is consistent across the Gondwanan margin to the Antarctic Peninsula, where they occur in distal facies in the late Albian Alexander Island flora.
The rise of flowering plants during the mid-Cretaceous is not well documented in eastern Gondwanan macrofloras, but angiosperms are important in Late Cretaceous floras, including those of New Caledonia. The first Australian angiosperms appeared toward the end of the Early Cretaceous51 and are widespread in Late Cretaceous assemblages. Late Cretaceous angiosperms from New Caledonia are similar to those of the Winton Formation of Queensland in general leaf physiognomy and especially in the predominance of small-toothed morphotypes. This contrasts with approximately coeval floras from New Zealand such as from the Clarence Series, which contains larger and entire-margined leaves, although it is unclear if this difference reflects contemporaneous spatial heterogeneity in climate and community composition or floras of slightly different ages. The presence of angiosperms in these communities is also associated with pronounced evidence of insect herbivory in the form of leaf damage, although no insect body fossils have been recovered from New Caledonia.
Although we have no fossil evidence from the Oligocene obduction interval, when fossil floras appear again in New Caledonia in the early Miocene, they are fundamentally different from those of the Late Cretaceous and are much more similar to those of modern New Caledonian vegetation. The early Miocene Nepoui flora is especially important as the first known post-obduction terrestrial paleobiota. Our results suggest this deposit preserves a highly diverse angiosperm community growing in a more humid climate than is found on the western coast of Grande Terre today, which is consistent with reconstructed paleoclimates32. The rich and diverse flora, together with the numerous traces of interactions with arthropods, are indicative of a complex paleobiota, suggesting extensive re-colonisation of the island potentially only ca. four million years following its final re-emergence. This Miocene fossil material also directly establishes the presence of some present-day groups in New Caledonia (e.g., Gymnostoma). The younger outcrops of the Fluvio-Lacustrine Formation document a rich, diverse, and complex tropical paleobiota. The diversity of leaf morphologies in the Fluvio-Lacustrine Formation, from those typical of wet forests at the Pont des Japonais to those at La Madeleine that are nearly identical to modern species living in ultramafic ‘maquis’ communities, demonstrate the existence of a diverse set of communities in southern Grande Terre during the Neogene, at least well before the arrival of humans. Such a mosaic landscape is still present today, in part maintained by wildfires before the last 50,000 yr52-53 or, more recently, by fires of human origin.
Based on new fossil evidence, it is clear that the modern vegetation of New Caledonia differs fundamentally in composition from its Late Cretaceous plant communities, as well as those from eastern Gondwana more generally, and not only in a general shift away from conifer dominated lowland settings to diverse angiosperm forests. The New Caledonian flora then appears to have undergone similar kinds of changes as the broader biogeographic region in which it is situated, at least in terms of a basic taxonomic turnover towards angiosperm-dominated communities over long geological periods54. Present fossil evidence does not rule out the possibility that some of the characteristic deep-branched lineages endemic to New Caledonia (e.g., Amborella) are relicts of earlier ecosystems (viz. those of Zealandia) that potentially survived obduction on neighbouring now-drowned islands, as suggested by geological studies55-56 and biogeographic analyses based on fossil and present-day taxa2-3,57-59. Nevertheless, assembly of the modern New Caledonian biota in the Oligocene to early Miocene is most consistent with the ages inferred by molecular dating for many extant clades6 and the idea that much of the current biota was assembled via dispersal from elsewhere. Our fossil evidence demonstrates that highly diverse angiosperm communities, very similar to those found on New Caledonia today, were established on the island by the early Miocene. It also shows that these communities evolved during the Miocene. These fossil assemblages provide an important new minimum age for the assembly of the modern New Caledonian flora. On the basis of the current material and further study of these sites, as well as investigation of potential new sites, it will be possible to develop a detailed inventory of the New Caledonian Miocene flora and entomofauna, which will aid the molecular dating of modern clades and provide insight into the taxa that disappeared during regional aridification over the last 20 MY60.