Biodiversity of ecosystems in an arid setting: The late Albian plant communities and associated biota from eastern Iberia

Diversified local vegetation. • Coastal settings with Araucariaceae Pollen of Araucariaceae is consistently recorded in the palynofloras of the three studied out- crops (S1 Appendix), albeit in low abundance (<5% of the total miospores). However, the presence and relative abundance of these conifers in the local vegetation may be underesti- mated since both modern [158, 159] and fossil Araucariaceae pollen [136] are believed to pres- ent low dispersal capabilities. Several researchers [158] noted that modern palynological assemblages collected from Kauri (Agathis australis) forests only include relatively low per- centages (<5–10%) of Araucariaceae pollen. Palaeobotanical and geochemical evidence suggests a clear Araucariacean affinity for the late Albian amber recovered in the Basque-Cantabrian Basin [90, 160]. However, a geochemi- cal analysis of the amber from the San Just outcrop seems to support an araucariacean-cheiro- lepidiacean affinity [161]. The ubiquity of Araucariaceae in palynological assemblages and the record of presumed specialised insect pollen feeders (nemonychid beetles) in amber inclusions from the San Just outcrop [162] would represent additional circumstantial evidence linking Araucariaceae to the resin production. The precise role of Araucariaceae in Cretaceous vegetation and the structure of the forest types they formed still need to be ascertained, given the characteristics of the habitats colonised by modern forms. Apart from isolated occurrences in savanna-like vegetation growing on

mafic-ultramafic soils of New Caledonia (i.e., Agathis ovata; [163]), modern araucarians are often part of evergreen rainforests where they act as emergents (Agathis microstachya in Aus- tralia, A. ovata and Araucaria schmidii in New Caledonia, Agathis australis in New Zealand), or constitute small thickets and discrete stands in low altitude forests of New Zealand (A. aus- tralis) and Australia (Araucaria heterophylla) [164, 165]. Species distributed in low-fertility soils, ridgetops, and steep slopes also reflect probable competitive exclusion by angiosperms, i.e., a derived, relictual distribution (a realised niche much more restricted than a potential niche sensu [166]). Some Recent Araucariaceae, such as Araucaria columnaris, A. luxurians and A. nemorosa are coastal trees able to withstand the influence of salt wind [74, 167], and this ecological habit was probably already exhibited by representatives of the family during the Mesozoic. The presence of Microphorites utrillensis (Dolichopodidae: Microphorinae) in amber from the SJ outcrop [168] would support a sandy coastal habitat for Araucariacites- producers, as living microphorines are known to inhabit this type of habitats [169] (Figs 16 and 17). It is interesting to mention the association of modern Araucariaceae with other conifers, mainly Podocarpaceae, but also Cupressaceae in the evergreen forests of New Zealand [165]. Associations among conifers, probably fortuitous within the context of modern vegetation dominated by angiosperms, should have been more frequent during most of the Mesozoic.

The association of Araucariaceae with Cheirolepidiaceae and Exesipollenites tumulus in PCA (Fig 10) would support the existence of mixed-gymnosperm communities in the area. The spa- tial proximity of conifers from distinct families is also suggested by the megaremain assem- blages where Brachyphyllum cf. obesum (Fig 11I), a species that indicates mesophytic to xerophytic conditions attributed to Araucariaceae, has been found together with the Cheirole- pidiaceae Frenelopsis justae (Fig 11E). Two types of plant formations involving Araucariaceae can be hypothesised: (i) thickets of limited lateral extent involving one or a few species of Araucariaceae as the dominant ele- ments. This hypothesis, still not supported by in situ palaeobotanical evidence, could explain the low dispersion of pollen (i.e., underrepresentation in palynological assemblages), the PLOS ONE Late Albian desert biota from Iberia PLOS ONE | https://doi.org/10.1371/journal.pone.0282178 March 2, 2023 35 / 51