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

plants preferring well-drained substrates based solely on palynological evidence is complex due to the similarity of the pollen produced by both groups of plants. By analogy with modern pollen of Taxodium, fossil grains presenting a papilla such as Taxodiaceaepollenites are assumed to have been produced by trees preferring saturated soils and wetlands subjected to periodic floodings. Taxodioideae pollen is frequent in Cenozoic deposits and has also been found in high numbers in Cretaceous deposits from high latitude locations [135, 145]. These pollen grains have also been recorded in mid-latitude assemblages associated with spores pro- duced by water ferns [90]. However, they are scarcely represented in the successions from the Maestrazgo Basin (S1 Appendix). Perinopollenites halonatus, pollen with two distinct exine layers related to Taxodioideae [77], neither reaches a significant representation in the studied sections (S1 Appendix). Therefore, most Cupressaceae pollen recovered is assumed to have been produced by trees or shrubs that preferred well-drained substrates. The presence of Widdringtonites sp. in SJ (Fig 11J) would support the presence of Cupressa- ceae linked to well-drained substrates as the specimens assigned to this taxon present well- developed Florin rings in their stomata, reflecting xerophytic conditions. Interestingly, the genus Widdringtonites has been related to the extant Widdringtonia, which presents species adapted to very hot, dry summers and recurring wildfires in South Africa [146].

If an arid/semi-arid environment can be inferred from the large number of Cupressaceae pollen and Classopollis as well as regional sedimentology, the presence of localised thickets of Taxodioideae colonising the wettest parts of the landscape (i.e., wet interdunes, see below), while unlikely, cannot be discarded. The palynofloral data presented here and indicating the prevalence of Cupressaceae in local vegetation confirm the results obtained from the fossil wood dataset for the Albian (T2 time bin in [5]). Cheirolepidiaceae and Cupressaceae are often the co-dominant families in the studied paly- nofloras. Fluctuating frequencies of these taxa could, however, reflect drastic changes in vege- tation which are difficult to ascertain. While Cheirolepidiaceae usually dominate in AP and CA (Figs 13 and 14), Cupressaceae remains are more abundant in SJ palynofloras and the amber-bearing levels of AP and CA (Figs 12–14). These two taxa have previously recorded similar dominance of the palynofloras in numerous mid-latitude Jurassic and Cretaceous deposits [25, 90, 104, 136, 142]. Present-day growth habits of Cupressaceae, as well as whole plant reconstructions of fossil representatives of Cheirolepidiaceae, suggest that these plants were dominantly, if not exclusively, arborescent during the Cretaceous. The corresponding shrubs and trees might have represented the sub-dominant and dominant components of open vegetation (savanna–woodlands; Fig 16) similar to those observed in modern arid to semi-arid climates [143]. This conifer-dominated plant community was very different from

the Jurassic and Cretaceous fern savanna biome characterised by the families Anemiaceae and Matoniaceae [75, 147–151] which might have also grown on dunes. The diversity of angiosperm pollen types and their varying abundance pattern (S2 Appen- dix) suggest that producers had distinct ecophysiologies allowing them to colonise not only moister habitats and, probably, water-logged, if not aquatic, environments [12, 35, 152–154] but also well-drained and/or dry edaphic settings. Originally associated with moist environ- ments [155], angiosperms may have already radiated into more arid settings by the late Albian, as their presence in deposits associated with the desert system could suggest [14]. It is likely that the representation of flowering plants in pre-Cenomanian ecosystems is underestimated owing to morpho-functional and taphonomic biases (i.e., low production and dispersal of entomophilous pollen [156]). While the exact phenology of early angiosperms is poorly under- stood, the existence of short life cycles allowing flash flowering [152, 157] could explain the localised high abundance of this pollen. PLOS ONE Late Albian desert biota from Iberia PLOS ONE | https://doi.org/10.1371/journal.pone.0282178 March 2, 2023 34 / 51