Early Cretaceous angiosperm radiation in northeastern Gondwana: Insights from island biogeography theory

The Early Cretaceous palynological successions record one of the earliest waves of angiosperm radiation in several low-latitude localities, including the Arabian Plate. The palynological analysis of 59 core and cuttings samples from the late Barremian–early Aptian Biyadh and Shu’aiba formations, offshore Saudi Arabia, reveals remarkably high angiosperm diversity. These Early Cretaceous angiosperms inhabited northeastern Gondwana together with a diverse flora that also included bryophytes, lycopodiopsids, ferns, conifers, and other gymnosperms. A review of the dispersal capacity of this Gondwanan flora suggests that most mainland taxa had potentially colonized the carbonate islands and island arc archipelagos on both sides of the Neotethys Ocean. Two models are postulated to explain the late Barremian–early Aptian radiation of angiosperms. During sea-level lows, angiosperms would be easily dispersed to carbonate islands contiguous to northeastern Gondwana. Early birds and flying reptiles acted as the main dispersal agents for the early flowering plants, together with a diverse set of abiotic factors. The sub-aerial exposures of oceanic seamounts would serve as stepping-stones for early birds and flying reptiles, allowing a two-way dispersal of angiosperm propagules to distant volcanic islands. During sea-level highs, angiosperm gene flow between the mainland pool and the island inhabitants would be severely impeded because of the drowning of the coastal regions, many carbonate islands, and seamounts, thus promoting insular allopatric speciation and adaptive radiation. During these periods of geographical isolation, the remaining emergent Neotethyan archipelagos would act as ‘insular centers of diversification’ generating angiosperm island endemics, which in turn would be dispersed to the mainland during the following sea-level lows. Additionally, the gene flow of early aquatic angiosperms colonizing the scattered water bodies on northeastern Gondwanan wetlands would be limited during sea-level lows, turning these sites into ‘wet centers of diversification’.