Question

For decades, paleobotanists held that the rapid expansion of angiosperms (flowering plants) during the Early Cretaceous period was primarily driven by co-evolution with specialized insect pollinators, such as early beetles and bees. This classical view posits that mutualistic plant-insect interactions accelerated speciation rates by enhancing reproductive isolation among plant populations. However, recent fossil discoveries in northern mid-latitude formations challenge this single-factor model. Newly analyzed charcoalified plant remains reveal that early angiosperms possessed anatomical adaptations—such as highly reduced leaves and dense epidermal trichomes—indicative of adaptation to high-frequency, low-intensity fire regimes.

Researchers hypothesize that the mid-Cretaceous was characterized by elevated atmospheric oxygen levels and widespread seasonal droughts, conditions exceptionally conducive to wildfire propagation. In fire-prone environments, gymnosperms, which dominated earlier Mesozoic ecosystems, regenerated relatively slowly due to their prolonged reproductive cycles and reliance on wind pollination over large distances. Conversely, early angiosperms matured rapidly, producing abundant, easily dispersed seeds capable of opportunistically colonizing fire-cleared canopy gaps before gymnosperm saplings could establish dominance. Therefore, the researchers conclude that fire-driven habitat disturbance, rather than insect pollination alone, was the primary ecological catalyst for the initial diversification of angiosperms.

Which of the following is an unstated assumption on which the researchers' conclusion depends?