Mid‐Cretaceous Hothouse Climate and the Expansion of Early Angiosperms

Abstract

The remarkable transition of early angiosperms from a small to a dominant group characterized the terrestrial ecosystem of the Cretaceous. This transition was instigated and promoted by environmental changes. Mid‐Cretaceous is characterized by major geological events that affected the global environment. δ18O, palaeothermometer TEX86, and other climatic indices from marine sediments suggest rapid temperature increase during mid‐Cretaceous despite occasional short cooling events. Simultaneously, terrestrial deposits in East Asia changed from coal‐bearing to shale, then to red beds and evaporites. Plant assemblages and other paleoclimate indicators point to rapid aridification for mid‐Cretaceous terrestrial environments. In addition, the wildfires were frequently spread all over the earth by the numerous charcoal evidence during the Mid‐Cretaceous. Thus, we speculate that the seasonally dry and hot conditions of mid‐Cretaceous created a fiery hothouse world. Early angiosperms increased in abundance and diversity and evolved from a few aquatic species to terrestrial herbaceous and then to the diversified flora of today. Angiosperms showed rapid physiological evolution in vein density and leaf area that improved photosynthesis and water absorption. These ecophysiological changes made early angiosperms well adapted to the hot and dry environment in mid‐Cretaceous. Moreover, these physiological changes facilitated the fire–angiosperm cycles in mid‐Cretaceous that likely further stimulated the early angiosperm evolution.

Dinosaur biodiversity declined well before the asteroid impact, influenced by ecological and environmental pressures

“…(ii) Biotic controls: Ecological interactions with rapidly expanding clades are increasingly recognized as important macroevolutionary drivers 56 , 155 . Dinosaurs experienced a drastic floristic change in the mid-Cretaceous with the origin and rapid radiation of angiosperms 16 , 161 , 162 at the expense of a decline in diversity of gymnosperms and non-Polypodiales ferns, while polypodialean ferns expanded after the Cretaceous. The rise and dominance of angiosperms may have contributed to altering the dietary regimes of herbivorous dinosaurs, which could in turn have affected carnivorous dinosaurs by a cascading effect.…”

How deep is the conflict between molecular and fossil evidence on the age of angiosperms?

“…Although these data imply that very rare eudicots existed in Southern Laurasia since the late Barremian, the consistent early Albian appearance of reticulate and striate tricolpates, which occurred in Northern Gondwana since the late Barremian and early Aptian, respectively, appears to represent a major migrational influx of eudicots, possibly due to global warming (Heimhofer et al ., ; Coiffard & Gomez, ; Zhang et al ., ). The Potomac leaf record shows a consistent pattern.…”

Coal petrology of the Yimin Formation (Albian) in the Hailar Basin, NE China: Paleoenvironments and wildfires during peat formation

“…The fossil record show that angiosperms were increasingly important plants during the Cretaceous and became dominant in the Late Cretaceous (Brown et al, 2012;Tao et al, 2013). Early flowering angiosperms were characterized by understory herbs and shrubs with little woody tissue until at least the Albian (Bond and Scott, 2010;Zhang et al, 2018a). Angiosperms had much higher maximum leaf vein densities and maximum photosynthetic rate than ferns and gymnosperms during the Cretaceous (Brodribb and Feild, 2010).…”

“…Angiosperms had much higher maximum leaf vein densities and maximum photosynthetic rate than ferns and gymnosperms during the Cretaceous (Brodribb and Feild, 2010). Angiosperms had typically faster life cycles than gymnosperms so that the angiosperms could make fuller use of the essential nutrients released by wildfires (Zhang et al, 2018a). Rapid accumulation of biomass from angiosperms with high rates of primary productivity during the Early Cretaceous (Brown et al, 2012;Zhang et al, 2018a) would provide ample fuel for the occurrence and high frequency of wildfires under high atmospheric oxygen concentrations (He and Lamont, 2018), further facilitating the spread of angiosperms.…”

“…Angiosperms had typically faster life cycles than gymnosperms so that the angiosperms could make fuller use of the essential nutrients released by wildfires (Zhang et al, 2018a). Rapid accumulation of biomass from angiosperms with high rates of primary productivity during the Early Cretaceous (Brown et al, 2012;Zhang et al, 2018a) would provide ample fuel for the occurrence and high frequency of wildfires under high atmospheric oxygen concentrations (He and Lamont, 2018), further facilitating the spread of angiosperms. Therefore, frequent fires would have increased the mortality rates and reduced the re-growth rates of trees.…”