A critical transition in leaf evolution facilitated the Cretaceous angiosperm revolution

Area of Science:

• Plant Biology
• Evolutionary Biology
• Paleobotany

Background:

• Angiosperm radiation during the Cretaceous period transformed global biodiversity.
• The precise mechanisms driving this rapid diversification remain unclear.

Purpose of the Study:

• To investigate the role of leaf venation patterns in angiosperm diversification.
• To identify critical leaf traits that facilitated angiosperm adaptation and dominance.

Main Methods:

• Analysis of leaf vein density in fossil and extant angiosperms.
• Development of a leaf transport model to assess water and CO2 pathways.
• Ecological modeling to simulate competitive interactions.

Main Results:

• A critical leaf vein density (2.5-5 mm mm(-2)) was identified, beyond which water transport path length became shorter than CO2 transport path length.
• Surpassing this critical density allowed for more efficient gas exchange (higher carbon return for equal water loss) via smaller, more numerous stomata.
• This efficiency facilitated adaptation to decreasing Cretaceous CO2 levels.

Conclusions:

• Leaf vein density is a key evolutionary innovation that promoted angiosperm radiation.
• Efficient gas exchange enabled angiosperms to outcompete conifers and dominate Cretaceous ecosystems.
• This finding provides a mechanistic link between leaf morphology, physiology, and macroevolutionary patterns.