Abstract
Despite more than a century of research, some key aspects of habitat preference and ecology of the earliest angiosperms remain poorly constrained. Proposed growth ecology has varied from opportunistic weedy species growing in full sun to slow-growing species limited to the shaded understorey of gymnosperm forests. Evidence suggests that the earliest angiosperms possessed low transpiration rates: gas exchange rates for extant basal angiosperms are low, as are the reconstructed gas exchange rates for the oldest known angiosperm leaf fossils. Leaves with low transpirational capacity are vulnerable to overheating in full sun, favouring the hypothesis that early angiosperms were limited to the shaded understorey. Here, modelled leaf temperatures are used to examine the thermal tolerance of some of the earliest angiosperms. Our results indicate that small leaf size could have mitigated the low transpirational cooling capacity of many early angiosperms, enabling many species to survive in full sun. We propose that during the earliest phases of the angiosperm leaf record, angiosperms may not have been limited to the understorey, and that some species were able to compete with ferns and gymnosperms in both shaded and sunny habitats, especially in the absence of competition from more rapidly growing and transpiring advanced lineages of angiosperms.
Highlights
The inferred ecological preferences of the earliest angiosperms have varied extensively over the last century
Sensitivity analysis indicates that environmental factors have a far greater impact on leaf temperature with increasing leaf size, indicating that habitat preference for sun or shade would be more critical in larger leaved species
Modelled fossil leaf temperature indicates that the small size of many early angiosperm leaves could have compensated for their low transpirational cooling capacity and enabled survival in full sun
Summary
The inferred ecological preferences of the earliest angiosperms have varied extensively over the last century. Low vein density is a uniform feature of all Zone I leaves from the Potomac Group, and characterizes taxa belonging to basal lineages and taxa related to eudicots and magnoliids, which today are characterized by higher vein densities than ferns, gymnosperms and basal angiosperm lineages [12,20,21] This implies that low vein density, and by extension preference for shaded habitats, was a feature of most or all early angiosperms, and that it was lost by many lineages later in the Cretaceous [19]. To address apparent conflicts in the early angiosperm record we use a leaf energy balance model to predict leaf temperature and evaluate proposed ecological roles of early angiosperms This approach has been used to examine the evolutionary origin of terrestrial planate leaves [27,28] and potential size-dependent high temperature injury to leaves across the Triassic–Jurassic boundary [23]. Through use of equation (2.1) [29, eq (14.6)], the temperature of fossil leaves (Tl) can be modelled as a function of air temperature (Ta), radiation load, leaf area, wind speed, relative humidity (RH)
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