Fossil evidence for Cretaceous escalation in angiosperm leaf vein evolution Taylor S. Feilda,1, Timothy J. Brodribbb, Ari Iglesiasc, David S. Chateletd, Andres Baresche, Garland R. Upchurch, Jr.f, Bernard Gomezg, Barbara A. R. Mohrh, Clement Coiffardh, Jiri Kvaceki, and Carlos Jaramilloe aDepartment of Ecology and Evolutionary Biology, University of Tennessee, Knoxville, TN 37996; bDepartment of Plant Science, University of Tasmania, Hobart, 7001 Tasmania, Australia; cFacultad de Ciencias Naturales y Museo, Universidad Nacional de La Plata, 1900 La Plata, Argentina; dDepartment of Ecology and Evolutionary Biology, Brown University, Providence, RI 02912; eSmithsonian Tropical Research Institute, Balboa, Panama; fDepartment of Biology, Texas State University, San Marcos, TX 78666; gUniversité Lyon 1, Centre National de la Recherche Scientifique-Unité Mixte de Recherche 5125, Paléoenvironnements et Paléobiosphère, Villeurbanne, France; hCollections, Museum of Natural History, 10 115 Berlin, Germany; and iNational Museum, Prague, Czech Republic Edited by Peter Crane, Yale School of Forestry and Environmental Studies, New Haven, CT, and approved April 4, 2011 (received for review September 26, 2010) The flowering plants that dominate modern vegetation possess sperm takeover (3, 4, 6, 9–15). In addition, increasing terrestrial leaf gas exchange potentials that far exceed those of all other productivity furnished by the rise of angiosperms with highly living or extinct plants. The great divide in maximal ability to photosynthetically active leaves has been posited as a large-scale exchange CO2 for water between leaves of nonangiosperms and driver for the mid-Cretaceous turning point when terrestrial angiosperms forms the mechanistic foundation for speculation biodiversity eclipsed that of the marine realm (16). However, the about how angiosperms drove sweeping ecological and biogeo- actual timing for when angiosperms first evolved their high chemical change during the Cretaceous. However, there is no capacities for leaf photosynthetic gas exchange remains poorly empirical evidence that angiosperms evolved highly photosyn- resolved (4, 5, 17–19). thetically active leaves during the Cretaceous. Using vein density A fundamental shift in how angiosperm leaf venation func- D ( V) measurements of fossil angiosperm leaves, we show that tions has been proposed as a key mechanism in the evolution of the leaf hydraulic capacities of angiosperms escalated several- high photosynthetic gas exchange potential (4, 5). Evidence for EVOLUTION fold during the Cretaceous. During the first 30 million years of a shift in angiosperm venation structure was reconstructed from angiosperm leaf evolution, angiosperm leaves exhibited uni- living phylogenetic diversity as a surge in the vein length density formly low vein DV that overlapped the DV range of dominant (DV) of angiosperm leaves that rose above the densities found Early Cretaceous ferns and gymnosperms. Fossil angiosperm vein across all other vascular plants during the mid-Cretaceous (5). D densities reveal a subsequent biphasic increase in V.Duringthe In contrast to nonangiosperms, leaves of modern ecologically fi D fi rst mid-Cretaceous surge, angiosperm V rst surpassed the dominant angiosperms routinely exhibit tremendous vein lengths, D upper bound of V limits for nonangiosperms. However, the up- which are evenly reticulated through the lamina (8). Leaves of per limits of DV typical of modern megathermal rainforest trees fi D tropical pioneer angiosperms, which operate at the upper limits rst appear during a second wave of increased V during the of C gas exchange for woody vegetation, possess an average of Cretaceous-Tertiary transition. Thus, our findings provide fossil 3 13 mm and a maximum of 20 mm of vein length within a single evidence for the hypothesis that significant ecosystem change square millimeter of leaf area (4). By comparison, all known ex- brought about by angiosperms lagged behind the Early Creta- tant and extinct nonangiosperm leaves fall within the range of 0.1– ceous taxonomic diversification of angiosperms. 6 mm of vein length per square millimeter (4). Increased vein branching furnishes greater photosynthetic capacity because add- angiosperm evolution | plant evolution | transpiration | tropical ing more veins brings the xylem-based water supply closer to the rainforest | venation sites of evaporation in the leaf. Hence, DV defines the hydraulic supply limit of water vapor exchange that secondarily constrains hotosynthesis and transpiration by leaves fundamentally in- fl maximum CO2 assimilation by leaves (4, 5, 8, 20, 21). P uence the cycling of carbon and water in the terrestrial Vein density evolution in the angiosperms has been argued to realm. Consequently, evolutionary changes in the rates at which represent an innovation that transformed global terrestrial bio- leaves exchange water for carbon bear on the origin and main- geochemistry and biodiversity (3–5, 15, 16). However, evidence tenance of biodiversity by varying the size and resource stoichi- for the timing of this Earth-changing shift in leaf function ometry of the primary productivity base. How leaves exchange remains untested in the fossil record and is limited to phyloge- gases also shape climate and atmospheric gas composition by netically based reconstruction of vein evolution from living changing the amounts of water vapor and carbon in the atmo- angiosperms (5). Clarifying when densely veined angiosperm – sphere (1 3). Recent evidence has suggested that the evolution leaves became the dominant fixture of the vegetation is critical fl of owering plants involved a sharp rise in the capacity of leaves for disentangling how Mesozoic environmental change affected to transport water and extract CO2 from the atmosphere (4, 5). angiosperm evolution and how innovations in early angiosperm The evolution of unrivaled CO2 uptake and transpirational function restructured Cretaceous ecosystem function (6, 11–16). output by angiosperm leaves form the mechanistic cornerstone Fossil angiosperm leaves of Cretaceous age often faithfully for a multitude of hypotheses citing angiosperms as agents of expansive ecosystem change during the Cretaceous (6–8). These i fi hypotheses include ( ) intensi ed mineral weathering by angio- Author contributions: T.S.F. designed research; T.S.F., A.I., D.S.C., A.B., G.R.U., B.G., B.A.R.M., sperms that decreased global atmospheric CO2 concentration; C.C., J.K., and C.J. performed research; T.S.F., T.J.B., and A.I. analyzed data; and T.S.F., (ii) heightened transpirational input to the atmosphere that in- T.J.B., A.I., G.R.U., B.G., B.A.R.M., C.C., J.K., and C.J. wrote the paper. creased regional rainfall and favored the spread and diversity of The authors declare no conflict of interest. tropical rainforest vegetation; (iii) the nearly complete compet- This article is a PNAS Direct Submission. itive exclusion by angiosperms of diverse gymnosperms and ferns 1To whom correspondence should be addressed. E-mail: [email protected]. iv from high-productivity sites worldwide; and ( ) the spread of This article contains supporting information online at www.pnas.org/lookup/suppl/doi:10. novel fire regimes that entrained a positive feedback on angio- 1073/pnas.1014456108/-/DCSupplemental. www.pnas.org/cgi/doi/10.1073/pnas.1014456108 PNAS | May 17, 2011 | vol. 108 | no. 20 | 8363–8366 Downloaded by guest on September 25, 2021 preserve the fine details of leaf venation (17, 22). Thus, vein Middle-Late Albian (106–100 Ma; Fig. 1). At 108–94 Ma (Late −2 densities of fossil leaves may offer signatures for ancient maximal Albian-Cenomanian), mean angiosperm DV (5.55 mm mm ± leaf hydraulic limits and their evolutionary changes during the 1.79 SD, n = 72) became ∼2× greater than that of non- early angiosperm radiation (4). Interestingly, there are reports angiosperms. Maximum DV values for Late Albian-Early Cen- that early angiosperms possessed low vein densities, but when omanian angiosperms increased to ∼40% greater than the highest −2 high DV leaves that dominate modern angiosperm forests first DV values (∼6mmmm ) measured for nonangiosperms and appear remains unknown (22). In this paper, we analyze fossil were nearly 4× greater than the nonangiosperm mean. The timing angiosperm leaves through the Cretaceous to determine the and magnitude for increased fossil angiosperm DV during the mid- evolutionary timing of angiosperms’ DV innovation. We mea- Cretaceous compared well to previous ancestral state recon- sured vein densities of 307 taxa (approximate morphotypes) of structions of DV evolution across extant angiosperms (Fig. 2). fossil early angiosperm leaves as well as diverse gymnosperms By the latest Cretaceous (Maastrichtian)-Paleocene, mean DV − and ferns from the early phases of angiosperm diversification and values of angiosperms (mean = 9.76 mm mm 2 ± 3.0 SD; n =78) ecological expansion (Early Cretaceous to earliest Tertiary) and increased 74% above the nonangiosperm sample through time, plotted vein density patterns through this crucial period. and the mean was 66% greater than the Hauterivian-Early Albian angiosperm mean. Several angiosperm DV values from this Results timeframe became comparable to the vein densities found across DV values of the oldest angiosperm fossil leaves sampled (Hau- extant canopy-dominants of the megathermal rainforest biome − terivian to Early Albian, 136–112 Ma) averaged 3.31 mm mm 2 ± (Fig. 1). 0.93 SD (n = 29). These angiosperm leaf fossils nested within the range of diverse