title: Fossil evidence for Cretaceous escalation in angiosperm leaf vein evolution.
abstract: The flowering plants that dominate modern vegetation possess leaf gas exchange potentials that far exceed those of all other living or extinct plants. The great divide in maximal ability to exchange CO(2) for water between leaves of nonangiosperms and angiosperms forms the mechanistic foundation for speculation about how angiosperms drove sweeping ecological and biogeochemical change during the Cretaceous. However, there is no empirical evidence that angiosperms evolved highly photosynthetically active leaves during the Cretaceous. Using vein density (D(V)) measurements of fossil angiosperm leaves, we show that the leaf hydraulic capacities of angiosperms escalated several-fold during the Cretaceous. During the first 30 million years of angiosperm leaf evolution, angiosperm leaves exhibited uniformly low vein D(V) that overlapped the D(V) range of dominant Early Cretaceous ferns and gymnosperms. Fossil angiosperm vein densities reveal a subsequent biphasic increase in D(V). During the first mid-Cretaceous surge, angiosperm D(V) first surpassed the upper bound of D(V) limits for nonangiosperms. However, the upper limits of D(V) typical of modern megathermal rainforest trees first appear during a second wave of increased D(V) during the Cretaceous-Tertiary transition. Thus, our findings provide fossil evidence for the hypothesis that significant ecosystem change brought about by angiosperms lagged behind the Early Cretaceous taxonomic diversification of angiosperms.
authors.affiliation: Department of Ecology and Evolutionary Biology, University of Tennessee, Knoxville, TN 37996, USA. hedyosmum@gmail.com
authors.forename: Taylor S
authors.initials: TS
authors.lastname: Feild
authors.forename: Timothy J
authors.initials: TJ
authors.lastname: Brodribb
authors.forename: Ari
authors.initials: A
authors.lastname: Iglesias
authors.forename: David S
authors.initials: DS
authors.lastname: Chatelet
authors.forename: Andres
authors.initials: A
authors.lastname: Baresch
authors.forename: Garland R
authors.initials: GR
authors.lastname: Upchurch
authors.forename: Bernard
authors.initials: B
authors.lastname: Gomez
authors.forename: Barbara A R
authors.initials: BA
authors.lastname: Mohr
authors.forename: Clement
authors.initials: C
authors.lastname: Coiffard
authors.forename: Jiri
authors.initials: J
authors.lastname: Kvacek
authors.forename: Carlos
authors.initials: C
authors.lastname: Jaramillo
mesh_terms: D005075:Biological Evolution; D019521:Body Patterning / Q000235:genetics; D017753:Ecosystem; D005580:Fossils; D019684:Magnoliopsida / Q000033:anatomy & histology / Q000145:classification / Q000235:genetics*; D018515:Plant Leaves / Q000033:anatomy & histology / Q000235:genetics