Fossil evidence for low gas exchange capacities for Early Cretaceous angiosperm leaves

20 Pages·2011·1.35 MB·English

Fossil evidence for low gas exchange capacities for Early
Cretaceous angiosperm leaves
Author(s): Taylor S. Feild, Garland R. Upchurch Jr., David S. Chatelet, Timothy
J. Brodribb, Kunsiri C. Grubbs, Marie-Stéphanie Samain, and Stefan Wanke
Source: Paleobiology, 37(2):195-213. 2011.
Published By: The Paleontological Society
DOI: 10.1666/10015.1
URL: http://www.bioone.org/doi/full/10.1666/10015.1
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Paleobiology,37(2),2011,pp.195–213
Fossil evidence for low gas exchange capacities for Early
Cretaceous angiosperm leaves
TaylorS.Feild,GarlandR.UpchurchJr.,DavidS.Chatelet, Timothy J.Brodribb,
Kunsiri C. Grubbs, Marie-Ste´phanie Samain, and Stefan Wanke
Abstract.—The photosynthetic gas exchange capacities of early angiosperms remain enigmatic.
Nevertheless,manyhypothesesaboutthecausesofearlyangiospermsuccessandhowangiosperms
influenced Mesozoic ecosystem function hinge on understanding the maximum capacity for early
angiosperm metabolism. We applied structure-functional analyses of leaf veins and stomatal pore
geometrytodeterminethehydraulicanddiffusivegasexchangecapacitiesofEarlyCretaceousfossil
leaves.AllofthelateAptian–earlyAlbianangiospermsmeasuredpossessedlowveindensityandlow
maximal stomatal pore area, indicating low leaf gas exchange capacities in comparison to modern
ecologicallydominantangiosperms.GasexchangecapacitiesforEarlyCretaceousangiospermswere
equivalentorlowerthanfernsandgymnosperms.FossilleaftaxafromAptiantoPaleocenesediments
previouslyidentifiedasputativestem-lineagestoAustrobaileyalesandChloranthaleshadthesame
gasexchangecapacitiesandpossiblyleafwaterrelationsoftheirlivingrelatives.Ourresultsprovide
fossil evidence for the hypothesis that highleaf gas exchangecapacity is aderived feature of later
angiosperm evolution. In addition, the leaf gas exchange functions of austrobaileyoid and
chloranthoidfossilssupportthehypothesisthatcomparativeresearchonthebiologyoflivingbasal
angiospermlineagesrevealsgenuinesignalsofEarlyCretaceousangiospermecophysiology.
Taylor S. Feild.* School of Biological Sciences, Monash University, Clayton Campus, Australia. E-mail:
[email protected]
David S. Chatelet. Department of Ecology and Evolutionary Biology, University of Tennessee, Knoxville,
Tennessee37996
TimothyJ.Brodribb. DepartmentofPlantScience,UniversityofTasmania,Hobart,Tasmania,Australia
GarlandR.Upchurch. DepartmentofBiology,TexasStateUniversity,SanMarcos,Texas
KunsiriC.Grubbs. DepartmentofBiology,WinthropUniversity,RockHill,SouthCarolina
Marie-Ste´phanieSamain. GhentUniversity,DepartmentofBiology,ResearchGroupSpermatophytes,B-9000
Ghent,Belgium
StefanWanke. TechnischeUniversita¨tDresden,InstitutfurBotanik,01062Dresden,Germany
*Correspondingauthor
Accepted: 29September2010
Introduction (Ko¨rner 1995; Lusk et al. 2003; Brodribb et al.
2007; Boyce et al. 2009; Brodribb and Feild
Evolutionary transitions in the ways orga-
nismsprocessenergyandresourcesrepresent 2010).Theevolutionofhighleafgasexchange
pivotal turning points in the history of life capacity may have stabilized or increased
(Vermeij 1999). In particular, evolutionary gross primary productivity of the vegetation
changes in plant metabolic function cascade under conditions of falling CO2 during the
across a network of planet-wide biogeochem- Cretaceousand/orlaterCenozoic(Volk1989;
ical and hydrological processes. Of the major Robinson 1994; McElwain et al. 2005; Bond
transitions in plant gas exchange function, and Scott 2010). In addition, the functional
diverse lines of evidence point to the emer- prerequisites for constructing leaves capable
gence of highly photosynthetically active ofhighratesofphotosynthesisandfunctional
angiosperm leaves as a transformative shift by-products of such leaves as they transpire
underpinning the assembly of modern eco- and eventually decompose may have elicited
system processes (Boyce et al. 2009; Brodribb enhanced positive feedbacks on vegetation–
and Feild 2010). For example, maximal leaf hydrological cycle interactions, fire frequen-
gas exchange capacities that define today’s cy, nutrient cycles, and weathering and
ecologically dominant angiosperms far out- biomineralization processes that changed
strip those of all known non-angiosperms climates and/or formed new selective inter-
’2011ThePaleontologicalSociety.Allrightsreserved. 0094-8373/11/3702–0002/$1.00
196 TAYLORS.FEILDETAL.
faces for diverse organisms during the Creta- synthesis and transpiration, as they dominat-
ceous(Volk1989;Robinson1994;Martin1995; ed forest canopies and open disturbed zones.
Boyceetal.2009;Tayloretal.2009;Bondand Under this hypothesis, the rise in ecological
Scott 2010; Brodribb and Feild 2010). Howev- abundance of angiosperms evolved synchro-
er, an unresolved issue is when high leaf gas nously with their ability to expand into a
exchange of angiosperm leaves first evolved, range of high photosynthetic capacities pre-
and how this functional transition influenced viously unexplored by other vascular plants
early angiosperm evolution (Wing and Bou- (Brodribb and Feild 2010). However, there is
cher1998;Feildetal.2009;BrodribbandFeild no known fossil evidence supporting the
2010). hypothesis that the earliest angiosperms
Aninfluentialhypothesisisthattheearliest functionedwith lowleafgasexchangepoten-
angiosperms functioned with high photosyn- tial. Indeed, any literal reading of early
thetic gas exchange capacities (Stebbins 1974; angiosperm leaf gas exchange evolution
Doyle and Hickey 1976; Hickey and Doyle based on extant basal angiosperm taxa is
1977;RetallackandDilcher1981,1986;Taylor fraught with uncertainty. Uncertainty exists
and Hickey 1996). Paleoecological interpreta- because these lineages experienced consider-
tions of the ecomorphology and depositional able range contraction over the last 100 Myr,
environments of Early Cretaceous fossil an- they may have become ecologically modified
giosperm leaves suggested that high photo- by later angiosperm evolution or shifts in
synthetic capacity and high rates of weedy global environment, and they may fail to
growth favored initial angiosperm success in sample extinct early angiosperm functional
sun-exposed point-bar zones along fast mov- diversity (Feild et al. 2009; Royer et al. 2010).
ing rivers (Stebbins 1974; Doyle and Hickey Recent discoveries on how leaf vein and
1976; Hickey and Doyle 1977; Retallack and stomatal pore anatomy determine leaf gas
Dilcher 1981, 1986; Taylor and Hickey 1996). exchange capacity offer new potential for
Thus, the evolution of high leaf gas exchange testing hypotheses on early angiosperm leaf
capacity by angiosperms did not spark the gas exchange function in the fossil record
global ecological sweep of the angiosperms. (Brodribb et al. 2007; Boyce et al. 2009;
Such is the case because the earliest angio- Brodribb and Feild 2010; McKown et al.
sperms, while capable of high productivity, 2010). These new approaches are potentially
remained rare and ecologically confined to informativebecausepreviousinferenceswere
early successional zones for nearly 20 Myr basedontraits,includingleafsize,leafshape,
after their first fossil appearance (Hickey and majorveinarchitecture patterns,andestimat-
Doyle 1977; Wing and Boucher 1998; Heim- ed leaf mass per area of fossil leaves, which
hofer et al. 2005). Later evolution of other cannotaccuratelyspecifywhereaspeciesfalls
functions in the reproductive system and/or along a spectrum of low to high leaf gas
mutualisms with animals gave considerable exchange capacity nor specify the hydraulic
lift to angiosperm ecological success and costs of leaf photosynthesis (Doyle and
allowed the export of high leaf gas exchange Hickey 1976; Hickey and Doyle 1977; Retal-
capacity to diverse environments. lack and Dilcher 1981, 1986; Taylor and
By contrast, comparative ecophysiological Hickey 1996; Ackerly and Donoghue 1998;
evidence from extant early diverging angio- Wing and Boucher 1998; Royer et al. 2010).
sperm lineages motivated a hypothesis that The first goal of our investigation was to
the first angiosperms functioned with low explore how venation and stomatal pore
metabolic capacity (Feild et al. 2004, 2009). structure are linked to leaf gas exchange
Low leaf gas exchange capacity was associat- capacity and to use these mechanistic linkag-
ed with the early diversification of angio- es to test previous hypotheses on how early
sperms in low-evaporative-demand, shady angiosperm fossil leaves functioned. The
habitats underneath forest canopies formed focus is on fossil Zone I leaves from the
by gymnosperms and ferns. Later, angio- Potomac Group of North America because
sperms evolved greater capacities for photo- these fossils represent one of the oldest
EARLYANGIOSPERMLEAFGASEXCHANGE 197
known records of early angiosperm leaves, Papua New Guinea, Thailand, United States,
and these fossils formed the conceptual andVietnam.Tenindividualsperspecieswere
cornerstone of the ancestral weed hypothesis sampled. In addition, we studied 18 species,
(Doyle and Hickey 1976; Hickey and Doyle represented by three to ten individuals, in an
1977; Taylor and Hickey 1996). The second outdoor garden collection in Knoxville,
goal was to determine whether gas exchange Tennessee. Plants were watered using a
capacities of fossil leaves related to extant timed drip irrigation system, and all plants
terrestrial basal angiosperm lineages, specifi- receivedfertilizationeverytwotothreeweeks
cally Austrobaileyales and Chloranthales, fell to ensure healthy leaves for physiological
outside the range of extant relatives. The measurements. The species sampled encom-
purposeofthesecomparisonswastoevaluate passed the modern diversity of life form, life
the hypothesis that similar leaf gas exchange zone,andregenerationecologyfoundinextant
capacitieshavebeenconservedinextantbasal terrestrial basal angiosperms (Appendix I,
angiosperm leaves since the Cretaceous. To online) (Feild et al. 2004, 2009). Diverse lines
do so, we determined the relations among of phylogenetic and paleobotanical data
veinandstomatalporeanatomywithleafgas support these lineages as having diverged
exchange capacity across a broad sample of nearthebaseofextantangiospermphylogeny
extant basal angiosperm leaves and used (Jansenetal.2007;Mooreetal.2007;Saarelaet
them to interpret fossil leaf function. al. 2007; Feild et al. 2009; Endress and Doyle
2009).
Methods
To examine intraspecific variation of leaf
Abbreviations.—D , vein density (mm structure in relation to light, we sampled sun
v
mm22); g STOMA, maximum stomatal conduc- and shade leaves of each species when
c
tancetowatervaporcalculatedfromstomatal possible. Sun and shade environments were
pore geometry (mmol H O m22 s21); g VEIN, designatedbyfieldobservations(Keelingand
2 c
maximal stomatal conductance to water va- Phillips2007).Sunleavesweretakenasthose
por calculated from vein density (mmol H O that fully expanded under greater than 70%
2
m22 s21); g , measured maximum stomatal exposure to open sky. Shade leaves were
m
conductance to water vapor (mmol H O taken as those that fully expanded in the
2
m22 s21); P , measured maximum photosyn- forest understory, defined as less than 5%
C
thetic capacity on leaf area basis (mmol CO exposure to open sky. For shade-demanding
2
m22 s21); Y, water potential (MPa); Y , leaf and short-lived pioneer species, only shade
leaf
waterpotential(MPa);Y ,soilwaterpoten- and sun leaves were available for sampling,
soil
tial (MPa); S , stomatal density (number respectively.
D
mm22); S , stomatal guard cell length (mm); Detailsonthelocalities,ages,andproposed
L
S stomatal guard cell width (mm); Pore , systematics of the sampled fossils are sum-
W L
stomatal pore length (mm); Pore , stomatal marized in Appendix II (online). The fossils
D
pore depth (mm); SPA, stomatal pore area at representedsevenlocalitiesspanningapprox-
maximal aperture (m2); VPD, vapor pressure imately 60 Myr (early Aptian to earliest
deficit (kPa). Paleocene). Two groups of fossils were
Extant Species and Fossil Leaf Collections.— focused on (1) leaf fossils from Zone I of the
The comparative investigations of leaf struc- Potomac Group (Doyle and Hickey 1976;
ture–function focused on 87 species of extant Hickey and Doyle 1977) and (2) leaf fossils
basalangiosperms,includingAmborella,Nym- that previous systematic investigations iden-
phaeales,Austrobaileyales,andChloranthales tified as possible stem-lineage relatives to
(Appendix I in the supplementary material extant Austrobaileyales and Chloranthales,
online at http://dx.doi.org/10.1666/10015. referred hereafter as austrobaileyoids and
s1). Most species studied were in natural chloranthoids, respectively (Appendix II, on-
populations in Australia, Costa Rica, China, line). Appendix III (online) presents evidence
Dominican Republic, French Polynesia, Fiji, for stratigraphic ages and systematic place-
Jamaica, New Caledonia,New Zealand, Peru, ments for the fossils measured.
198 TAYLORS.FEILDETAL.
Maximum Photosynthetic Rates and Stomatal sites of water evaporation in the leaf (Brod-
Conductances.—To test how leaf venation and ribb et al. 2007; McKown et al. 2010). Hence,
stomatal pore structure related to photosyn- D definesthehydraulicsupplylimitofwater
v
thetic gas exchange, water vapor and CO vapor exchange that secondarily dictates
2
exchange fluxes were measured with a maximum CO assimilation by leaves (Brod-
2
photosynthesis infrared gas analyzer (LiCOR ribb et al. 2007; Brodribb and Feild 2010).
6400XT, Li-COR Biosciences, Lincoln, NB, D of extant taxa was measured on leaves
v
United States of America). We chose 30 basal cleared using sodium hydroxide, sodium
angiosperm species to obtain a broad sam- hypochlorite, and heat (Hudson et al. 2010).
plingofphylogeneticandecologicaldiversity Veins were stained in safranin and measured
(Appendix I, online) (Feild et al. 2004, 2009). using an upright microscope over approxi-
Stomatal water vapor conductance (g , mately 6 mm2 of area (Axio-Imager, Carl-
m
mmol H O m22 s21) and photosynthetic Zeiss, Germany). Digital images were cap-
2
capacity (P , mmol CO m22 s21) were mea- tured with an AxioCam camera (Carl-Zeiss,
C 2
sured on clear mornings (0900–1130 h) to: (1) Germany) and processed using ImageJ
avoid heterogeneities on leaf gas exchange (http://rsb.info.nih.gov/ij/; NIH, Bethesda,
due to passing clouds, (2) ensure leaves were Maryland) to measure D . D was measured
v v
dry, and (3) ensure that maximal stomatal oncompressionfossilsbytracingveinlengths
opening and light-induction of photosynthe- using ImageJ. Images were obtained with a
sis occurred before midday stomatal closure. digital camera and macro lens (Nikon D300S
Microclimate around leaves during measure- with Nikkor 60 mm lens, Nikon,Japan) or by
mentswascontrolledat2561.5uC,1300mmol digitizing 10.1 3 12.7 cm photographic
quanta m22 s21 photosynthetic photon flux negatives and prints of fossils at 600 dpi.
density (PPFD), vapor pressure deficit (VPD) Three D measurements on each sampled
v
between 0.9 and 1.1 kPa, and 380 6 5 mL L21 fossilforeachspeciesweretaken.Onlyfossils
CO . The light intensities provided saturated with well-preserved venation were sampled
2
photosynthesis but did not induce photoinhi- (Fig. 1). Details on how fossils were selected
bition(Feildetal.2004).Measurementsunder for measurements are provided in Appendix
these optimal conditions served as the pho- III (online).
tosynthetic and stomatal conductance maxi- We used a published model to reconstruct
ma attainable for a species. Photosynthetic photosynthetic capacity (P ) and maximum
C
gas exchange capacity for each species was stomatal conductance to water vapor of fossil
based on a sample of five undamaged and leaves from D (see Brodribb et al. 2007;
v
fully expanded leaves from each of five BrodribbandFeild2010).Themodelassumes
individuals. that under non-limiting conditions of soil
Vein Density (D ) of Extant and Fossil Leaves water availability, maximum leaf hydraulic
v
to Calculate Leaf Gas Exchange Capacity.—Vein conductance(K )canbecalculatedfromthe
leaf
density is the length of veins ramifying in a distance water must flow from the vein
given amount of leaf area (mm mm22). Data terminals to the sites of evaporation (d ;
m
from living plants demonstrated that photo- Brodribb et al. 2007). By knowing K one
leaf
synthetic capacity and stomatal conductance cancalculatemaximumstomatalconductance
to water vapor can be predicted from D to water vapor from vein density (g VEIN),
v c
(Brodribb et al. 2007; Boyce et al. 2009; which provides a basis for calculating maxi-
Brodribb and Feild 2010; McKown et al. mum leaf P . To emphasize the impact of
C
2010). D is a major determinant of leaf vein evolution on gas exchange, we used
v
CO /H O exchange because water transport fixed concentrations of CO and O (current
2 2 2 2
andphotosyntheticgasexchangearecoupled ambientconcentrations)toreconstruct photo-
(Sperry 2003). Leaf hydraulic capacity is set synthetic capacity. Our emphasis was on
by vein structure because increased vein diffusive-hydrodynamic constraints to leaf
branching brings xylem tissues that are gas exchange rather than biochemical con-
specialized for water transport closer to the straints upon photosynthesis such as maxi-
EARLYANGIOSPERMLEAFGASEXCHANGE 199
(140mm)endsfortherangeofvein-epidermal
thicknessesformesophyticangiospermleaves
(Brodribb et al. 2007; D. S. Chatelet and T. S.
Feild unpublisheddata2009). This procedure
was applied because leaf thickness cannot be
measured in compression fossils.
Measurements of Stomatal Apparatus Geome-
tryinLivingandFossilLeavesandCalculationof
Maximum Stomatal Conductance to Water Va-
por.—An accepted equation to calculate the
maximum diffusive conductance of the sto-
mata (g ) was used (Parlange and Wag-
stoma
goner 1970; Van Gardenigen et al. 1989;
Kaiser 2009):
g ~1=f½ðd=p(cid:1)a(cid:1)bÞ(cid:5)
stoma
zlnð4a=bÞ=p(cid:1)a(cid:5)=D(cid:1)Ng ð1Þ
where a 5 guard cell pore length/2, b 5
guard cell pore width/2, d 5 guard cell pore
depth;D5diffusivityofwatervaporinair,N
5guardcelldensity.Measurementsofdwere
made on cross-sections of FAA fixed leaves
embedded in plastic resin (JB-4, Polysciences
FIGURE 1. Comparison of fossil preservation of minor
veinsinFicophyllumcrassinerve(USNM192353),aputative Inc., Warrington, Pennsylvania), and sec-
austrobaileyoid stem-lineage with a vein density of tioned at 5-mm thickness on a rotary micro-
3.52 mm mm22 (A) and a sun leaf of extant Amborella
trichopodawithveindensityof3.77mmmm22(B).Scale tome (RM2245, Leica Microsystems, Ger-
bars,1.5mm. many). From each species, 20 guard cell
pores were imaged at 4003 and measured
mum electron transport and carboxylation with ImageJ. a and N were determined on
rates. These enzymatic processes are not macerated cuticles stained in safranin. Cuti-
preserved in fossils. Instead of attempting a cles were macerated using acetic acid and
highly uncertain reconstruction leaf gas ex- hydrogenperoxide.Wemeasuredfive4-mm2
changecapacitiesacrossanuncertainrangeof sheets of cuticle for N at 2003. Geometric
cross-varying Cretaceous ambient CO and landmarks for assessing guard cell pore
2
O concentrations (McElwain et al. 2005; length and depth followed previous criteria
2
Fletcher et al. 2008; Barclay et al. 2010; (Lawson et al. 1998). We assumed that
Glasspool and Scott 2010; Royer 2010), atmo- maximum guard cell width (b) was approx-
spheric gas concentrations were constrained imated by one-third of the guard cell pore
to allow a comparison of gas exchange length (Osborne et al. 2004). Using the length
capacities under standard conditions. Param- and width of the stomatal pore, and approx-
eters for VPD and water potential gradient imating the pore as an ellipse, we calculated
acrosstheleafweresetat1kPaand20.5MPa, stomatalporeareaatmaximalaperture(SPA,
respectively. These physiological conditions m2).
are associated with maximal gas exchange The stomata of extant basal angiosperms
capacity for terrestrial basal angiosperms and the fossil stomata investigated possessed
(Feild et al. 2009). Leaf thickness affects prominent peristomatal rims over the guard
estimates of maximum CO /H O gas ex- cell pores (Upchurch 1984a,b, 1995). Vesti-
2 2
change by varying d (Brodribb et al. 2007). bules could lengthen the diffusional path
m
Thus, leaf gas exchange capacities were length from the stomatal pore to the bulk
calculated at lower (70 mm) and upper phase. As a result, the accuracy of equation
200 TAYLORS.FEILDETAL.
(1) will be affected. The calculated maximum calculate stomatal pore depth. Stomatal ves-
conductance of the stomatal pore complex tibule depth of fossil leaf cuticles was ap-
(g STOMA mmol m22 s21) to water vapor is: proximated in the calculations as the average
c
g STOMA~1=ðr zr Þ ð2Þ depth (5.48 6 2.6 SD mm, n 5 113) found
c stoma vestibule across all of the extant angiosperms sampled.
whererstomaistheresistanceofthestomaand Phylogenetic Analyses.—To compare leaf
rvestibule is the resistance of the vestibule atop fossils to reconstructed nodes of trait evolu-
a stoma. The resistances of each term were tion across extant angiosperm phylogeny, we
calculated by substituting the length, width, assembled a composite phylogenetic tree by
and depth of the vestibule or the stoma into grafting species-level trees onto two well-
equation (1). Vestibule depth was measured supported backbone topologies using Mes-
at 4003 from the same cross-sections as quite version 2.6 (Maddison and Maddison
described above, and vestibule length and 2008). The species-level phylogenies included
widthdeterminedfromcuticlemacerationsat the following clades: Chloranthales, Illicia-
4003. To improve the accuracy of vestibule ceae, Schisandraceae, Trimenia, and Nym-
geometrymeasurements,weobservedstoma- phaeales (Jansen et al. 2007; Moore et al.
tal vestibules of some species with scanning 2007; Saarela et al. 2007; Endress and Doyle
electron microscopy on critically dried leaves 2009; Feild et al. 2009). Published consensus
(observations not shown). An approximation treeswereused,andthepolytomiestreatedas
of the cuticular vestibule and stomatal pore soft. The composite tree was pruned to leave
resistors as additive in series makes three the species sampled.
assumptions: (1) the architectures can be Ancestral state values for functional traits
approximated as connected pores of simple were reconstructed using weighted squared-
cylindrical geometry; (2) diffusion within the change parsimony over species terminals
two components does not involve exchange using Mesquite. This method minimizes the
through the walls of each; and (3) other sum of squared change along all branches of
resistors to water vapor diffusion, such as the tree to reconstruct the values of internal
internalcuticleandintercellularconductance, nodes from the trait values of the species’
arenotsignificant.Assumptionsoneandtwo terminals (Maddison and Maddison 2008).
are reasonable because vestibules consist of a For species with sun and shade leaves,
thick cuticle that is likely to be impermeable ancestral state trait reconstructions were
to water vapor. Assumption three is valid performed using the maximum trait values.
because maximum conductance was calculat- Then, a separate analysis was run using the
ed (Kaiser 2009). average of sun and shade values to define a
On fossil leaf cuticles, measurements were species trait value. In cases where the phylo-
made on previous cuticular preparations genetic relations were uncertain, Mesquite
from four localities (Appendix III, online) was used to randomly resolve phylogenetic
(Upchurch 1984a,b, 1995; Upchurch and relations within clades (100 times), and the
Dilcher 1990). Guard cell pore length as well trait values for nodes within the tree were
as the length and width of the vestibule averaged (Appendix IV, online).
aperture were measured at 4003. Guard cell
Results
width was approximated as one-third of the
pore length. The taxon means are from ten Leaf Venation Density and Gas Exchange
stomata. Stomata pore or vestibule depths Capacity in Extant and Extinct Leaves.—Leaf
could not bemeasured on the prepared fossil photosynthetic capacity (P ) and maximum
C
cuticles. However, stomatal pore depth cor- stomatal conductance to water vapor (g )
m
related with stomatal pore length (n 5 113; r2 increasedlinearlywithincreasingveindensity
5 0.87) across a broad range of stomatal size (D ; Fig. 2A,B). Nymphaeales, however, rep-
v
(i.e., 18 mm to ,100 mm in maximum length resented exceptions with high P and g but
C m
[D.S.Chateletunpublisheddata2010]).Thus, with comparatively low D (Fig. 2A). Across
v
stomatal length measurements were used to allspecies,P increasedwithg (Fig. 2C).
C m
EARLYANGIOSPERMLEAFGASEXCHANGE 201
Vein densities of extant Chloranthales
ranged from 1.62 mm mm22 for shade leaves
of Hedyosmum orientale to 5.06 mm mm22 for
sun leaves of Ascarina maheswari. Extant
Austrobaileyales D ranged from 1.94 mm
v
mm22 for shade leaves of Austrobaileya scan-
dens to 6.35 mm mm22 for sun leaves of
Kadsuraheteroclita.MeanD ofallZoneIfossil
v
species measured from the Potomac Group
extended over approximately 50% of the
extant terrestrial basal angiosperm range,
varying from 2.8 mm mm22 6 0.36 SD in
Proteaephyllum reniforme to 4.1 mm mm22 6
0.18SDin‘‘Sapindopsis’’elliptica.Themeanfor
Zone I leaves (3.38 mm mm22 6 0.35 SD; n 5
12 species) did not differ significantly from
the extant terrestrial basal angiosperm mean
(results not shown). Zone I fossil leaves also
exhibitedD valuessimilartothereconstruct-
v
ed D trait values over the base of extant
v
angiosperm phylogeny (Fig. 2; Supplemental
Table 1 and Fig. S1).
Vein densities for chloranthoids averaged
3.58 mm mm22 6 0.62 SD (n 5 11 species;
Fig. 3)andrangedfrom2.47mmmm2260.08
SD in Reynoldsiophyllum nebrascense (Rose
Creek, Late Albian) to 4.61 mm mm22 6 0.46
in Crassidenticulum cracendentis (Courtland,
Cenomanian). D of austrobaileyoid leaves
v
averaged 3.61 mm mm22 6 0.46 SD (n 5 5)
ranged from 3.12 mm mm22 6 0.21 SD in the
Fredericksburg sample of Eucalyptophyllum
oblongifolium (Upper Zone I, Potomac Group)
to 4.36 mm mm22 6 0.32 SD in Longstrethia
varidentata (latest Albian, Rose Creek; Fig. 3).
FIGURE2. Coordinationofstructure-functionrelationsof
thevenationsystemandleafgasexchangecapacityacross In addition, mean vein densities for chlor-
35extantbasalangiospermspecies.A,Relationbetween anthoids and austrobaileyoids did not differ
leafphotosyntheticcapacity(PC,mmolCO2m22s21)and significantly (results not shown). No trend in
leaf vein density (D , mm mm22; excluding Nym-
phaeales, y 5 2.444x,vr2 5 0.69). B, Relation between Dv across the sampled fossil species was
measured maximum stomatal conductance to water observed through time (Fig. 3). Finally, max-
vapor (gm mmol H2O m22 s21) and Dv (y 5 36.1x, r2 5 imum CO exchange and stomatal conduc-
0.62). C. Relation of P to g (with Nymphaeales, y 5 2
0.059x,r250.79;excludCingNymmphaeales,y50.065x,r25 tances to water vapor calculated from vein
0.42). Standardized measurement environmental condi- density (g VEIN) for chloranthoid and austro-
c
tions are described in ‘‘Methods.’’ Symbols denote baileyoid species through time fell within the
growth forms for A–C: #, shrubs and trees; e lianas;
%,sub-shrubs;n,terrestrialherbs;,,floatingleavesof 10th and 90th percentiles’ extant species and
aquatic Nymphaeales herbs. Points are the averages of reconstructedancestralnodetraitvaluesover
threeobservationsmadeonfivetotenleavesperspecies, the base of extant phylogeny (Fig. 3).
anderrorbarsdenotestandarddeviationaboutthemean.
Stomatal Pore Anatomy and Gas Exchange
ThedottedlinesinAandBrefertomodeledphotosyn-
thetic capacities and maximum stomatal conductances Capacity in Leaves of Extant and Extinct
basedonveindensityat140and70mmleafthicknesses. Species.—Stomatal size measured on fossil
SpeciesinformationanddataareprovidedinAppendix
I(online). cuticles encompassed approximately 60% of
202 TAYLORS.FEILDETAL.
FIGURE3. Leafveindensity(Dv,mmmm22)andleafCO2/H2Ogasexchangecapacities(photosyntheticcapacity,PC,
mmol CO m22 s21 and stomatal conductance to water vapor, gVEIN) calculated from a coupled venation hydraulic-
2 c
photosyntheticmodelwithCO concentrationsetat380mLL21CO forearlyangiospermleaffossils.Grayregionson
2 2
thetimescalerefertothestratigraphicagerangeforthefossillocalities(ZoneIPotomacGroup[lowerupperAptian],
BraunRanchandRoseCreek[upperAlbian],Courtland[Cenomanian]).P andgVEINwereparameterizedusingtwo
C c
vein-to-epidermalhydraulicdistancesfrom70mmto140mmtoproduceaspanofpredictedvaluesencompassingthe
likelymorphologicalvariabilityinthethicknessesofangiospermleaves.ThetwopointsforP andgVEINforeachfossil
N C c
leaf taxon reflect these values. Symbols refer to previously hypothesized systematic affinities of fossil leaves: stem-
lineageaustrobaileyoids(&)andchloranthoids( ),andfossilleavesofuncertainsystematicplacement(e).Numbers
refer to the species: 1, Proteaephyllum reniforme; 2, Celastrophyllum latifolium; 3, Eucalyptophyllum oblongifolium
Fredericksburg sample; 4, Celastrophyllum sp. Drewry’s Bluff; 5, Drewry’s Bluff Leaf Type #1 (Moutonia); 6,
Celastrophyllumsp.(C.obovatum);7,Vitiphyllummultifidum;8,Quercophyllumtenunerve;9,Eucalyptophyllumoblongifolium
Drewry’s Bluff; 10, Ficophyllum crassinerve; 11, Rogersia angustifolia; 12, cf. Ficophyllum; 13, Sapindopsis elliptica; 14,
Reynoldsiophyllum nebrascense; 15, Crassidenticulum landiase; 16, Crassidenticulum decurrens Braun Ranch samples; 17,
Longstrethiaaspera;18,DennsinervumkauliiRoseCreeksamples;19,CrassidenticulumdecurrensRoseCreeksamples;20,
Longstrethiavaridentata;21,DensinervumkauliiCourtlandsample;22,Crassidenticulumcracendtis.Thedashedlinesacross
thethreefossildatapanelsindicatethe10thand90thpercentilesofgSTOMAacrosstheextantspecies.Theboxplots
c
depictthevariationwithinextantcladeswiththebottomandtopoftheboxindicatingthe25thand75thpercentiles,
respectively,thetwowhiskersthe10thand90thpercentiles,respectively,andthehorizontallinewithinthebox,the
medianvalue.Symbolsbeyondthewhiskersareoutliers.
thestomatalsizeanddensityrangeforextant 53.57; n 5 104; r2 5 0.8). Similarly, g STOMA
c
AustrobaileyalesandChloranthales(Fig. 4A). increased with greater SPA in fossil cuticles
Across extant terrestrial basal angiosperms [(g STOMA) 5 34.36 (SPA) 2 75.73, n 5 10, r2 5
c
(excluding the sun leaves of Kadsura coccinea 0.82; Fig. 4B].
that possessed unusually large stomata), Maximumcalculatedstomatalconductance
maximum calculated stomatal conductance from pore geometry ranged from 105 mmol
from pore geometry (g STOMA) increased with H O m22 s21 in the austrobaileyoid Ficophyl-
c 2
maximal stomatal pore area (SPA) for diffu- lum crassinerve to 327 mmol H O m22 s21 in
2
siveexchange(Fig. 4B;g STOMA530.15SPA2 Celastrophyllum latifolium (Figs. 4B, 5). Over
c
EARLYANGIOSPERMLEAFGASEXCHANGE 203
thefossilspeciessampled,notime-dependent
pattern in g STOMA was found (Fig. 5). Maxi-
c
mum calculated stomatal conductance from
pore geometry of fossil cuticles nested within
the range of extant species (Figs. 4B, 5).
Consistent with the result that SPA drives
muchofg STOMA,watervaporconductancesof
c
stomatal vestibules were three to six times
greaterthancalculatedstomatalconductances
ofjustthestomataporeacrossalloftheextant
and extinct leaves sampled (Fig. 4C).
g STOMA was related positively to maximum
c
stomatal conductance (g ) as measured by
m
leaf gas exchange analysis across terrestrial
basalangiospermspecies(g STOMA51.632(g )
c m
+ 9.04, n 5 30; r2 5 0.64; Fig. 6A). g STOMA
c
values, however, were greater than g in
m
terrestrialbasal angiosperm species (Fig. 6A).
By comparison, maximal stomatal conduc-
tance to water vapor calculated from vein
density (g VEIN) yielded estimates closer to
c
measured stomatal conductance maxima on
extantleaves(g VEIN50.514(g )257.56,n5
c m
30; r2 5 0.60; Fig. 6A). Nymphaeales leaves,
however, exhibited much greater g in
m
comparison to g STOMA and g VEIN (Fig. 5B). D
c c v
and SPA were unrelated, as were g STOMA and
c
g VEIN (Fig. 6C).
c
Discussion
FossilEvidenceforLowGasExchangeCapacity
in Early Angiosperm Leaves.—All Zone I
Potomac Group fossil angiosperm leaves
measured had structural traits related to
lower gas exchange capacities, in contrast to
modern ecologically dominant angiosperms.
D values of late Aptian–early Albian fossil
v
angiosperm leaves occurred in the narrow
rangeoflowdensitiesfoundacross fernsand
gymnosperm clades that dominated plant
communities during the Early Cretaceous
FIGURE 4. Relations among stomatal morphology and
maximum stomatal conductance to water vapor (mmol (Lupia et al. 1999; Boyce et al. 2009). The
H2O m22 s21) for extant basal angiosperms and fossil uniform pattern of low vein densities across
cuticles. A, Stomatal size (mm2) versus stomata density
ZoneIangiospermfossilleavescontrastswith
(numbermm22ofleafarea)forextantbasalangiosperms
and fossil cuticles. B, Maximal stomatal conductance to the widespread dominance of high vein
water vapor calculated from stomatal pore dimensions
(gSTOMA,mmolHOm22s21)versusstomatalporesurface
c 2
area(m23106)acrossextantandextinctfossilcuticles.C, r
Calculated maximal water vapor conductance of the
N
stomatalvestibule(gVESTIBULE,mmolHOm22s21)versus (includingAmborella,Austrobaileyales,Chloranthales,and
c 2
gSTOMAforfossilandextantcuticles.Thedashedlineshows Nymphaeales); and , fossil cuticles (see Appendix II,
c
theone-to-onerelation.#,extantbasalangiospermleaves online,forthetaxasampled).