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The emergence of core : rspb.royalsocietypublishing.org new floral evidence from the earliest Late Cretaceous

Else Marie Friis1, Kaj Raunsgaard Pedersen2 and Peter R. Crane3,4 Research 1Department of Palaeobiology, Swedish Museum of Natural History, Stockholm, Sweden 2 Cite this article: Friis EM, Pedersen KR, Crane Department of Geoscience, University of Aarhus, Aarhus, Denmark 3Yale School of Forestry and Environmental Studies, New Haven, CT, USA PR. 2016 The emergence of core eudicots: 4Oak Spring Garden Foundation, Upperville, VA, USA new floral evidence from the earliest EMF, 0000-0003-2936-2761 Late Cretaceous. Proc. R. Soc. B 283: 20161325. http://dx.doi.org/10.1098/rspb.2016.1325 Eudicots, the most diverse of the three major clades of living angiosperms, are first recognized in the latest Barremian–earliest Aptian. All Early Cretaceous forms appear to be related to species-poor lineages that diverged before the rise of core eudicots, which today comprise more than 70% of angiosperm Received: 11 June 2016 species. Here, we report the discovery of a well-preserved , Caliciflora Accepted: 14 November 2016 mauldinensis, from the earliest Late Cretaceous, with unequivocal core eudicot features, including five sepals, five petals and two whorls of stamens borne on the rim of a floral cup containing three free carpels. Pollen is tricolporate. Carpels mature into follicular fruitlets. This character combination suggests a phylogenetic position among , but more specific assignment is Subject Areas: precluded by complex patterns of character evolution among the very large evolution, palaeontology, structural biology number of potentially relevant extant taxa. The whorled floral organization is consistent with ideas that this stable pattern evolved early and was a pre- Keywords: requisite for more integrated patterns of floral architecture that evolved fossil flower, rosids, asterid, SRXTM, later. However, limited floral synorganization in Caliciflora and all earlier synchrotron X-ray microtomography, eudicot recognized so far, calls into question hypotheses that substan- tial diversification of core eudicots had already occurred by the end of the tricolporate pollen Early Cretaceous.

Author for correspondence: Else Marie Friis 1. Introduction e-mail: [email protected] Hypotheses of relationships among living angiosperms recognize a species-poor basal grade, within which are embedded three major clades; eumagnoliids, mono- cots and eudicots. Soon after their first appearance in the fossil record about 135 Ma, Early Cretaceous angiosperms include diverse extinct taxa related to basal grade angiosperms (Austrobaileyales, Chloranthaceae and Nymphaeales), certain eumagnoliids (Laurales, Magnoliales and Piperales), early monocots (Alismatales) and basal grade eudicots (e.g. [1–4]). These Early Cretaceous assem- blages contrast markedly with Late Cretaceous angiosperm assemblages that are dominated by fossils related to lineages of core eudicots. Core eudicots comprise more than 70% of living angiosperm species and both major clades within the group are well represented in the Late Cretaceous. Late Cretaceous rosids include a rich record of early Fagales (e.g. [5]) as well as diverse fossils related to other clades [1]. Late Cretaceous asterids include many taxa related to extant Cornales and Ericales (e.g. [1,6–9]), the two earliest diverging lineages of the group. The transition from Early Cretaceous floras, dominated by basal grade lineages of angiosperms, eumagnoliids and early eudicots, to Late Cretaceous floras dominated by core eudicots, occurred sometime between the mid-Albian and the Turonian–Santonian. Here, we describe a new flower from the earliest Late Electronic supplementary material is available Cretaceous (earliest Cenomanian) of eastern North America with distinctive fea- online at https://dx.doi.org/10.6084/m9.fig- tures of core eudicots. Together with an unnamed fossil flower of approximately share.c.3588719. & 2016 The Authors. Published by the Royal Society under the terms of the Creative Commons Attribution License http://creativecommons.org/licenses/by/4.0/, which permits unrestricted use, provided the original author and source are credited. Downloaded from http://rspb.royalsocietypublishing.org/ on January 18, 2017

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Figure 1. SEM images of flower bud of Caliciflora mauldinensis gen. et sp. nov. from the Late Cretaceous (earliest Cenomanian) Mauldin Mountain locality, MD, USA; holotype (PP53985; sample Mauldin Mountain 116). (a–c) Lateral (a) and apical (b,c) views of stalked flower bud showing subtending (br), two prophylls (pp), revolute-valvate sepals (s) and petals (p). (d) Detail of petal surface showing dense indumentum of interlocking stellate hairs. (e) Detail of sepal surface showing a stellate hair. similar age [10], this new discovery provides the earliest direct After SRXTM the flower bud was remounted on an alu- evidence of floral structure in early core eudicots and has impli- minium stub for SEM scanning electron microscopy (SEM), cations for understanding the evolution of floral structure sputter coated with gold, and examined using a Hitachi S-4300 within this hyperdiverse clade of extant angiosperms. Field Emission Scanning Electron Microscope at 2 kV. The four other specimens were also prepared for SEM in the same way. 2. Material and methods 3. Results The fossil material described here consists of one complete flower bud, one anthetic flower and three fragments of post-anthetic Angiospermae flowers, one of which has the remains of a stamen. All five speci- Core eudicots mens were recovered from samples of Potomac Group sediments . Caliciflora gen. nov. (Mauldin Mountain samples 022, 116, 117) collected from the Type species designated here. Caliciflora mauldiniensis Elk Neck Beds at the Mauldin Mountain locality on the Elk Neck sp. nov. 00 00 Peninsula, northeastern Maryland, USA (39829’15 N, 75859’44 Generic diagnosis. Flower small, sessile, with an associ- W). The Elk Neck Beds of earliest Cenomanian age have also ated bract and two prophylls borne on a stout stalk. yielded inflorescences and flowers of Mauldinia mirabilis Drinnan, Staminate and pistillate organs in the same flower. Floral cup P. R. Crane, E. M. Friis & K. R. Pedersen [11], which is related to distinct. Perianth with five sepals and five petals borne on extant Lauraceae, fruits of Couperites mauldinensis K. R. Pedersen, P. R. Crane & E. M. Friis [12], which are of uncertain affinity the rim of the floral cup. Sepals free, thick, with broad base among early diverging angiosperms, and pistillate and stami- and acute apex; sepal aestivation revolute-valvate. Petals free, nate flowers of Spanomera mauldinensis Drinnan, P. R. Crane, broadening distally from a narrower base, keeled, with a thin E. M. Friis & K. R. Pedersen [13], which are related to extant Bux- lamina and median rib; petal aestivation open below, quincun- ales. For further details on the geology and age of the Mauldin cial above. Indumentum dense on the outer surfaces of the Mountain assemblage, see [11]. floral cup, sepals and petals composed of interlocking stellate The specimens are coalified and were treated following hairs. Stamens in two whorls, minute with dithecate, tetraspor- standard methods for Cretaceous mesofossils [1]. The flower bud angiate and dorsifixed anthers. Pollen minute, tricolporate, was mounted on a brass stub for synchrotron radiation X-ray psilate. Orbicules present. Gynoecium trimerous with three microtomography (SRXTM) at the Tomcat beamline of the Swiss free carpels borne on the inside of the floral cup. Light Source, Paul Scherrer Institute, Switzerland [14]. It was Specific diagnosis. As for the genus. measured using a 20 objective with isotopic pixel size of 0.325 mm at 10 keV using a sCMOS detector and a 20 mm thick Etymology. Generic name from calice (Latin for cup) and LAG:Ce scintillator screen and the specimen was vertically stacked flos (Latin for flower) and specific name from the Mauldin (for more details on the technique, see [15]). Data derived from the Mountain locality where the fossils were collected. SRXTM [16] were reconstructed and imaged using AVIZO (v. 6.3, Holotype designated here. PP53985 (sample Mauldin 7.1, 9.0.1, 9.1.1) software for computed tomography. Mountain 116); figure 1–4d,h. Downloaded from http://rspb.royalsocietypublishing.org/ on January 18, 2017

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Figure 2. SRXTM volume renderings of flower bud of Caliciflora mauldinensis gen. et sp. nov. from the Late Cretaceous (earliest Cenomanian) Mauldin Mountain locality, MD, USA; holotype (PP53985; sample Mauldin Mountain 116). (a) Volume rendering of apical view of entire floral bud showing subtending bract (br), prophylls (pp) and five revolute-valvate sepals (s). (b–g) Transverse views of flower bud at different levels from apex (b) to middle of floral cup (g) showing subtending bract (br), two prophylls (pp), five revolute-valvate sepals (s), five petals (p), three antesepalous stamens (arrowheads), five antepetalous stamens (asterisks) and three carpels; note three closely appressed stigmas (c) and the free ventral margins of carpels below the stigmatic region ( f,g)(a, not cut; b, section at level of orthoslice xy0766; c, section at level of orthoslice xy1756; d, section at level of orthoslice xy1916; e, section at level of orthoslice xy2044; f, section at level of orthoslice xy2233; g, section at level of orthoslice xy2427). (h,i) Longitudinal views of flower bud in median section showing floral cup surrounding the carpels and with antesepalous stamens (arrowheads) and antepetalous stamens (asterisks) on the rim of the floral cup (h, section at level of orthoslice xz1112; i, section at level of orthoslice yz0983).

Paratypes. PP34773 (sample Mauldin Mountain 116), fragments preserved post-anthesis (PP54159–PP54161). The PP54159 (sample Mauldin Mountain 117), PP54160, specimens are linked mainly by their identical stellate tri- PP54161 (sample Mauldin Mountain 022). chomes, similarities in the shape of the sepals, the presence Type locality. West of Mauldin Mountain, Elk Neck of a floral cup and identical pollen grains in situ in specimens Peninsula, MD, USA, (39829’1500 N, 75859’4400 W). PP34773 and PP54161 and on the surface in PP54160. Sepals Type horizon and age. Elk Neck Beds, Potomac Group; and petals are well-developed in the floral bud indicating that Late Cretaceous (Early Cenomanian; lowermost palynological it was close to anthesis when fossilized. Zone III). The floral bud (figure 1–4d,h; electronic supplementary Description. The taxon is based on a single flower bud material, 1), the anthetic flower (figure 4a) and one of the (PP53985), one anthetic flower (PP34773) and three flower post-anthetic flower fragments have both carpels and Downloaded from http://rspb.royalsocietypublishing.org/ on January 18, 2017

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Figure 3. SRXTM reconstructions (orthoslices) of flower bud of Caliciflora mauldinensis gen. et sp. nov. flower from the Late Cretaceous (earliest Cenomanian) Mauldin Mountain locality, MD, USA; holotype (PP53985; sample Mauldin Mountain 116). (a) Detail of perianth in transverse section below the apex of the flower bud showing the thicker sepals with a layer of thin-walled hypodermal cells (asterisks), thin lamina of a folded petal with an almost glabrous inner surface and a dense indumentum on the outer surface; note the especially clear stellate hair (arrowhead) on the keel of the outer surface of the petal (orthoslice xy1200). (b) Detail of perianth in transverse section close to the apex of the floral bud showing folded petal with dense indumentum on the outer surface and scattered hairs on the inner surface (arrowhead) (orthoslice xy1050). (c) Longitudinal sections of sepals (s) and petals (p) showing dense indumentum; note sepals with dense, infilled cells towards the outside, and a layer of thin-walled hypodermal cells (asterisks) (orthoslice xz1485). (d) Detail of perianth and androecium in transverse section close to the rim of the floral cup showing sepals (s) with dense indumentum of stellate hairs on the outer surface and sepal margins (arrowhead), narrow base of petal (p, arrow) and an antepetalous anther (an, arrow) (orthoslice xy1900). (e) Detail of sepal (s) showing glabrous inner surface and dense indumentum of stellate hairs on the outer surface and sepal margins; note thin-walled cells on the inner surface of the sepal (asterisk) (orthoslice xy1950).

stamens. However, the bud is unusual in having carpels that bases and acute apices. The post-anthetic flower fragments appear partly open, with no signs of ovule initiation. It is show that the sepals are persistent. On their ventral surface, therefore possible that the flower was functionally unisexual, the sepals have a conspicuous hypodermis of thin-walled, and that only stamens developed to maturity. Alternatively, empty cells. This hypodermis is two to several cell layers ovule development may have been interrupted by insect thick in the median-basal region, but only one cell layer thick damage. There are burrows in the bud and an insect larva distally and along the sepal margins (figure 3a,c–e). Hypoder- is present between the bract and sepals. mal cells on the dorsal surface of the sepals are distinctly The flower and the associated bract and two prophylls are different with amorphous contents that obscure the anatomical borne on a short, stout stalk (figures 1a and 2h,i). There is no details of the sepal lamina, including the number of vascular other information about how the flower was borne on the bundles. The amorphous contents may indicate that these . The flower bud is about 1.32 mm long and 0.75 mm in cells were mucilaginous or perhaps tanniferous (figure 3a,c–e). diameter, and has both staminate and pistillate organs preserved. Petals are present in the floral bud and in the specimen pre- There is also a distinct floral cup, about 0.17 mm deep and about served at anthesis, but not in the post-anthetic specimens. 0.3 mm in diameter. The anthetic flower (figure 4a)issimilarin Petals do not have a distinct claw, but broaden distally from size, about 1.2 mm in diameter, somewhat compressed and a narrow base into a broad, keeled lamina with a prominent with fully formed staminate and pistillate organs. median rib and a single vascular bundle (figure 4d). Petals The perianth is differentiated into an outer whorl of sepals are folded longitudinally over the midrib and at least the and an inner whorl of petals. Sepal aestivation is revolute- inner petals appear conduplicate (figure 3a,b). The petals are valvate (figures 1a–c,2a–f and 4a) and sepals have broad narrow at the base, and widely separated by large interspaces. Downloaded from http://rspb.royalsocietypublishing.org/ on January 18, 2017

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Figure 4. SEM (a–c, e–g, i–l ) and SRXTM (d, h) images of carpels, ovules/seeds, stamens and pollen of Caliciflora mauldinensis gen. et sp. nov. from the Late Creta- ceous (earliest Cenomanian) Mauldin Mountain locality, MD, USA. (a) Anthetic specimen in apical view showing five sepals, remnants of a keeled petal (arrow), six stamens and three carpels (PP34773, sample Mauldin Mountain 116). (b,c) Fragment of post-anthetic specimen in apical view (b) showing remains of three carpels (ca) and three elongated, anatropous, reticulate ovules/seeds (arrowhead) in the most complete carpel; note also remains of the slightly bulging margins of the two other carpels (arrows) (PP54159; sample Mauldin Mountain 117). (d) Surface rendering of adaxial surface of petal from specimen in figures 1–3 showing the gradual expansion of the thin petal lamina from the base and the distinct groove, which corresponds to the keel on the abaxial surface (PP53985; sample Mauldin Mountain 116). (e,f) Fragment of post-anthetic specimen showing the dense indumentum of stellate hairs on the surface of the sepals (e) and scattered on the papillate surface of the carpel (f) (PP54160; sample Mauldin Mountain 022). (g) Detail of the apical portion of an anther from anthetic flower in (a) showing numerous pollen grains in situ (PP34773, sample Mauldin Mountain 116). (h) Volume rendering of the floral bud cut at level of orthoslice xy1905 to show stamens with filament attached to dorsal side of the anthers (arrow heads) (PP53985; sample Mauldin Mountain 116). (i–l ) Pollen grains are psilate to weakly regulate with three long colpi that reach almost to the pole; a narrow raised area (bridge) is seen at the middle of the colpi (arrows): note tiny, spherical orbicules (arrowheads) with a central depression adhering to the inner surface of the thecae and pollen grains (PP54161; sample Mauldin Mountain 022).

Petal aestivation is open at the base (figures 2d and 3d), but short in both of the more mature specimens (PP34773, quincuncial above (figures 2b,c and 3a,b). PP54161). In the floral bud, the three stamens that are opposite The androecium consists of two whorls of stamens. the sepals (antesepalous) are borne on the rim of the floral cup. Anthers are almost sessile in the bud and filaments are also Five stamens opposite the petals (antepetalous) are borne Downloaded from http://rspb.royalsocietypublishing.org/ on January 18, 2017

slightly below this level (figure 2c–e,h,i). Anthers are tetra- molecular data (e.g. [17–24]). A further problem is that floral 6 sporangiate, dithecal and dorsifixed (figure 4a,h). They are morphology among rosids exhibits considerable variation, and rspb.royalsocietypublishing.org very small, up to about 0.15 mm long in the bud and about sometimes there are strong similarities in unusual features 0.4 mm long in the anthetic flower. Pollen grains are not between taxa that are widely separated phylogenetically (e.g. obvious in the flower bud. Anthers in two of the more flowers of (, : fabids) mature specimens have abundant minute pollen grains, and (, : COM)) [25]. These about 10 mm long and 5 mm in equatorial diameter. Grains difficulties, coupled with uncertainties over deep relationships are prolate-rhomboidal in equatorial view (figure 4i,k,l ) and in the rosids, complicate characterization of major clades using semi-angular in polar view (figure 4j). Pollen grains have flower morphology [22,26–28] and there are no recent compre- three long colpi, each with a small bridge in the middle of hensive attempts to evaluate the evolution of floral characters the colpi (figure 4i,k,l ) indicating that the grains are tricolpo- in the group as a whole. We therefore compare Caliciflora to rate. The tectum is psilate-imperforate with weakly rugulate extant rosids only in general terms based on the extensive litera- B Soc. R. Proc. ornamentation. Tiny, rounded orbicules, about 1 mm in diam- ture, as well as the few discussions of floral structure that have eter, often with a small central depression, occur on the inside taken into account new molecular models of angiosperm of the theca wall and the surface of the pollen grains relationships (e.g. [25,26,28–41]). (figure 4i,k), most probably indicating a secretory tapetum. One of the most distinctive characters of Caliciflora is the 283 The three free carpels (figures 2c–i and 4a,h) are borne on floral cup, which is also characteristic of many rosids, where the inside of the floral cup above the free space of the floral it occurs scattered in three major clades (N-fixing clade, COM 20161325 : apex. In the flower bud, the carpels are fully open along clade and malvids), as well as in Myrtales [28,42–45] and the ventral suture below the stigmatic region (figure 2c–g). Crossomostomatales [33]. In the N-fixing clade, a floral cup is In this specimen, no ovules are developed and there is no present in certain [40,41] and Cucurbitales [31], while indication of placentation. However, in the more mature in the COM clade, it occurs in Celastrales [32], Oxalidales specimens (figure 4b,c) placentation is ventral and apparently [30] and [29,35–37,46]. Among malvids, a floral concentrated to the middle part of each carpel. cup occurs in certain Brassicales [47], Huerteales [28] and Ovules/seeds are exposed in one mature carpel [48,49]. Actinomorphic flowers with free perianth (figure 4b,c). They are small, elongated, anatropous, with a reti- parts are also characteristic of many rosids, and typically culate outer surface, and do not fill out the locule. The fruitlets such flowers are pentamerous with a well-differentiated are dry follicles that open along their ventral sutures. The inner calyx and corolla as in Caliciflora. The revolute-valvate sepal part of the fruit wall consists of transversely oriented fibrous aestivation of Caliciflora is also a feature of certain rosids and sclereids. The outer epidermis of the fruit wall consists of occurs scattered in the Cucurbitales (Anisophylleaceae, [31]), equiaxial cells, each with a central papilla (figure 4f); no sto- Malpighiales ( and Erythroxylaceae, [36]) and mata have been observed on the carpel or fruit wall. also in Oxalidales (Cunoniaceae and , [30]). The outer surface of sepals and petals, as well as the outer A characteristic feature of many rosids is flowers with surface of the floral cup is covered by a dense indumentum the same number of floral organs in all whorls (isomerous) of interlocking stellate hairs each composed of up to about 10 including the gynoecial whorl. However, monocarpellate, long unicellular elements that radiate from a central point bicarpellate and tricarpellate forms often occur in typically iso- (figures 1a–e,2a–i,3a–e and 4e). Scattered stellate hairs also merous groups, and some groups of rosids, for example, many occur along the margins of the sepals (figure 3d) and on the Malpighiales [46] are characterized by heteromerous flowers inner surface of the petals in the apical region (figures 2i and with one to three carpels. Also common in several groups of 3b,c). The inner surface of the sepals and the median and rosids is apocarpy and in the N-fixing clade it occurs in certain basal parts of the petals are glabrous (figure 3a,c–e). , as well as Surianaceae and Quillajaceae (both fabids). In Rosaceae, dry, follicular fruitlets with several seeds borne centrally on ventral placentae, as well as open ventral sutures, 4. Discussion are known in some members of subfamily [50]. The androecial features of Caliciflora are less informative (a) Systematic assessment systematically, but are also consistent with the androecial struc- The combination of characters in Caliciflora, including the ture of some members of the N-fixing clade. For example, while actinomorphic organization with an open floral cup, penta- most flowers of Rosaceae have 15 or more stamens, flowers merous whorls of free sepals and petals, two whorls of free with fewer stamens occur in the North American cushion stamens, tricolporate and tectate-imperforate pollen, one plant, Kelseya uniflora (S. Wats.) Rydb. [51]. Reduction of whorl of three free carpels and follicular fruitlets, unequivo- stamen number in one of the stamen whorls, as seen in the cally place the fossil taxon among core eudicots. In Caliciflora flower bud, occurs also in Surianaceae (). addition, while flowers with a floral cup, together with free Pollen morphology among rosids is diverse, but small, tri- petals (choripetalous) and an apocarpous gynoecium, are colporate grains similar to those of Caliciflora are common in found in several groups of rosids, to our knowledge this com- many taxa. Often the tectum is reticulate, microreticulate or bination of features is not recorded among asterids or in any foveolate, but grains with a finely rugulate tectum as in other group of core eudicots. Caliciflora occur in many taxa in the N-fixing clade, including Resolving the phylogenetic position of Caliciflora among in (e.g. [52–55]), Surianaceae and Quillajaceae [56], extant rosids is complicated by the vast number of living species and Rosaceae (e.g. [57,58]). (around 70 000 extant species, [17]), many of which have not The stellate hairs of Caliciflora are distinctive. Similar hairs been studied in detail, combined with the absence of clear mor- occur on the outer surface of sepals and petals in Rhizophora- phological synapomorphies that correspond to the higher level ceae, and in the closely related Ctenolophonaceae [36], as well groupings, orders and families recognized based largely on as on the outer surface of the sepals in some Cunoniaceae and Downloaded from http://rspb.royalsocietypublishing.org/ on January 18, 2017

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Figure 5. Floral diagrams of Caliciflora mauldinensis gen. et sp. nov. from the Late Cretaceous (earliest Cenomanian) Mauldin Mountain locality, MD, USA (a) and the Rose Creek Flower [10] from the latest Early Cretaceous or earliest Late Cretaceous (latest Albian–earliest Cenomanian) Rose Creek locality, NE, USA (b). 283 20161325 : Tremandraceae [30,59]. In flower buds of Rhizophoraceae and be related to early diverging lineages of angiosperms [1], Ctenolophonaceae, the perianth parts are congenitally con- including early diverging lineages of eudicots (taxa related to nected by the hairs in the overlapping regions [36], which Lardizabalaceae and other members of Ranunculales, Buxales, also appears to be the case for Caliciflora. Congenital connection Platanaceae and Nelumbonaceae [1,3,73]). Similarly, among of sepals by hairs is also reported for Cunoniaceae [30]. Stellate the 21 species of fossil reported from the Rose Creek hairs similar to those of Caliciflora are not widespread in locality, 70% of the species and 90% of the leaf specimens Rosaceae, but occur in a few taxa (e.g. Sorbaria) [60]. appear to be related to magnoliids or magnoliid grade angios- perms [74]. Four species were unassigned. Only two species (b) Implications for the origin of core eudicots were assigned to core eudicots [74]. Caliciflora mauldinensis from the earliest Cenomanian, very close to the Early–Late Cretaceous boundary, establishes a minimum age of about 100 Myr for the origin of core eudicots. 5. Conclusion Setting aside the report of a rosid flower from Burmese amber Caliciflora [61], the age of which is uncertain (possibly Cenomanian), the The characters of unequivocally place this new fossil only other mid-Cretaceous core eudicot flower currently angiosperm flower among core eudicots, most likely among the rosids rather than asterids, perhaps near the base of the known from around the Early–Late Cretaceous boundary is the Rose Creek flower reported from the Dakota Formation N-fixing clade. Flowers with a floral cup, heteromerous organ- [10], which is of broadly similar age, or perhaps very slightly ization, tricolporate, rugulate-tectate pollen and an apocarpous gynoecium are common at this level of angiosperm evolution, older [62]. While Caliciflora and the Rose Creek flower differ dramatically in size, their floral architecture is fundamentally including in certain Rosaceae, and also in the fabalean family similar. Both are regular actinomorphic and bisexual flowers Surianaceae. More precise systematic placement is precluded by incomplete knowledge of floral structure over a very large with whorled, pentamerous organization and free floral parts (figure 5a,b), a prerequisite for the development of more inte- number of extant species as well as problems of defining the grated patterns of floral architecture [63] that accompanied currently recognized rosid lineages based on floral features alone. Caliciflora and the Rose Creek flower establish a mini- the rapid diversification of core eudicots through the Late Cretaceous and Cenozoic [1,64]. mum age for core eudicots that is broadly in agreement with Subsequent to the earliest Cenomanian there are only few ideas based on molecular data that suggest a major radiation of core eudicots in the mid-Cretaceous [17,75–77]. However, fossil floras of Late Cenomanian or Turonian age. Currently no core eudicot flowers have been described from the Late Cenoma- hypotheses suggesting that a significant diversification of nian flora of the Bohemian Basin [65–67] or from the core eudicots was already underway in the Early Cretaceous [78] are more problematic. Direct fossil evidence of core eudi- Cenomanian–Turonian Sarbay flora [68,69]. The mesofossil flora from Old Crossman Clay Pit of possible Turonian age (e.g. cots prior to around the Early Cretaceous–Late Cretaceous is [8,70]) does contain abundant flowers of core eudicots. However, currently lacking. the Old Crossman flora is very similar in many respects to Data accessibility. The specimens are housed in the palaeobotanical collec- younger mesofossil floras from the Santonian–Campanian of tions of the Field Museum of Natural History, Chicago, USA (PP). Raw North America [1]. The interval between the first appearance of data from the SRXTM study are stored at the Swedish Museum of early core eudicot flowers (Caliciflora, Rose Creek flower) and Natural History, Stockholm, Sweden (raw data PP53985a_B1_B2_). later flowers with more pronounced synorganization (e.g. Rarita- Competing interests. We declare we have no competing interests. niflora tomentosa Crepet, Nixon & Daghlian) is therefore a Funding. Financial support was provided by the Swiss Light Source minimum of about 10, or possibly as much 15–20 Myr. (European Union FP6 projects 20130185), by the Swedish Research The Late Cretaceous record of both mesofossils and palyno- Council and by the Edward P. Bass Distinguished Visiting Fellowship at Yale University. floras both suggest an extensive post-Cenomanian radiation of Acknowledgements. We thank Marco Stampanoni, Federica Marone and core eudicot angiosperms [1,71,72]. However, at the level of Anna Lindstro¨m for help with the SRXTM analyses performed at the Caliciflora and the Rose Creek flower, and also in older sedi- Swiss Light Source, Paul Scherrer Institute, Villigen, Switzerland, and ments, all other angiosperms that are known so far appear to Pollyanna von Knorring for the linedrawings. Downloaded from http://rspb.royalsocietypublishing.org/ on January 18, 2017

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