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Archaenthus: An Early Angiosperm From the Cenomanian of the Western Interior of North America
Abstract and Figures
Archaeanthus linnenbergeri Dilcher & Crane, gen. et sp. nov., a multifollicular angiosperm fruit, is described from the mid-Cretaceous (uppermost Albian-mid-Cenomanian) Dakota Formation of central Kansas. Clusters of follicles were borne terminally at the apex of a stout branch with helically arranged leaves. Each cluster comprised 100-130 helically arranged follicles on an elongated receptacle. The follicles were stalked with a short, rounded tip and dehisced along a single adaxial suture. Ovules were numerous and borne along either side of the suture; 10-18 seeds matured in each follicle. The receptacle below the gynoecium shows three groups of scars, an upper group of numerous small scars interpreted as those of stamens with six to nine larger scars immediately below, interpreted as those of inner perianth parts. The flower is delimited at the base by three large, narrowly elliptical scars interpreted as those of outer perianth parts. A prominent scar below the base of the flower is thought to mark the position of floral bud-scales. Archaeanthus is linked with perianth parts (Archaepetala beekeri Dilcher & Crane, gen. et sp. nov. and Archaepetala obscura Dilcher & Crane, sp. nov.), bud scales (Kalymmanthus walkeri Dilcher & Crane, gen. et sp. nov.), and leaves (Liriophyllum kansense Dilcher & Crane, sp. nov.) on the basis of association evidence and structural agreement in the presence of distinctive resin-bodies. Liriophyllum populoides Lesq. is shown to be a separate species. The reconstructed Archaeanthus plant is most closely related to Recent Magnoliidae and in some features comes close to the hypothetical angiosperm archetype predicted by magnoliid floral theory; it demonstrates that many of the characters interpreted as primitive from neontological evidence are also ancient. Archaeanthus does not predate other kinds of angiosperm reproductive structure in the fossil record but conclusively demonstrates the existence of magnoliid-like plants and flowers early in angiosperm evolution.
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... The bisexuality and proximal arrangement of androecium and gynoecium in Bennettitales were compared to those in Magnoliales and interpreted as evidence favoring the Euanthial Theory. This conclusion was further repeatedly strengthened by several ensuing publications of influence published in the past century, including anatomic work of Canright [9], studies of fossil early angiosperm Archaeanthus by Dilcher and Crane [10], Archaefructus by Sun et al. [11], and Monetianthus by Friis et al. [12]). These works initially were interpreted as favoring the Euanthial Theory. ...
... This case is not peerless. Instead similar case also occurred in some studies of fossil angiosperms: Dilcher and Crane [10] drew ovules of Archaeanthus on the ventral margins of carpels in their Figure 60h (just as the Euanthial Theory expected), their own Figure 24 indicated that at least some of the ovules were borne on the dorsal fruit margin [10]. In addition, the famous early angiosperm Archaefructus was initially described as bearing "follicles", implying that ovules were borne on the ventral margin of carpels in Archaefructus [11]. ...
... This case is not peerless. Instead similar case also occurred in some studies of fossil angiosperms: Dilcher and Crane [10] drew ovules of Archaeanthus on the ventral margins of carpels in their Figure 60h (just as the Euanthial Theory expected), their own Figure 24 indicated that at least some of the ovules were borne on the dorsal fruit margin [10]. In addition, the famous early angiosperm Archaefructus was initially described as bearing "follicles", implying that ovules were borne on the ventral margin of carpels in Archaefructus [11]. ...
Angiosperms are frequently assumed to constitute a monophyletic group. Therefore, the origin of angiosperms is a key question in systematic botany since the answer to this question is hinged with many questions concerned with angiosperm evolution. Previously, the lack of fossil evidence undermines the robustness of related hypotheses, and explains the instability of the systematics of angiosperms in the past century. With increasing evidence of early angiosperms, the origin and early evolution of angiosperms become approachable targets. However, reaching a strict consensus is still a mission impossible now: there are too many issues open to debate. A good sign in research is that palaeobotanists started addressing the issue of criterion identifying angiosperms, this would bring order in studies of early angiosperms. Several flaws in fundamental concepts inflicting botany require efforts to elucidate and remedy. The author here opens a discussion on these problems, hoping that more botanists will join to discuss and clarify previously blurry concepts and place a solid foundation for future development in botany.
... The latest progresses include 1) recognizing the branch-terminal position of seeds in gymnosperms [6,7], 2) treating ovule and carpel (wall) separately, which become tightly synorganized, in extant angiosperms [8]. Although these moves are in the right direction, the controversy over nature of carpels continues, partially due to on-going debates on ovule/seed position in fruits of early angiosperms, including famous Archaefructus [9][10][11][12] and Archaeanthus [13,14] (Figure 3). In these debates, the seeds were interpreted as borne either on the adaxial or the abaxial margin (two opposite positions) of the fruits, and neither of the confronting schools could convince the other. ...
... The most intriguing feature of Xenofructus may be its placenta positioning. Unlike previously reported fossil plants including Archaefructus [9][10][11][12], Sinocarpus [21,37], Archaeanthus [13,14], in which the seeds/ovules were interpreted as borne either on adaxial or abaxial margin of fruits, the seeds in Xenofructus are clustered around an axis independent of both adaxial and abaxial margins. This phenomenon is so far never seen in extant angiosperms. ...
Carpels are a reproductive feature restricted to angiosperms, therefore they are a focus of many botanical studies. However, there are controversies over the nature of carpels. A reason underly-ing these controversies is mixing implications given by conflicting interpretations on fossil carpels in early fossil angiosperms from the Cretaceous. These controversies hinder a clear understanding of angiosperm evolution and systematics. A key to these questions is older fossil fruits bearing concerned information. Here we report a new fossil fruit, Xenofructus dabuensis gen. et sp. nov, from the Middle Jurassic of Liaoning, China. Unlike previously reported fruits of early angio-sperms that were interpreted as bearing seeds either on adaxial or abaxial margin by various au-thors, our fossil demonstrates clearly that the seeds in Xenofructus are neither borne on the adaxial nor abaxial margin of the fruit, instead the seeds of Xenofructus are borne on an axis positioned between two margins. This new feature implies that a placenta in carpels is an ovule/seed-bearing axis, a carpel is a composite organ comprising an enclosing leaf (fruit wall) and an axis (placenta). The adaxial or abaxial position of ovules/seeds frequently seen in fossil and extant angiosperms is a consequence derived through long time evolution (coalescence of placenta with either margin of fruits). Carpels can be taken as foliar structures enclosing their associated ovulate branches.
... Most of the angiosperm fossil record from this period consists of sterile twigs or isolated leaves or flowers (Friis et al., 1994;Friis et al., 2011;Crepet and Nixon, 1994;Mohr and Eklund, 2003;Poinar et al., 2017;Pessoa et al., 2021). Among the most complete fossil taxa are Archaefructus G. Sun, Dilcher, Zheng & Zhou (Sun et al., 1998), Archaeanthus Dilcher & Crane (Dilcher and Crane, 1984), Callianthus Wang & Zheng (Wang et al., 2021), Gansufructus B. Du (Du et al., 2021), Leefructus G. Sun, Dilcher, H.S.Wang & Z.D.Chen (Sun et al., 2011) and Santaniella Gobo, Coiffard, Bachelier, L. Kunzmann & Iannuzzi (Gobo et al., 2022;Pessoa et al., 2023), but even for these, the phylogenetic position among extant lineages is still subject to debate (Pessoa et al., 2023). ...
... It is important to highlight that lobed leaves, as the ones found in A. florifera, are uncommon among extant taxa of this clade (e.g. only found in some Aristolochia L., Gyrocarpus Jacq., Illigera Blume, Lindera Thunb., Liriodendron L. and Sassafras L.), but it seem to have been much more frequent among early angiosperms during the Cretaceous (Dilcher and Crane, 1984;Pessoa et al., 2021;Gobo et al., 2022;Coiro et al., 2020). Opposite leaves, as originally described by Mohr and Eklund (2003) for A. florifera, are more common among Laurales and some Piperales (Christenhusz et al., 2017), whereas alternate leaves in a spiral arrangement represent the most widespread phyllotaxis condition among magnoliid taxa. ...
The angiosperm Araripia florifera was originally described based on a fossil specimen including a few lobed leaves and floral buds. Although nothing was known of the internal structure of the flowers, based on the external similarity of the flower buds, it was compared with Calycanthaceae (Laurales). A new fossil from the type locality provides the first morphological evidence for the gynoecium of A. florifera. An androecium was not found, but new evidence is still needed to determine whether the flowers are unisexual. The gynoecium of this fossil species is syncarpous or monomerous, the ovary is superior, globose and distinctly stipitate, and the style is elongated and relatively thick. This new floral information disagrees with suggestions that Araripia belonged to the family Calycanthaceae, because this extant family is characterized by perigynous flowers, with apocarpous gynoecium and ovaries that are never stipitate. We also question the leaf arrangement, originally proposed as opposite, but alternate in this new fossil. We provide a phylogenetic hypothesis combining morphological and DNA sequence data using Bayesian inference. Our phylogenetic analyses indicate that A. florifera is more likely an extinct lineage in the stem group of Laurales, and is hereby placed in its own family Araripiaceae.
... These are also similar to recent representatives of ANA grade taxa producing apocarpous berries with ethereal oil-containing cells in the mesocarp -Austrobaileya, Kadsura and Schisandra. On the other hand, it should be mentioned that various Early Cretaceous and mid-Cretaceous follicles have been described, including Archaeanthus linnenbergeri Dilcher & Crane (Dilcher and Crane, 1984) with close affinities to the family Magnoliaceae Doyle and Endress, 2010;Romanov and Dilcher, 2013), as well as Lesqueria elocata (Lesq.) Crane & Dilcher and ...
Premise
The representatives of the ANA‐grade of angiosperms demonstrate a diverse pattern of morphological characters, but their apocarpous gynoecium (except in Nymphaeaceae), composed of at least partly ascidiate carpels, the four‐nucleate and four‐celled female gametophyte, and the diploid endosperm (except in Amborella ) are inferred to be plesiomorphies. Since the structure of fruits in Austrobaileyales is under‐investigated, this research aims to fill this gap in these data, describing the carpological characters of ANA‐grade taxa, and potentially illuminating the ancestral fruit and seed types of angiosperms.
Methods
The pericarp and seed coat anatomy was studied with light microscopy. The character optimization was carried out using WinClada software.
Results
The fruits of Austrobaileya, Trimenia, Kadsura and Schisandra are determined to be apocarpous berries of the Schisandra type, with a parenchymatous pericarp and mesotestal ( Austrobaileya ) or exomesotestal seeds (other genera). Most inferred scenarios of fruit evolution
indicate that the apocarpous berry is either the most probable plesiomorphic fruit type of all angiosperms, or that of all angiosperms except Amborellaceae. This suggests the early origin of the berry in fruit evolution. The plesiomorphic seed type of angiosperms according to reconstructed scenarios of seed type evolution was either a seed lacking a sclerenchymatous layer or an exotestal seed.
Conclusions
The current research indicates that an apocarpous berry, and not a follicle, is a probable plesiomorphic character of the ANA grade taxa and of angiosperms as a whole.
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... Atkinson et al., 2015), impressions/compressions (e.g. Dilcher & Crane, 1984;Mohr & Eklund, 2004), and amber inclusions (e.g. Gandolfo et al., 2018). ...
Flowers are the complex and highly diverse reproductive structures of angiosperms. Because of their role in sexual reproduction, the evolution of flowers is tightly linked to angiosperm speciation and diversification. Accordingly, the quantification of floral morphological diversity (disparity) among angiosperm subgroups and through time may give important insights into the evolutionary history of angiosperms as a whole.
Based on a comprehensive dataset focusing on 30 characters describing floral structure across angiosperms, we used 1201 extant and 121 fossil flowers to measure floral disparity and explore patterns of floral evolution through time and across lineages.
We found that angiosperms reached their highest floral disparity in the Early Cretaceous. However, decreasing disparity toward the present likely has not precluded the innovation of other complex traits at other morphological levels, which likely played a key role in the outstanding angiosperm species richness.
Angiosperms occupy specific regions of the theoretical morphospace, indicating that only a portion of the possible floral trait combinations is observed in nature. The ANA grade, the magnoliids, and the early‐eudicot grade occupy large areas of the morphospace (higher disparity), whereas nested groups occupy narrower regions (lower disparity).
Flowers are attractive to many people mainly due to their colourful and conspicuous perianth, which is closely related to successful insect pollination in extant angiosperms. The Lower Cretaceous Yixian Formation (125 million years ago) in Northeastern China is famous for its great diversity of early angiosperms. However, unlike typical flowers in extant angiosperms, the previously documented fossil flowers are usually famous for their "nakedness", namely, they do not have typical petals (corolla), suggesting a pollination strategy for early angiosperms that is different than that of the extant angiosperms. However, without fossil evidence, whether there are petals (corolla) and whether androecium and gynoecium are protected in early flowers are rarely addressed questions. Here, we document a flower bud fossil, Archaebuda cretaceae sp. nov., from the Yixian Formation, which is the second case in the Formation. While reinforcing the truthful occurrence of fragile flower buds in the Early Cretaceous, new fossil evidence demonstrates the existence of gynoecium and possible androecium within the flower bud, suggestive of the possible bisexuality and the protection function of petals in Archaebuda, just as in extant flowers. In addition, it is very likely that conspicuous petals may have played an important role in attracting insects for successful pollination in early angiosperms. Therefore, the occurrence of petals (corolla) in Archaebuda cretaceae sheds a new light on the reproduction of early angiosperms.
The plant fossil record during the Cretaceous documents a major transition in the dominant group of terrestrial autotrophs, as plant communities from the earlier Mesozoic were transformed by the appearance and rapid diversification of angiosperms. This transformation began in the Early Cretaceous, continued through the Late Cretaceous, and led ultimately to the dominance of angiosperm in most terrestrial ecosystems today, which had profound consequences for the other organisms inhibiting terrestrial ecosystems and perhaps the planet as a whole. Our understanding of angiosperm diversification has been greatly improved over the past 50 years by integrated studies of fossil assemblages containing angiosperm pollen and leaves, but especially by new information from mesofossil floras that have provided previously unanticipated detail on floral form in Cretaceous angiosperms and have allowed the recognition of key dispersed pollen types in situ . Information from fossil flowers has greatly facilitated meaningful comparisons with living plants and integration with phylogenetic analyses of extant angiosperms based on DNA evidence. The combined insights from these discoveries provide a broadly consistent and coherent picture of angiosperm evolution through the Cretaceous, which comprises more than half of their entire evolutionary history.
Vertebrate‐mediated seed dispersal is a common attribute of many living plants, and variation in the size and abundance of fleshy diaspores is influenced by regional climate and by the nature of vertebrate seed dispersers among present‐day floras. However, potential drivers of large‐scale variation in the abundance and size distributions of fleshy diaspores through geological time, and the importance of geographic variation, are incompletely known. This knowledge gap is important because fleshy diaspores are a key mechanism of energy transfer from photosynthesis to animals and may in part explain the diversification of major groups within birds and mammals. Various hypotheses have been proposed to explain variation in the abundance and size distribution of fleshy diaspores through time, including plant–frugivore co‐evolution, angiosperm diversification, and changes in vegetational structure and climate. We present a new data set of more than 800 georeferenced fossil diaspore occurrences spanning the Triassic–Oligocene, across low to mid‐ to high palaeolatitudes. We use this to quantify patterns of long‐term change in fleshy diaspores, examining the timing and geographical context of important shifts as a test of the potential evolutionary and climatic explanations. We find that the fleshy fruit sizes of angiosperms increased for much of the Cretaceous, during the early diversification of angiosperms from herbaceous ancestors with small fruits. Nevertheless, this did not cause a substantial net change in the fleshy diaspore size distributions across seed plants, because gymnosperms had achieved a similar size distribution by at least the Late Triassic. Furthermore, gymnosperm‐dominated Mesozoic ecosystems were mostly open, and harboured low proportions of specialised frugivores until the latest Cretaceous, suggesting that changes in vegetation structure and plant–frugivore co‐evolution were probably not important drivers of fleshy diaspore size distributions over long timescales. Instead, fleshy diaspore size distributions may be largely constrained by physical or life‐history limits that are shared among groups and diversify as a plant group expands into different growth forms/sizes, habitats, and climate regimes. Mesozoic gymnosperm floras had a low abundance of fleshy diaspores (<50% fleshy diaspore taxa), that was surpassed by some low‐latitude angiosperm floras in the Cretaceous. Eocene angiosperm floras show a mid‐ to high latitude peak in fleshy fruit abundance, with very high proportions of fleshy fruits that even exceed those seen at low latitudes both in the Eocene and today. Mid‐ to high latitude proportions of fleshy fruits declined substantially over the Eocene–Oligocene transition, resulting in a shift to more modern‐like geographic distributions with the highest proportion of fleshy fruits occurring in low‐latitude tropical assemblages. This shift was coincident with global cooling and the onset of Southern Hemisphere glaciation, suggesting that rapid cooling at mid‐ and high latitudes caused a decrease in availability of the climate conditions most favourable for fleshy fruits in angiosperms. Future research could be focused on examining the environmental niches of modern fleshy fruits, and the potential effects of climate change on fleshy fruit and frugivore diversity.
Lesqueria elocata (Lesq.) Crane & Dilcher, an early angiosperm fruiting axis, is described from the mid-Cretaceous (Upper Albian-Middle Cenomanian) Dakota Formation of central Kansas and the Woodbine Formation of northeastern Texas. The species is based on three-dimensional molds preserved in sandstone. The fruits (multifollicles) comprise 175-250 follicles borne helically, in a tight, more or less spherical or ovoid head. The receptacle below the gynoecium is cylindrical, elongated, and bears numerous, helically arranged, persistent laminar flaps. The bases of these flaps are diamondshaped at their attachment to the receptacle, but it is not known whether they are stamens or perianth parts. The follicles are short stalked, with two terminal prolongations, and dehisced along a single adaxial suture. The follicles contained 10-20 seeds arranged in two longitudinal rows. The former assignment of L. elocata to Williamsonia (Bennettitales) is rejected and Lesqueria is shown to be most similar to Recent magnoliid angiosperms. Lesqueria is one of a diverse group of extinct mid-Cretaceous magnoliid flowering plants.
The primitive angiosperms in the South Pacific exhibit three modes of pollination. The flowers of Drimys are pollinated by a wide variety of Diptera. One species of thrips (Thysanoptera) pollinate the flowers of Belliolum. Coleoptera play an important role in the reproductive biology of Zygogynum (plus primitive moths) and Degeneria. The flowers of all genera examined are adapted for outcrossing. Coleoptera-pollinated flowers display the most intricate floral movements. It is suggested that Diptera as well as Coleoptera may have been the pollinators of the early angiosperms.
The hypothesis that angiosperms are primitively vesselless was tested in the context of a cladistic analysis of selected families of the Magnoliidae, Ranunculidae, and Hamamelididae. Results of this analysis show that the most parsimonious explanation of the absence of vessels in dicotyledons is that in all instances it is a derived feature. This conclusion is supported by three taxon out-group comparisons and does not alter our well-founded understanding of vessel member evolution within the angiosperms. I suggest that presence of vessel members with scalariform perforation plates and scalariform intervascular pitting, such as those in Austrobaileya, is the "primitive" condition within the angiosperms. I also suggest that in the earliest angiosperms vessels were restricted to secondary xylem. Absence of vessels in the eleven extant genera of "primitive" woody dicotyledons is a secondary loss, and it is suggested that the xylem of these genera is paedomorphic.