Archaenthus: An Early Angiosperm From the Cenomanian of the Western Interior of North America

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. Hypotheses of flowering plant phylogeny are inextricably linked to the evolutionary interpretation of angiosperm reproductive organs and thus to concepts of the primitive angiosperm flower. Traditionally such ideas have been based on comparative studies of living plants, and the fossil record has made little contribution. In recent years, however, knowledge of early angiosperm reproductive diversity has increased, and paleobotanical data relevant to these problems have begun to accumulate. In this paper we describe a new species of mid-Cretaceous angiosperm known from multifollicular fruits, perianth parts, bud-scales, and leaves, and discuss its relevance to concepts of floral evolution in flowering plants. The earliest speculations on the nature of the primitive angiosperm flower developed from the pre-Darwinian classifications of the eighteenth and nineteenth centuries and polarized into two principal hypotheses: either the simple, unisexual and predominantly wind-pollinated flowers of the Amentiferae were primitive (Engler, 1897; Strasburger et al., 1898; Rendle, 1925, 1930; Wettstein, 1935) or the bisexual and predominantly insect-pollinated flowers of the magnoliid (Ranalean) angiosperms most closely approximate to the 'ancestral' flower (Bessey, 1897, 1915; ' We thank C. Beeker, W. L. Crepet, G. J. Gastony, M. B. Farley, E. M. Friis, D. Hattin, E. Kauffmann, and K. Longstreth for their advice and assistance at various stages of the work; H. Reynolds, M. Walker, and F. Potter, Fort Hays State University, Kansas, for invaluable help in locating and collecting specimens; the Missouri Botanical Garden and the United States National Museum for the loan of specimens; A. Linnenberger and E. Linnenberger for permission to collect material; and Megan Rohn for drawing figures 69 and 70, and Polyanna Quasthoff for drawing figure 60. This research was supported by NSF grant DEB 77-04846 to D. L. Dilcher. Travel funds provided for P. R. Crane during the early part of the study by the British Council and Reading University Research Board are gratefully acknowledged. 2 Department of Biology, Indiana University, Bloomington, Indiana 47405. 3Department of Geology, Field Museum of Natural History, Roosevelt Road at Lake Shore Drive, Chicago, Illinois 60605. ANN. MISSOURI BOT. GARD. 71: 351-383. 1984. This content downloaded from 170.140.26.180 on Mon, 07 Mar 2016 23:56:15 UTC All use subject to JSTOR Terms and Conditions 352 ANNALS OF THE MISSOURI BOTANICAL GARDEN [VOL. 71 Arber & Parkin, 1907). Subsequent investigations and interpretations, particularly of anatomical (Bailey, 1944; Eames, 1961; Dickison, 1975) and palynological evidence (Wodehouse, 1935, 1936; Walker 1974a, 1974b, 1976), led to the widespread acceptance of the second hypothesis, that the Magnoliidae are the most primitive living group of flowering plants and exhibit the most primitive floral morphology. This hypothesis is central in most of the putatively 'phylogenetic' classifications of flowering plants that have been proposed in the last 50 years (Hutchinson, 1959; Cronquist, 1968, 1981; Takhtajan, 1969; Stebbins, 1974; Thorne, 1976; Dahlgren, 1980), and although alternative viewpoints have been suggested (Corner, 1949; Melville, 1962, 1963; Meeuse, 1966), none have been widely accepted. Along with the development of the magnoliid hypothesis has come the recognition of 'evolutionary trends' for a wide range of characters. The primitive states in these trends have been combined into a concept of a hypothetical angiosperm morphotype (Takhtajan, 1969). Until recently, direct paleobotanical evidence relevant to these evolutionary hypotheses has been conspicuously absent. The last two decades, however, have seen considerable advances in angiosperm paleobotany and an increasing awareness of the relevance of fossil material to concepts of flowering plant evolution (Doyle & Hickey, 1976; Hickey & Doyle, 1977; Hughes, 1976; Doyle, 1978; Dilcher, 1979). A major radiation is regarded as having occurred during the Barremian to Cenomanian stages of the midCretaceous, followed by further diversification throughout the Upper Cretaceous and Tertiary. No unequivocal angiosperms have so far been reported from pre-Barremian rocks, although there are many earlier, potentially relevant fossil plants about which we know too little (Doyle, 1978; Hill & Crane, 1982; R. A. Scott et al., 1960). Despite some insight into the timing of angiosperm evolution, the systematic origin of the group remains a mystery that continues to stimulate a variety of speculations (Melville, 1962, 1963; Meeuse, 1966; Retallack & Dilcher, 1981a). Such conjectures inevitably reduce to discussions of homology, particularly of reproductive structures, and have been severely limited by inadequate knowledge of early flowering plants. Few of the mid-Cretaceous angiosperms are known in detail from flowering or fruiting specimens, and even fewer are known from both vegetative and reproductive material. The species described in this paper is currently one of the more completely understood of all early angiosperms. We propose the name Archaeanthus linnenbergeri for multifollicular fruits and the vegetative branches on which they are borne. This species is linked with two kinds of 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.) as parts of a single fossil plant on the basis of association and the anatomical similarity that all these organs contain the same distinctive resin-bodies. Liriophyllum populoides Lesq. is shown to represent a different species. The reconstructed Archaeanthus plant is closely allied to the Magnoliidae sensu lato, and in some features comes close to the hypothetical angiosperm morphotype predicted by magnoliid floral the-