Investigations of angiosperms from the Eocene of North America: a mimosoid inflorescence

Six examples of a spicate inflorescence from the Middle Eocene Claiborne Formation in western Tennessee have been investigated. Individual flowers are small, alternately arranged, nearly sessile, and perfect. The style protrudes 2 mm beyond the floral envelope and terminates in a slightly swollen, rounded stigma. Both the style and the stigma are hairy. Ten stamens are exserted and extend 5 mm beyond the floral envelope. Anthers are small, versatile, and dehisce longitudinally. Pollen grains are tricolporate, tectate, and are in permanent tetrahedral tetrads 32 gm in diameter. Comparison of the fossil inflorescences with those of extant families having multiple pollen grain configurations suggests that the fossil inflorescences are most closely allied to the subfamily Mimosoideae of the Leguminosae. These are the first structurally documented remains of the Mimosoideae from the Middle Eocene. THE NUMBER of reports of vegetative and reproductive remains of the Mimosoideae from the fossil record is extensive and encompasses a long period of time. Leaflets with suggested affinities to the Mimosoideae have been reported from strata as early as the Lower Cretaceous (Fontaine, 1889) and there are several reports of Upper Cretaceous leaflets (e.g., Lesquereux, 1892; Berry, 1914). By the Middle Eocene there are numerous reports of mimosoid leaflets (e.g., Berry, 1916, 1930), fruits (e.g., Bowerbank, 1840; Berry, 1916, 1930), seeds (e.g., Bowerbank, 1840), and even a report of an isolated flower with suggested affinities to the Mimosoideae2 (Antholithes mimosaformis Berry, 1930). 1-Received for publication 8 October 1976; revision accepted 21 February 1977. Research supported by NSF Grant DEB76-02886 and the University of Connecticut Research Foundation Grant 35-845 to W. L. Crepet and NSF Grant BMS75-02268 to D. L. Dilcher. The authors gratefully acknowledge the assistance of Dr. T. Elias of the Cary Arboretum of the New York Botanical Garden for commenting on the manuscript; Dr. K. M. Piel of the Union Oil Company of California for a computer survey of the paleo-palynological literature on polyads, tetrads, the Mimosoideae, and the Leguminosae; Dr. L. J. Hickey of the U.S. National Museum for the loan of type specimens; Ms. Mary M. Hubbard, University of Connecticut for the reconstruction of Eomimosoidea; and especially to Mr. P. Irwin of Bloomington, Indiana for his excellence in the field. 2 The type specimen of Antholithes mimosaformis was obtained from the U.S. National Museum for study as part of this investigation. Although the flower appears single in the illustration of Berry (1930, Plate 25), it is actually attached sessilely to an axis-possibly an inflorescence axis. The morphology of the flower is similar to the general floral morphology of the flowers of the Mimosoideae, but certain features of the calyx and corolla are not clear in this specimen and the style and stigma are not well defined. Transfer preparations of the Post-Ecoene reports of the Mimosoideae are common and include leaflets, wood, seeds, and fruits (e.g., Goeppert, 1855; Lesquereux, 1878; Felix and Nathorst, 1893; Knowlton, 1898; Nikitin, 1935). None of the Cretaceous or early Tertiary megafossils has been the object of careful morphological, cuticular, or fine venation analysis and little is known about the fine structure and actual affinities of these fossils. Consequently, it is impossible to evaluate the significance of these reports to mimosoid evolution, and the status of the Mimosoideae during the Cretaceous and early Tertiary is poorly understood. The Mimosoideae are palynologically interesting because pollen may be shed at maturity in either monads (e.g., Prosopis, Desmanthus, Pentaclethra, Leucana), tetrads (e.g., Mimosa, Schrankia, Dinizia, Fillaeopsis), or polyads of various configurations and numbers of grains (e.g., Dichrostachys, Acacia, Calliandra, Xylia) (Sorsa, 1969; Guinet, 1969). Despite the variability and potential taxonomic value of pollen in the modern Mimosoideae (Sorsa, 1969), the palynological record of the group has contributed little to the understanding of its history and evolution. The palynological record of the Mimosoideae is fragmentary and reports of pollen with mimosoidean affinities from the early Tertiary through the Miocene are rare and limited almost exclusively to grains arranged in polyads-the most distinctive pollen configuration in the subfamily. The earliest report of a mimosoid polyad is from the Upper Eocene (Thomson and Pflug, 1953) and anthers revealed that there was no pollen preservation. Because of the structural information that is lacking in this fossil it is unreasonable to suggest its possible affinities with modern families.