Little, Stefan A. , Manchester, Steven R. , Smith, Una R. .
Should Palaeoaster be expelled from the angiosperms?
The fossil Paleoaster porosa (Lesquereux) U.R. Smith, traditionally interpreted as an angiosperm fruit, occurs in Upper Cretaceous floras across the western interior of the USA. We examined anatomically preserved specimens from Maastrichtian ammonite-bearing concretions of the Fox Hills Formation in western South Dakota using serial thin sections and cellulose acetate peels. The stalked specimens are ellipsoidal, 5-6 cm long and 2.5-3 cm wide. Fractures, as well as successive transverse and longitudinal sections, reveal a complex morphology with 9-10 radial cuneiform fertile segments (like “citrus wedges”) surrounded by a corresponding number of alternate longitudinal valves. In numerous other specimens the thick, narrow outer valves are splayed open, presumably dehisced and mature. The outer valves and central axis contain tracheary elements with annular, helical and scalariform wall thickenings, in some cases with simple perforation plates. The cuneiform segments contain numerous tiny elongate seeds oriented with their micropyles pointing obliquely inward to the central axis and directly to their point of attachment on radial partitions between segments. Each seed has a central, ellipsoidal body (1.8 mm long, 0.5 mm long, sometimes containing a dicotyledonous embryo) and a long micropyle extending at least 1/3 of the length of the seed body. The nucellus forms an elongate plug within the micropyle and a peg-like structure at the chalaza, beyond which is a conspicuous elongate parenchymatous extension (up to 2 mm long, 0.5 mm wide). The integument and nucellus are free from each other in the chalazal region. Pollen and vegetative parts of this plant remain unknown. Some aspects of seed structure, including the nucellar peg, nucellar plug, and elongate micropyle, are remarkably similar to Bennettitales and not typical of angiosperms. However, this plant lacks interseminal scales and has a more complex placentation than known in Bennettitales. The suite of characters now available raises the question whether Palaeoaster may represent a complex gymnosperm reproductive structure.
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1 - University of California Davis, Department of Plant Sciences, One Shields Avenue, Davis, California, 95616, USA
2 - University of Florida, Department of Botany, Florida Museum of Natural History, P.O. Box 117800, Gainesville, Florida, 32611-7800, USA
3 - Pajarito Environmental Education Center, PO Box 547, Los Alamos, NM, 87544, USA
Presentation Type: Oral Paper:Papers for Sections
Location: 555A/Convention Center
Date: Tuesday, August 3rd, 2010
Time: 11:15 AM