Krings, M. (Bayerische Staatssammlung für Paläontologie und Geologie und GeoBio-Center^LMU, Richard-Wagner-Straße 10, D-80333 Munich, Germany), S. D. Klavins, T. N. Taylor, E. L. Taylor (Department of Ecology and Evolutionary Biology, and Natural History Museum and Biodiversity Research Center, The University of Kansas, Lawrence, KS 66045), R. Serbet (Division of Paleobotany, Natural History Museum and Biodiversity Research Center, The University of Kansas, Lawrence, KS 66045), and H. Kerp (Forschungsstelle für Paläobotanik am Geologisch-Paläontologischen Institut, Westfälische Wilhelms-Universität Münster, D-48143 Münster, Germany). Frond architecture of Odontopteris brardii (Pteridospermopsida, ?Medullosales): new evidence from the Upper Pennsylvanian of Missouri, U.S.A. J. Torrey Bot. Soc. 133: 33–45. 2006.—Exceptionally well-preserved adpressions of the Late Pennsylvanian seed fern Odontopteris brardii (Brongniart) Sternberg are described from the Bonner Springs Shale (Kansas City Group, middle Missourian) of western Missouri, U.S.A. The fossils indicate that O. brardii fronds are bipinnate and similar in architecture to those seen in Lescuropteris (= Odontopteris) genuina (Grand'Eury) Remy et Remy from the Stephanian of France. This seems to contradict the widely accepted opinion that O. brardii is conspecific with Odontopteris minor f. zeilleri Potonié, because fronds of the latter taxon are asymmetrically tripinnate. We suggest that heteroblastic development occurred in O. brardii and O. minor f. zeilleri, and bipinnate fronds were produced by juvenile plants. On the other hand, intraspecific differences in frond architecture may also have been a mechanism of adaptation. As a result, frond architecture can be used only in a limited sense for species circumscription in odontopterid seed ferns until the mechanisms underlying heterophylly in these plants are more fully understood.