Aroid Seeds from the Middle Eocene Princeton Chert (Keratosperma allenbyense, Araceae): Comparisons with Extant Lasioideae

Abstract

A reinvestigation of more than 200 new specimens of Keratosperma allenbyense Cevallos-Ferriz et Stockey from the Middle Eocene Princeton chert allows a more detailed comparison with living Araceae and provides evidence for the evolution of the lasioid clade. The anacampylotropous seeds possess a warty seed coat, single dorsal ridge, and two lateral ridges. Idioblasts that may have contained raphides are scattered in the outer integument. The seed has a thin micropylar cover and an epistase, with evidence of mucilage in the space between these two structures. The raphe is embedded in the seed coat and parallels the concave surface of the seed. At the chalazal end of the seed, there is a prominent hypostase and podium. Endosperm tissue with dark contents and monocotyledonary embryo are present, but in most specimens these tissues are replaced by fungal hyphae. Anatomical comparisons with the extant lasioid taxa Cyrtosperma merkusii (Hassk.) Schott and Urospatha sagittifolia (Rudge) Schott were made. A reconstruction of K. allenbyense constructed from serial sections allowed the fossil to be compared with the extant genera of Araceae, subfamily Lasioideae, on the basis of external seed morphology of 22 taxa. Keratosperma appears to be a distinct genus among aroids having many shared characters with extant Lasioideae and represents the oldest known member of the lasioid clade.