Limnobiophyllum Krassilov — a fossil link between the Araceae and the Lemnaceae

Abstract

Limnobiophyllum Krassilov includes fossil free-floating stoloniferous plants each with one or two sessile suborbicular to reniform leaves of different size as well as numerous simple and one or two longerbranched roots on a reduced main stem. The venation consists of campylodromous curved primaries, up to 14 in number, among which irregular reticulate veins of higher orders can be preserved. There are no signs of lateral pouches characteristic of the Lemnaceae. However, aerenchyma and pigment cells are well developed in Limnobiophyllum. Two species are recognized - Limnobiophyllum scutatum (Dawson) Krassilov (latest Cretaceous to Oligocene of western North America and Palaeocene of East Asia) and Limnobiophyllum expansum (Heer) Kvaček, comb. n. (Miocene of Europe). Turion-like bodies are associated with L. scutatum. No fruits have been found in connection, but numerous isolated ribbed seeds are associated with L. expansum. They resemble some Araceae and also Lemna L. and Spirodela Schleid. Such seeds are known as Lemnospermum Nikitin from the Tertiary of Eurasia. Limnobiophyllum resembles in some respects Spirodela, but it is larger and the roots and the habit are more like young plants of Pistia L. (Araceae). It differs from Hydromystria Mey. (i.e. Limnobium Rich.) by venation, root system and habit; the Lemnaceae, although similar, differ by lateral pouches of leafy fronds and the lack of branched roots and higher-order veins. Limnobiophyllum is considered as an extinct link related to Pistia (Araceae), from which Spirodela (Lemnaceae) may have evolved by reduction.