The Fossil Record of Basal Monocots

The fossil record of basal monocots (Acorales and Alismatales) extends back to the Cretaceous in the Northern Hemisphere. While many fossils were originally assigned to these basal groups, rigorous paleobotanical studies show many of them to be misidentified. Acarus fossils have been reliably reported from the Eocene while those of Alismatales extend back to the early Cretaceous. The fossil record of basal monocots is usually represented by leaves, fruits, and seeds; however, some localities preserve stems with attached leaves and roots and even whole plants. A detailed examination of leaf venation patterns in alismatids has recently allowed the description of a new taxon from the Upper Cretaceous of Alberta based on leaves attributed to Limnocharitaceae. Anatomically preserved alismatid petioles (Heleophyton helobiaeoides) and well-preserved flowers/fruits are known from the Middle Eocene Princeton chert of British Columbia. A complete developmental sequence from flower to fruit is known, and this material has good possibilities for whole plant reconstruction. The extinct floating aquatic Limnobiophyllum (Araceae!Lemnoideae) and the genus Pistia have been the subject of morphological cladistic analyses and competing hypotheses of relationships among aroids and duckweeds. The fossil record and recent molecular studies support separate origins of Pistia and the duckweeds from within Araceae. The fossil taxon "Pistia" corrugata has been reexamined in light of new evidence and indicates the presence of a new genus that shows leaf morphology unlike that seen in extant Pistia, but with a similar growth habit. Fossil evidence indicates that the floating aquatic habit probably arose at least three times within Araceae.