The Families of Gymnosperms
Comments. The 9 genera and 65 species of extant cycads are the remnants of the great cycadophyte assemblage, which has been in existence for at least 200 million years, and which once dominated much of the land flora worldwide. Their leaves are so common in Mesozoic deposits that before by the turn of the 19th Century, the period had become known as the age of cycads; and they provided raw material for a classic example of early palaeobotanical/taxonomic detective work, when comparative studies of leaf cuticles from fossils and living cycads revealed the two distinct lineages now recognized as the Orders Cycadales and Bennettitales (see illustrations).
The living members are all referred to the Cycadales, which are represented in early Mesozoic deposits by plants closely resembling their present day descendants in both general form and details of leaf anatomy. Fossil material of Bennettitales first appears more or less contemporaneously with that of Cycadales in the early Mesozoic, but they evidently surpassed them in relative abundance and diversity for some 120 million years before disappearing from the record in the mid-Cretaceous. In fact, the majority of fronds from the age of cycads exhibit the epidermal characteristics of Benettitales. The best known bennettitalean forms combined gymnspermous ovules with reproductive structures superficially resembling angiosperm flowers.
To view the illustrations with detailed captions, go to the interactive key. This also offers full and partial descriptions, diagnostic descriptions, differences and similarities between taxa, lists of taxa exhibiting or lacking specified attributes, and distributions of character states within any set of taxa.
Cite this publication as: ‘Watson, L., and Dallwitz, M.J. 2008 onwards. The families of gymnosperms. Version: 6th November 2009. http://delta-intkey.com’.