Evolutionary history

The earliest tree-like organisms were tree ferns, horsetails and lycophytes, which grew in forests in the Carboniferous period. The first tree may have been Wattieza, fossils of which have been found in New York State in 2007 dating back to the Middle Devonian (about 385 million years ago). Prior to this discovery, Archaeopteris was the earliest known tree.[81] Both of these reproduced by spores rather than seeds and are considered to be links between ferns and the gymnosperms which evolved in the Triassic period. The gymnosperms include conifers, cycads, gnetales and ginkgos and these may have appeared as a result of a whole genome duplication event which took place about 319 million years ago.[82] Ginkgophyta was once a widespread diverse group [83] of which the only survivor is the maidenhair tree Ginkgo biloba. This is considered to be a living fossil because it is virtually unchanged from the fossilised specimens found in Triassic deposits.[84] During the Mesozoic (245 to 65 million years ago) the conifers flourished and became adapted to live in all the major terrestrial habitats. Subsequently the tree forms of flowering plants evolved during the Cretaceous period. These began to dominate the conifers during the Tertiary era (65 to 2 million years ago) when forests covered the globe. When the climate cooled 1.5 million years ago and the first of four ice ages occurred, the forests retreated as the ice advanced. In the interglacials, trees recolonised the land[clarification needed] only to be driven back again at the start of the ne

t ice age. The order Cyatheales, which includes the tree ferns, is a taxonomic division of the fern class, Polypodiopsida.[1] No clear morphological features characterize all of the Cyatheales, but DNA sequence data indicate the order is monophyletic. Some species in the Cyatheales have tree-like growth forms, but others have creeping rhizomes (stems). Some species have scales on the stems and leaves, while others have hairs. However, most plants in the Cyatheales are tree ferns and have trunk-like stems up to 20 meters tall. It is unclear how many times the tree form has evolved and been lost in the order.[2] Equisetum (pron.: /kwsi?t?m/; horsetail, snake grass, puzzlegrass) is the only living genus in the Equisetaceae, a family of vascular plants that reproduce by spores rather than seeds.[2] Equisetum is a "living fossil", as it is the only living genus of the entire class Equisetopsida, which for over one hundred million years was much more diverse and dominated the understory of late Paleozoic forests. Some Equisetopsida were large trees reaching to 30 meters tall;[3] the genus Calamites of family Calamitaceae for example is abundant in coal deposits from the Carboniferous period. A superficially similar but entirely unrelated flowering plant genus, mare's tail (Hippuris), is occasionally misidentified and misnamed as "horsetail". It has been suggested that the pattern of spacing of nodes in horsetails, wherein those toward the apex of the shoot are increasingly close together, inspired John Napier to discover logarithms.[4]