From a BBC article from October 2010:
Paleobotanists have recorded evidence of at least five different types of non-vascular plants - similar to Liverworts - in the form of cytospores from the Early/Middle Ordovician of Argentina.
From the article:
..."These spores, dating from between 473 and 471 million years ago, come from plants belonging to five different genera - groups of species.
"That shows plants had already begun to diversify, meaning they must have colonised land earlier than our dated samples," said Dr Rubinstein, who made the discovery with scientists at the National University of Cordoba, Argentina and the University of Liege, Belgium.
The researchers' best estimate is that the colonisation of land could have occurred during the early Ordovician period (488 to 472 million years ago) or even during the late Cambrian period (499 to 488 million years ago)."...
..."The previous record holder of the earliest known land plants were small liverwort cryptospores found in Saudi Arabia and the Czech Republic.
These were dated at 463 to 461 million years old."...
A bit more:
"The cryptospores from Argentina hint at where land plants originated.
"It most probably happened on Gondwana, as already demonstrated by previous discoveries, but very far, at least 5000km, from the Saudi Arabian and the Czech Republic, where previous earliest traces of land plants were found," said Dr Rubinstein.
As land plants matured, they evolved from liverworts into mosses, and then into plants known as hornworts and lycopods.
Then ferns appeared before seed plants, of which there are many species today, finally evolved."
