Oldest land plants?

The evolution of land plants had a huge impact on planet Earth.  Land plants irreversibly altered our climate, created soil, and set the stage for the movement of animals onto land.  Researchers (Rubinstein, et. al. 2010. Early Middle Ordovician evidence for land plants in Argentina (eastern Gondwana). New Phytologist 188: 365-369) recently discovered evidence of the oldest plant fossils known this far - cryptospores of liverwort-type plants from Argentina a bit over 470 million years old (473-471 Ma, the early Middle Ordovician Period).

Here's a BBC News article.

Back during this time, Argentina was in the eastern part of a southern supercontinent known as Gondwana (in yellow below) and the continents were barren places.

 Into this harsh environment, on the shores of the ancient freshwater streams, green algae evolved into liverwort-type plants.  Liverworts (Marchantiophyta) belong to a primitive group of plants called the bryophytes.  Unlike tracheophytes, the vascular plants dominant in our world today, bryophytes (like liverworts and the more familar mosses) lack vascular tissue to transport water and reproduce by spores. 

Reproduction cannot take place without the presence of water and they are thought to be the evolutionary bridge between aqueous green algae and true vascular plants.

The Argentian researchers discovered what they interpret to be cryptospores from five different genera of liverworts some 470 Ma.  This pushes back the age of these plants some 10 million years - not a lot, geologically, but interesting because five different genera shows a diversity implying an older history for these plants, certainly into the Early Ordovician and possibly back into the Cambrian Period.

Without these modest green plants, we would not be here today!