Friday, April 17, 2009

Here We Go Again: Land Plants Evolved Earlier Than Thought

This type of news makes it to the science headlines all the time. Something is thought to have originated at time line A. Now a new finding suggests it could have originated at time line B; "much earlier than previously thought". This is supposed to fundamentally change that field of study. The latest to make this news blurb is vascular plant life. Vascular plants are thought to have made their first appearance in the early Silurian. Now some spores that have been interpreted to represent vascular plants have been found in the late Ordovician.

Life has a low probability of being preserved, more so in terrestrial environments. Destructive agencies are all too common in sedimentary environments. Organic material can get eaten up, oxidized or just physically pulverized. The sediment layers also might not get preserved in the rock record.

As a result by and large fossils sample populations that have already become widespread and numerous.

When you say the earliest evidence of vascular land plants comes from the Silurian it really means that vascular land plants had become common enough to have a fair chance of being fossilized by the early Silurian. There is absolutely nothing surprising in the discovery that pioneers may have existed in the mid-late Ordovician a few million years earlier.

Think of this problem the other way round, the last appearance of some organism. If tomorrow someone finds a fossil of a non-avian dinosaur in the earliest Cenozoic sediment the media would have a field day. But despite their best attempts to portray the discovery as one that shakes the foundation of evolutionary theory and stratigraphy, the finding would not have that profound an implication. It would be surprising but not revolutionary. After all another group of dinosaurs- birds- did survive the K-T mass extinction period. There is always the chance that a small population of non avian dinosaurs hung on to dear life for a while into the Cenozoic. Their rarity means that the chances of fossils being preserved are slim!

Coming back to the land plants, the physical and chemical signatures of mid-late Ordovician sediments indicate that vascular plant life if it did exist was very rare. Sometime back I wrote about field expressions of mid-late Ordovician discontinuities. They just don't have any signs of disruptive features caused by root action and by organic acid dissolution. There is no discernible geochemical signature of significant land plant carbon input either. Plants preferable take in the lighter carbon isotope C12. Fresh water calcite cements which contain this plant carbon should show depletion relative to the marine baseline. No sign of that in late Ordovician meteoric cements. Other studies of paleosols developed on clastic sediments of Late Ordovician from the Appalachians also suggest minimal influence of vascular land plants. I am also not aware of any discovery of plant impressions or other physical remains of vascular plants from Ordovician anoxic terrestrial environments where they would have had the best chances of preservation.

Despite all this "absence of evidence" a finding that vascular land plants originated earlier in the Mid-Late Ordovician is not that surprising and maybe even inevitable given that the sudden first appearance of a complex feature in the fossil record - in this case vascular systems in the early Silurian- indicates a relatively longish period of evolution of that feature preceding its first widespread appearance as a fossil.


  1. When I was in exploration we found amazing petrified wood, fern fronds, and cycad fossils in "mid-ordovician" sediments.

    But we called it a state survey mapping error instead of a discovery.

    Our fossil consultant said early Cretaceous.

  2. apparently here the age of the sediment is not in doubt. but whether those spores are really from vascular plants is still being questioned by some.