In search of Pangaea

Readers of a nervous disposition may wish to skip this post. This week we venture into the thickets on the edge of the forest of academia, a dark and magical place from which many never return.

the thickets of academia

 

Once upon a very, very long time ago (about 200 million years ago) virtually all the continents were joined, forming the vast supercontinent of Pangaea. Or at least, that’s what the scientists tell us.

And such are the marvels of modern technology that these days anyone with the time, technology, inclination and english language skills can access the experts’ views – superficially at least – with a minimum of effort.

A recent study* of the Cupressaceae family and its distribution after the break up of Pangaea is a landmark because although these studies have been conducted before for fauna, this is the first equivalent study for flora. The study analyses the phylogenetic relationships of Cupressaceae (162 species in 32 genera) in relation to the break-up of Pangaea into the two supercontinents: Laurasia in the north and Gondwana in the south.

Now I am no scientist, and much of the six page paper gives me a headache. I cannot pretend to understand taxons, clades or Bayesian co-estimation of topology and divergence time (attractively abbreviated to BEAST, but I’m not sure why). I can only dimly make out what the sentence “Likelihood Ancestral Areas Ranges were implemented under the dispersal-extinction-cladogenesis model in Lagrange” might mean. All this is a direct result, no doubt, of my decision to forsake ‘O’ level biology at the age of 14 in preference for german. (A mistake, with the benefit of hindsight – but predicting history is a tricky business).

But what I can grasp is this: tree species have been evolving and migrating for hundreds of millions of years; adventurous scientists are working to track these wanderings, undaunted by the 200 million year head start the trees have been given; and – for this study at least – the scientists seem confident of their findings. “The dense taxon sampling and large amount of sequence data used here yielded a solidly supported phylogeny for the Cupressaceae”. Relief all round!

Their conclusions, and I hope I’ve got this right, are that the humble cypress family has been around and identifiable as a separate tree family for over 200 million years (ie before Pangaea split apart), and that its seven major lineages (sub-families) date back to the Triassic and Jurassic periods, pre-dating or coinciding with Laurasia and Gondwana’s wanderings. Like so many of the world’s wonders, this family seems to have originated in (what is now) Asia, with some descendants heading east into (what would become) North America during the late Jurassic (say, 150 million years ago), with relatively fewer heading south into (what would become) Africa and India, before those two land masses were shunted north again, up against the Eurasian land mass. So who says migration is a modern thing?

The paper goes on to explore through the fossil record the subsequent local extinction of some sub-families, in some regions, after continental separation. Its conclusions include the observation that “In Cupressaceae, the evidence for widespread extinction and range shrinkage is particularly strong. For example [the sub-families] Cunninghamioideae and Taiwanioideae were widely distributed in the northern hemisphere during the Cretaceous [period] but now are restricted to Asia… the Ice Ages of the past 2 million years further contributed to population extinction and reductions in species range.”

So why, I hear you ask, should anyone care about all this arcane stuff unless you happen to be a biogeography expert? (in which case you would hardly be bothering yourself with this post, unless perhaps to criticize or correct it). The answer comes in three parts, and is worth a moment of reflection:

1. The natural location of our planet’s trees is not an accident. It came about as a result of evolution, migration and climate change over a vastly long period far in excess of man’s engineering powers or planning horizons.

2. Although we tend to view trees as “static” (in our terms at least) this perception is a flawed one – caused only by our very short attention span. They really do get around.

3. Like all complex systems, the outcomes of external interventions (including those of mankind) may be predicted to some extent, but never with much hope of certainty. CAVEAT MUTATIO!

Now this all got me thinking about petrified forests (like you do), and once again the internet came up trumps. Browsing through the Wiki entry (another internet wonder!) for petrified forests, I was amazed to discover that one of the 20 or so countries suitable for exploring fossilized trees is… you guessed it… Saudi Arabia. There is a petrified forest “just north of Riyadh”.

The “Edge of the World” in Saudi Arabia
(photo courtesy of Saudi Arabian Fossils Homepage)

So now all I have to do is wait for the summer heat to abate, grab my trusty panama to shield me from the blistering arabian sun, and head off to the edge of the world in search of the start of the world.

Into the desert in search of  Pangaea…

wish me luck

.

* “Distribution of living Cupressaceae reflects the breakup of Pangaea” PNAS journal, 1st May 2012.    http://www.pnas.org/content/109/20/7793.full.pdf+html?sid=9ab3865e-dcd7-4a0b-9ddb-769c717fd37f

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1 comment
  1. Fascinating places – petrified forests: hard to grasp the age of them. We have a couple that I know of here in Oman. One of them gets covered/uncovered by shifting sands, so one never knows what will be seen on a visit.

    David.

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