Crabs expose colliding continents

ResearchBlogging.orgEvery high school student now learns that plate tectonics slowly drive our continents in different directions. Since only the most uncontroversial scientific knowledge finds its way to high school text books, it’s hard to imagine that when the theory of continental drift was proposed by Alfred Wegener in 1912, it was firmly rejected by the majority of geologists.

Established geologists criticized Wegeneir’s theories for lacking a plausible mechanism of how the continents drifted over our earth (he theorized that the continents moved through the oceans like a snow plow, which was quickly shown to be impossible). They had found and attacked the weakest point in Wegener’s argument, for Wegener hadn’t built his theory upon clever mechanisms, but on careful observations. He compared his evidence driven approach to a trial in court:

At a specified time, the earth can have had just one configuration. But the earth supplies no direct information on this. We are like a judge confronted by a defendant who declines to answer, and we must determine the truth from circumstantial evidence.

~ Alfred Wegener, The Origin of Continents and Oceans, (1929, 4th edition, English translation)

In hindsight, the circumstantial evidence that Wegener collected is incredibly convincing. The first piece of evidence comes from the shape of the different continents. Some landmasses fit together almost like the pieces of a jigsaw puzzle. With a little bit of fantasy it is possible to combine continents and islands like South America, India, Madagascar and Africa into a single landmass. Additionaly, if you link these former neighbours together, it becomes clear that they share several distinct geological features. The Appalachian mountains of Northern America match the Scottish highlands in age and shape, for example.

Fossil patterns across continents: hard to explain without continental drift.

Another line of evidence comes from fossil patterns. Fossils like that of the Triassic Cynognathus can be found in places that are thousands of kilometers apart, like South Africa, Argentina and Antarctica. Nobody would suggest that these creatures independently evolved on these different continents, or that they have swum across the South Atlantic ocean.

These remarkable patters were well known at the beginning of the 20th century. Geologists that rejected continental drift explained these patterns by dreaming up huge continental land bridges wherever they needed one. Different species supposedly migrated over these bridges, until they later became submerged by the rising seas (that’s why we find no traces of them now). Instead, Wegener correctly concluded that such animals had spread over these continents in a time where they still had been joined together.

The German zoologist Von Ihering was one of Wegener’s few supporters who advocated the use of both extinct and extant life to reconstruct past earths:

[I hold to the] conviction that only the history of life on this earth enables one to grasp the geographical transformations of the past.

~ Herman von Ihering, quoted in The Origin of Continents and Oceans, (1929, 4th edition, English translation)

While I wouldn’t want to gloss over the achievements of geophysicists in ‘grasping the geographical transformations of the past’ , I do agree with von Ihering that by placing the evolution of life in the context of our planet’s geological history, we can discover patterns which would be impossible to explain on their own.

A recent BMC paper contains a great example of this approach. In this study, German researchers reconstructed the evolutionary history of freshwater crabs to shed some light on India’s journey from Gondwanaland to Eurasia. Gondwana was the supercontinent formed by Africa, South America, India and Australia. At some point it slowly broke apart, with India drifting all the way to its current position in Asia. While conventional research indicates that India collided with Asia around 50 million years ago, there is new evidence that this collision happened much later, about 35 million years ago. Exactly how India came into collision is still somewhat unclear. Either the Indian subcontinent directly cruised for it’s current position near Tibet, or it had a bumpier ride and collided with an island like Sumatra or Burma first.

Two different scenarios for the position of India 45 million years ago. In scenario A, India first collided with a Southeast Asian peninsula. Scenario B describes India as heading straight for Tibet. Figure 1 from reference.

When continents collide, ecosystems get shaken up. Species from both sides will take the opportunity to explore and exploit the new environments. If India and a Southeast Asian island collided before India hit the Tibetan region, this collision would have left some marks behind on the flora and fauna of both landmasses.

This is where the freshwater crab family Gecarcinucidae comes in. Species of this family can currently be found from India to Australia and everywhere in between. By studying the evolution of these crabs, German scientists retraced the path that India took millions of years ago.

Time and place are the two most important ingredients for such a reconstruction. To get a grip on time, the team first established the full family tree of these freshwater crabs. With the molecular clock they then crudely estimated how many million years ago the ancestral crabs split into different crab species. The geography comes into the mix by overlaying the family tree with the distribution of crab species across India and Southeast Asia today. From this overlay it is possible to estimate the distribution of the ancestral crabs.

They found that the Indian and Southeast Asian crabs living today shared their last common ancestor before the main collision between Tibet and India 35 million years ago. Moreover, the biogeographical models reveal that the crabs spread from India to Southeast Asia. Since freshwater crabs rarely cross open seas, these results suggest that India and Southeast Asia were in contact at some point before the main collision. The crabs clearly favour the double collision hypothesis!

A well hidden paddyfield's crab (Paratelphusa) on West Java.

But there’s more: the researchers noticed that the entire family of Gecarcinucidae was unrelated to crab species from ancient Gondwana. This could mean that the Gecarcinucidae evolved on India itself while it was on its way to Asia! The authors suggest that the marine Gecarcinucidaean ancestor arrived on the Indian subcontinent from the Tethys Ocean, the ancient ocean between Gondwana and Laurasia. While the Indian subcontinent never achieved the degree of isolation that Australia did, it was underway long enough to give rise to some unique species of its own!

I think Wegener would have liked these entrepreneurial crabs. They are the silent witnesses in his hypothetical trial, revealing tiny chapters in the history of our earth. Alfred Wegener never lived to see the acceptance of his theories: he died on an expedition in Greenland, in 1930. But in a perfect example of poetic justice, Wegener is proving his critics wrong even after his death: his grave in Greenland is slowly drifting away from his home country, Germany. We sure live in a strange, drifting world.

I have submitted this blog post to a contest for a travel award to the Science Online conference 2011, granted by NEScent.

Klaus S, Schubart CD, Streit B, & Pfenninger M (2010). When Indian crabs were not yet Asian–biogeographic evidence for Eocene proximity of India and Southeast Asia. BMC evolutionary biology, 10 PMID: 20849594

Picture sources 1 and 3

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