Icons of Plate Tectonics

30 years ago, yesterday, I started breathing. 101 years before that, dude on the left, Alfred Wegener, showed up* which seems as good a reason as any to throw up a post that I have been meaning to put together forever.

Nearly every contemporary introductory geology or paleontology textbook–usually right next to the passing mention of Wegner’s frigid death collecting weather data on a Greenland glacier–reproduces some version of this figure:

It is a fantastic picture, one that helps to demonstrate one classic piece of evidence that helped to seal the paradigm shift of plate tectonics. It can be employed to address general concepts like the utility of fossils for addressing broader geologic questions; or to illustrate the actual relationship of the continents approximately 250 million years ago when most of the world’s landmasses were assembled into the massive supercontinent of Pangea.

It made several appearances in my History of Life and Paleobiology lectures this summer.

But there is something about it that has always bothered me: it’s totally bogus.

Well perhaps that is overstating it a bit. The nested ovals and zig-zags purport to show the distribution of several key Gondwanan fossil taxa across the landmasses that now constitute South America, Africa, Asia, India, Antarctica and Australia. The ranges of these species–terrestrial or freshwater plants and animals unlikely to cross major ocean barriers–strongly suggest that the southern continents were long ago connected permitting easy movement between land masses today separated by oceans. The fact that these fossils range from Permian to Triassic in age even gives us an approximate time range for this continental configuration.

But scrutinizing the actually ranges shown in the picture some problems arise. Mesosaurus–something of a poster-fossil for plate tectonics–has been found in Africa and modern day Brazil this map places it in Argentina. Cynognathus on the other hand is known from South Africa, Argentina and Antarctica, while this map appears to show it in Peru and central Africa. Glossopteris has been found across much of South America, India, Africa, Antarctica, Australia and beyond, however this map shows it in a very narrow zig-zag belt across these continents. One version of the figure maintains the overall shapes, but juggles the labels to bring it slightly more into accord with reality.

So what gives?

The map has its origins in a figure printed in Wandering Lands and Animals (1973) written by Edwin Colbert:

Colbert was one the paleontologists that uncovered the Triassic fossils in Antarctica represented in the figure. Clearly the map is meant to be diagrammatic, illustrating, as the original caption states “the paleontological links that bind it [Gondwana] together,” rather than a realistic depiction of the actual ranges of these fossils.

No harm, no foul, I guess although past experience dictates that we simplify the stories fossils tell at our own peril. Creationists (and other science deniers) have a thing for rigid literalism.

For fun, I thought I would fire up the Paleobiology Database to have a look at what the actual distribution of these fossils might look like. Among an assortment of fun tools, PBDB allows you to plot fossil distributions on paleomaps to get a sense for how the ranges of fossil taxa maps on to former continental configurations.

So I punched in Glossopteris, Cynognathus, Lystrosaurus and Mesosaurus, set the map to show the approximate location of the continents at the Permo-Triassic boundary, and here is what I came up with:

Lots of overlapping colored dots. Obviously it needs some more beautification to really bring out the story. So I decided to follow the overall look of the “iconic” version and place polygons over each species’ range:

I left off Glossopteris since it would effectively cover all the land shown in the map. I’m also choosing to sweep under the rug the case of the Chinese Lystrosaurus and Glossopteris which would require a more detailed discussion than I can feasibly get into here. That or I am part of a global tectono-evolutionary old-earth conspiracy that uses free publicly accessible data to deceive the masses.

Er, anyway. You can see the problem here. Glossopteris, Mesosaurus, Lystrosaurus and Cynognathus all overlapped extensively in space (though they were not all contemporaries) and any figure that accurately illustrates their ranges looses the striking aspect of Colbert’s map with four fossils stitching the southern continents together.

But when I started this post today was tomorrow and  I am too lazy to go and fix that first paragraph. Time marches onward, the continents wheel about the planet, species evolve and ebb away. The great Glossopteris forests have evaporated, Rasmus Villumsen (dude on the right) lies buried under an estimated 100 meters of snow, Antarctic ice sheets peel backward revealing a long-lost Triassic menagerie. And I am going to bed.

*We share a birthday with Coco Crisp, Lyle Lovett and Bo Bice. True story.