Here’s just a quick supplement on yesterday’s article, showing Earth’s historical biodiversity graph correlates with different phases of plate tectonics.
Yesterday we learned that apart from simply stating Earth has experienced 5 [or 4, or 8] mass extinctions, it’s also good to note that a mass extinction is defined as a relative decline of (major) species, of at least 75 percent. However, the absolute numbers (after extinction recovery – millions of years) are far from constant, throughout Earth’s history. In fact, we showed, biodiversity seems to do well with time – as the total number of genera increased about tenfold over the ±500 million years from the mid-Cambrium (when complex life erupted to the Tertiary (Paleogene & Neogene) Period. We also see however timeframes of structural decline (Devonian-Triassic, about 200 million years) and rapid increase (Jurassic-Tertiary, again roughly 200 million years).
An interesting publication in the Proceedings of the National Academy of Sciences (PNAS) last month by two geoscientists of the University of Wisconsin–Madison, comes up with a broad explanation for the pattern. Plate tectonics play a key role. Not just as a key geological factor leading to mass extinctions (for instance the flood basalts that triggered the Permian-Triassic Mass Extinction and the massive rifting volcanism that led to the Triassic-Jurassic Mass Extinction) but also in less catastrophic ways, the assembly and disassembly of the supercontinent Pangaea.
Having a one superocean + one supercontinent situation is detrimental for global (marine) biodiversity, the authors think. For biodiversity it is good to have many small continents, connect a few with some land bridges, separate them again, have islands, and as many coastlines and shallow waters as you can image. Variety of landscape is good for variety of life.
And indeed if we link the downward biodiversity trend of the early Phanerozoic to the (rapid) increase from the Jurassic Period to the formation and subsequent breakup of Pangaea… well, it fits the hypothesis. An interesting more ‘recent’ example is the lesser marine extinction event of the Paleocene-Eocene Thermal Maximum 55 million years ago. That was when the world lost India as a separate continent – due to plate tectonics.
© Rolf Schuttenhelm | www.bitsofscience.org