Mention "Mass Extinction" and most people will immediately think of the extinction that killed the dinosaurs.

To be fair, this was pretty big, as far as extinctions go. Not only did it kill all of the non-avian dinosaurs, it also finished off the ammonites, belemnites, all of the large swimming reptiles, and many, many others. It's almost like all mammals being killed today.

So yes, pretty big. The K-T extinction, as it's called, ranks among the top 5 greatest extinctions in Earth's paleozoic history.

But it's peanuts to the P-T extinction.

To put it in perspective, at the K-T extinction, about 60% of life on Earth died out. At the P-T extinction, it was about 95%. So it's fair to say that this was when the Earth nearly died.

As for what caused it, there is no simple explanation. It's always been difficult to study this period, literally because of a lack of rock sequences to study. Most people agree that there was a global anoxic event - oxygen was literally drained from the oceans, and that the event wasn't exactly short and sweet; it had many phases and pulses. But what caused the anoxia, and whether that was the sole reason for the scale of the extinction, remains to be seen.

I could fill a whole post - or even a whole textbook - about what could have caused the extinction, and the global ecosystem collapse. What is equally as interesting, though, is how the Earth recovered after this event. The interesting thing is, according to this paper by Brayard et al. in Nature Geoscience is: surprisingly quickly. Geologically quickly, anyway. As in, a mere 1.5 million years is their estimate.

Because 85% of marine species died, it's safe to say that the P-T event forced a widespread restructuring of marine ecosystems - that's despite the fact that a few members from many major groups managed to hold on through the event through to the Mesozoic.

It's always been thought that life didn't exactly flourish after the event. The four million years of the early triassic, the period after the event, has traditionally been viewed as a bit of a botched recovery; weak growth, interspersed with many brief crises. A bit like the UK economy following the credit crunch (ho ho).

But some recent finds have started to challenge this. Recently we've found some sequences that show that the fast-evolving ammonoids and conodonts rapidly diversified after the event, albeit still punctured by numerous crises. This is barely 2 million years after the event.

What we're really interested in is reefs, though. Metazoan reefs, as in, those built by animals, are the hallmark of a settled benthic ecosystem, and we can take their presence to mean that the recovery has ended.

This is exactly what Brayard et al. have found, barely 1.5 million years after the P-T extinction ended, cutting the size of the Early Triassic Reef Gap drastically. The very earliest reefs formed are simply sponges encrusting some shells, which are then joined with some serpulid worm traces, with some other shelly reef builders. These occasionally have tempestites, storm deposits, dumped on them, but then recover. We then get the development of a bone fide sponge-microbe supported reef system, teeming with many different organisms.

a, Abundant sponges encrusting bivalves. b,c,h, Coalesced spheroid sponges identified as lyssacine hexactinellids. dg, Cavities, chambers or in situ spiculitic networks of various incrusting, planar and branched sponges. ik, Sponges encrusting bivalves co-occurring with lenticular to irregular sponge structures with centimetre-thick walls.

You're probably thinking: "but where are the corals?" It is true that corals haven't always been the major reef builders. It is also true that they could be present, but just be non-calcifying forms. However, corals later go on to great heights as the major reef builders in the Triassic.

The likelyhood is that corals just couldn't hack it. The sponge species found are those usually accepted to be stress-tolerant. This period was probably typified by high and rapidly fluctuating temperatures, a nightmare for calcareous organisms due to the way that it dissolves skeletons, and there was likely low pH, low O2 and high H2S as a hangover from the polluting Siberian Traps volcanism. In fact, it may be that even reefs like these would have been rare at the time, restricted to little pockets of milder conditions on particular shores of Pangaea. Obviously, to find out how widespread this early recovery was, we need to do some more collecting.

Though I rubbished 1.5 million years as being a geologically short period earlier on, on reflection, it's actually a long, long time. That's a long time for the Earth to be in this fragile state. I wonder what it would have taken to finish life off for good? It must be near impossible, save for the sun engulfing the planet. I think if there's any testament to the resilience of life, its the PT extinction.



Main citation:

Arnaud Brayard, Emmanuelle Vennin, Nicolas Olivier, Kevin G. Bylund, Jim Jenks, Daniel A. Stephen, Hugo Bucher, Richard Hofmann, Nicolas Goudemand,&Gilles Escarguel (2011). Transient metazoan reefs in the aftermath of the end-Permian mass extinction Nature Geoscience, 4 : doi:10.1038/