You can imagine that whatever the causes of the great extinction at the end of the Permian, the effects continued at least a while into the Triassic. It looks like it took maybe 4 million years or so for the recovery to be clearly underway. Extinctions are double-edged swords – while they decimate many species and eliminate countless individuals, they also clear the slate, opening up ecological niches, so that when conditions allow, the survivors that can change and adapt to the new circumstances have great opportunities to expand into vacated ecological realms. Extinctions favor the opportunistic, to some degree at least.
So what were the conditions during the Triassic that life had to adapt to? To a large extent, Pangaea was still assembled into one big continent. We heard about small blocks rifting off to form the Cimmerian continent, and there is evidence that Pangaea had started to rift apart between today’s Greenland and Scandinavia – the first hints of the North Atlantic Ocean. But on the whole, it was still one big continent, and its tropical interior, distant from the sea was in many places pretty hot and arid. The red beds that characterize much of the Triassic are clear evidence for this.
On average, carbon dioxide levels in the atmosphere were high. Depending on which geochemical model you use, the level could have been as much as 1500 parts per million, 4 times today’s value. And oxygen levels had plummeted during the Permian, to possibly as low as 15% of the total versus 21% today. That continued during the Triassic, increasing slowly through the period. The overall average temperature was higher than today, probably at least 3ºC higher, and perhaps more. All of the geochemical models and actual measurements indicate hot, dry, carbon-dioxide-rich settings during the Triassic, and that meshes well with the kinds of rocks – commonly, red beds and evaporites – and terrestrial life, adapted to such conditions, that we actually observe.
Arid and dry does not necessarily mean conditions like the middle of a modern desert. There had to be some water, both for life and to give the alternating wet-dry conditions that produce red beds and evaporites. And even modern deserts, with some relatively small exceptions, support life. The Triassic seems to have been an intensely seasonal time – in part because of the world ocean, Panthalassa, which would have generated monsoons bringing seasonal rains to at least the coastal parts of Pangaea. Large woody trees, hit hard by the Permian extinction, appear to have recovered to a large extent within about 6 million years, about 245 million years ago during the early Triassic.
You can and should imagine that we would not expect the 50-million-year time span of the Triassic climate to be uniform and boring. There was always variety, depending on where you were on the globe, and there was plenty of variation in time as well. There’s even evidence that at times during the Triassic the climate became much wetter. We’ll talk about one such period in the late Triassic later this month.
—Richard I. Gibson