September 9. Recovery from extinction

The extinction at the end of the Permian decimated marine invertebrates, with 90 to 95% of all species disappearing. The ammonites, the shelled cephalopods related to octopuses and squids and the modern chambered nautilus, were almost wiped out completely, but the few survivors – perhaps just a single lineage – adapted and increased in diversity almost explosively during the Triassic, but underwent another decline late in the period.  

Encrinus, Triassic crinoid from Germany. Photo by Ghedoghedo under GFDL. 

Rocks from the first few million years of the Triassic, right after the extinction, contain few marine invertebrate fossils, as you might expect. Some of the groups that had been prominent during the Paleozoic, including brachiopods and crinoids, never really rebounded from the extinction, although both survive to this day in small numbers and without much biodiversity.

Corals and clams seem to have adapted pretty well to the new conditions and their recovery from the extinction was accomplished within about 10 million years or so – but during the early Triassic, there may have been strong variations in the carbon cycle, affecting carbon dioxide in the atmosphere as well as in sea water, and that would have had a big impact on things like corals and clams that secrete calcium carbonate. Link (carbon cycle) Despite this, a new important branch of corals called the scleractinians evolved during the Triassic. They are also called stony corals and they represent the modern reef-building corals we have today. They probably evolved from relatives of the rugose corals that lived in the sea from Ordovician to Permian time, but rugose corals didn’t survive the end Permian extinction.

There are no known coal beds from the early Triassic, so much so that it’s called the global coal gap. Probably the best theory to explain this is the idea that land plants in enough abundance to make coal were severely affected by the Permian extinction, and they did not really recover for at least 6 and more like 20 million years – at least that’s how long it took for coal to appear in the rock record again in much volume. They would have had to evolve adaptations to the new conditions, and that takes time – probably more time for relatively long-lived things like trees than for short-lived ammonites. A further explanation for why it took so long for trees to recover is the suggestion that temperatures were “lethally hot” during the early Triassic, at least in tropical zones, maybe even too hot for trees to survive. The high temperatures may have extended to tropical sea water, with one report suggesting surface sea temperatures of 40°C, 15 degrees higher than today. That could certainly have affected sea life.  I have links below to several technical papers that address these issues if you are interested in more detail.

—Richard I. Gibson

Links:
Coal Gap 

Lethally hot temperatures (Sun et al. 2012) 

Early Triassic ocean life (Bottjer 2012)

Encrinus, Triassic crinoid from Germany. Photo by Ghedoghedo under GFDL.