In Episode 5, we talked about the K-Pg mass extinction – the one that ended the Age of Dinosaurs. This episode, we discussed the extinction that started the Age of Dinosaurs: The end-Triassic mass extinction.
In the News:
A Giant in Colorful Armor
That gorgeously-preserved ankylosaur from a few episodes ago is now published! Its name is Borealopelta, and it appears to have had a common form of camouflage! [Report]
A Post-Extinction Apex Predator
A 6-foot fish discovered in Nevada lived only a million years or so after the Permian mass extinction, suggesting that food webs bounced back surprisingly quickly. [Report by David]
Variation: More than Just Genes
These cichlid fish jaws come in a variety of shapes, but not all this diversity is genetic. A new study explores how the fish’s behavior changes how their jaws develop, a different source of variation for evolution to act upon. [Report]
Fast Evolution: Big-Headed Lizards
In Brazil, flooding split a species of lizard onto several islands, and over just 15 years they’ve evolved larger heads as an adaptation to feed on the newly-available variety of insect prey! [Report]
The Triassic Period
The Triassic Period began around 250 million years ago, in a world ravished by the Permian mass extinction (we’ll do an episode about that one, too, someday!). Much of the diverse lifeforms of the Paleozoic Era were gone, and the world had entered the Mesozoic: the Age of Reptiles.
Throughout the Triassic, evolution produced a huge variety of reptilian creatures. On land, a variety of croc-cousins (and croc-like non-croc-cousins) vied for dominance with the earliest dinosaurs and their relatives, while the earliest turtles, pterosaurs, lizards, and more scuttled about nearby. In the aquatic realm, many lineages of reptiles took to the sea, including the first ichthyosaurs and plesiosaurs. This time also saw the first mammals, while a wide variety of their ancient cousins ruled the landscape alongside the big reptiles.
But around 200 million years ago, that diversity took a huge hit. Plankton, plants, marine invertebrates, land animals … altogether, hundreds of families of life disappear at the end of the Triassic Period. World ecosystems were dramatically changed. Most famously, with so much ecological space freed up on the land, one group of reptiles was able to rise to dominance – with the start of the Jurassic Period, the Age of Dinosaurs truly began.
So, what happened?
It’s always difficult to say for sure what caused a mass extinction, and the end-Triassic is a particularly challenging puzzle since not much is known about this time period compared to, say, the end-Cretaceous. There’s even a good deal of research over whether this was one big burst of extinction or several.
But we do know that the end-Triassic was a time of major environmental change, with evidence showing high global temperatures, high carbon dioxide levels, low ocean oxygen, and high ocean acidity, among other things. And we also know of a few major geologic events that may have been involved in these dramatic shifts.
Here are the main candidates:
Volcanoes – the Central Atlantic Magmatic Province (CAMP)
During the end-Triassic, the great supercontinent Pangaea was in the process of breaking in half, and the rift zone that would eventually become the Atlantic Ocean was a region of major volcanic activity, stretching 6,000 miles across the landmass. Today, the cooled lava left behind forms one of the most extensive Large Igneous Provinces in the world, covering millions of cubic kilometers over parts of North America, South America, Africa, and Europe.
Volcanic activity can release all sorts of gases that can mess with the atmosphere, including CO2 that can cause such effects as rising temperatures, oxygen-poor waters, and acidified oceans. And there’s some evidence that CAMP volcanism occurred in several major pulses over hundreds of thousands of years.
But this isn’t a sure-fire extinction-forcer. For one thing, there’s some dispute over the timing of the eruptions, with various studies suggesting that they happened too early or too late to have been the main cause. Then again, some other studies find the timing matches up well – the conversation continues!
An asteroid impact can cause many of the same effects as intense volcanic activity, mainly through the release of various gases into the atmosphere. And there is some evidence of impact-minerals in various parts of the world, as well as a big hole in Canada called the Manicougan Crater, all dating to around the end-Triassic. Much like the more familiar end-Cretaceous event, it may be that an asteroid strike kicked off the worldwide collapse of ecosystems.
But again, the picture isn’t totally clear. And again, timing is a big factor: there is some debate over whether or not the crater, the impact minerals, and the beginning of extinction line up in the record. If not, then maybe this isn’t our extinction spark after all.
Sea Level Change
The time surrounding the end-Triassic boundary is a period of major rise and fall in sea level, including lots of smaller fluctuations along the way. Shifting seas can re-arrange coastal habitats and wreak havoc on ecosystem balance. We don’t know if these changes would have been big enough to cause a mass extinction by themselves, but they could certainly have made the world’s ecosystems vulnerable.
All Together Now
It’s also possible that none of these events could have done the job alone. Maybe the combination of volcanism, asteroid impact, and changing habitat space came together to make this burst of disappearing species worthy of a spot on the list of the Big Five Mass Extinctions. Exactly how – and if – these big changes were related to the mass extinction is a subject of ongoing study.
For more information on the end-Triassic extinction, check out these links: