Mystery of Multiple Extinctions in the Jurassic Solved

In the Cretaceous, meanwhile, why did the T-rex have such short arms?

Ruth Schuster
Ruth Schuster
Gondwana 420 million years ago. View centered on the South Pole.
Gondwana 420 million years ago. View centered on the South Pole.Credit: Fama Clamosa
Ruth Schuster
Ruth Schuster

The only halfway realistic aspect of “Jurassic Park” may be that the land was thronged with life, much of which with fangs. One can hardly generalize about the weather 200 to 145 million years ago, but the Jurassic by and large seems to have been warm and wet. The fact is that a lot of new species emerged in that time, from the lumbering stegosaurs and gigantosaurs to the earliest known proper birds – yes, your duck had ancestors back then. Tiny rodents began to emerge. In the Late Jurassic, the fearsome allosaurus arose too. And no, it isn’t a T-rex.

But there were also a number of mass extinctions during the Jurassic, and the question is what caused them.

One theory involved massive volcanism lasting a million years in the Jurassic, in the southern part of the Gondwana supercontinent. But that correlates to one mass extinction event, called the Toarcian, around 183 million years ago. It cannot explain Jurassic extinctions before or after.

Now, a new paper published in the journal Gondwana Research
reports on dating multiple events of massive volcanism, each lasting millions of years, during the early and middle Jurassic from 186 million to 178 million years ago. These can explain the multiple events of mass extinction during the Jurassic, the team says.

By the way, those mass extinctions would theoretically have freed up niches that could be exploited by newly arising species, leading to the rise of all those animals.

The Deccan Traps in India.Credit: Cj.samson

These volcanic events took place in a single (vast) area of the Gondwana supercontinent, which has since split up into the continents we know and love. Today, lands with remnants of this Jurassic volcanic mayhem lie in southern Africa, Australia and Antarctica – the Karoo province and Ferrar territory, respectively.

The age of the magmatic masses were measured at the Nordsim Laboratory in Stockholm using the “uranium-lead method” on tiny zircon crystals in volcanic rocks. The production of lead from the radioactive decay of uranium provides the most reliable chronometer for dating ancient geological processes, the researchers explained.

“Our results provide strong support for the view that episodic magmatism in the Karoo province may have been the culprit of repeated Jurassic environmental and biological crises,” stated lead author Arto Luttinen from the Finnish Museum of Natural History.

The Karoo-Ferrar igneous province in Jurassic Gondwana is one of the biggest igneous provinces known to date; what caused this magmatic unrest is not entirely clear – possibly, a team suggested in 2017, a tectonic plate oddly named Phoenix slid beneath southern Gondwana.

The tectonic plates of the Pacific Ocean in the Early Jurassic.Credit: Fama Clamosa

Before this paper, Karoo volcanism had been dated to that narrow phase of a million years, 183 to 182 million years ago, which coincided with the Toarcian mass extinction. Now Luttinen and colleagues argue that Karoo-Ferrar volcanism lasted for millions of years.

Their conclusion is based on analysis of crystals in a 1,000-kilometer-long (621 miles) lava zone in Mozambique. That did show the previously established peak of volcanism 183 to 182 million years ago, which is good. And it showed volcanic spasms from 190 to 185 million years ago and another 181 to 178 million years ago. That was then followed not by quiet but by reduced volcanism that would last millions of years more. And this, they say, plausibly explains the multiple extinctions in the Jurassic that could not be explained by that one-million-year-long volcanic vomiting.

To be clear, Karoo-Ferrer was not the only “Large Igneous Province,” or LIP, in Gondwana. At its heyday, Gondwana was more than 100 million kilometers in area, encompassing what are today Africa, South America, much of Australia and Antarctica, the Indian subcontinent, Arabia and Madagascar. During the 300 million years it existed, it experienced multiple volcanic periods that massively impacted the global climate and environment. Remnants of Gondwanan LIPs can be found today in the Americas, Africa, Australia, the Antarctic, Zealandia (the eighth continent recently announced, almost all of which is now submerged) and the Indian subcontinent.

The Indian subcontinent’s most famous LIP is the Deccan Traps, but that was far, far after the Jurassic: the Deccan eruptions are dated to about 65 million years ago, the Cretaceous – and why yes, that is also thought to have caused mass extinctions irrespective of the Chicxulub meteor blamed for killing most of the dinosaurs.

The Deccan Traps are estimated to have been created by eruptions over 30,000 years, as opposed to millions of years in the Jurassic. The Siberian Traps, another example of volcanism of a degree we cannot imagine, lasted about one to two million years, happening about 250 million years ago – way before the Jurassic, in northern Pangaea. We mention the Deccan and Siberian traps because they are very well-known and could confuse the issue. The Siberian Traps are thought to have been responsible for the massive Permian extinction.

The Deccan Traps in India.Credit: Ssriram mt
The Deccan Traps in India.Credit: Nicholas (Nichalp)

So there we have the story. There were multiple extinctions in the Jurassic and now science may have found the smoking volcanism behind them.

Meanwhile in the Cretaceous

Which brings us to another story making the news, which has absolutely nothing to do with volcanism or the Jurassic but to dinosaurs in the Cretaceous. Dinosaurs is the only connection.

The question is, why did the T-rex have such short arms? It arose from a quadruped with presumably four functional locomotory limbs, but a 45-foot-long rex might have forelimbs all of 3 feet long. That is the equivalent of a 6-foot human with 5-inch arms, it has been pointed out.

It bears qualifying that not all think rexian arms are vestigial, and some think they had their purposes – even if those are hard to fathom millions of years after the event. Maybe they served a slashing function when in close proximity, or a function in mating. Who knows.

Illustration of T-rex with babyCredit: Herschel Hoffmeyer / Shutterstock

Now, paleontologist Kevin Padian at the University of California, Berkeley, suggests a new theory, published in Acta Palaeontologica Polonica: adaptation to feeding frenzies. T-rex arms shrank in the course of T-rex evolution because those with long arms had them bitten off by their colleagues as they engaged in riotously dismembering their dinner, so they died and did not procreate as much as the ones with shorter arms.

Padian admits he can’t prove his theory any more than anybody with other theories has been able to. We note that rex relatives also had relatively puny arms, so if he’s right, this feeding frenzy impetus began early on.

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