The largest extinction on Earth began on land and not in the ocean

The great event of mass extinction, popularly called the Great Dying, which wiped out almost 70% of the Earth’s land animals 252 million years ago, took place at different moments of history on land and at sea, according to a new research on fossils from South Africa and Australia collected in the magazine Nature Communications. The delay between land extinction in the southern hemisphere and marine extinction in the northern hemisphere suggests different causes.

The work, carried out by scientists from Colby College and the University of California, Berkeley (U.S.A), found new ages for fossilized vertebrates that lived just after the disappearance of the fauna that dominated the last epoch of the Permian 251 to 299 million years ago.

"Most scientists thought that the Earth’s collapse began at the same time as the marine collapse, and occurred at the same time in the southern hemisphere and in the northern hemisphere," stated Cindy Looy from the University of California. "The fact that the big changes were not synchronous in the northern and southern hemispheres has a great effect on the hypothesis of what caused the extinction. An extinction in the ocean, per se, does not have to have the same cause or mechanism as an extinction on land." 

Resetting the timeline

According to palaeontologists and sedimentologists, changes in the ecosystem during this mass extinction event in which most species disappeared due to a series of volcanic eruptions that shed harmful greenhouse gasses, it all began hundreds of thousands of years earlier on the mainland than at sea.

So, just over 250 million years ago, when dinosaurs were barely in their prime, the Earth was going through an incredibly complicated time. There are signs that tectonic activity near what is now Siberia (Russia) generated eruption pulses that coincided with the loss of approximately 90% of all species, according to chemical changes in the geological record, as well as in the rapid decline of relatively common fossils and animals, such as Daptocephalus, and the increase of bones of opportunistic species, such as the Lystrosaurus, which excelled in the lower Triassic. All of them now extinct.

The subsequent marine extinction, in which almost 95% of ocean species disappeared, may have occurred in the space of tens of thousands of years later. In the ocean, the final extinction of the Permian period is perhaps best associated with the disappearance of the trilobites.

Uranium-lead dating

Experts estimated the ages of zircon mineral crystals in a fairly well-preserved volcanic ash deposit in the Karoo basin of South Africa (which was part of the continental mass of Gondwana, the southern half of the supercontinent known as Pangea that would later be separated into the present continents). They found that the sediments several metres above the dated layer were devoid of Glossopteris pollen (a shrub at the end of the Paleozoic), suggesting that these seed ferns, which used to dominate the flora of the late Permian Gondwana, became extinct around that time. At 252.24 million years old, zircons, microscopic silicate crystals that form in the ascending magma inside volcanoes and are thrown into the atmosphere during eruptions, 300,000 years older than the dates previously indicated for the Permian-Triassic limit in China, confirming that life on earth was already beginning to fade shortly after being deposited.

"Our new zircon date shows that the base of the Lystrosaurus area predates marine extinction by several hundred thousand years, similar to the pattern in Australia," Looy said. This means that both floral and faunal rotation in Gondwana is not synchronized with the marine biotic crisis in the northern hemisphere.

Was the Permian extinction event much slower than we imagined? It would seem so.

"For some years now, we have known that, in contrast to the massive marine extinction, the pulses of disturbance of life on earth continued in the Triassic period. But that the fact that the beginning of the terrestrial renovation occurred long before the marine one, was a surprise ".

"Nuestra nueva fecha del circón muestra que la base de la zona Lystrosaurus es anterior a la extinción marina con varios cientos de miles de años, similar al patrón en Australia", dijo Looy. "Esto significa que tanto la rotación floral como faunística en Gondwana no está sincronizada con la crisis biótica marina del hemisferio norte".

¿El evento de extinción del Pérmico fue mucho más lento de lo que nos imaginábamos? Eso parece.

"Desde hace algunos años, hemos sabido que, en contraste con la extinción masiva marina, los pulsos de perturbación de la vida en la tierra continuaron en el período Triásico. Pero que el hecho de que el comienzo de la renovación terrestre ocurriera mucho antes de que la marina, fue una sorpresa ".

Reference: Robert A. Gastaldo et al. The base of the Lystrosaurus Assemblage Zone, Karoo Basin, predates the end-Permian marine extinction, Nature Communications (2020). DOI: 10.1038/s41467-020-15243-7