Episode 64: When life nearly died

Jack Sepkoski’s famous curve (1978), indicating the diversity of marine faunas throughout the Phanerozoic eon. The ‘big five’ extinction events are indicated in yellow: Ordovician-Silurian, Late Devonian, Permian-Triassic, Triassic-Jurassic and Cretaceous-Palaeogene. The PTME (3) shows the greatest decrease in the number of marine families. And from that point onwards, we lose the ‘Cambrian fauna’ and the ‘Paleozoic fauna’ never recovers its former diversity.

Summary of the environmental changes and biodiversity variations from the latest Permian to Middle Triassic. Note the onset of the Siberian Traps large igneous province (STLIP) eruption and the effect this had on the environment and ecosystems throughout the whole Early Triassic. From Chen & Benton 2012.

The current consensus is that the PTME was caused by the eruption of the STLIP, a large volcanic expanse estimated to have been up to 7 million km². It wasn’t the lava itself that caused the extinction, but the large amounts of CO₂ released into the atmosphere and the effect that had on the environment.

From 2004, Prof. Benton participated in the Bristol-Saratov Permo-Triassic Boundary Expedition to the South Urals, Russia. Pictured is Uaz (Ulyanovski Avtombilski Zavod, the Ulyanovsk Car Factory) which carried the team, plus tons of rocks and equipment. It is parked in front of a Soviet-era sign for Gavrilovka, the village nearest their base camp.

The Permo-Triassic boundary in Russia. The Kulchomovskaya Svita (latest Permian) below the ledge, and the Kopanskaya Svita (basalmost Triassic) above, in the Korolki Ravine, near Sol-Iletsk, on the south-western margin of the Urals, Asiatic Russia. The team (left to right), Mikhail Surkov, Michael Benton and Valentin Tverdokhlebov inspect the sandstone at the boundary.

Another section of the Permo-Triassic boundary in Russia. The rock formation at the top of Sambulla hill, near Saraktash, Orenburg region, European Russia, is a massive conglomerate deposited close to the western margin of the Ural Mountains at the very beginning of the Triassic. The grassy slope covers fine-grained latest Permian (Kulchomovskaya Svita) rocks, and the conglomerate marks the base of the Kopanskaya Svita (basalmost Triassic). This higher-energy deposit of terrestrially-derived sediments can be seen in similar sections all around the world. This increase in erosion is attributed to the stripping of vegetation from the land.

The latest Permian Vyatskian fauna from Russia. At the back, the gorgonopsian Inostrancevia looks speculatively at the plant-eating pareiasaur, Scutosaurus. A dicynodont stands at the water’s edge, while the flesh-eating synapsid Annatherapsidus sits on a log, with Dvinia below. The temnospondyl amphibian Chroniosuchus sits on a sand bank, with Kotlassia in the water. In the foreground, the little procolophonid Microphon is to the left, the temnospondyl Raphanodon to the right. (Drawing by John Sibbick.)

Global diversity (dashed line) and mean alpha (community level) diversity (solid line) of Permo-Triassic tetrapod families. Extinctions are labelled as 1, Olson’s extinction; 2, end-Guadalupian extinction; and 3, end-Permian extinction. Even before the PTME, tetrapods had been through several relatively short-spaced extinctions. With poor sampling resolution, these separate extinctions could be misinterpreted as a single event. From Sahney & Benton 2008.

Cosmopolitanism of tetrapods through the Carboniferous, Permian and Triassic. Cosmopolitanism (C), a measure of how wide-spread a group is, is measured as mean alpha diversity ð
T Þ divided by global diversity (Tt ), according to the formula CZ
T =Tt. Note the overall decline of cosmopolitanism through this time interval, perhaps related to increasing taxonomic and ecological diversity of tetrapods, but also note the coupled rises and falls in cosmopolitanism following major extinction events, especially (1) Olson’s extinction, (2) the end-Guadalupian extinction and (3) the end-Permian extinction. This graph shows that at each extinction, tetrapod groups became much more cosmopolitan. Diversity was lost and opportunistic/generalist ‘disaster taxa’, such as Lystrosaurus, were able to spread globally. From Sahney & Benton 2008.

The PTME had a profound affect on marine life in particular. Reconstructed marine ecosystems before and after the end-Permian mass extinction in south China. A. Pre-extinction marine benthic ecosystem in the latest Permian; low abundance, high diversity and dominated by brachiopods, corals, crinoids and fusulinid foraminifers. Scale bar, 10 cm. B. Microbe-dominated ecosystem immediately after the PTME in early Griesbachian (early Induan); primary producers dominate. Scale bar, 10 cm. C. Opportunist-dominated ecosystem in Griesbachian–Dienerian (Induan); high abundance, low diversity and dominated by disaster taxa (for example, the bivalve Claraia). Scale bar, 5 cm. D. Tracemaker-dominated ecosystem in Spathian (late Olenekian), indicating recovery of tracemakers. Scale bar, 6 cm. E. Mid Anisian (Middle Triassic) benthic ecosystem; low abundance, high diversity and dominated by brachiopods and crinoids. Scale bar, 8 cm. F. Mid–late Anisian ecosystem; dominated by marine fishes and reptiles, marking the rebuilding of top-predator trophic structure. Scale bar, 10 cm. Drawings © John Sibbick. From Chen & Benton 2012.

These ecosystems would eventually recover, but they had changed dramatically. Image: Keichosaur killer of Xingyi; on a shallow reef in the latest Middle Triassic (Ladinian) of Xingyi, Guizhou. The deadly Nothosaurus youngi grabs its smaller relative, the abundant Keichousaurus hui, ducking beneath the incredible neck of Tanystropheus cf. longobardicus to do so. Other reef denizens flee in terror: swarms of shrimps (Schimperella acanthocercus) and the fishes Peltopleurus orientalis (numerous small brown fishes), Guizhoubrachysoma minor (short blue), Guizhouamia bellula (slender with orange stripe) and Asialepidotus shingyiensis (brown with vertical bands). Ammonoids (Protrachyceras sp.) and the wingfinned Potanichthys xingyiensis hover unconcerned in the surface waters. Large open-water predators cruise in the distance: the serpentine Anshunsaurus wushaensis and a pair of longnecked Yunguisaurus liae. Painting by Brian Choo © 2013. From Benton et al. 2013.