Yamaguchi University and others discover fossils in Chinese sedimentary rock from a barren period that clearly indicates marine ecosystem collapse

The end-Permian mass extinction, which occurred around 250 million years ago, was the third of five mass extinction events on Earth. At the boundary of this event, species such as trilobites, brachiopods and Fusulinida, which had previously flourished, became extinct, and bivalves and conches diversified. The Early Triassic, which came immediately after the end-Permian mass extinction, yields few fossils, to the extent that paleontologists call it "the most boring period". It was a time that saw ecosystems at their lowest biodiversity due to the aftermath of the mass extinction. It is thought that the end-Permian mass extinction was caused by large-scale volcanic activity in the north-east of the supercontinent Pangaea, in modern-day Siberia. However, the recovery process after this extinction event was oddly slow, and the reason for the delay in ecosystem recovery was unclear. Moreover, as the fossil record is scarce, the effects of Early Triassic Period volcanic activity on ecosystems of the time were also uncertain.

A joint research group consisting of Associate Professor Ryosuke Saito of the Graduate School of Sciences and Technology for Innovation, Yamaguchi University, and members of China University of Geosciences, Tohoku University, Nagoya University, and other institutions, collected end-Permian-Early Triassic Period sedimentary rock from strata in China, and analyzed its molecular fossils. As a result, they discovered molecular fossils known as C33 normal alkyl cyclohexanes (C₃₃-ACH). C₃₃-ACH are molecular fossils specifically found in strata dating from the end-Permian mass extinction. The group's research outcomes were published in the online version of Palaeogeography, Palaeoclimatology, Palaeoecology.

At the time of the end-Permian mass extinction, global warming progressed rapidly, resulting in a lack of oxygen in the ocean and increasingly severe heavy metal pollution. On the land, acid rain and ultraviolet rays streamed down due to the destruction of the ozone layer. Eukaryotic organisms could not prosper in this environment, and a mass extinction occurred both in the sea and on the land.

It is thought that the organisms from which C₃₃-ACH originated appeared in the sea to support oceanic primary production in place of other algae, which had declined, acting as the last lifeline of primary producers. C₃₃-ACH does not appear in strata from the periods following the end-Permian mass extinction around 250 million years ago, and as they are fossils specific to a time when many ocean creatures were in decline, they were used in strata records as markers of the collapse of the marine ecosystem during the end-Permian mass extinction.

However, with this discovery, researchers have learned that they also appeared in the Early Triassic Period immediately after the mass extinction. The Early Triassic was the period of the recovery process from the end-Permian mass extinction, when biodiversity was not yet restored. As the fossil record is scarce, the relationship between environmental changes during the Early Triassic and biodiversity is not clearly understood, with the exception of the partial fossil record. Now, the fact that fossils used as a marker of marine ecosystem collapse have come from the Early Triassic Period indicates that environmental degradation severe enough for marine ecosystem collapse was also occurring during this period.

Researchers believe it is likely that the simultaneity of large-scale volcanic activity and markers of marine ecosystem collapse is due to a recurrence of the large-scale volcanic activity thought to have caused the end-Permian mass extinction, which led to repeated degradation of the global environment in the Early Triassic, and curtailed biodiversity recovery.

These molecular fossils have only been discovered in one area in South China. In the future, researchers say they will clarify the relationship between the biodiversity recovery process after the end-Permian mass extinction and global environmental changes by investigating the geographic distribution of the spread of the marine ecosystem collapse, whether the effects of the large-scale volcanic activity were local only to South China or worldwide, and the scale of these effects.

Journal Information
Publication: Palaeogeography, Palaeoclimatology, Palaeoecology
Title: Biomarker evidence for the prolongation of multiple phytoplankton blooms in the aftermath of the end-Permian mass extinction
DOI: 10.1016/j.palaeo.2022.111077