Shrimps, worms and similar bottom-dwellers recovered first after history’s largest mass extinction

Researchers studying ancient seabed burrows and trails have discovered that bottom-burrowing animals were among the first to bounce back after the mass extinction at the end of the Permian period 252 million years ago.

In a new study, published this week in Science Advances, the researchers from China, the United States and the United Kingdom, studying trace fossils from the seafloor, reveal how life in the sea recovered from the event, which killed more than 90% of the Earth’s ocean species.

After the end-Permian mass extinction, millions of years passed before Earth’s biodiversity returned to pre-extinction levels. But by examining ancient seabed trails and burrows from sedimentary rocks in South China, the international team was able to piece together sea life’s revival by pinpointing what animal activity was happening when.

“One of the most remarkable aspects of the South China data is the breadth of ancient environments we could sample,” said David Bottjer, professor of Earth sciences, biological sciences and environmental studies at the USC Dornsife College of Letters, Arts and Sciences.

Study collaborator Michael Benton of the University of Bristol said the team examined samples “representing 7 million crucial years of time, and showing details at 400 sampling points,” which the team used to reconstruct the various recovery stages of all animals that dwelled in, on or near the seafloor.

Bottjer said their findings show a variety of ways different groups of seafloor dwellers responded to changing environmental conditions over time, and how that may have played a more important role in the evolution and ecology of species as life recovered than previously understood.

Study lead Xueqian Feng of the China University of Geosciences, who’s research focuses on ancient burrows and trails, elaborated on Bottjer’s observation.

“Trace fossils such as trails and burrows document mostly soft-bodied animals in the sea,” he explained, but the details of these trace fossils show how the various species — most of which had little or no skeleton — interact with each other and their environment, and how their behaviour affected the evolution of other species, including those that had skeletons.

“The trace fossils show us when and where soft-bodied, burrowing animals flourished in this Early Triassic greenhouse world,” added study director Zhong-Qiang Chen, also of the China University of Geosciences. Chen said it took about 3 million years for ecological recovery of soft-bodied animals to match pre-extinction levels of these types of creatures.

“The first animals to recover were deposit feeders such as worms and shrimps,” said USC Dornsife PhD student and study collaborator Alison Cribb. “The recovery of suspension feeders such as brachiopods, bryozoans and many bivalves took much longer. Maybe the deposit feeders were making such a mess of the seafloor that the water was polluted with mud, the churned mud meant suspension feeders could not properly settle on the seafloor, or the muddy water produced by those deposit feeders just clogged the filtering structures of suspension feeders and prohibited them from feeding efficiently.”

According to study lead Feng, understanding mass extinctions of the distant past and how soft-bodied species recovered can provide important insights relevant to the present and future.

“The end-Permian crisis — which was so devastating to life on Earth — was caused by global warming and ocean acidification, but trace-making animals may be selected against by the environment in a way that skeletal organisms were not,” he said.

Their data reveals how resilient soft-bodied animals were to high carbon dioxide levels and warming, he said, suggesting that these ecosystem engineers may have played an important role in biodiversity recovery,possibly triggering the evolutionary innovations and radiations that occurred in Early Triassic oceans.

About the study

Authors on the study include Feng, Chen, Benton, Chunmei Su, Bottjer, Cribb, Ziheng Li, Laishi Zhao, Guangyou Zhu, Yuangeng Huang, and Zhen Guo.

The study was supported by grants from the National Natural Science Foundation of China.