Bottom burrowing animals were among the first to recover after the end-Permian mass extinction, according to researchers studying ancient sea bed burrows and trails. Researchers from China, the United States, and the United Kingdom reveal how life in the sea recovered from the event that killed over 90 percent of all species on Earth in a new study published in the journal Science Advances based on their observations of trace fossils.
Life on Earth was devastated by the end-Permian mass extinction 252 million years ago, and it took millions of years for biodiversity to recover to pre-extinction levels. The international team, however, was able to piece together sea life’s revival by pinpointing what animal activity was happening when by examining trails and burrows on the South China sea bed.
“The end-Permian mass extinction and the recovery of life in the Early Triassic are very well documented throughout South China,” said Professor Michael Benton of the University of Bristol’s School of Earth Sciences, a collaborator on the new paper.
“We were able to look at trace fossils from 26 sections through the entire series of events, representing seven million crucial years of time, and showing details at 400 sampling points, we finally reconstructed the recovery stages of all animals including benthos, nekton, as well as these soft-bodied burrowing animals in the ocean.”
The trace fossils show us when and where soft-bodied, burrowing animals flourished in this Early Triassic greenhouse world. For example, elevated temperatures and extended anoxia coincided with low values of behavioral and ecologic diversities across the Permian-Triassic boundary, and it took about 3 million years for ecological recovery of soft-bodied animals to match the pre-extinction levels.
Professor Zhong-Qiang Chen
The study was led by Dr. Xueqian Feng of the China University of Geosciences in Wuhan, and his focus was on ancient burrows and trails. “Trace fossils such as trails and burrows document mostly soft-bodied animals in the sea,” he explained. Most of these soft-bodied animals had no or poor skeletons.
“There are some incredible localities in South China where we find a large number of beautifully preserved trace fossils, and the details can show infaunal ecosystem engineering behaviors, as well as their feedback effects on biodiversity of skeletonized animals.”
Professor David Bottjer, a collaborator in the study from the University of Southern California, added: “One of the most remarkable aspects of the South China data is the breadth of ancient environments we could sample.
“Differential responses of infaunal ecosystems to variable environmental controls may have played a significant but heretofore little appreciated evolutionary and ecologic role in the recovery in the hot Early Triassic ocean.”
Dr. Chunmei Su, another collaborator, said: “The mass extinction killed over 90 percent of species on Earth, and we see that in the catastrophic reduction in the ecological function of the surviving animals in the ocean.
“At first, there were only a few survivors, and recovery began in deeper waters. The recovery of nekton occurred at the same time as the full rebound of infaunal ecosystem engineering activities.”
Alison Cribb, a collaborator in the study from the University of Southern California, added: “The first animals to recover were deposit feeders such as worms and shrimps. The recovery of suspension feeders such as brachiopods, bryozoans and many bivalves took much longer.
“Perhaps the deposit feeders made such a mess of the seafloor that the water was polluted with mud, the churned mud meant suspension feeders couldn’t properly settle on the seafloor, or the muddy water produced by those deposit feeders simply clogged the filtering structures of suspension feeders and prevented them from feeding efficiently.”
“And some animals, such as corals, had completely vanished,” Professor Chen added. Coral reefs did not reappear for a long time.” Dr. Feng concludes, “Why is it important to understand these great geological mass extinctions?”
“The answer is that global warming and ocean acidification caused the end-Permian crisis, which was so devastating to life on Earth, but trace-making animals may have been selected against by the environment in a way that skeletal organisms were not.”
“Our trace fossil data show that soft-bodied animals are resilient to high CO2 levels and warming.” These ecosystem engineers may have played a role in the recovery of benthic ecosystems following mass extinctions, potentially triggering evolutionary innovations and radiations in the Early Triassic.”
