New study pushes back origin of complex animal behavior by almost 10m years

BEIJING -- A group of fossilized tunnels dug by animals about 550 million years ago, discovered in the upper reaches of the Yangtze River in China, have offered new insights into how the earliest animal activities began to shape the seafloor environment.

Previous research has shown that during a pivotal evolutionary period around 539 million years ago, known as the Ediacaran-Cambrian transition, animals began to shift from simply moving across the seabed to actively exploring deeper layers of sediment. The transition from 2D movement to 3D burrowing turned the ocean bottom from a single, uniform environment into a complex, animal-engineered habitat, fundamentally changing marine ecosystems.

Now, a discovery from Central China's Hubei province indicates that this revolutionary change had started much earlier.

Researchers from the Nanjing Institute of Geology and Palaeontology of the Chinese Academy of Sciences conducted a study on the Shibantan Biota, a treasure trove of ancient fossils in Hubei. This fossil biota dates back approximately 550 to 543 million years. They identified several types of burrows made by worm-like animals, indicating that complex animal behaviors were already shaping the seafloor environment nearly 10 million years earlier than previously thought.

These findings were on Thursday published in the journal Science Advances. Researchers involved in the study noted their discovery of Treptichnus streptosus, a new type of fossilized, zig-zag burrow made by an ancient worm.

"Its structure suggests that trace-making creatures were capable of rhythmic and repeatable movement and exploring, which reflects a well-developed nervous system and muscular control," lead researcher Chen Zhe told Xinhua.

Chen also mentioned other known traces found at the site, such as tadpole-shaped burrows and complex, compound tunnels created by ancient animals as they walked, searched for food and temporarily dwelled.

These adaptations indicate that animal behaviors were already diverse and complex at that time -- earlier than the rapid burst of life known as the Cambrian explosion, Chen explained, while adding that the new study extends the timeline of animal evolutionary sophistication further back than before.

Complex digging behaviors inferred from trace fossils, notably, also caused a lot of disturbance in the sediment.

They not only broke up the microbial mats covering the seafloor but also damaged the habitat of Ediacara organisms that lived there, which may have led to their decline, said co-author Liu Yarong, a PhD student at the institute.

This means that early animal burrowing may have contributed to the first known mass extinction event -- the decline of the Ediacara biota around 550 million years ago.

However, this disturbance also created new opportunities for other types of animals to evolve and diversify. Along with additional biological and environmental factors, early animal burrowing helped drive the major ecosystem transformation that took place at the dawn of the Cambrian period, according to the researchers.

In 2018, paleontologists from China and the United States reported the discovery of trace fossils that represented some of the earliest known evidence for animal appendages or legs in the Shibantan Biota in the Yangtze Gorges area.

The new study confirms that this biota, with its rich variety of trace and body fossils -- offers a unique window into one of the most important turning points in the history of life, namely the transition from simple life forms to complex ecosystems.

"Animals are not only shaped by their environment, but they also play a vital role in shaping it," Liu said.