The seabed today offers a rich biotope for worms, snails, mussels and many other creatures. But when did animals first burrow into the sediment? In a fossil deposit in China, researchers have found traces that indicate that animals conquered the seabed around 550 million years ago – around ten million years earlier than previously thought. The discovered tunnels in the rock date back to the Ediacaran period. They suggest that the prehistoric animals’ burrowing activities profoundly changed the living conditions on and in the seabed and thus created the basis for the transition to a new era, the Cambrian.
Long before the first animals developed bones or shells that could be preserved as fossils, living things were already leaving traces. Their fossilized impressions provide paleontologists with important clues about the biology and behavior of early organisms and provide insights into the ecosystems many millions of years ago. A well-known location for trace fossils is the Shibantan deposit in southeast China. The rock layers date from around 550 to 543 million years ago and thus provide an insight into the final phase of the Ediacaran. During this era, the first tissue animals lived in the sea and on the seabed. The sediment, on the other hand, was previously believed to be the realm of microorganisms, which formed dense, uniform mats there.
Passages in the sediment
However, newly discovered trace fossils from the Shibantan deposit now suggest that worm-like creatures ventured into the seabed as early as the Ediacarian – and thus significantly disrupted the previously unmolested microorganisms. Zhe Chen and Yarong Liu from the Chinese Academy of Sciences in Nanjing discovered three-dimensional tunnels in the rock layers that are up to 550 million years old, some of which lead vertically into the ground and others that run parallel to the surface. The researchers assigned these trace fossils to a new species, which they named Treptichnus streptosus.
“Our discovery reveals important behavioral innovations from simple horizontal locomotion to complex sediment penetration,” explain the researchers. Other trace fossils also show that more and more worm-like animals penetrated the sediment at that time. This new behavior likely had serious consequences for the ecosystem at the time: “The advent of three-dimensional sediment exploration fundamentally changed benthic ecodynamics and disrupted the stability of microbial mats,” said the team.
Foundation stone for a new era
As a result, many of the organisms that had shaped the Ediacaran died out – including not only various species of mat-forming microorganisms in the seafloor, but also species that depended on them. “These innovations laid the foundation for animal-sediment interactions and catalyzed a crucial ecological transition that preceded and made possible the Cambrian species explosion,” the researchers write.
The formal boundary between the Ediacaran and Cambrian is currently considered to be the appearance of another trace fossil of the same genus, Treptichnus pedum. But the current study shows that T. pedum was far from the first species to burrow into the ground. In addition to the newly discovered species T. streptosus, the authors also identified numerous other Treptichnus species. According to this, worm-like creatures were already traversing the seabed with their burrows around ten million years earlier than previously thought.
Source: Zhe Chen & Yarong Liu (Chinese Academy of Sciences, Nanjing, China), Science Advances, doi: 10.1126/sciadv.adx9449