The researchers examined and did field work on thousands of meters of rock. "We collected samples from Australia, Newfoundland, the western United States, Scandinavia and Namibia," said Soren Jensen, co-author of the PNAS paper and a postdoctoral researcher in the department of earth sciences at UC Riverside. "These samples of ancient marine rocks were then carefully inspected, x-rayed, and thin-sectioned for microscopic examination to provide us with an even closer look."
The authors attribute the exceptional preservation of Early Cambrian trace fossils to the low levels of sediment mixing, which resulted in relatively firm substrates less prone to resuspension. Close inspection of these fossils could help determine exactly when bilaterian animals emerged, a topic of much controversy.
"There have been reports of trace fossils as old as 1 billion years old," said Droser. "But these records are scarce and, on critical examination, are not convincing. On examining the trace fossil record from 565 million years ago through until 535 million years ago, we found that the substrate conditions -- for example, the bottom of the ocean -- were such that if animals were burrowing or walking or crawling along the seafloor, their traces would have likely been preserved. We see a gradual increase in diversity and complexity of trace fossils from about 555 million years ago, known as the Cambrian Explosion and which likely reflects the appearance of bilaterians. We found no evidence for a long history of large animals befo
Contact: Iqbal Pittalwala
University of California - Riverside