A recently published study was based on an analysis of rocks that are 518-million-years-old and contain the oldest collection of fossils that scientists currently have on record. According to the study, the ancestors of many creatures alive today may have lived more than 500 million years ago in modern-day China.
In Yunnan, southwest China, scientists discovered one of the oldest groupings of animal fossils now known to science, containing the remnants of more than 250 species.
It’s an important record of the Cambrian Explosion, which saw the rapid spread of bilaterian species – creatures that, like modern animals and humans, possessed symmetry as embryos, meaning they had a left and right side that are mirror images of each other.
Fossils discovered at the 518-million-year-old Chengjiang Biota include worms, arthropods (ancestors of living shrimps, insects, spiders, and scorpions), and even the earliest vertebrates (ancestors of fish, amphibians, reptiles, birds, and mammals). The findings of the recent study revealed for the very first time that this environment was a shallow marine delta that was rich in nutrients and was impacted by storm floods.
Although the area is presently on land in the mountainous province of Yunnan, the team examined rock core samples that revealed evidence of marine currents in the environment that existed in the past.
“The Cambrian Explosion is now universally accepted as a genuine rapid evolutionary event, but the causal factors for this event have been long debated, with hypotheses on environmental, genetic, or ecological triggers,” said senior author Dr. Xiaoya Ma, a palaeobiologist at the University of Exeter and Yunnan University.
“The discovery of a deltaic environment shed new light on understanding the possible causal factors for the flourishing of these Cambrian bilaterian animal-dominated marine communities and their exceptional soft-tissue preservation.”
“The unstable environmental stressors might also contribute to the adaptive radiation of these early animals.”
Co-lead author Farid Saleh, from Yunnan University, said: “We can see from the association of numerous sedimentary flows that the environment hosting the Chengjiang Biota was complex and certainly shallower than what has been previously suggested in the literature for similar animal communities.”
Changshi Qi, another co-lead author and a geochemist at the Yunnan University, added: “Our research shows that the Chengjiang Biota mainly lived in a well-oxygenated shallow-water deltaic environment.”
“Storm floods transported these organisms down to the adjacent deep oxygen-deficient settings, leading to the exceptional preservation we see today.”
Co-author Luis Buatois, a paleontologist and sedimentologist at the University of Saskatchewan, said: “The Chengjiang Biota, as is the case of similar faunas described elsewhere, is preserved in fine-grained deposits.”
“Our understanding of how these muddy sediments were deposited has changed dramatically during the last 15 years.”
“Application of this recently acquired knowledge to the study of fossiliferous deposits of exceptional preservation will change dramatically our understanding of how and where these sediments accumulated.”
The findings of the research are significant because they indicate that majority of the early species were able to adapt to challenging environments such as salinity fluctuations and large volumes of sediment deposition.
This contradicts the findings of earlier studies, which suggested that animals with identical characteristics colonized deeper waters and marine environments with greater stability.
“It is hard to believe that these animals were able to cope with such a stressful environmental setting,” said M. Gabriela Mángano, a paleontologist at the University of Saskatchewan, who has studied other well-known sites of exceptional preservation in Canada, Morocco, and Greenland.
Maximiliano Paz, a postdoctoral fellow at the University of Saskatchewan who specializes in fine-grained systems, added: “Access to sediment cores allowed us to see details in the rock which are commonly difficult to appreciate in the weathered outcrops of the Chengjiang area.”
The paper, published in the journal Nature Communications, is entitled: “The Chengjiang Biota inhabited a deltaic environment”