Spectacular fossil treasure trove pushes back origins of complex animals

Part of the research team from the University of Oxford and Yunnan University during June 2024 fieldwork in the section of Jiangchuan Biota. From left to right, Dr. Wenwen Wen, Professor Peiyun Cong, Dr Frances Dunn, Associate Professor Luke Parry, Associate Professor Fan Wei and Dr Gaorong Li. Image credit: Gaorong Li.

A newly discovered fossil site in southwest China has transformed our understanding of how complex animal life emerged on Earth, revealing that many key animal groups had already evolved before the start of the Cambrian Period. The study, led by researchers at Oxford University’s Museum of Natural History and Department of Earth Sciences as well as Yunnan University in China, has been published today in Science.

One of the most transformative events in Earth’s history was the rapid diversification of animal life, resulting in a dramatic increase in complexity and diversity from simpler life forms. Up to now, this was thought to have occurred at the start of the Cambrian Period, in an event known as the Cambrian explosion, starting around 535 million years ago. The new study, however, shifts this timeframe back by at least 4 million years, to the end of the Ediacaran period.

Lead author Dr Gaorong Li (Yunnan University at the time of the study, now Museum of Natural History, Oxford University), said: “Our discovery closes a major gap in the earliest phases of animal diversification. For the first time, we demonstrate that many complex animals, normally only found in the Cambrian, were present in the Ediacaran period, meaning that they evolved much earlier than previously demonstrated by fossil evidence.”

The discovery comes from the Jiangchuan Biota in Yunnan Province, southwest China, where more than 700 fossil specimens were recovered, aged between 554 and 539 million years old. The fossil site revealed a diverse community of Ediacaran organisms - both new, undescribed animal forms and groups known from the Cambrian period. Most strikingly, the international team identified fossils thought to be the oldest known relatives of deuterostomes – the broader group that today includes vertebrates such as humans and fish. The new fossils push the fossil record of deuterostomes back into the Ediacaran Period for the first time.

A deuterostome cambroernid fossil from the Jiangchuan Biota (~554-539 million years old) and artist’s reconstruction, scale bar: 2mm. Image credit: Gaorong Li & Xiaodong Wang.

Among these fossil specimens were ancestors of modern starfish and their closest relatives, the acorn worms (the Ambulacraria). These fossils have a U-shaped body and were attached to the seafloor with a stalk, with a pair of tentacles on their head used to catch food. 

Co-author Dr Frankie Dunn (Museum of Natural History, Oxford University) said: “The presence of these ambulacrarians in the Ediacaran period is really exciting. We have already found fossils which are distant relatives of starfish and sea cucumbers and are looking for more. The discovery of ambulacrarian fossils in the Jiangchuan biota also means that the chordates – animals with a backbone – must also have existed at this time.”

Other ancestral groups among the fossils included worm-like bilaterian animals (having bilateral symmetry), some with complex feeding adaptations, alongside rare fossils interpreted as early comb jellies.

Many specimens showed novel combinations of anatomical features (such as tentacles, stalks, attachment discs, and feeding structures that can be turned inside out) that do not match any known Ediacaran or Cambrian species. “For instance, one specimen looks a lot like the sand worm from Dune!” Dr Dunn added.

"This discovery is extremely exciting because it reveals a transitional community: the weird world of the Ediacaran giving way to the Cambrian, the following time period where the animals are much easier to place in groups that are alive today. When we first saw these specimens, it was clear that this was something totally unique and unexpected.”

- Associate Professor Luke Parry (Oxford Earth Sciences), co-author of the study

The new findings help to resolve a long-standing puzzle in evolutionary biology. While molecular studies and trace fossils suggested that animal lineages diversified well before the Cambrian explosion, up to now fossils of many of these groups of complex animals have been missing from the Ediacaran period.

The Haootia-like fossil (an early cnidarian – the phylum that includes jellyfish, sea anemones and corals) from the Jiangchuan Biota (~554-539 million years old) and artist’s reconstruction. Scale bar: 2mm. Image credit: Gaorong Li & Xiaodong Wang.

Unlike most Ediacaran fossil sites, which preserve organisms mainly as impressions on sandstone surfaces, the Jiangchuan Biota fossils are preserved as carbonaceous films, a mode of preservation more typical of famous Cambrian sites such as the Burgess Shale in Canada. This exceptional preservation reveals anatomical details such as feeding structures, guts and locomotory organs.

Co-author Associate Professor Ross Anderson (Museum of Natural History, Oxford University) said: “Our results indicate that the apparent absence of these complex animal groups from other Ediacaran sites may reflect differences in preservation rather than true biological absence. Carbonaceous compressions like those at Jiangchuan are rare in rocks of this age, meaning that similar communities may simply not have been preserved elsewhere.”

Reconstruction of Jiangchuan biota (~554-539 million years ago). Image credit: Xiaodong Wang.

The new fossils were discovered by a research group in Yunnan University, China, led by Professor Peiyun Cong and Associate Professor Fan Wei, who have spent nearly ten years looking for diverse Ediacaran animal fossils. The rocks from Eastern Yunnan were already known to contain fossils but previously had yielded only remains of algae and not animals.

Associate Professor Fan said: “After years of fieldwork, we finally found several sites with the right conditions where animal fossils are preserved together with the abundant algae.”

Professor Feng Tang from the Chinese Academy of Geological Science, Beijing, whose previous work on the site inspired the team’s decade-long fieldwork effort, said: “The new fossils provide the most compelling evidence for the presence of diverse bilaterian animals at the end of the Ediacaran, evidence people have searched for across decades.” 

The study ‘The dawn of the Phanerozoic: a transitional fauna from the late Ediacaran of Southwest China’ is available to read in the journal Science at https://doi.org/10.1126/science.adu2291.