Chinese researchers have uncovered a ‘palaeonursery’ of fossils in the country’s southern Yunnan province, offering detailed insight into the Cambrian period when life on Earth first began to explode in diversity.
The deposit revealed 2846 fossil specimens of early vertebrates and other soft-bodied organisms. More than 50% were in the larval and juvenile stages of development, many still with appendages intact and soft tissues visible, and even some eggs were found.
“It’s just amazing to see all these juveniles in the fossil record,” says Julien Kimmig from Penn State University, US. “Juvenile fossils are something we hardly see, especially from soft-bodied invertebrates.”
Kimmig is the co-author of a new paper published in Nature Ecology and Evolution. The research was led by Xianfeng Yang, a palaeobiologist at Yunnan University, China, who, along with a Chinese team, collected and recorded fossils at the site, known as the Haiyan Lagerstätte.
The paper reports a total of 118 species (with 17 previously unknown ones), including the ancestors of today’s insects and crustaceans, worms, trilobites, algae, sponges and precursors to jawless fish. All were marine species, because all life at this stage – approximately 518 million years ago in the Early Cambrian – was in the ocean.
The fossils add to the diverse Chengjiang biota, found in the Maotianshan Shales of Yunnan in 1984. Like the fossils of the younger Burgess Shale of British Columbia, Canada, the Maotianshan Shales preserves both hard skeletal parts of organisms and non-mineralised skeletal parts and softer tissues.
The new specimens include body parts never seen before in early deposits, such as compound eyes.
Studying juvenile fossils also offers palaeontologists the opportunity to understand how these ancient animals developed.
“We’ll see how different body parts grew over time, which is something we currently do not know for most of these groups,” Kimmig says. “And these fossils will give us more information on their relationships to modern animals. We will see if how these animals develop today is similar to how they developed 500 million years ago, or if something has changed throughout time.”
The accumulation of so many juveniles at one site has researchers wondering whether this place was a ‘palaeonursery’, where, as modern animals do, species chose to lay their eggs because it offered protection from predators.
On rare occasions, these nurseries are preserved in the fossil record – one example is in South Australia’s Billy Creek Formation, where an assemblage of juvenile trilobites has been suggested to be a palaeonursery, as they were found in the intertidal zone while adult specimens were found in deeper water.
But there is another possible explanation for the juvenile party in the Haiyan section, involving rapid environmental change.
“The site might represent a place where habitable and inhospitable conditions fluctuated rapidly, perhaps due to variations in ambient oxygen availability, salinity or other external factors,” the researchers explain in the paper.
“In such a setting it is possible that some specimens may have invaded and reproduced, thus explaining the abundant larvae, juveniles and subadults. Rapid environmental change resulted in the death of the specimens before the population could reach a steady state. A catastrophic event such as this would skew the fossil record towards juvenile specimens.”
Either way, studying these fossils helps us understand the diversity and rates of evolutionary change in one of the most important moments in the history of life.