Huayuan biota decodes Earth’s first Phanerozoic mass extinction

(Press-News.org) Around 540 million years ago, Earth's biosphere underwent a pivotal transformation, shifting from a microbe-dominated world to one teeming with animal life, as nearly all major animal phyla appeared abruptly in the fossil record over a very short geological time interval. This landmark evolutionary event is known as the Cambrian Explosion. However, this surge in animal diversity was cut short around 513 million years ago by the Phanerozoic eon's first mass extinction, the Sinsk Event—with an extinction rate on par with the planet's five most severe mass extinctions, the so-called "Big Five." In its aftermath, global biodiversity remained low for around 50 million years, until the onset of the Great Ordovician Biodiversification Event.

For decades, scientific understanding of the Sinsk Event has been hampered by gaps in the shelly fossil record across the extinction boundary, where existing fossils preserve only the skeletonized parts of ancient organisms. By contrast, rare soft-bodied fossil deposits offer a far more complete snapshot of ancient ecosystems.

While paleontologists have uncovered dozens of such Cambrian soft-bodied fossil sites—including China's early Cambrian Chengjiang biota in Yunnan and Canada's middle Cambrian Burgess Shale biota, the most famous examples of their kind—no equivalent top-tier soft-bodied fossil deposit had ever been found from the critical post-Sinsk Event time interval.

That changed over the past five years, however, with the discovery of the Huayuan biota—a world-class soft-bodied fossil deposit dating to shortly after the Sinsk Event. The deposit, located in Huayuan County, Hunan Province, was identified by a research team from the Nanjing Institute of Geology and Palaeontology of the Chinese Academy of Sciences (CAS), whose findings were published in Nature on January 28.

The researchers have collected more than 50,000 specimens from a single fossil quarry and formally classified 8,681 specimens. Of these, 153 animal species have been documented, belonging to 16 phylum-level groups—with 59% of these species representing newly described taxa.

Numerous fossils from the Huayuan biota exhibit exquisitely preserved soft-bodied anatomical structures, including digestive, respiratory, and nervous systems. The presence of diverse active predators and abundant pelagic tunicates points to a deep-water faunal community with a complex food web and fully functional biological carbon pump mechanisms.

The Huayuan biota boasts an extraordinary abundance and diversity of soft-bodied fossils, exceptional fidelity of soft-tissue preservation, and complex ecosystem structures, establishing it as a top-rank Burgess Shale-type fossil deposit, rivaling China's Chengjiang biota and Canada's Burgess Shale biota.

Dating to the critical post-extinction interval following the Phanerozoic's first mass die-off, the Huayuan biota provides crucial insights into the processes and consequences of this early extinction crisis. Notably, the biota contains unexpected representatives of taxa known from the Burgess Shale biota—fossils that evidence transoceanic dispersal events in the aftermath of the extinction and highlight the role of ocean currents in shaping the biogeographic patterns of early marine animals.

Furthermore, the research team compiled a comprehensive global dataset of Cambrian soft-bodied biotas and conducted quantitative comparative analyses between the Huayuan biota and other major fossil deposits. These analyses reveal a fundamental reorganization of global marine communities across the Sinsk Event extinction boundary. The findings further suggest that deep-water outer shelf environments acted as critical refugia for faunal migration, biological survival, and evolutionary innovation during this pivotal post-extinction transition.

The research was led by CAS academician Prof. ZHU Maoyan, in collaboration with researchers from the Hunan Museum, the Chengdu Center of China Geological Survey, Nanjing University, Guizhou University, and Linyi University.

The study was supported by the National Key Research and Development Program of China and the National Natural Science Foundation of China, among other sources.

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