BEIJING, Oct. 2 (Xinhua) -- A new study has identified trace fossils of a large horseshoe crab species dating back more than 240 million years, from the shallow reaches of long-vanished waters in east China.
These findings offer valuable clues to ocean life after the most severe extinction event in Earth's history.
The fossils were preserved in a rock formation in Chaohu City, Anhui, which is now an inland province. Researchers identified nine complex trails and tracks, curved or straight, up to 5 meters long. They found the traces were marked by parallel scratch marks. After comparing them with fish and invertebrate animals, such as insects and spiders, they believed the trace-maker was a horseshoe crab.
Horseshoe crabs are marine arthropods, which are invertebrate animals with an external skeleton and jointed legs. They are named for their horseshoe-shaped bodies but are not true crabs at all. They are actually more closely related to scorpions and spiders. Because their basic structure has changed very little over hundreds of millions of years, horseshoe crabs are often referred to as "living fossils."
In the new study, published in the Journal of Paleontology, researchers said the size of the traces suggested the animal was a true giant, potentially over 70 cm long, making it one of the largest horseshoe crabs ever identified.
Co-author Xing Lida, of China University of Geosciences (Beijing), said previously discovered fossilized horseshoe crabs were mostly small. Records include larger footprints found in France, whose maker was estimated to be about 85 cm long, and records from north China, which indicated a body length of up to 89 cm.
Based on the shape of the traces, the new study inferred that the ancient horseshoe crab lived in a shallow sea and swam or moved near the seafloor. Its appendages or parts of its carapace occasionally brushed the bottom, leaving the distinctive scratches.
Interestingly, modern horseshoe crabs swim upside down, with their belly up. However, the traces imply a back-up swimming posture, indicating that the ancient horseshoe crabs may have behaved differently from living species.
"That's a big surprise and raises new questions about how these animals behaved in the past," said co-author Gabriel E. B. de Barros, at Brazil's Federal University of Sao Carlos. He added that the finding highlights how trace fossils can unlock secrets of animal behavior.
Barros told Xinhua that his main contribution to the international study was comparing the traces with other known traces and determining the likely trace-maker. He has worked on trace fossils from glacial and lake environments since undergraduate studies.
By revealing how extinct horseshoe crabs differed from living ones, the study offers rare evidence of the recovery of shallow marine ecosystems after the Permian-Triassic mass extinction, a global catastrophe that occurred about 250 million years ago, the study noted.
The research team also includes several members from Anhui, home to Chaohu Lake, China's fifth-largest freshwater lake. Before this study, the region had already yielded the Chaohu Fauna, a highly diverse marine community of the Early Triassic period. The presence of large horseshoe crabs adds a new species to this fauna, further enriching the region's biodiversity at that time.
"We believe the future holds many more discoveries in the region," the researchers said. ■
