Researchers at the Chinese Academy of Sciences identified a giant fossilized dipluran called Sinojapyx giganteus from 165 million years ago. Found in China, this rare six-legged predator helps experts understand early insect evolution. "Daohugou has produced hundreds of thousands of insect fossils, but no two-pronged bristletail had ever been found there."
Insects belong to a broader group of six-legged animals called hexapods. This group also includes other less well-known, small animals, including diplurans or two-pronged bristletails, wingless relatives of insects with two rear projections called cerci that resemble feelers or pincers.
The ancestors of diplurans branched off early in the evolution of hexapods. As these animals are rarely preserved as fossils, their evolution remains poorly understood.
Researchers at the Chinese Academy of Sciences and the University of Valencia examined a fossilized dipluran from the Daohugou fossil assemblage in northeastern China. Their paper, published in Proceedings of the Royal Society B, describes a newly identified giant dipluran called Sinojapyx giganteus, which lived approximately 165 million years ago.
"Our work grew out of the Nanjing Institute of Geology and Paleontology's long-term research on the origin and evolution of modern ecosystems, with the Middle Jurassic Daohugou Biota as one of our key windows," Diying Huang, senior author of the paper, told Phys.org. "Daohugou has produced hundreds of thousands of insect fossils, but no two-pronged bristletail had ever been found there."
Studying a rare fossilized Jurassic giant
The fossil analyzed by Huang and his colleagues was collected about 20 years ago. It remains the only dipluran found in the Daohugou fossil assemblage despite decades of fieldwork and screening of hundreds of thousands of fossils, which makes it a particularly rare and valuable specimen.
"Our primary objectives were to describe this remarkable dipluran and place it in a phylogenetic framework (i.e., a framework that maps the inferred evolutionary relationships between organisms), to use its anatomy to test long-standing ideas about hexapod mouthpart evolution, and to reconstruct its ecological role in the Jurassic soil ecosystem," said Huang. "We named it Sinojapyx giganteus and established a new family, Sinojapygidae."
The exceptionally well-preserved fossil analyzed by the authors comes from deposits associated with the Middle Jurassic Daohugou Biota in Inner Mongolia, China, an assemblage of organisms that lived approximately 165 million years ago. The Middle Jurassic is a division of geological time within the Jurassic period.
"We prepared the fossil carefully and examined it under a stereo microscope, using detailed photographs to document its mouthparts, jaws, tail pincers and abdomen," explained Huang. "We then compared these features with those of living and fossil diplurans and ran a phylogenetic analysis to work out where it sits in the family tree."
The team's analysis revealed that the ancient dipluran had partly exposed mouthparts, structures used to handle and consume food. Unlike those of living diplurans, its jaws were not fully enclosed within its head. The team's observation of this feature could help to better understand how enclosed mouthparts evolved.
"This challenges the traditional view that only insects had exposed mouthparts and that the three non-insect groups shared a single origin of hidden mouthparts," said Huang. "We propose instead that the common ancestor of all hexapods had exposed mouthparts, and that the 'hidden' condition evolved independently in proturans, springtails and two-pronged bristletails."
The fossil analyzed by the researchers also captures a transitional stage in the evolution of the pincer-like cerci of Japygoidea, a group of diplurans with specialized rear pincers. The symmetrical pincers of the analyzed specimen lacked small toothlike projections, offering clues to how cerci evolved from sensory structures into more specialized hunting tools.
Toward further analyses of Daohugou fossils
The dipluran analyzed by Huang and his colleagues had a body length exceeding 5 cm (2 inches) and is the largest dipluran fossil recorded to date. The researchers hypothesize that it was a top predator within the Jurassic soil ecosystem, based on its large, powerful jaws and robust rear pincers.
"We will now continue our broader research on the origin and evolution of modern ecosystems, using the Daohugou Biota and other exceptionally preserved fossil deposits as our main windows," said Huang.
In their earlier analyses of fossils from Daohugou, the researchers uncovered some of the earliest known fleas, webspinners (i.e., insects that produce silk) and gladiators (i.e., wingless predators). They expect that ongoing screening of the fossil assemblage and their future analyses will lead to more unexpected discoveries.
"Specifically for diplurans and early hexapods, we want to search for additional specimens that can further test our hypotheses about body pattern evolution," added Huang.
"We also plan to integrate morphological, phylogenetic and ecological evidence to better reconstruct how early hexapod groups diversified and interacted in deep time. International collaboration, including with colleagues at the University of Valencia, will remain an important part of this work."