The rise of complex ecosystems on land was one of the major turning points in the history of life. As organisms expanded beyond the oceans, terrestrial environments became home to increasingly diverse communities. Insects, which today make up the most species-rich animal group on Earth, eventually became central to these ecosystems. Yet important parts of their early transition from water to land have remained unclear.
An international team of researchers has now identified a stem group insect from the Late Mississippian period, about 324 million years ago. By studying this fossil alongside other puzzling specimens from much earlier and later periods, the researchers reconstructed a critical stage in early insect evolution.
Their findings suggest that insects did not become fully terrestrial immediately after beginning to colonize land. Instead, early forms retained and modified features inherited from aquatic ancestors, spending an extended period in a semi-aquatic, amphibious way of life.
The research was led by Prof. Chenyang Cai of the Nanjing Institute of Geology and Paleontology of the Chinese Academy of Sciences (NIGPAS) and Erik Tihelka, a joint training Ph.D. student at the University of Cambridge. They worked with researchers from the United States, Spain, and other countries.
The findings were published in Nature on August 26.
A New Species From 324 Million Years Ago
The researchers describe a new species called Chosha praecursor. They also reexamined mysterious stem insect fossils from the Early Devonian chert deposits of Britain and the Late Carboniferous Mazon Creek biota in the United States.
Together, these fossils help close major gaps in the early history of insects. They also challenge long-standing ideas about how the insect body plan developed and how pancrustacean ancestors made the transition onto land. Most importantly, the specimens provide direct fossil evidence that the shift from aquatic to terrestrial life occurred gradually.
Molecular clock studies have suggested that hexapods split from their marine crustacean relatives and began adapting to terrestrial environments as early as the Cambrian-Ordovician interval. The fossil record, however, has told a less complete story.
Undisputed hexapod body fossils first appear in the Early Devonian Rhynie Chert around 405 million years ago. Clearly recognizable insect fossils do not become abundant until the Late Carboniferous. That leaves an approximately 80 million year gap in the fossil record.
Fossils documenting the intermediate steps have also been extremely rare. Scientists have had little direct evidence showing how early insects moved from aquatic and semi-aquatic habitats into terrestrial environments, or how their bodies changed during that process. As a result, the evolutionary path, anatomical innovations, and ecological adaptations involved in insect terrestrialization have remained poorly understood.
A Fossil Once Mistaken for a Crustacean
The newly described fossils of C. praecursor were found in calcareous claystone concretions in the Tesnus Formation of the Marathon Uplift in western Texas. They preserve unusually fine anatomical details.
Using cross-polarized light imaging, the researchers were able to identify distinctive features that revealed the fossils had been misclassified for years. Rather than crustacean larvae, the specimens were adult female insects.
The bodies measured 32.09 millimeters long. When the median caudal filament and two cerci were included, the total length reached 49.66 millimeters. Their streamlined, fusiform bodies combined features characteristic of hexapods and insects with more primitive ancestral traits.
Detailed analysis showed that C. praecursor possessed an ovipositor and a terminal caudal filament. It also had a segmented trunk and six walking legs, the basic body organization associated with insects.
But its abdomen contained something far more unusual.
Segments 1 through 9 carried segmented appendages, and the rear abdominal limbs had been modified into paddle-like structures. No living crown group insect has comparable abdominal appendages.
Geological reconstructions indicate that the sediments containing the fossils formed in near-shore, shallow-water delta coastal environments. That setting, combined with the animal’s anatomy, suggests C. praecursor lived an amphibious, semi-aquatic lifestyle in humid habitats along the boundary between water and land.
Filling an 80 Million Year Gap
The team also reevaluated three enigmatic Paleozoic hexapods using detailed comparisons of their anatomy and evolutionary relationships. These included Leverhulmia from the Early Devonian of Scotland, an unnamed hexapod from the Mazon Creek biota of Illinois, USA, and C. praecursor.
The researchers concluded that all three belonged to a primitive stem lineage of insects.
Together, they represent the oldest documented assemblage of insects known so far and help fill a major missing interval in the group’s evolutionary history.
The findings could significantly change how scientists understand the rise of terrestrial insects. By filling the long-standing 80 million-year hexapod gap, the work pushes the early diversification of insects back from the Late Carboniferous into the Early Devonian.
That shift brings the fossil record somewhat closer to estimates produced by molecular clock studies. The newly recognized Late Mississippian fossil also provides a valuable snapshot of an intermediate stage in insect evolution.
How Insects Lost Their Extra Limbs
The fossils shed light on another major evolutionary transformation: the emergence of the modern insect body plan.
Living hexapods have six walking legs, all attached to the thorax, while appendages on the abdomen have been almost completely lost. Paleozoic stem insects were different. Many still possessed segmented limbs along the abdomen.
The new evidence suggests that gradually reducing these abdominal appendages was an important adaptation to terrestrial life.
As insects evolved away from their crustacean ancestors, swimming appendages were progressively simplified and lost. Over time, that process produced the body arrangement familiar in modern insects.
The fossils therefore help document the anatomical transition from pancrustacean ancestors to hexapod insects.
Another important feature preserved in C. praecursor is its ovipositor. Its presence indicates that early insects had already developed different adaptations for laying eggs. Such structures could later have helped insects exploit a wide variety of terrestrial microhabitats and may have provided an anatomical foundation for their enormous later diversification.
Early Insects Lived Between Water and Land
The researchers also reconstructed how these ancient insects may have lived.
Stem group insects appear to have combined aquatic adaptations for movement and respiration with body structures increasingly suited to life on land. They probably fed on humus, decaying plant material, and fungal spores.
That would have allowed them to serve several ecological roles, including acting as decomposers and consumers in environments where aquatic and terrestrial ecosystems met. In doing so, early insects may have played an important part in the development and increasing complexity of Paleozoic land ecosystems.
The fossils of C. praecursor and related Paleozoic stem insects provide a new view of how the most species-rich animal group on Earth began its move onto land. They also challenge previous ideas about changes in insect body size, ecological adaptation, and the way insects evolved alongside terrestrial ecosystems.
Rather than making a sudden transition from water to land, insects appear to have undergone a prolonged evolutionary process. Features inherited from aquatic ancestors were retained, repurposed, simplified, and eventually lost as insects became increasingly adapted to terrestrial life.
An amphibious stage may therefore have been a crucial bridge between aquatic and fully terrestrial environments. The fossils offer new clues to the origins and early evolution of the insect body plan while also revealing how the rise of insects contributed to the emergence of increasingly complex ecosystems on land.