The formation of complex terrestrial ecosystems was a significant innovation in the evolutionary history of life on Earth. This event signified the expansion of life beyond marine environments and marked the beginning of a new phase in the diversification of terrestrial biota. As the most species-rich animal group on our planet, insects played an important role in this evolutionary transition to land. However, significant gaps in the relevant research have long existed.
Now, an international research team has discovered a new stem-group insect from the Late Mississippian period, approximately 324 million years ago. They combined this discovery with previously enigmatic fossil material to reconstruct a pivotal stage in the early evolution of insects. The fossils suggest that insects did not immediately become fully terrestrial after colonizing land. Instead, they retained and remodeled ancestral aquatic features and passed through a prolonged semi-aquatic, amphibious phase.
The study was led by Prof. CAI Chenyang from the Nanjing Institute of Geology and Palaeontology of the Chinese Academy of Sciences (NIGPAS) and Erik Tihelka, a joint-training Ph.D. student at the University of Cambridge, who collaborated with researchers from the United States, Spain, and other countries.
The findings were published in Nature on August 26.
This study describes a new species, Chosha praecursor. The researchers also investigated enigmatic stem‑insect material from the Early Devonian chert biota of Britain and the Late Carboniferous Mazon Creek biota of the United States.
These insect fossils fill critical gaps in early insect evolution. The fossils also revise long-standing interpretations of insect body-plan transformation and pancrustacean terrestrialization. Furthermore, they furnish key empirical evidence for the gradual aquatic-to-terrestrial evolutionary transition of insects.
Molecular‑clock reconstructions suggest that hexapods diverged from their marine crustacean relatives and began adapting to life on land as early as the Cambrian–Ordovician interval. However, undisputed hexapod body fossils only appear in the fossil record in the Early Devonian Rhynie Chert about 405 million years ago. Unambiguous insect fossils do not become abundant until the Late Carboniferous. This creates an 80-million-year gap in the fossil record.
Moreover, direct fossil evidence documenting the gradual adaptation of early insects to terrestrial habitats from aquatic and semi-aquatic settings, as well as the transformation of their body plans, has been scarce. Consequently, the evolutionary pathways, morphological innovations, and ecological adaptive mechanisms underlying insect terrestrialization have been poorly understood.
The newly described C. praecursor fossils were recovered from calcareous claystone concretions in the Tesnus Formation of the Marathon Uplift in western Texas and exhibit exquisite, well‑preserved anatomical details.
Using cross-polarized light imaging, the researchers identified the species' distinctive morphological traits and corrected the longstanding misinterpretation that these specimens were crustacean larvae. The fossils, representing adult females, measured 32.09 millimeters in body length. With a median caudal filament and two cerci included, their total body length reached 49.66 millimeters. The fusiform body displayed derived traits diagnostic of hexapods and insects while retaining ancestral plesiomorphic features.
Systematic analyses revealed that C. praecursor had an ovipositor and a terminal caudal filament, while it had a segmented trunk and six walking legs—the basic body organization characteristic of insects.
Most strikingly, however, segments 1–9 of the abdomen bore segmented appendages, with the posterior abdominal limbs modified into paddle-like structures. Such abdominal appendages are unknown in living crown-group insects. Paleoenvironmental reconstructions indicate that the host strata represent near‑shore, shallow‑water delta‑coastal settings, suggesting that this early stem‑insect lived a semi-aquatic, amphibious lifestyle, inhabiting humid environments at the interface between water and land.
Based on detailed comparative morphology and phylogenetic analyses, the researchers reassessed three enigmatic Paleozoic hexapod fossils: Leverhulmia from the Early Devonian of Scotland, an unnamed hexapod from the Mazon Creek biota of Illinois, USA, and C. praecursor. They determined these hexapods to be members of a primitive insect stem clade. Representing the oldest documented insect assemblage globally, this clade substantially fills a gap in the early evolutionary history of insects.
This study provides important scientific insights that potentially reshape our framework for understanding the terrestrial evolution of insects. It fills the long‑persistent 80‑million‑year hexapod gap. In this way, the early diversification of insects is pushed back from the Late Carboniferous into the Early Devonian, reconciling to some degree discrepancies between molecular‑clock estimates and the body-fossil record. The newly recognized Late Mississippian fossil provides a critical record of the intermediate stages of insect evolution.
The fossils also illuminate one of the most fundamental changes in insect evolution: the transformation of the ancestral body plan.
Modern hexapods have only six walking legs, all of which are attached to the thorax; abdominal appendages have almost entirely been lost. Paleozoic stem insects, by contrast, commonly retained segmented abdominal limbs.
The new fossils suggest that the reduction of abdominal appendages was a key evolutionary innovation for terrestrial adaptation. These structures were progressively simplified as the swimming appendages of crustacean ancestors were lost, ultimately yielding the body organization seen in modern insects. These findings clarify the morphological transition from pancrustacean ancestors to hexapod insects.
Furthermore, the ovipositor preserved in C. praecursor demonstrates that early insects already possessed diverse oviposition adaptations, providing the structural foundation for the subsequent colonization of heterogeneous terrestrial microhabitats and later insect radiations.
In addition, the researchers reconstructed ecological scenarios for early insects. Stem-group insects combined aquatic locomotor and respiratory adaptations with terrestrial body architectures. Their diets likely included humus, plant detritus, and fungal spores. Thus, early insects fulfilled multifunctional ecological roles as decomposers and consumers within aquatic–terrestrial ecotones. They were pivotal in driving the maturation of Paleozoic terrestrial ecosystems.
Fossils of C. praecursor and related Paleozoic stem insects reveal the early trajectory of terrestrialization for the most species-rich animal group on Earth, challenging interpretations of body size evolution, ecological adaptation, and coevolution with terrestrial ecosystems. The colonization of land by insects was gradual; the retention, remodeling, and reduction of ancestral aquatic structures permeated their early evolutionary history.
An amphibious phase appears to have been an important step in this transition, providing insects with a bridge between aquatic and terrestrial environments. These results provide new insights into the origins and early body-plan evolution of insects and offer a broader perspective on the development of complex terrestrial ecosystems on Earth.
