The establishment of complex terrestrial ecosystems represents a milestone innovation in the evolutionary history of life on Earth. It marks the definitive release of organisms from marine constraints and inaugurates a brand-new phase for the diversification of terrestrial biota. Evidence from molecular‑clock estimates, trace fossils and exceptional‑preservation biotas indicates that the terrestrialization of arthropods stretches back to the Cambrian‑Ordovician, far earlier than inferred from conventional body-fossil records. As the most species‑rich animal group on our planet, insects have long posed major gaps in research regarding their early origins and the evolutionary transition to land.
Paleontology has long grappled with the well-known hexapod gap. Molecular‑clock reconstructions suggest that hexapods diverged from marine crustacean relatives and initiated terrestrial adaptation as early as the Cambrian‑Ordovician. However, globally undisputed hexapod body fossils are only documented from the Early Devonian Rhynie Chert (ca. 405 Ma), and unambiguous insect fossils do not occur in abundance until the Late Carboniferous. This creates an 80-million-year gap in the fossil record. Moreover, direct fossil evidence documenting how early insects gradually adapted to terrestrial habitats from aquatic and semi‑aquatic settings, as well as the transformation of their body plans, has remained scarce. Consequently, evolutionary pathways, morphological innovations and ecological adaptive mechanisms underlying insect terrestrialization have remained poorly constrained.
Recently, an international research team including Prof. CAI Chenyang (Nanjing Institute of Geology and Palaeontology, Chinese Academy of Sciences) and Erik Tihelka (joint-training PhD student, University of Cambridge), together with collaborators from the United States, Spain and other countries, reported a new stem‑group insect, Chosha praecursor Tihelka, Engel & Cai, 2026 (Fig. 1), from the Late Mississippian (~324 Ma) of Texas, USA. The team 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 (Fig. 2). These insect fossils fill critical gaps in early insect evolution, revise long‑standing interpretations of insect body-plan transformation and pancrustacean terrestrialization, and furnish key empirical evidence for the gradual aquatic‑to‑terrestrial evolutionary transition of insects. The findings were published online 26 August 2026 in Nature.
Fossils of Chosha praecursor derive from calcareous claystone concretions within the Tesnus Formation of the Marathon Uplift, western Texas, and exhibit exquisite, well‑preserved anatomical details. Using cross-polarized light imaging, the team resolved its distinctive morphological traits and corrected long‑standing misinterpretations that these specimens represented crustacean larvae. The studied material corresponds to adult females with a body length of 32.09 mm; a median caudal filament plus two cerci extend the total length to 49.66 mm. The fusiform body displays derived traits diagnostic of hexapods and insects, while retaining plesiomorphic ancestral features (Fig. 1).
Systematic analyses demonstrate that Chosha praecursor possesses hallmark insect structures including an ovipositor and terminal caudal filament. Its thorax bears a segmented trunk and six walking legs, conforming to the canonical insect body plan. Most strikingly, however, segments 1‑9 of the abdomen bear segmented appendages; posterior abdominal limbs are modified into paddle‑like structures — a morphology unknown among extant crown‑group insects (Fig. 1). Palaeoenvironmental reconstructions indicate that the host strata represent near‑shore shallow‑water delta‑coastal settings, confirming that this early stem‑insect led a semi‑aquatic, amphibious lifestyle, occupying humid microhabitats across aquatic‑terrestrial interfaces.
Based on detailed comparative morphology and phylogenetic analyses, the researchers re‑evaluated three enigmatic Palaeozoic hexapod fossils: Leverhulmia from the Early Devonian of Scotland, and an unnamed hexapod from the Mazon Creek biota, USA. Phylogenetic results recover Chosha praecursor together with these taxa as a primitive insect stem clade. Representing the oldest documented insect assemblage globally, this clade substantially connects the evolutionary genealogy of early insects (Fig. 3).
This study delivers paradigm-shifting scientific advances that reshape our framework for understanding insect terrestrial evolution. First, it fills the long‑persistent 80‑million‑year hexapod gap. Reliable evidence for insect origins and early diversification 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. Prior models assumed that insects evolved fully terrestrial body plans immediately following land colonization. The paddle‑shaped abdominal appendages and amphibious habit demonstrated here prove that insect terrestrialization was not an abrupt leap, but unfolded via a prolonged semi-aquatic amphibious transitional phase (Fig. 4).
Second, the fossils illuminate pivotal transformations in the insect body plan. Extant hexapods retain only six thoracic legs; abdominal appendages are almost entirely lost. By contrast, Palaeozoic stem-insects commonly preserve segmented abdominal limbs. This confirms that reduction of abdominal appendages constituted a key evolutionary innovation for terrestrial adaptation. These structures were progressively simplified and lost from the swimming appendages of crustacean ancestors, ultimately yielding the body organization seen in modern insects. The findings clarify the morphological transition from pancrustacean ancestors to hexapod insects (Figs 1, 4). Furthermore, the plesiomorphic ovipositor preserved in Chosha praecursor demonstrates that early insects already possessed diverse oviposition adaptations, providing the structural foundation for subsequent colonization of heterogeneous terrestrial microhabitats and later insect radiations.
In addition, the research reconstructs 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. Early insects thus fulfilled multifunctional ecological roles as decomposers and consumers within aquatic‑terrestrial ecotones, acting as pivotal components driving the maturation of Palaeozoic terrestrial ecosystems (Figs 2, 4).
Fossils of Chosha praecursor and related Palaeozoic stem-insects reconstruct the early terrestrialization trajectory of Earth’s most species‑rich animal group, revising interpretations of body-size evolution, ecological adaptation and co‑evolution with terrestrial ecosystems. Terrestrial colonization by insects was a gradual process; retention, remodelling and reduction of ancestral aquatic structures permeated their early evolutionary history. An amphibious transitional phase formed the critical evolutionary bedrock enabling insects to conquer land. These results supply invaluable fossil evidence for deciphering hexapod origins and body‑plan evolution, and offer fresh perspectives on the origin and early diversification of complex terrestrial ecosystems on Earth.
This research was supported by the National Key Research and Development Program of China and the National Natural Science Foundation of China. Mr. Chao Tan prepared the palaeoecological reconstructions.
Reference: Tihelka E., Vásquez C., Engel M.S., Schram F.R., Lozano‑Fernandez J., Cai C., 2026. Amphibious stem‑insect sheds light on colonization of land. Nature. https://doi.org/10.1038/s41586‑026‑10961‑2.
Fig. 1 The stem-group insect Chosha praecursor Tihelka, Engel & Cai, 2026 from the Carboniferous Tesnus Formation (ca. 324 Ma), Texas, USA.
Fig. 2 Comparative anatomical details of stem-insects and extant apterygote insects.
Fig. 3 Phylogeny of early hexapods and the evolution of key morphological traits.
Fig. 4 Palaeoecological reconstruction of the Carboniferous stem-insect Chosha praecursor.
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