Amphibious stem-insect sheds light on colonization of land

Molecular clock analyses^1-3, trace fossils⁴, exceptionally preserved amphibious stem-myriapods⁵ and sea scorpions⁶ have recently challenged classical assumptions about the timeline of the animal colonization of land⁷, suggesting that terrestrial ecosystems may date back as far as the Cambrian period. However, the early evolutionary history of the most successful clade of animals, the insects, has remained elusive, as the period of their early diversification constrained by time divergence estimates falls within the notorious 'hexapod gap' in the fossil record⁸. Here we describe Chosha praecursor gen. et sp. nov. from the Carboniferous period (Late Mississippian epoch, approximately 324 million years ago) Tesnus Formation in Texas. Cross-polarized light imaging reveals that C. praecursor possesses insect apomorphies including an ovipositor and a terminal filament but differs from crown-group insects, most notably with respect to the presence of multisegmented abdominal legs with paddle-like modifications. Our phylogenetic reconstruction recovers C. praecursor as a hexapod, strongly favouring its placement as an early-diverging insect. A revision of three other enigmatic hexapod fossils, the Devonian period Leverhulmia and two undescribed Carboniferous fossils from the Mazon Creek Lagerstätte, demonstrates diverse body organization in Palaeozoic wingless insects. Together, these stem-group fossils represent the earliest uncontested insects and partly reconcile the incongruence between molecular clock estimates and the fossil record. The unusual abdominal gill-like appendages of Chosha suggest a semiaquatic mode of life in at least some stem-insects and revise our understanding of the assembly of insect morphological organization.