Simplified phylogeny with hypothetical evolutionary stages of modern avian reproduction. Stage 1: Plesiomorphic dinosaurian condition: abnormal multilayered eggshells due to egg retention can occur34, dinosauroid eggshell microstructure35, possible sexual dimorphism75, strong histological evidence for sexual maturity occurring prior to skeletal maturity63. Stage 2: Possible appearance of medullary bone4,8. Stage 3: Possible appearance of ornithoid eggshell microstructure35. Stage 4: Possible appearance of cuticular nanostructuring21. Stage 5: Strong evidence for sexual dimorphism (inferred sexually dimorphic feathers, e.g., ref. 65). Stage 6: In Avimaia: presence (not necessarily first occurrence) of cuticular nanostructuring, medullary bone, and a plesiomorphic non-avian dinosaur-like timing of sexual maturity within Aves. Further support for the presence of sexually dimorphic plumage. Stage 7: Avian-like timing of sexual maturity, skeletal maturity occurs prior to sexual maturity (probably evolved multiple times in parallel in Ornithuromorpha64)
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