What fossil evidence shows that insect wings were fully developed by the late Carboniferous Period?

By the late Carboniferous Period (specifically the Pennsylvanian subperiod), the fossil record confirms that insects possessed fully developed wings.12

Evidence for this includes:

• Abundance and Diversity: By the Pennsylvanian, both dragonflies and mayflies were abundant and had reached significant sizes, with some early ancestors of modern dragonflies (Protodonata) exhibiting wingspans of approximately 70 cm (28 inches).2
• Advanced Wing Mechanics: Fossils from this period demonstrate the presence of more advanced insects capable of folding their wings, such as cockroaches, which are well represented in Pennsylvanian rock strata.2
• Consistent Venation: The consistent pattern of wing venation observed in these fossils suggests that wings evolved only once, with all Pterygota arising from a single ancestral stem.1

While winged insects likely appeared earlier in the Carboniferous, there is no fossil evidence documenting the transitional stages of their evolution.12