FOSSIL INVERTEBRATES/Insects 299 Figure Orders through time and key events Dashed line represents extrapolation, dots represent equilibrium (saturation) value Key: Pl, Pliocene; M, Miocene; O, Oligocene; E, Eocene; P, Paleocene; K, Cretaceous; J, Jurassic; Tr, Triassic; P, Permian; C, Carboniferous; D, Devonian Numerals 1, 2, and represent Lower, Middle, and Upper subperiods and epochs; I, earliest hexapods; II, earliest pterygotes; III, near modern range of plant feeding strategies; IV, evolution of holometabolous insects; V, earliest insectan parasites and parasitoid radiation; VI, earliest amberized and definite social insects Table Principal groups of insects in the fossil record Principal group Order Age/description Apterygota Collembola (springtails) Diplura (two pronged bristletails) ‘Thysanura’ (three tailed bristletails) Lower Devonian Recent Upper Carboniferous Recent This order is split into two groups: Archaeognatha (Middle Devonian? Recent) and Zygentoma (Upper Carboniferous Recent; e.g., silverfish) The extinct order Monura (Upper Carboniferous Permian) with single ‘tails’ is probably related to the Archaeognatha, whereas Zygentoma is related to the Pterygota Palaeodictyopteroid group An Upper Carboniferous Upper Permian group of three or four extinct orders (Palaeodictyoptera, Permothemistida, Megasecoptera, Diaphanopterodea) with beak like mouthparts and an additional pair of ‘winglets’ Upper Carboniferous Recent Upper Carboniferous Triassic, including the giant dragonflies with wingspans up to 70 cm, the largest insects of all time Upper Carboniferous Recent Lower Carboniferous Triassic A taxonomic wastebasket of early neopterans Pterygota Paleoptera Ephemeroptera (mayflies) Protodonata Neoptera Odonata (dragonflies, damselflies) ‘Protorthoptera’ Continued