The earliest known holometabolous insects

Fossils of four insects and one larva from the Carboniferous Pennsylvanian epoch are described; these are very small relative to other known Palaeozoic-era insects, and reveal a previously unknown diversity of early eumetabolan insects, although the lineage radiated more successfully only after the mass extinctions at the end of the Permian period. Diversity in early insect populations The discovery of a group of previously unknown insect species that lived around 300 million years ago reveals a remarkable level of diversity in early insect populations. André Nel et al . identify the oldest representatives of the holometabolan groups Hymenopterida and Coleoptera, together with the oldest accurate holometabolous larva, oldest Psocodea and oldest Hemiptera in the Late Carboniferous. These are all tiny fossils, revealed by stereomicroscopy, living at the same time as the more easily studied and hence familiar Palaeozoic-era 'giant' insects. The fossils described here include very early examples of the groups which would one day evolve into beetles, true bugs and hymenopterans — the ants, bees and wasps: together, the most species-rich of any modern animal group. The Eumetabola (Endopterygota (also known as Holometabola) plus Paraneoptera) 1 have the highest number of species of any clade, and greatly contribute to animal species biodiversity 2 , 3 . The palaeoecological circumstances that favoured their emergence and success remain an intriguing question 3 , 4 , 5 , 6 . Recent molecular phylogenetic analyses have suggested a wide range of dates for the initial appearance of the Holometabola, from the Middle Devonian epoch (391 million years (Myr) ago) to the Late Pennsylvanian epoch (311 Myr ago 7 , 8 , 9 , 10 , 11 , 12 ), and Hemiptera (310 Myr ago 13 ). Palaeoenvironments greatly changed over these periods, with global cooling and increasing complexity of green forests 14 . The Pennsylvanian-period crown-eumetabolan fossil record remains notably incomplete 15 , 16 , 17 , 18 , 19 , particularly as several fossils have been erroneously considered to be stem Holometabola 1 , 15 , 20 , 21 ( Supplementary Information ); the earliest definitive beetles are from the start of the Permian period 21 , 22 . The emergence of the hymenopterids, sister group to other Holometabola, is dated between 350 and 309 Myr ago 8 , 9 , 12 , incongruent with their current earliest record (Middle Triassic epoch) 1 , 20 . Here we describe five fossils— a Gzhelian-age stem coleopterid,