A modern type of ant-like stone beetle larva preserved in 99-million-year-old Kachin amber

Abstract

Scydmaeninae, the group of ant-like stone beetles, was previously named “Scydmaenidae”, but is today recognized as an ingroup of Staphylinidae. The group has a more or less global distribution and includes 4,900 formally described species. The fossil record of the group seems well studied with almost 40 formally described species so far. However, all the described fossil specimens are adults, which once again demonstrates the rarity of reports of larvae. This fact also applies for the extant counterparts: even the larvae of some modern lineages of Scydmaeninae have not been identified yet. Here we contribute to the fossil record of ant-like stone beetles with the first report of a relatively modern-appearing fossil larva. In the center of the study is a single specimen preserved in 99-million-year-old Kachin amber, Myanmar. The fossil shares multiple characteristics (for example, long and slender antennae, maxillary palps, and walking legs) with modern representatives of second stage larvae of the species Stenomastigus longicornis. This similarity indicates a closer relationship to this species, and the fossil is therefore likely a representative of the group Mastigini. In the light of the new find, we discuss phylogenetic implications and the evolution of developmental patterns within Scydmaeninae