Past “giants” and extant dwarfs, more like the opposite: deep-time investigation of bristly millipedes reveals reduced sizes during the Cretaceous

Abstract Bristly millipedes face many challenges regarding fossilisation (i.e., soft body, hidden living environments) and are clearly under-represented in the literature. Surprisingly, they are quite common in amber, offering an exceptional window to investigate their evolutionary history of the past 100 million years. Hundreds of inclusions in amber that cover the almost complete ontogenetic sequence of both Polyxenidae and Synxenidae have been investigated, and for each specimen distances between ten landmarks are measured. In contradiction to previous expectations, representatives of both Polyxenidae and Synxenidae have significantly changed through time, going from “dwarfs” during the Cretaceous (~ 100 Ma) to “giants” during the Eocene (~ 35 Ma); extant representatives are found in-between. This first attempt in investigating morphological changes in bristly millipedes may suggest the potential influence of the environment on the evolutionary trajectories within this group. The Cretaceous was a time of warmer climates, with tropical rainforests on the Burma Terrane, while the Eocene and modern-day representatives are from equable and temperate regions. Potentially, the environmental conditions on the Burma Terrane pressured bristly millipedes to reach “dwarf” sizes. The observed signals may be an indication of the Temperature-Size Rule, favouring quick development while slow growth in warmer ecosystems.