Extinct detritivorous mites and a bradytelic bristly millipede together frozen in time.

Fossiliferous tree resin, known as amber resin, is a unique material that offers the rare opportunity to observe organisms from deep time, sometimes preserved in exceptional quality. Thanks to the quick encapsulation of the animals, it facilitates the unique preservation of past ecological interactions, offering a glimpse into how past ecosystems looked like up to 100 million years ago. We here investigated a syninclusion preserved in Eocene Baltic amber (~40 mya) in which two astigmatan mite specimens and a bristly millipede were fossilised in close proximity. The two mites are identified to be adult representatives of Glaesacarus rhombeus, one of them female. This is a species of mites that is frequently found in Baltic and Rovno amber, considered to be now extinct. The bristly millipede is also a female adult, identified as a representative of Polyxenus, exhibiting remarkable morphological similarities to the extant species Polyxenus lagurus; indicating that they have probably been thriving since the Eocene. This syninclusion provides an interesting interaction, with one of the mites seemingly surrounded by the defensive setae of the millipede, but it also provides clues on the shared microhabitat of these two groups in Eocene forests. The striking persistence of morphological traits in representatives of Polyxenus over 40 million years suggests a stable ecological niche and effective adaptive strategies, highlighting the evolutionary stability, resilience, and long-term success of the group. Although the two lineages here represented used to share the same habitat in deep time, despite being commonly entrapped, they displayed different evolutionary trajectories.