When opposites attract: a syninclusion of extinct mites and a bradytelic bristly millipede in Baltic amber

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Abstract Amber resin is known for its exceptional preservations thanks to quick encapsulation of animals and plant material. Fast encapsulation not only helps preserve detailed organisms, but it also offers a glimpse into “frozen behaviors” from up to hundreds of million years, which provide information into the configuration of past ecosystems. We here investigated a syninclusion preserved in Baltic amber (~40 mya) consisting of a fossilized bristly millipede and two mite specimens, located in close proximity to the millipede’s ventral side. The two mites are representatives of Glaesacarus rhombeus , a frequently reported species of mite in Eocene amber whose lineage is considered to be now extinct. The bristly millipede is a female adult identified as a representative of Polyxenus . The specimen exhibits remarkable morphological similarities to the extant species Polyxenus lagurus , indicating the possibility of P. lagurus thriving since the Eocene. The striking persistence of morphological traits over millions of years suggests a stable ecological niche and effective adaptive strategies in Polyxenus . This unexpected association sheds new light on the ecosystem of Eocene forests and the potential effectiveness of defensive mechanisms, already present at that time, since one of the two mites is touching and seemingly surrounded by the posterior defensive setae of the millipede. Together, these findings enhance our understanding of the life of fossil Polyxenida and the evolutionary stability of Polyxenus , highlighting their long-term evolutionary resilience and success.

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