Syninclusions reveal "ant mosaic" in the Eocene amber forest.

Ants (Hymenoptera: Formicidae) are key ecosystem engineers in modern terrestrial environments, where their communities are structured by competition, niche partitioning, and dominance hierarchies. Whether such complex organisation existed deep in geological time remains unclear due to the rarity of direct fossil evidence for species co-occurrence. Here, we analyze eusyninclusions — fossilized assemblages comprising several ant species preserved together within individual pieces of Baltic amber from the late Eocene (~ 34–38 Ma). Among 3246 fossil ant specimens examined, we identify 110 eusyninclusions. Eocene ant assemblages show a strong prevalence of negative co-occurrence patterns, indicating non-random, competitively structured communities consistent with a deep-time ant mosaic. Two dominant species, †Ctenobethylus goepperti (Mayr, 1868) and †Lasius schiefferdeckeri Mayr, 1868, anchor distinct interaction clusters reflecting divergent ecological strategies. †C. goepperti exhibits low interspecific co-occurrence and high dominance, consistent with a territorial arboreal lifestyle, whereas †L. schiefferdeckeri displays frequent associations and a broader ecological tolerance, indicating a eurytopic subordinate role. Our results provide the first empirical evidence that ant mosaic–like community organisation was already established in the Eocene. These results highlight the deep evolutionary roots of ant community organisation and underscore the importance of preserving habitat structural complexity in the face of modern global warming.