| Summary: | <p>Abstract</p> <p>Background</p> <p>The remarkable mutualistic associations between termites and protists are in large part responsible for the evolutionary success of these eusocial insects. It is unknown when this symbiosis was first established, but the present study shows that fossil termite protists existed in the Mesozoic.</p> <p>Results</p> <p>A new species of termite (<it>Kalotermes burmensis </it>n. sp.) in Early Cretaceous Burmese amber had part of its abdomen damaged, thus exposing trophic stages and cysts of diverse protists. Some protists were still attached to the gut intima while others were in the amber matrix adjacent to the damaged portion. Ten new fossil flagellate species in the Trichomonada, Hypermastigida and Oxymonadea are described in nine new genera assigned to 6 extant families. Systematic placement and names of the fossil flagellates are based on morphological similarities with extant genera associated with lower termites. The following new flagellate taxa are established: <it>Foainites icelus </it>n. gen. n. sp., <it>Spiromastigites acanthodes </it>n. gen. n. sp., <it>Trichonymphites henis </it>n. gen., n. sp., <it>Teranymphites rhabdotis </it>n. gen. n. sp., <it>Oxymonas protus </it>n. sp., <it>Oxymonites gerus </it>n. gen., n. sp., <it>Microrhopalodites polynucleatis </it>n. gen., n. sp., <it>Sauromonites katatonis </it>n. gen., n. sp., <it>Dinenymphites spiris </it>n. gen., n. sp., <it>Pyrsonymphites cordylinis </it>n. gen., n. sp. A new genus of fossil amoeba is also described as <it>Endamoebites proterus </it>n. gen., n. sp. Fourteen additional trophic and encystid protist stages are figured and briefly characterized.</p> <p>Conclusion</p> <p>This represents the earliest fossil record of mutualism between microorganisms and animals and the first descriptions of protists from a fossil termite. Discovering the same orders, families and possibly genera of protists that occur today in Early Cretaceous kalotermitids shows considerable behaviour and morphological stability of both host and protists. The possible significance of protist cysts associated with the fossil termite is discussed in regards the possibility that coprophagy, as well as proctodeal trophallaxis, was a method by which some termite protozoa were transferred intrastadially and intergenerationally at this time.</p>
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