A late-surviving Triassic protomonaxonid sponge from the Paris Biota (Bear Lake County, Idaho, USA).

7 pagesInternational audienceProtomonaxonid sponges are a major group of Cambrian and Ordovician fossils in exceptionallypreserved (especially Burgess Shale-type) faunas, but are rare thereafter. Rare examples of apparentsurviving lineages are known from the late Palaeozoic and Mesozoic, but by this time more derivedgroups of sponges have generally displaced them in at least shallow-water (shelf depth) ecosystems. Theearly Spathian (Early Triassic) Paris Biota includes abundant material of a new leptomitidprotomonaxonid, Pseudoleptomitus advenus Botting nov. gen., nov. sp., distinguished by having anunbundled longitudinal skeleton and very weak transverse component. This is the first post-Ordovicianleptomitid known, and indicates long-term survival of the group in unknown environments. Itsoccurrence near storm wave base is similar to the preferred environment of earlier examples of thefamily, suggesting either ecological rarity or taphonomic reasons for their 200-million-year absencefrom later Palaeozoic rocks