A late-surviving Triassic protomonaxonid sponge from the Paris Biota (Bear Lake County, Idaho, USA).
Résumé
Protomonaxonid sponges are a major group of Cambrian and Ordovician fossils in exceptionally
preserved (especially Burgess Shale-type) faunas, but are rare thereafter. Rare examples of apparent
surviving lineages are known from the late Palaeozoic and Mesozoic, but by this time more derived
groups of sponges have generally displaced them in at least shallow-water (shelf depth) ecosystems. The
early Spathian (Early Triassic) Paris Biota includes abundant material of a new leptomitid
protomonaxonid, Pseudoleptomitus advenus Botting nov. gen., nov. sp., distinguished by having an
unbundled longitudinal skeleton and very weak transverse component. This is the first post-Ordovician
leptomitid known, and indicates long-term survival of the group in unknown environments. Its
occurrence near storm wave base is similar to the preferred environment of earlier examples of the
family, suggesting either ecological rarity or taphonomic reasons for their 200-million-year absence
from later Palaeozoic rocks.