Solenopora is a Chaetetid Sponge, not an Alga

For over one hundred years the Ordovician fossil Solenopora Dybowski has been widely considered to be a calcified red alga. The type species, Solenopora spongioides, consists of tubes with longitudinally flexuous walls, lobate‐petaloid cross‐sections 30–175 μm across with septal projections, and sporadic cross‐partitions. This internal micromorphology is not characteristic of calcified red algae, but is consistent with the original interpretation of Solenopora as a chaetetid, and with subsequent recognition of chaetetids as sponges. Solenopora is widely misidentified in Silurian and younger rocks. Removal of Solenopora from the algae underscores the need to comprehensively reassess the palaeoecological and phylogenetic significance of numerous disparate Ordovician–Miocene fossils currently classed as solenoporaceans.