Reassessment of the problematic Burgess Shale sponge Takakkawia lineata Walcott, 1920
The monotypic Burgess Shale sponge genus Takakkawia Walcott, 1920, has been previously assigned to the Protomonaxonida, despite showing several unique features. A reassessment of the genus, including restudy of previously described material and the addition of new material that includes partially disarticulated specimens, has allowed a greatly modified understanding of its structure. The sponge possessed four primary columns (each divided into two) of highly modified spicules, with an outer organic layer extended into a unique, complex flange that shows a fine microstructure. The flanges within one column overlap to give the impression of radial fins, and each column is surmounted by an enlarged marginal spicule of similar morphology. Multi-rayed spicules are present both within the primary column architecture and as marginalia lining the osculum. The architecture most closely resembles Metaxyspongia Wu et al., 2005, which is placed into the Takakkawiidae. This restudy supports an interpretation of the family as a very early-branching sponge lineage, diverging within the early stem group of Silicea or prior to the separation of Calcarea and Silicea. Resume : Le genre d'eponge monotypique du schiste de Burgess Takakkawia Walcott, 1920, a deja ete affecte aux Protomonaxonides et ce, malgre le fait qu'il presente plusieurs caracteres uniques. Une reevaluation du genre, incluant un re- examen de materiel deja decrit et l'ajout de nouveau materiel dont des specimens partiellement desarticules, a considerable- ment modifie la comprehension de sa structure. L'eponge possedait quatre colonnes primaires (chacune etant divisee en deux) de spicules fortement modifies dotes d'une couche organique externe se prolongeant en une frange complexe et parti- culiere presentant de fines microstructures. Les franges au sein d'une meme colonne se superposent pour donner l'impres- sion d'ailettes radiales et chaque colonne est surmontee d'un spicule marginal plus grand que les autres, mais de morphologie semblable. Des spicules a plusieurs rayons sont presents dans l'architecture des colonnes et comme elements marginaux bordant l'oscule. L'architecture s'apparente le plus a Metaxyspongia Wu et al., 2005, qui est affecte aux Takak- kawiides. Le present reexamen appuie une interpretation voulant que cette famille constitue une lignee d'eponges represen- tant un embranchement tres precoce, divergeant a l'interieur du groupe souche precoce des Silicea ou avant la separation des Calcarea et des Silicea. (Traduit par la Redaction)