An echiuran from the Carboniferous of Mazon Creek and the evolution of spoon worms (Annelida: Thalassematidae)

Echiurans (Thalassematidae), the spoon worms, are an elusive group of unsegmented marine annelids with a convoluted taxonomic history. Their limited fossil record and ambiguous internal relationships present a challenge for reconstructing echiuran evolution. Here we describe the first unequivocal fossil member of the extant spoon worm subfamily Bonelliinae, and the second spoon worm fossil from the Palaeozoic. Mazobonelliia chenyangcaii gen. et sp. nov. from the middle Pennsylvanian Mazon Creek in Illinois possess a characteristic elongated proboscis with a bifid tip and, along with the only other previously known fossil echiuran Coprinoscolex ellogimus, documents that crown spoon worms diversified by the Carboniferous. The most extensive molecular dataset for echiurans published to date sampling five genes for 58 ingroup taxa was reanalysed using a better-fitting site-heterogeneous model to re-evaluate the phylogeny of the group and patterns of character evolution. Thalassematini was recovered a monophyletic, but Echiurini in its current state is paraphyletic and forms a clade with Bonelliinae. A revised classification of spoon worms is proposed, elevating Urechini stat. nov. and Echiurini sensu. nov. to tribal rank placed within a revised Bonelliinae sensu nov.