Per Ahlberg

The acrotretide Acrotreta socialis von Seebach, 1865 has suffered a long and tumultuous history since its original description from the middle Cambrian (Guzhangian Stage) Andrarum Limestone of Bornholm, Denmark. Uncertainties regarding key morphological characters have resulted in numerous revisions and redescriptions of the species with much of the taxonomic shuffling occurring before the mid-20(th) century. A major problem with the early documentation of acrotretide brachiopods is that many species were described in the days before the advent of the Scanning Electron Microscope (SEM), resulting in dubious interpretations of significant morphological characteristics, like the position of the pedicle foramen and the internal morphology of both valves. A detailed revision of Acrotreta species resulted in the proposal that the genus is restricted solely to the Ordovician with the Bornholm acrotretide species subsequently left floating in taxonomic limbo. Recent taxonomic suggestions have been provisional, awaiting morphological information based on type or topotype material. The examination of topotype material in addition to supplementary material collected from southern Sweden has elucidated many of the previous ambiguous morphological characteristics of the species confirming that the original description was based on more than one brachiopod species. The new morphological information acquired here has resulted in the erection of a new acrotretid genus, Clupeafumosus gen. nov., to accommodate the material previously described as Acrotreta socialis. Co-occurring with Clupeafumosus socialis in southern Sweden is another new acrotretide brachiopod, Monophthalma andersoni sp. nov., which is additionally described.