Early orthid brachiopods from the Tremadocian (Lower Ordovician) of northwestern Argentina

Two brachiopod species of two of the earliest rhynchonelliformean families are described from one of the first studied Ordovician fossiliferous localities of northwestern Argentina. One is a new species of the family Euorthisinidae (Orthida, Plectorthoidea), Notorthisina musculosa Villas and Herrera. It displays peculiar features not observed in the family before, as well as a combination of internal and external features previously considered diagnostic of different genera, leading to emendment of the familial and generic diagnoses. Its dorsal muscle field, best impressed in large specimens, resembles that of Kvania Havlicek (Nanorthidae) in having four scars arranged radially. This feature reinforces the phylogenetic proximity of Kvania to the euorthisinids. The second studied species, Nanorthis bifurcata Harrington, has not been revised since its superficial description in 1937-1938, its type having been lost. Its description herein, based on a new collection from its type locality, allows a better knowledge of the group and helps clarify its supra-familial assignment.