Systematics and phylogenetics of the family Streptacididae (Gastropoda: Heterobranchia) from Devonian through Triassic marine strata

Streptacididae is among the earliest families in the subclass Heterobranchia, an abundant and ubiquitous subclass of fossil and modern Mollusca. In the Paleozoic, the subclass Heterobranchia contains one informal group with two genera and two species in the Early and Middle Devonian, one family with one genus and one species in the Permian, and the family Streptacididae with seven genera and about 85 species from the Middle Devonian to the Triassic and possibly from the Cretaceous. The family Streptacididae is characterized by a heterostrophic protoconch, small size (0.4 to 14 mm), and an elongate shape.; Streptacididae survived both the Late Devonian and Permo-Triassic extinctions. Heteroloxonema is known from Devonian strata in Germany. Streptacis and Donaldina are known from the Mississippian of Australia; the Pennsylvanian of the USA and Russia; the Permian of the USA, China, and Uzbekistan; and the Triassic of China. Donaldina is known also from the Mississippian and Triassic of Europe. Laxella is known from the Pennsylvanian of the USA and China, the Permian of China, and the Triassic of China. Jiangxispira is known from the Triassic of China.; The family Streptacididae is a basal member of the superfamily Pyramidelloidea based on cladistic analyses. Streptacis is paraphyletic to Ebala, Jiangxispira, and Laxella. Splitting the genus Donaldina into additional genera is not supported by cladistic analyses. The following taxonomic assignments are based on protoconch morphology, spiral ornamentation, and other shell characters. The genus Mapesella is placed in synonymy with Streptacis . The genera Neodonaldina, Rovalella, Texasella, Yoospira, and Heteroaclisina are placed in synonymy with Donaldina. Streptacis and Donaldina are considered to be distinct at the genus rather than family rank, and family Streptacididae is assigned to the superfamily Pyramidelloidea.