Plectorthoid brachiopods from the Lower Ordovician of north-western Argentina; phylogenetic relationships with Tarfaya Havlíček and the origin of heterorthids.

The re-study of species previously attributed to the genusNanorthisfrom the Tremadocian of north-western Argentina has led to the recognition of the new generaGondwanorthis(type speciesNanorthis calderensisBenedetto), to which the Iranian speciesNanorthis bastamensisGhobadi Pour, Kebriaee-Zadeh & Popov is also referred, andLampazarorthis(type speciesEoorthis bifurcataHarrington), which includesNanorthis brachymyariaBenedetto andLampazarorthis alatasp.nov.New extensive collections from the Cordillera Oriental support reassignment of the upper Tremadocian speciesNanorthis purmamarcaensisto the genusTarfaya. The speciesNanorthis carinataLaurie, from Tasmania, is reassigned here toTarfaya. According to the present taxonomic revision,Nanorthisis not cosmopolitan but is a pan-tropical genus, confined to Laurentia, the Laurentian-derived Precordillera terrane, and probably to Baltica, Siberia and Kazakhstanian terranes, whereasGondwanorthisandTarfayaare Gondwanan and peri-Gondwanan endemic taxa. Cladistic analysis of early orthoids, plectorthoids and dalmanelloids, including those recorded from mid to high latitude Gondwana successions, portraysGondwanorthisandLampazarorthisas a sister group basal to the tarfayid + heterorthid clade, whereas the South American and North AfricanIncorthisappears as a basal member of this clade. Morphological and stratigraphical evidence provided in this study, along with the results of the cladistic analysis, supports the view that heterorthids could have arisen from the tarfayids by the end of Floian. The enlargement and incipient bilobation of the cardinal process inTarfaya grandisleaves this species as a potential ancestor ofTissintia, which occurs in Darriwilian beds of the Central Andean Basin (Bolivia), but could have been present in this basin since the late Floian. We speculate that punctae in the Heterorthidae evolved independently from aTarfaya-like ancestor, and that the transition from an impunctate to a punctate shell may have occurred repeatedly through rhynchonelliform evolution. http://zoobank.org/urn:lsid:zoobank.org:pub:FFE818F6-FA92-4B26-AE76-077617C3D9EA [ABSTRACT FROM AUTHOR]