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

摘要

The re-study of species previously attributed to the genus Nanorthis from the Tremadocian of north-western Argentina has led to the recognition of the new genera Gondwanorthis (type species Nanorthis calderensis Benedetto), to which the Iranian species Nanorthis bastamensis Ghobadi Pour, Kebriaee-Zadeh & Popov is also referred, and Lampazarorthis (type species Eoorthis bifurcata Harrington), which includes Nanorthis brachymyaria Benedetto and Lampazarorthis alata sp. nov. New extensive collections from the Cordillera Oriental support reassignment of the upper Tremadocian species Nanorthis purmamarcaensis to the genus Tarfaya. The species Nanorthis carinata Laurie, from Tasmania, is reassigned here to Tarfaya. According to the present taxonomic revision, Nanorthis is not cosmopolitan but is a pan-tropical genus, confined to Laurentia, the Laurentian-derived Precordillera terrane, and probably to Baltica, Siberia and Kazakhstanian terranes, whereas Gondwanorthis and Tarfaya are Gondwanan and peri-Gondwanan endemic taxa. Cladistic analysis of early orthoids, plectorthoids and dalmanelloids, including those recorded from mid to high latitude Gondwana successions, portrays Gondwanorthis and Lampazarorthis as a sister group basal to the tarfayid + heterorthid clade, whereas the South American and North African Incorthis appears 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 in Tarfaya grandis leaves this species as a potential ancestor of Tissintia, 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 a Tarfaya-like ancestor, and that the transition from an impunctate to a punctate shell may have occurred repeatedly through rhynchonelliform evolution.