The plectambonitoid brachiopod Ahtiella Öpik in the Ordovician of Gondwana: phylogenetic and macroevolutionary implications

BENEDETTO, J.L.

San Luis

Jornada; Reunion Anual de Comunicaciones de la APA; 2017

Asociacion Paleontologica Argentina

Ahtiella Öpik is a distinctive resupinate and variably geniculate plectambonitoid brachiopod described originally from Norway and Sweden but subsequently recognized in Wales, Central Newfoundland and South America. Here Monorthis coloradoensis Benedetto from NW Argentina is reassigned to the genus Ahtiella and new species of this genus are identified from the Famatina and the Precordillera basins. Stratigraphic and paleogeographic evidence strongly suggests that Ahtiella originated in the Andean region of Gondwana during the Floian and further dispersed to Avalonia, Baltica and Cuyania where it underwent successive speciation events. Contrary to previous assumptions, the fossil record from the Famatina volcaniclastic succession supports that Ahtiella evolved from the hesperonomiid orthoid Monorthis transversa Benedetto recorded from underlying strata. Comparative morphology shows that differences between Monorthis and basal species of Ahtiella are subtle leading difficult their distinction. Given the striking resemblance in multiple traits, along with the gradation in some features, a case of morphological convergence seems highly improbable. Phylogenetic analysis of Ahtiella species shows that the new Famatinan species and the Peruvian Ahtiella zarelae Villas are not only the earliest species of the genus but also are morphologically intermediate between Monorthis and the younger and more derived species of Ahtiella from Baltica and Cuyania. If, as empirical evidence presented here shows, the plectambonitoid Ahtiella originated from the orthoid Monorthis, then the impressive morphological gap among orthides and strophomenides was bridged through a series of cladogenetic events, suggesting that there is no significant discontinuity between micro and macroevolutionary processes.