LAUSR

Abstract. Modiomorphid bivalves from the Silurian and Devonian beds of the Central Andean Basin of Western Gondwana are herein presented. This paper is not only a systematic updated revision of certain species of modiomorphids but also includes the description of newly found ones in Bolivia. They were collected from 17 scattered outcrops along Eastern Cordillera, Eastern Altiplano, Interandean, and Subandean regions of Bolivia. A total of 55 specimens, well- to regularly-preserved, were studied. The Sphenotomorpha orientalis new species from the Interandean of Mataral is described. The genus Sphenotomorpha is reported for the first time from Ludlowian layers of the Central Andean Basin with Sphenotomorpha sp. A. Other species described and systematically discussed are Modiomorpha concentrica, Modiomorpha cf. pimentana, Sphenotomorpha ulrichi and Sphenotomorpha cf. bodenbenderi? Forms classified as Modiomorphidae indet. A, B, and C are studied and some discussions about their previous and current taxonomic designations are herein presented. The biostratigraphic distribution of the two cosmopolitan genera, Sphenotomorpha, 1916, and Modiomorpha, 1869, is proposed. From the Eifelian to the Frasnian, endobyssal forms, such as Modiomorpha concentrica, coexisted with foot forms like Modiomorphidae indet. C and Sphenotomorpha sp. B, which would have developed a discreet byssus. This research contributes to the still inaccurately known Silurian and Devonian palaeogeography of the Central Andean Basin.