Taxonomia de brachiopoda (Família Discinidae Gray, 1840) da Formação Manacapuru (Siluro-Devoniano), Bacia do Amazonas, sudoeste do Pará

Abstract

Discinoids are inarticulate brachiopod exclusively marine with two organophosphate valves that arise during Ordovician and can be divided into four living genera. The remarkable marine transgressions at northwest Gondwana during Silurian-Devonian transition collaborated to make discinoids so abundant in South America during Devonian. Despite this big radiation during Devonian, rare records of this group are studied in Amazon and Parnaíba basins. In layers of Manacapuru Formation (Amazon Basin Siluro-Devonian), Ererê Formation (Amazon Basin Meso-Devonian) and Pimenteiras Formation (Parnaíba Basin Eifelian-Frasnian) there are occurrences with no taxonomic detailing. On the other hand, discinoids are easily found in Devonian deposits of Paraná Basin, especially in Ponta Grossa and São Domingos formations, as pointed by well advanced studies. This work aims the taxonomic study of brachiopods (Family Discinidae) of Manacapuru Formation, at the south border of the Amazon Basin, collected during “Paleontological Patrimony Rescue Program” of Belo Monte Hydroelectric Plant, at Vitória do Xingu city, Pará state. Four sampling points (C3P1, C9P1, C13P1 e C14P1) compose the stratigraphic profile of the study area, that consists, from bottom to top, of crystalline basement followed by a nearly 0.5 meters layer of massive fine grained sandstone, interlayered with clay lenses, but discinoids occur only in sandstone portions. Above that, a fine-grained sandstone layer display incipient lamination with disseminated discinoids. Finally, there is a nearly 2.1 meters layer of laminated siltstones where discinoids are concentrated at the bottom, associated with Rhynchonelliformeas; at the top of the layer, isolated lingulids occur. 272 brachiopods samples were analyzed: 205 Discinidae, 57 Rhynchonelliformea and 10 Lingulids. The focus of this research lies in Linguliformeas brachiopods that belongs to Family Discinidae. Taxonomic studies in 205 discinoids samples pointed to three species of Orbiculoidea d’Orbigny, 1847: O. baini Sharpe, 1856, (10 specimens), O. bodenbenderi Clarke, 1913 (5 specimens) and O. excentrica Lange, 1943 (34 specimens). In addition, O. sp. 1 (18 specimens) and O. sp. 2 (19 specimens) were preliminarily identified as Orbiculoidea, but the nomenclature was kept open; another 99 Orbiculoidea samples classification remained undefined due to poorly preserved samples. 20 specimens of Gigadiscina Mergl & Massa, 2005 also remained with open classification. Although some papers mention Orbiculoidea genera in Manacapuru Formation, this is the first record of O. baini, O. bodenbenderi, O. excentrica and Gigadiscina genera in this formation, as well as the first documented occurrences of these species and genera in North of Brazil. The discinoids association studied in this paper (Gigadiscina? sp., O. baini, O. bodenbenderi, O. excentrica, O. sp. 1 and O. sp. 2) is the oldest (Lochkovian) record in South America (e.g. Paraná Basin / Pragian-Givetian; Alto das Garças sub-Basin / Givetian; Parecis Basin / Pragian; Chacoparanense Basin / Pragian; Pre-Argentinian Range / Pragian) and this can be explained by two main reasons: Laurasia and Gondwana (the main continental blocks) were apparently close enough to allow the cosmopolite invertebrate larvae (e.g. Orbiculoidea) to cross the oceans more easily. So, Amazon Basin was closer to Laurasia during Eodevonian, what made easier for these organisms to firstly accommodate in this basin. The other reason is that the global eustatic sea level increased during Eodevonian, leading to the great marine transgressions that reached many portions of Gondwana, favoring the emergence of shallow seas at northwest Gondwana, which is a favorable environmental condition for inarticulate brachiopods colonization, represented by the discinoids in marine sediments of Manacapuru Formation described in this paper. Shallow marine coast environments are the main habitat of Orbiculoidea genera. This affirmation relies, in a certain proportion, in the current distribution of discinoids in less than 30 meters depths: 92,7% of Orbiculoidea fossil records are associated to shallow marine conditions. Therefore, the presence of O. baini, O. bodenbenderi, O. excentrica, and Gigadiscina? sp. in Manacapuru Formation layers suggest a shallow marine environment, in agreement with what is already proposed for the upper portion of this formation.