FIRST FOSSIL CLAM SHRIMPS FROM THE PENSYLVANIAN OF ITAITUBA FORMATION, AMAZONAS BASIN, BRAZIL

GALLEGO, O.F.; JIMENEZ, V.; MONFERRAN, M.D.; SCOMAZZON, A.; NASCIMENTO, S.

Buenos Aires

Congreso; XII Congreso de la Asociación Paleontológica Argentina; 2021

Asociación Paleontológica Argentina

Fossil clam shrimps (conchostracans, spinicaudatans) are the most common freshwater invertebrates found in the continental environments throughout geological time. With a record ranging from the Middle Devonian to Recent, they have around 390 Ma of paleobiological history. The fossil record of South American clam shrimps has been known since the 1800’s with the contributions of different authors. Conversely to their abundance in the late Permian to the Mesozoic from South America, the record of late Paleozoic (except for the late Permian) conchostracans is very scarce with only three incomplete mentions from the Carboniferous. The subject of this contribution is to report the presence of species probably assignable to the genus Pseudestheria from the Itaituba Formation, Amazonas Basin, Brazil. The latter is dated based on conodonts of an age ranging from upper Bashkirian to lower Moscovian (Lower to Middle Pennsylvanian, Atokan—according to North American Stages). The studied material comes from three levels corresponding to three cores from drilling wells respectively (T4-950–954 m, T10-1110.60 m and T12-2552.51 m). The descriptions and measurements of the clam shrimps were based on the analysis of only 11 casts and impressions (possibly with preservation of original carapace material) due to poor preservation and fragmentation of the abundant studied material. Taphonomically, studied specimens show different colorations (varying from brown, black to white enameloid or porcelanous) between the three analyzed levels, possibly due to the influence of a geothermal gradient. The collection is deposited in the Departamento de Paleontologia e Estratigrafia/UFRGS/Porto Alegre, Brazil, under catalog prefix MP-M. In particular, the Itaituba species resembles some Pseudestheria forms identified for European Carboniferous conchostracan assemblages as the Megasitum form Markenkamp-Pseudestheria form Hoya assemblage zone Westfalian C (Bolsovian, middle Moscovian) and the Pseudestheria limbate-Pseudestheria rimosa-Lioestheria form Köllerbach assemblage zone (Stephanian A, Kasimovian) coinciding with the age assigned based on the conodonts of this unit. Also, this record represents the first fossil clam shrimps from the Carboniferous of Brazil and even from South America.