FIRST FOSSIL CLAM SHRIMPS FROM THE PENNSYLVANIAN OF MONTE ALEGRE ANS ITAITUBA FORMATIONS, AMAZONAS BASIN, BRAZIL

GALLEGO, OSCAR FLORENCIO; JIMENEZ, VICTORIA CECILIA; MONFERRAN, MATEO DANIEL; SCOMAZZON ANA KARINA; SILVA DO NASCIMENTO

Otro; Reunión de Comunicaciones de la Asociación Paleontológica Argentina; 2022

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 the1800’s with the contributions of different authors. Conversely to their abundance in the late Permian to the Mesozoic of South America, the record of late Paleozoic (except for the late Permian) clam shrimps is very scarce with only three incomplete mentions from the Carboniferous. The subject of this contribution is to report the presence of species probablyassignable to the genus Pseudestheria from the Itaituba and Monte Alegre formations—marine portion of Tapajós Group, Amazonas Basin, Brazil. This succession (or section) 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 materialcomes from three levels corresponding to two cores from the drilling well 2BIST-1-AM respectively (T17-1340.97 m Monte Alegre Formation, and T10-1110.60 m and T10-1110.21 m Itaituba Formation). The descriptions and measurements of the clam shrimps were based on the analysis of only 11 casts and impressions (possibly with preservation of original carapacematerial) 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 dePaleontologia e Estratigrafia/UFRGS/Porto Alegre, Brazil, under catalog prefix MP-C-33, MP-C-25, MP-C-24, respectively assigned to the abovementioned well core. In particular, Itaituba-Monte Alegre specimens resemble 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 limbata-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.