EM - Escola de Minas

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High-resolution taphonomy of the Lower Cretaceous “Amargosa Biota”, Central Tucano Sub-Basin, Bahia, Brazil : implications for the paleoenvironmental dynamics of a new Konservat-Lagerstätte.
(2022) Souza, T. G. L.; Silva, Suzana Aparecida Matos da; Varejão, Filipe Giovanini; Rodrigues, Mariza Gomes; Ribeiro, Alexandre Cunha; Freitas, Bernardo Tavares; Warren, Lucas Veríssimo; Assine, Mario Luis; Simões, Marcello Guimarães
We report the Amargosa Biota from the middle part of the Lower Cretaceous Marizal Formation (Central Tucano Sub-Basin, NE Brazil), as a new Konservat-Lagerstatte ̈ . Exceptionally preserved fossils are confined to the lower part of an up to 15-m-thick, mud-dominated succession, named Amargosa Bed. Seven bedding planes (L0-L6) with distinct sedimentological and taphonomic attributes were identified in the type section (Amargosa Village, Euclides da Cunha County, Bahia State), distributed in an ~1-m-thick succession of well-laminated claystone, mudstone, siltstone, and very fine-grained sandstone. These contain ostracods, spinicaudatan carapaces, palae- monid shrimps, fish, and comminuted plant remains. Fossils occur in high concentration on at least four bedding planes (i.e., L2, L3, L5, and L6), forming polytypical assemblages that are dominated by one of the fossil groups. Assemblages are formed mainly by autochthonous to parautochthonous elements, representing variable, but limited, temporal mixing. A key attribute of some fossil-rich strata (L3, L5, and L6) is the preservation of poorly biomineralized organisms and/or of complete soft-bodied parts, which are typically prone to destruction due to rapid decay or bioturbation. The polytypical nature of these fossil assemblages, interbedded with non- fossiliferous intervals, suggests mass mortality events, probably caused by abrupt changes in water parameters (anoxia, salinity, pH, among others). The dark greenish gray color (yellowish when weathered), and the finely laminated nature of the claystone, siltstone, and mudstone containing members of the Amargosa Biota indicates that the benthic infaunal life was absent or, at least, very scarce in a locally, relatively deep, oxygen-poor lake bottom. Anoxia and high salinity, linked with local semi-arid conditions during the Lower Cretaceous may have played key roles in the exceptional preservation of some fossils (shrimps, fish). Finally, our data provide a more comprehensive understanding of the temporal distribution of taxa and taphonomic processes associated with the complex genesis of the fossil-bearing interval of the Amargosa Bed in its type locality.
Towards an actualistic view of the Crato Konservat-Lagerstätte paleoenvironment : a new hypothesis as an Early Cretaceous (Aptian) equatorial and semi-arid wetland.
(2021) Ribeiro, Alexandre Cunha; Ribeiro, Guilherme Cunha; Varejão, Filipe Giovanini; Battirola, Leandro Dênis; Pessoa, Edlley Max; Simões, Marcello Guimarães; Warren, Lucas Veríssimo; Riccomini, Claudio; Poyato-Ariza, Francisco José
An alternative hypothesis concerning the paleoecological and paleoenvironmental depositional conditions of the Crato Konservat-Lagerstatte ̈ (CKL), Crato Formation, Aptian, NE Brazil, one of the most extraordinary Gondwana fossil sites, is proposed. Following an actualistic approach, the ecology of extant relatives of the most abundant and diverse fossil groups recorded in the CKL (i.e., vascular plants, arthropods, fishes, and tetrapods) is considered. Data is based on an extensive literature review followed by a re-examination of recently collected fossils. This approach allowed a detailed appraisal of the stratigraphic/ecological distribution of the main fossil groups preserved in the CKL. Plant and animal groups are recorded in three main stratigraphic intervals, named Intervals I–III in ascending order. Most fossils are to be considered autochthonous to parautochthonous and have been preserved in distinct stages of base-level fluctuations within a shallow lacustrine depositional system, subject to periodic flooding in large, depressed areas. Exceptional preservation in such environments was mediated by microbially-induced processes (i.e., microbial mat entombment), mostly in the coastal areas of the alkaline lake. Based on the distinct sedimentary facies and autecological attributes of dominant paleo- bioindicators, a new paleoenvironmental model for the CKL is proposed, encompassing a seasonal, semi-arid, shallow lacustrine wetland. Faunal and floral content were ecologically arranged in long-lasting aquatic zones, surrounded by periodically flooded mesophytic ecotones and outer xeric habitats, as in the modern alkaline lake Chad in Africa. Our data show the relevance of multiproxy analyses (i.e., paleontological, sedi- mentological, geochemical, and stratigraphic) of exceptional fossil sites for assessing paleoenvironmental con- ditions in depositional settings subject to continuous base-level changes, such as those existing in complex, present-day wetland ecosystems. The recognition of key parameters in ancient wetlands is of great importance concerning the formation of non-marine Konservat-Lagerst ̈ atten in the geological record.
The branchial skeleton in Aptian chanid fishes (Gonorynchiformes) from the Araripe Basin (Brazil) : autecology and paleoecological implications.
(2020) Ribeiro, Alexandre Cunha; Poyato-Ariza, Francisco José; Varejão, Filipe Giovanini; Bockmann, Flavio Alicino
Gonorynchiformes are a small, but morphologically diverse group of teleost fishes with an extensive fossil record. Most extant gonorynchiforms are efficient filter feeders, bearing long gill rakers and other morphological specializations, such as microbranchiospines and an epibranchial organ. The analyses of the gill arch of the Brazilian gonorynchiform fishes Dastilbe crandalli and Tharrias araripis from the Aptian of the Araripe Basin, Northeast Brazil, demonstrate significant morphological variation suggestive of distinct feeding habitats as well as ontogenetic dietary shifts in these closely related gonorynchiforms.

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