Abstract
The Ipubi and Romualdo Formations are Cretaceous units of the Araripe Basin (Santana Group). The first and most ancient was deposited in a lake environment, and some fossils were preserved in shales deposited under blackish conditions. The second was deposited in a marine environment, preserving a rich paleontological content in calcareous concretions. Considering that these two environments preserved their fossils under different processes, in this work we investigated the chemical composition of two fossilized specimens, one from each of the studied stratigraphic units, and compared them using vibrational spectroscopy techniques (Raman and IR), X-ray diffraction and large-field energy-dispersive X-ray spectroscopy (EDS) mappings. Calcite was observed as the dominant phase and carbon was observed in the fossils as a byproduct of the decomposition. The preservation of hydroxide calcium phosphate (Ca10(PO4)6(OH)2, hydroxyapatite) was observed in both fossils. In addition, it was observed that there was a smaller amount of pyrite (pyritization) in the Romualdo Formation sample than in the Ipubi one. Large-field EDS measurements showed the major presence of the chemical elements calcium, oxygen, iron, aluminum and fluoride in the Ipubi fossil, indicating a greater influence of inorganic processes in its fossilization. Our results also suggest that the Romualdo Formation fossilization process involved the substitution of the hydroxyl group by fluorine, providing durability to the fossils.
Highlights
Palaemonid crustaceans in Brazil are known from tertiary outcrops, with species occurring in the Tremembé Formation (Oligocene, from São Paulo state) and tertiary deposits in the Marizal Formation (Martins-Neto & Mezzalira, 1991a; Martins-Neto & Mezzalira, 1991b)
It is difficult to observe the pleura of the second somite, the sample looks like a Caridea specimen because the pleura of the second somite is heavily rounded at the base
The amount of the element phosphorus found by energy dispersive spectroscopy (EDS) analysis in the Romualdo Formation fossil shrimp was quite sparse compared to the results found for the Ipubi fossil (Oliveira et al, 2015), confirming what was found by the other techniques employed in this work
Summary
Palaemonid crustaceans in Brazil are known from tertiary outcrops, with species occurring in the Tremembé Formation (Oligocene, from São Paulo state) and tertiary deposits in the Marizal Formation (Martins-Neto & Mezzalira, 1991a; Martins-Neto & Mezzalira, 1991b).How to cite this article Barros OA, Silva JH, Saraiva GD, Viana BC, Paschoal AR, Freire PTC, Oliveira NC, Paula AJ, Viana MS. 2019. The preservation of crustacean fossils is difficult because of their propensity for decomposition (Feldmann & Pole, 1994), what may explain (in part) why only a few species of shrimps are described in Brazilian sedimentary basins. The fossils from the Araripe Basin are very well known for their excellent state of preservation and paleobiological diversity (Martill, 1988). The knowledge about fossil shrimp in the Araripe Basin is basically restricted to taxonomic works (Martins-Neto & Mezzalira, 1991a; Martins-Neto & Mezzalira, 1991b; Maisey & Carvalho, 1995; Santana et al, 2013; Pinheiro, Saraiva & Santana, 2014; Saraiva, Pinheiro & Santana, 2018)
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