Crustáceos decapoda excepcionalmente fossilizados, formação crato, cretáceo, bacia do Araripe, Brasil, e suas implicações paleoambientais e paleoecológicas
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Data
2022-03
Autores
Queiroz, Hugo Gonçalves Dias
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Universidade Estadual Paulista (Unesp)
Resumo
A Formação Crato, Aptiano, Bacia do Araripe, NE do Brasil, é constituída por sucessão carbonática-siliciclástica mundialmente conhecida pela diversidade de fósseis excepcionalmente preservados. Estes são registrados nos calcários laminados, constituindo depósitos do tipo Konservat-Lagerstätte da porção basal da unidade. Dentre o diverso registro fóssil, apenas uma espécie de crustáceo decápode (Beurlenia araripensis) é conhecida na unidade, restrita aos calcários da região de Nova Olinda, CE. Por razões intrínsecas (e.g., natureza do exoesqueleto) os Decapoda possuem registro fóssil muito incompleto. Portanto, a preservação excepcional de camarões constitui fato notável, passível de investigação geológica e paleontológico. Exemplares preservados com ovos são, obviamente, ainda mais raros no registro fossilífero e nunca foram documentados para camarões, o que torna o material em estudo altamente importante do ponto de vista paleobiológico. O presente estudo teve por objetivo (1) determinar a natureza das estruturas esféricas, postuladas como ovos encontradas em espécimes de B. araripensis, (2) precisar sua localização anatômica e (3) calcular as dimensões e o número delas, a fim de discutir questões paleoambientais e biológicas relevantes. Os fósseis estudados pertencem à Coleção Científica do IGc/USP, sendo representados por “slabs” centimétricos a métricos contendo vários exemplares. Os espécimes estão preservados em laminitos, em 3D, completos e articulados. Estão dispostos paralelamente ao plano de acamamento, com os apêndices aparentemente presos à matriz sedimentar por calcita subédrica de granulação fina, aciculares e estruturas semelhantes a filamentos de bactérias. Os fósseis foram submetidos a diferentes técnicas de imageamento digital (dentro das possibilidades das limitações impostas pela pandemia de Covid-19) para a acurada interpretação das estruturas anatômicas e dos possíveis ovos. O imageamento digital confirma a presença dos objetos esféricos na porção abdominal, local onde camarões atuais carregam a ova, junto aos pleópodes. Apenas em 2 espécimes foram encontrados ovos. Desses, 24 ovos no total foram passíveis de mensuração, apresentando diâmetro médio de 0.427 ± 0.063 mm. Espécies atuais da mesma família (Palaemonidae) de água doce apresentam intervalo de 0.64-0.74mm, enquanto as marinhas de 0.48-0.65mm. As dimensões dos ovos de B. araripensis são semelhantes à de camarões marinhos atuais. Camarões de água doce apresentam ovos maiores à medida que se distanciam dos estuários, local onde as larvas se desenvolvem. Nesses casos as larvas apresentam lecitotrofia, nutrindo-se apenas das reservas dos ovos até que adquiram o hábito planctotrófico no estuário ou no mar. Já nas espécies marinhas o trajeto até o local de desenvolvimento é encurtado, com menor necessidade de reservas nutricionais. O ambiente deposicional hipersalino dos calcários laminados da Formação Crato pode ter contribuído para o padrão observado. A forma, número, dimensões e região anatômica onde os objetos esféricos estão preservados sugerem, fortemente, tratar-se de ovos. O tamanho destes pode auxiliar na interpretação dos paleoambientes e da paleobiologia de espécies pretéritas de Decapoda, servindo como parâmetro independente à interpretação de paleoambiente aquoso, salino, durante a deposição dos laminitos fossilíferos.
The Aptian Crato Formation, Araripe Basin, NE Brazil, is represented by a carbonate-siliciclastic succession known worldwide for the diversity of exceptionally preserved fossils. These are recorded in laminated limestones forming Konservat-Lagerstätte deposits in its the basal portion. Among the diverse fossil record, only one species of decapod crustacean (Beurlenia araripensis) is currently known and is recorded in limestones of the Nova Olinda region, Ceará State. Due to intrinsic properties (e.g., nature of the exoskeleton) the Decapoda crustaceans have a very incomplete fossil record. Therefore, the exceptional preservation of shrimps is a remarkable fact, subject to geological and paleontological investigations. Specimens preserved with eggs are, of course, even rarer in the fossil record and have never been documented for shrimps, which makes the material under study highly relevant from a paleobiological perspective. This study aimed to (1) determine the nature of the spherical structures, postulated as eggs found in specimens of B. araripensis, (2) specify their anatomical location, and (3) calculate their dimensions and number, in order to discuss relevant paleoenvironmental and paleobiological issues. The studied fossils belong to the Scientific Collection of the IGc/USP, being represented by centimetric to metric “slabs” containing several specimens. The specimens are preserved in laminites, in 3D, complete and articulated. They are arranged parallel to the bedding plane, with the appendages apparently attached to the sedimentary matrix by fine-grained subhedral calcite, acicular and bacterial filament-like structures. The fossils were subjected to different digital imaging techniques (within the possibilities of the limitations imposed by the Covid-19 pandemic) for the accurate interpretation of anatomical structures and possible eggs. Digital imaging confirmed the presence of spherical objects in the abdominal portion, the location where shrimps currently carry their eggs, along with pleopods. Eggs were found in only 2 specimens. Of these, a total of 24 eggs were measurable, with a mean diameter of 0.427 ± 0.063 mm. Current freshwater species of the same family (Palaemonidae) have a range of 0.64-0.74mm, while marine species of 0.48-0.65mm. The dimensions of B. araripensis eggs are similar to those of currently found in marine shrimps. Freshwater shrimps have larger eggs as they move away from estuaries, where the larvae develop. In these cases, the larvae are lecithotrophic, feeding only on the egg reserves until they acquire the planktotrophic habit in the estuary or in the sea. In marine species, the path to the development site is shortened, with less need for nutritional reserves. The hypersaline depositional environment of laminated limestones of the Crato Formation may have contributed to the observed pattern. The shape, number, dimensions and anatomical region where the spherical objects are preserved strongly suggest that they are eggs. Their size can help in the interpretation of paleoenvironments and paleobiology of past Decapoda species, serving as an independent parameter for the interpretation of the depositional history of the fossil-rich laminites of the Crato Formation.
The Aptian Crato Formation, Araripe Basin, NE Brazil, is represented by a carbonate-siliciclastic succession known worldwide for the diversity of exceptionally preserved fossils. These are recorded in laminated limestones forming Konservat-Lagerstätte deposits in its the basal portion. Among the diverse fossil record, only one species of decapod crustacean (Beurlenia araripensis) is currently known and is recorded in limestones of the Nova Olinda region, Ceará State. Due to intrinsic properties (e.g., nature of the exoskeleton) the Decapoda crustaceans have a very incomplete fossil record. Therefore, the exceptional preservation of shrimps is a remarkable fact, subject to geological and paleontological investigations. Specimens preserved with eggs are, of course, even rarer in the fossil record and have never been documented for shrimps, which makes the material under study highly relevant from a paleobiological perspective. This study aimed to (1) determine the nature of the spherical structures, postulated as eggs found in specimens of B. araripensis, (2) specify their anatomical location, and (3) calculate their dimensions and number, in order to discuss relevant paleoenvironmental and paleobiological issues. The studied fossils belong to the Scientific Collection of the IGc/USP, being represented by centimetric to metric “slabs” containing several specimens. The specimens are preserved in laminites, in 3D, complete and articulated. They are arranged parallel to the bedding plane, with the appendages apparently attached to the sedimentary matrix by fine-grained subhedral calcite, acicular and bacterial filament-like structures. The fossils were subjected to different digital imaging techniques (within the possibilities of the limitations imposed by the Covid-19 pandemic) for the accurate interpretation of anatomical structures and possible eggs. Digital imaging confirmed the presence of spherical objects in the abdominal portion, the location where shrimps currently carry their eggs, along with pleopods. Eggs were found in only 2 specimens. Of these, a total of 24 eggs were measurable, with a mean diameter of 0.427 ± 0.063 mm. Current freshwater species of the same family (Palaemonidae) have a range of 0.64-0.74mm, while marine species of 0.48-0.65mm. The dimensions of B. araripensis eggs are similar to those of currently found in marine shrimps. Freshwater shrimps have larger eggs as they move away from estuaries, where the larvae develop. In these cases, the larvae are lecithotrophic, feeding only on the egg reserves until they acquire the planktotrophic habit in the estuary or in the sea. In marine species, the path to the development site is shortened, with less need for nutritional reserves. The hypersaline depositional environment of laminated limestones of the Crato Formation may have contributed to the observed pattern. The shape, number, dimensions and anatomical region where the spherical objects are preserved strongly suggest that they are eggs. Their size can help in the interpretation of paleoenvironments and paleobiology of past Decapoda species, serving as an independent parameter for the interpretation of the depositional history of the fossil-rich laminites of the Crato Formation.
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Palavras-chave
Camarão, Tafonomia, Konservat-Lagerstätte, Carbonatos