Paleontologia da passagem Ediacarano-Cambriano na Formação Tagatiya Guazú, Grupo Itapucumi (Gondwana Ocidental), NE do Paraguai
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Data
2023-04-28
Autores
Inglez, Lucas [UNESP]
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Universidade Estadual Paulista (Unesp)
Resumo
Os estágios terminais do Período Ediacarano (565-538 Ma) foram palco de profunda reestruturação ecossistêmica, associada a irradiação de organismos multicelulares e o aparecimento de animais bilatérios no registro fóssil. Dentre as importantes novidades evolutivas ocorridas durante a transição Ediacarano-Cambriano, tem-se o aparecimento de táxons bentônicos esqueletizados e o aumento na abundância e disparidade morfológica (denotando maior complexidade ecológica e comportamental) das estruturas de bioturbação. Neste contexto, e com idade mínima de 536 Ma, a Formação Tagatiya Guazú (FTG) (Grupo Itapucumi) no nordeste do Paraguai, contém abundante conteúdo fóssil esqueletal, bem como inédita assembleia de icnofósseis. Deste modo, esta tese tem por objetivos principais: 1) caracterizar a diversidade taxonômica da assembleia fóssil; 2) posicionar estratigraficamente as associações fossilíferas, caracterizando sua distribuição em distintas fácies e estilos preservacionais; 3) caracterizar as estruturas de bioturbação em termos de sua icnodiversidade e 4) discutir as implicações paleoecológicas e bioestratigráficas da assembleia. O detalhamento taxonômico permitiu o reconhecimento de C. hartmannae, C. cf. carinata, Cloudina sp., cf. Zuunia chimidtsereni, Sinotubulites sp. Corumbella werneri, cf. Namacalathus hermanastes, além de thalli de vendotaeniáceas e outros restos de macroalgas de afinidade incerta. Os fósseis ocorrem preferencialmente em fácies lagunares microbialíticas (depositadas em ambientes de inframaré rasa a inter- e supramaré) e, sua caracterização tafonômica qualitativa sugere preservação controlada por eventos de rápido soterramento, e precoce cimentação carbonática. Ambos os processos são indicados como essenciais para a alta qualidade de preservação de carapaças predominantemente orgânicas e delicadas, e de organismos de corpo mole como compressões carbonificadas. Em sentido ao topo do intervalo analisado foi notada expressiva diminuição na abundância e diversidade de táxons esqueletais, associada a mudança abrupta nos padrões de bioturbação. Enquanto na base, apenas estruturas suspeitas, irregulares e morfologicamente simples foram observadas, no topo são registrados níveis com abundantes icnofósseis (1-9% de índices de bioturbação nos planos de acamamento), com considerável icnodisparidade. Estes incluem icnotáxons atribuíveis a Bergaueria hemisphaerica., Skolithos isp., Planolites isp., cf. Torrowangea isp., Treptichnus isp., Phycodes palmatus, abundantes treptinideos e estruturas helicoidais (tipo-Streptichnus) além de raras pistas bilobadas rusophyciformes. Esta diversidade de planos arquiteturais sugere padrões de bioturbação mais típicos de ecossistemas terranovianos, do que aqueles reportados para o Ediacarano terminal. Assim, considerando 1) a ausência de hiatos estratigráficos expressivos na sucessão avaliada, 2) sua idade mínima de ~536 Ma, 3) a proximidade vertical de intervalos bioturbados e com abundantes fósseis tipicamente Ediacaranos, é reforçado o caráter proeminentemente gradacional das transformações faunísticas que marcaram a transição Ediacarano-Cambriano.
The terminal Ediacaran Period, spanning from 565-538 million years ago, was a time of significant ecological restructuring, linked to the emergence of diverse multicellular marine organisms, and the first appearance of bilaterian animals in the fossil record. Among important evolutionary novelties the Ediacaran-Cambrian Transition was marked by the appearance of skeletonized benthic taxa and the increase in abundance and disparity (denoting further ecological and behavioural complexity) of biotirbation structures. Within this context, the Tagatiya Guazú Formation (TGF) (Itapucumi Group) in northeastern Paraguay contains abundant fossil skeletal remains and an undescribed ichnofossil assemblage that date back to ~536 Ma ago. Thus, the main objectives of this thesis are to 1) characterize the taxonomic diversity of the fossil assemblage, 2) stratigraphically position the fossil associations, 3) describe the bioturbation structures, and 4) discuss their paleoecological and biostratigraphic implications. The taxonomic assessment of body fossils indicates the presence of C. hartmannae, C. cf. carinata, Cloudina sp., cf. Zuunia chimidtsereni, Sinotubulites sp., Corumbella werneri, cf. Namacalathus hermanastes, abundant thalli of vendotaeniaceans, as well as macroalgal remains of uncertain affinity. The fossils are predominantly distributed within microbialitic lagoonal facies (from shallow sub- to inter- and supratidal settings) and were preserved through events of rapid sedimentation and early carbonate cementation. Both processes are indicated as key for the high-quality preservation of both delicate, and predominantly organic carapaces, as well as for the soft tissues of macroalga as carbonaceous compressions. Towards the top of the stratigraphic interval a decrease in the abundance and diversity of body fossils is observed, along with an abrupt shift in the patterns of bioturbation. While the lower and middle parts of the succession display only morphologically simple and inconsistent structures, the uppermost interval contains abundant ichnofossils (Bedding Plane Bioturbation Indexes of ~1-9%), with considerable ichnodisparity. These include ichnotaxa such as Bergaueria hemisphaerica, Skolithos isp., Planolites isp., cf. Torrowangea isp., Treptichnus pedum, Phycodes palmatus, abundant treptichniids, potential helicoidal structures (Streptichnus-like), besides rare hypichnial bilobed rusophyciform burrows. The diversity of architectural designs suggests a bioturbation pattern more typical of Terraneuvian ecosystems than those reported for the terminal Ediacaran. Thus, given the 1) absence of stratigraphical hiatuses and sharp faciological shifts along the succession, 2) the recent geochronological data providing minimum age of ~536 Ma for the unit, and 3) the vertical association between trace fossil-bearing intervals and abundant typically Ediacaran skeletal metazoans, results reinforce the prominent gradational character of the major faunal turnovers that marked the Ediacaran-Cambrian Transition.
The terminal Ediacaran Period, spanning from 565-538 million years ago, was a time of significant ecological restructuring, linked to the emergence of diverse multicellular marine organisms, and the first appearance of bilaterian animals in the fossil record. Among important evolutionary novelties the Ediacaran-Cambrian Transition was marked by the appearance of skeletonized benthic taxa and the increase in abundance and disparity (denoting further ecological and behavioural complexity) of biotirbation structures. Within this context, the Tagatiya Guazú Formation (TGF) (Itapucumi Group) in northeastern Paraguay contains abundant fossil skeletal remains and an undescribed ichnofossil assemblage that date back to ~536 Ma ago. Thus, the main objectives of this thesis are to 1) characterize the taxonomic diversity of the fossil assemblage, 2) stratigraphically position the fossil associations, 3) describe the bioturbation structures, and 4) discuss their paleoecological and biostratigraphic implications. The taxonomic assessment of body fossils indicates the presence of C. hartmannae, C. cf. carinata, Cloudina sp., cf. Zuunia chimidtsereni, Sinotubulites sp., Corumbella werneri, cf. Namacalathus hermanastes, abundant thalli of vendotaeniaceans, as well as macroalgal remains of uncertain affinity. The fossils are predominantly distributed within microbialitic lagoonal facies (from shallow sub- to inter- and supratidal settings) and were preserved through events of rapid sedimentation and early carbonate cementation. Both processes are indicated as key for the high-quality preservation of both delicate, and predominantly organic carapaces, as well as for the soft tissues of macroalga as carbonaceous compressions. Towards the top of the stratigraphic interval a decrease in the abundance and diversity of body fossils is observed, along with an abrupt shift in the patterns of bioturbation. While the lower and middle parts of the succession display only morphologically simple and inconsistent structures, the uppermost interval contains abundant ichnofossils (Bedding Plane Bioturbation Indexes of ~1-9%), with considerable ichnodisparity. These include ichnotaxa such as Bergaueria hemisphaerica, Skolithos isp., Planolites isp., cf. Torrowangea isp., Treptichnus pedum, Phycodes palmatus, abundant treptichniids, potential helicoidal structures (Streptichnus-like), besides rare hypichnial bilobed rusophyciform burrows. The diversity of architectural designs suggests a bioturbation pattern more typical of Terraneuvian ecosystems than those reported for the terminal Ediacaran. Thus, given the 1) absence of stratigraphical hiatuses and sharp faciological shifts along the succession, 2) the recent geochronological data providing minimum age of ~536 Ma for the unit, and 3) the vertical association between trace fossil-bearing intervals and abundant typically Ediacaran skeletal metazoans, results reinforce the prominent gradational character of the major faunal turnovers that marked the Ediacaran-Cambrian Transition.
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Paleontologia, Bioestratigrafia, Tafonomia, Icnologia, Ediacarano, Palentology, Bioestratigraphy, Taphonomy, Ichnology