Systematics of the bivalves (Mollusca) from the Crato Formation (Lower Cretaceous), NE Brazil and its paleoenvironmental significance

Author(s)

Victor Ribeiro da Silva

• Document type: Master's Dissertation
• Press: Rio Claro. 2019-10-14.
• Institution: Universidade Estadual Paulista (Unesp). Instituto de Geociências e Ciências Exatas. Rio Claro
• Defense date: 2019-09-27
• Advisor: Marcello Guimarães Simões

Abstract

The Aptian Crato Formation is one of the main litostratigraphic units of the Mesozoic sedimentary succession of the Araripe Basin, northeastern Brazil. This unit is widely known for containing one of the main Konservat-Lagerstätte deposits of the Gondwana. Such deposits are represented by laminated limestones enclosing exceptionally well-preserved fossils. Stratigraphic and paleontological investigations, combined with detailed mapping of the unit in the eastern border of the Araripe Basin, enabled the recognizing of a key interval encompassing shell-rich siltstones and claystones, with wide lateral distribution. The 0.3–2.25-m-thick interval is represented by tabular layers of intensely bioturbated grayish mudstones, intercalated in shales, fine sandstones and heterolithic facies. In this study, for the very first time, bivalve molluscs found within this layer are described, and their paleoenvironmental and paleobiogeographic implications are discussed. Specimens have been collected in four different locations (i.e., Três Irmãos, Batateira, Caldas and Estiva), and then submitted to careful analyses of key morphological characters (e.g., muscle scars, hinge and ornamentation). Two new genera are herein described (Cratonaia gen. nov. and Araripenaia gen. nov.), each one of them belonging to the superfamilies Silesunionoidea Skawina and Dzik, 2011 and Trigonioidoidea Cox, 1952, respectively. Both genera are monospecific and represented by the new species Cratonaia novaolindensis gen. et sp. nov. and Araripenaia elliptica gen. et sp. nov. The genus Monginella also seems to be present, represented by the new species ?Monginella bellaradiata sp. nov. This genus is typical of north African cretaceous mollusc assemblages. Therefore, bivalves of both the suborder Silesunionidina Skawina and Dzik, 2011 and the superfamily Trigonioidoidea are recorded for South American Early Cretaceous deposits for the first time. The studied fossils are generally represented by composite molds with variable preservation quality. They are commonly preserved with articulated valves (closed or splayed), suggesting short lateral transportation (i.e., autochtonous to parautochtonous fauna). The predominantly shallow-burrowing, or semi-infaunal, suspensivorous fauna paleoautoecological attributes, along with the available stratigraphic and sedimentological observations, suggest that the depositional environment inhabited by these molluscs consisted in a huge muddy flat of a freshwater to brackish lagoon. This means that the bivalve-rich fossiliferous layer records large-scale environmental changes, representing a short-timed event, immediately after the deposition of the laminated limestones, which, in turn, are an indicative of hypersaline lake conditions. Finally, the potential presence of the bivalve family Plicatounionidae Chen, 1987, may suggest affinities with mollusc assemblages from the Aptian of north Africa, more specifically, with Barremian-Aptian deposits of the Djoua Group, Algeria. (AU)

• FAPESP's process: 17/22036-8 - Systematics and paleoecology of the bivalves of the crato formation (Lower Cretaceous), Northeastern Brazil: paleoenvironmental and paleogeographic significance
• Grantee: Victor Ribeiro da Silva
• Support Opportunities: Scholarships in Brazil - Master