Abstract
The lower Miocene Pirabas Formation in the North of Brazil was deposited under influence of the proto-Amazon River and is characterized by large changes in the ecological niches from the early Miocene onwards. To evaluate these ecological changes, the elasmobranch fauna of the fully marine, carbonate-rich beds was investigated. A diverse fauna with 24 taxa of sharks and rays was identified with the dominant groups being carcharhiniforms and myliobatiforms. This faunal composition is similar to other early Miocene assemblages from the proto-Carribbean bioprovince. However, the Pirabas Formation has unique features compared to the other localities; being the only Neogene fossil fish assemblage described from the Atlantic coast of Tropical Americas. Phosphate oxygen isotope composition of elasmobranch teeth served as proxies for paleotemperatures and paleoecology. The data are compatible with a predominantly tropical marine setting with recognized inshore and offshore habitats with some probable depth preferences (e.g., Aetomylaeus groups). Paleohabitat of taxa particularly found in the Neogene of the Americas (†Carcharhinus ackermannii, †Aetomylaeus cubensis) are estimated to have been principally coastal and shallow waters. Larger variation among the few analyzed modern selachians reflects a larger range for the isotopic composition of recent seawater compared to the early Miocene. This probably links to an increased influence of the Amazon River in the coastal regions during the Holocene.
Highlights
The evolution of the Amazon River and its drainage basin are closely related to the uplift of the Andes at the northwestern coast of South America [1,2,3]
Taxonomic characteristics and oxygen isotope compositions of 72 teeth of sharks and rays were examined for sediments from the Pirabas Formation, Eastern Amazon, Brazil
Comparison between the fossil and Recent isotopic compositions led to interesting paleoecological propositions
Summary
The evolution of the Amazon River and its drainage basin are closely related to the uplift of the Andes at the northwestern coast of South America [1,2,3]. The sediments deposited onto the Precambrian rocks at the coastal margin of the Guyana and the Brazilian shields [4,5] are mainly biogenic carbonates and siliciclastic rocks with an exceptional abundance and diversity of a shallow marine fossil fauna [6,7]. These sedimentary sequences are linked to global sea-level variations, and two regionally transgressive episodes may be distinguished along the Brazilian coast: one in the Oligo-Miocene and another in the early to middle Miocene [5,8,9]. This event triggered changes in the coastal marine environment together with sea level variation through time [19]
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