Neogene sharks and rays from the Brazilian 'Blue Amazon'

Area of Science:

• Paleontology
• Marine Biology
• Geochemistry

Background:

• The lower Miocene Pirabas Formation in Northern Brazil experienced significant ecological niche changes.
• This formation was influenced by the proto-Amazon River and is characterized by marine, carbonate-rich beds.

Purpose of the Study:

• To investigate the elasmobranch fauna of the Pirabas Formation to understand early Miocene ecological changes.
• To reconstruct paleotemperatures and paleoecology using phosphate oxygen isotope analysis of elasmobranch teeth.

Main Methods:

• Systematic identification of elasmobranch taxa (sharks and rays).
• Analysis of phosphate oxygen isotope composition in elasmobranch teeth.
• Comparison of fossil fauna with modern assemblages and other Neogene fish assemblages.

Main Results:

• A diverse fauna of 24 elasmobranch taxa was identified, dominated by carcharhiniforms and myliobatiforms.
• Faunal composition resembles proto-Caribbean early Miocene assemblages but is unique for the Atlantic coast of the Americas.
• Isotope data indicate a tropical marine setting with inshore and offshore habitats, and probable depth preferences for certain taxa.

Conclusions:

• The Pirabas Formation provides a unique Neogene fossil fish assemblage from the Atlantic coast.
• Paleoecological reconstructions suggest predominantly coastal and shallow water habitats for key taxa.
• Variations in modern selachian isotope composition indicate increased Amazon River influence on coastal waters in the Holocene compared to the early Miocene.