Morphological and histological reviews of Acregoliath rancii Richter 1989 (Osteichthyes: Acregoliathidae) scales from the Acre Basin (Solimões Formation)

• Grant number: 22/06369-5
• Support Opportunities: Scholarships in Brazil - Master
• Effective date (Start): November 01, 2022
• Effective date (End): September 30, 2024
• Field of knowledge: Biological Sciences - Zoology - Paleozoology
• Principal Investigator: Annie Schmaltz Hsiou
• Grantee: Karina da Silva Alencar
• Host Institution: Faculdade de Filosofia, Ciências e Letras de Ribeirão Preto (FFCLRP). Universidade de São Paulo (USP). Ribeirão Preto , SP, Brazil

Abstract

The Acre Basin comprise the state of Acre and the Southern Amazonas, presenting a broad stratigraphic sequence, from the late Miocene of the Solimões Formation the most known and studied. The amniotes paleofauna from this geological unit has been well documented and studied since the late 19th century; however, the paleoichthyofauna records are still incipient, despite the rich fossil material available in public collections. The fish fossils of this formation are, for the most part, scales, vertebrae, teeth and bone fragments from small to medium size. Furthermore, large fossil scales from the enigmatic taxon Acregoliath rancii, described in the late 1980s, with about eight cm diameter are found in several sites in the Southwestern Brazilian Amazon. Morphologically, A. rancii scales are different from other Amazonian bonefishes, both living and fossil species. The structure of these scales consists of two bone tissues, a thin, laminar basal layer formed by isopedine, and a thick spongy layer, filled with pore channels and blood vessels, being covered by a vitreous tissue, identified as hyalodentin. Because the peculiar histological and morphological constitution, there are uncertainties about the systematic positioning of the taxon within Osteichthyes. Therefore, the objective of the macro and microscopic morphological reviews of A. rancii scales is to discuss their phylogenetic positioning among taxa of bony fish.