MicroCT reinvestigation of the only articulated fossil anostomid fish reveals synonymy of Arhinolemur Ameghino, 1898 and Megaleporinus Ramirez et al., 2017

Abstract

Arhinolemur scalabrinii† Ameghino, 1898 was originally described as a strepsirrhine primate (Mammalia) but has been recognized as an anostomid fish since 2012. It remains the only extinct anostomid species known from complete cranial material. The holotype and only known specimen come from the Ituzaingo Formation, Entre Ríos province, Argentina, dated to the Late Mio- cene. This study provides a comprehensive redescription of its cranial anatomy using micro-CT data and comparative osteology. Phylogenetic analyses combining morphological, cytogenetic, and molecular data place Arhinolemur scalabrinii within a clade that includes species previously assigned to Megaleporinus, supporting the treatment of Megaleporinus as a junior synonym of Arhinolemur. The species exhibits a unique combination of features—such as a rhomboidal-shaped mesethmoid, a distinct fontanelle, and specialized dentition—that justify the revalidation of the genus. Arhinolemur scalabrinii represents a key morphological reference for understanding Miocene diversification within Anostomidae and illustrates the paleontological value of microCT analysis of exceptional, articulated fossil fishes in reconstructing the evolutionary history of Neotropical freshwater ichthyofaunas.