Late Cretaceous invasion of Madagascan fresh waters by percomorph fishes (Teleostei: Acanthomorpha)

Abstract

Numerous isolated elements from freshwater Upper Cretaceous (Maastrichtian) deposits in the Mahajanga Basin of northwestern Madagascar are identified as belonging to acanthomorph fishes. Multiple specimens representing the same bony element, including vertebrae, fin spines, basioccipitals, dentaries, hyomandibulae, anguloarticulars, and premaxillae (with a few other elements such as a vomer and a quadrate being less numerous), document the presence of several different morphologies that are indicative of at least three species. The morphologies of the elements, however, are broadly similar to one another, and we interpret this to be a reflection of their shared ancestry and identify the multiple species as representing a single lineage. Therefore, the material likely documents a single invasion of Madagascan fresh waters by a marine lineage, followed by the diversification of this lineage into several taxa within the freshwater environment. A less parsimonious scenario would be that the lineage may have diversified first in marine waters followed by several independent invasions of freshwaters by the most closely related lineages, which seems less likely. Most known Cretaceous acanthomorphs belong to Paracanthopterygii or early-diverging groups of Acanthopterygii, with few belonging to Percomorpha. The material reported here documents the presence of percomorphs in Upper Cretaceous freshwater deposits of Madagascar, the first from Eastern Gondwana and only the second from all of Gondwana. Additionally, the Madagascan acanthomorphs are much larger than most of those previously reported from Albian through Campanian marine and freshwater deposits, indicating that by the latest Cretaceous acanthomorph fishes had evolved relatively large body forms, at least in Eastern Gondwana.