The tibiotarsus of a giant darter from the upper Miocene of Amazonia and weight estimates for fossil darters

Abstract

Fossil darters are known from every continent since the Oligocene. However, it was in the Miocene of South America that this group reached its maximum diversity. Macranhinga ranzii is the largest darter from the Solimões Formation, described based on the left and right femora as well as the distal end of a tarsometarsus and four vertebrae. Here, we describe for the first time the left tibiotarsus of this species by comparing it with the tibiotarsus of the extant Anhinga anhinga and other fossil anhingids. We also calculated and compared the body mass of Ma. ranzii to that of all known tibiotarsi fossil anhingids using two calculation methods. The tibiotarsus described here is morphologically similar to that of other anhingids, however, in addition to its large size and robustness, it presents diagnostic characters (e.g., torus linearis) that allow us to attribute it to the genus Macranhinga and differentiate it from other fossil species of this genus whose tibiotarsi are known. The estimated body mass of Ma. ranzii ranges from 14.39 to 19.1 kg, ranking it the highest estimated body mass among the darters where the tibiotarsi were found.