The origin and homology of the turtle shell are one of the most captivating topics in amniote evolution. In this contribution, we present a new species of turtle from the Late Triassic of Argentina whose peripheral plates question the homology of these bones in turtles. The external morphology of the peripheral plates of Waluchelys cavitesta gen. et sp. nov. (Testudinata: Australochelyidae) is as in any other turtle, however, appearances can be deceiving. Internally, these plates exhibit an unexpected internal cavity. The absence of structural similarities and of ontogenetic or phylogenetic transitional forms between the peripheral plates of W. cavitesta and other testudinatans might suggest that the periphery of turtles represents a case of deep homology. Furthermore, the present and recent findings suggest that the structure and ossification patterns of the periphery of the turtle shell were more plastic and subject to variation than other elements of the shell, at least in the earliest stages of turtle evolution. These findings also suggest that the “typical” mesochelydian turtle shell could have been acquired in a two-stage process.