The sauropod dinosaur clade Titanosauria includes the most massive terrestrial animals yet discovered1, 2. Nevertheless, with the exception of Futalognkosaurus—known from much of the vertebral column and pelvis3—all truly gigantic titanosaurs (sensu Sander et al.4; i.e., those with an adult body mass exceeding 40 metric tons) are represented by very fragmentary fossils5, 6, 7, 8, 9. The incomplete understanding of the anatomies of colossal titanosaurs has frustrated attempts to characterize important aspects of their evolutionary history and palaeobiology. For example, it is presently uncertain whether extreme gigantism evolved multiple times or only once within Titanosauria4. Furthermore, it is not even clear how large the largest titanosaurs were, with different studies yielding widely divergent estimates of body dimensions7, 10, 11, 12, 13.