Paleontological examinations of the invasion of land by vertebrates suggest that limb-like appendages likely originated in aquatic environments, but direct comparisons of the functional consequences of using early limbs with digits, rather than fins, for terrestrial locomotion had not previously been performed. Salamanders are used to model the general body form of early tetrapods (e.g., Paleozoic amphibians) since their morphology has remained essentially unchanged for at least 150 million years. Mudskippers are similar to early fossil precursors of the tetrapods: they use "crutching" movements on land similarly to the hypothesised locomotion of Ichthyostega, and their pectoral fins are similar to elpistostegalids, such as Tiktaalik.