Marsupials exhibit unique biological features that provide fascinating insights into many aspects of mammalian development. These include their distinctive mode of reproduction, altricial developmental stage at birth and the associated extreme skeletal heterochrony that is required for their crawl to the mother’s pouch and attachment to the teat. Marsupials are also an invaluable resource for mammalian comparative biology as they form a unique lineage distinct from the extant placental and egg-laying monotreme mammals. Despite their unique biology, marsupial resources are lagging behind those for placental mammals, largely due to the lack of a well-developed, laboratory-based marsupial model species. The fat-tailed dunnart (Sminthopsis crassicaudata) is an excellent candidate laboratory marsupial model that requires simple and robust husbandry, has a short reproductive cycle and is amenable to experimental manipulations. Here we present a detailed staging series for the dunnart, focusing on their extensive skeletal heterochrony with accelerated development of the forelimbs and jaws compared with the placental fetus. This study provides the first skeletal developmental series on the fat-tailed dunnart and provides a fundamental resource for future studies exploring mammalian diversification, development and evolution.