These differences in reproductive patterns have been tied to clear differences in sequence heterochrony between marsupials and placentals (S�nchez-Villagra, 2002; Smith, 2006). It has also been hypothesized that coordinated shifts in developmental timing occur among functionally- or developmentally-related structures, such as the forelimbs and oral apparatus in marsupials (Goswami, 2007). Here, we use new developmental sequence data for 11 marsupial and 14 placental species to assess the integration of developmental timing in the cranial and postcranial elements across mammals. Results demonstrate that marsupials and placentals differ markedly in the integration of developmental timing. Marsupials show significant modularity in the development of the postcranial skeleton, with independent forelimb and hindlimb developmental modules, while placentals show significant integration of the entire appendicular skeleton. There are few significant developmental modules in cranium, although placentals show greater integration in the facial region than marsupials. These differences in developmental modularity may relate to differences in morphological diversity between these clades and across the skeleton