Growth of the shoulderblade in marsupials and placentals. The ontogenetic trajectories of individual marsupial and placental species are plotted in a morphospace representing general scapular shape (PC2 and PC3 are the 2nd and 3rd principal component; the first principal component is just size). Marsupials, with the exception of the bandicoot Isoodon, are represented by solid black arrows (w, Macropus [Tammar wallaby] t, Trichosurus [Australian possum] m, Monodelphis [South American opossum]). Isoodon (i) is represented by the white arrow with the black border. Placentals are represented by solid gray arrows. The ontogenetic trajectories of marsupials, with the exception of Isoodon, are remarkably similar, whereas the trajectories of placentals vary greatly. Isoodon does not crawl to the pouch, and so its fetus does not experience the same developmental constraints as other marsupials. (Figure 7 from Karen E. Sears. 2004. "Constraints on the Morphological Evolution of Marsupial Shoulder Girdles," Evolution 58(10):2353–2370.)