On January 9, 2023, professor of biology Jennifer Mansfield and Ava Brent, a research scientist at Barnard College and currently a lecturer in biological sciences at Columbia University, published new research with Professor Emily Buchholtz of Wellesley College in the journal Current Biology. Their paper, titled “Evolutionary assembly and disassembly of the mammalian sternum,” uses developmental, genetic, and paleontological data to demonstrate that the sternum (i.e., the breastbone) of placental mammals was assembled from preexisting elements. Using imaging of the perinatal mouse, the researchers revealed two sternal elements, both composed primarily of cells with lateral plate mesoderm origin. Furthermore, they showed through evolutionary trait mapping that the elements were independent structures throughout most of synapsid history. The differentiation of rib articulation styles and subdivision of the sternal bands were key innovations likely associated with transitions in locomotor and respiratory mechanics. Their analysis also indicated that subsequent disassembly seen in the radically reduced sternum of mysticete cetaceans (baleen whales), was element-specific, reflecting the constraints that conserved developmental programs exert on composite structures. By examining the development of an extant species and evolutionary changes in the fossil record, the study provides insight into how anatomical structures are added and lost over time.