Evolutionary assembly and disassembly of the mammalian sternum

Evolutionary transitions are frequently associated with novel anatomical structures,1 but the origins of the structures themselves are often poorly known. We use developmental, genetic, and paleontological data to demonstrate that the therian sternum was assembled from pre-existing elements. Imaging of the perinatal mouse reveals two paired sternal elements, both composed primarily of cells with lateral plate mesoderm origin. Location, articulations, and development identify them as homologs of the interclavicle and the sternal bands of synapsid outgroups. The interclavicle, not previously recognized in therians,2 articulates with the clavicle and differs from the sternal bands in both embryonic HOX expression and pattern of skeletal matu-ration. The sternal bands articulate with the ribs in two styles, most clearly differentiated by their association with sternebrae. Evolutionary trait mapping indicates that the interclavicle and sternal bands were indepen-dent elements throughout most of synapsid history. The differentiation of rib articulation styles and the subdivision of the sternal bands into sternebrae were key innovations likely associated with transitions in locomotor and respiratory mechanics.3,4 Fusion of the interclavicle and the anterior sternal bands to form a presternum anterior to the first sternebra was a historically recent innovation unique to therians. Subse-quent disassembly of the radically reduced sternum of mysticete cetaceans was element specific, reflecting the constraints that conserved developmental programs exert on composite structures.

Automated summary

We examined the origins of the therian sternum using developmental, genetic, and paleontological data. Our findings showed that the therian sternum was assembled from pre-existing elements, namely the interclavicle and sternal bands. These elements articulates with the clavicle and ribs and were independent elements throughout most of synapsid history. The fusion of the interclavicle and anterior sternal bands to form a presternum was a unique innovation in therians.