Most aquatic amniotes descended from terrestrial ancestors with appendicular locomotion via an upright or sprawling posture, evolving one or a combination of axial and appendicular aquatic locomotion. To explore if ancestral posture and locomotion might influence secondarily aquatic transitions, forelimb elements of thirty transition-spanning cetaceans, mosasaurs, sauropterygians, and pinnipeds were compared. Linear measurements were taken between homologous landmarks of the scapula, humerus, ulna, corrected for size using geometric mean, and made into principal components for principal components analyses. The PCA morphospaces of cetaceans and mosasaurs, both of which have derived members utilizing axial aquatic locomotion, occupy a nearly identical range in PC1, heavily loaded to long bone length, but cetaceans occupy a significantly wider morphospace than mosasaurs along the PC2 axis, loaded to scapular measurements. This potentially suggests either constraint in certain forelimb aspects in mosasaurs, and/or a functional release in cetaceans, potentially pertaining to ancestral posture. Pinnipeds and sauropterygian forelimb morphospaces, which had different ancestral postures, do not overlap in forelimb morphospace at all, not even between early sauropterygians and pinnipeds, despite these groups being primarily appendicular swimmers and oft used as convergence examples. Further, the morphospaces of sauropterygians and mosasaurs, both ancestrally sprawling clades, but whose derived members use appendicular and axial aquatic locomotion respectively, do align and then diverge in the morphospace, potentially reflecting these locomotory changes.