Mammals are an extremely successful group of tetrapods, and the great diversity of limb morphologies and locomotor modes is underpinned by adaptations in their long bones. Elongation of long bones takes place at each end of the bone shaft, at the epiphyseal growth plate surface. Secondary ossification centres form bony epiphyses above these growth plates, eventually fusing with the plates and marking the end of bone elongation. The proximal growth plate surface in the humerus is not uniform, with varying distributions of peaks and grooves that appear to differ across Mammalia. Although the process of growth plate bone deposition is well understood, there has been limited research on the growth plate surface morphology and what factors may be influencing it, such as ecology, phylogeny, or ontogeny. Using synchrotron propagation phase-contrast X-ray microtomography to reconstruct 3D models of the growth plate surface of the proximal humerus. We applied a metric called ariaDNE to quantify and measure the curvatures of these surfaces. We studied several extant mammals and one fossil stem mammal. We found that there was a high diversity of growth plate morphologies across mammal species, but the overall morphology remained consistent within the same species. The morphology also appears to shift throughout different ontogenetic stages, with a trend towards higher complexity in later developmental stages. The fossil specimen, G. planiceps, has a growth plate morphology that is remarkably similar to the short-beaked echidna (Tachyglossus aculeatus). Preliminary statistical tests suggest that growth plate morphology may be influenced by phylogeny and locomotory mode, although our small dataset has likely introduced sampling bias and the sample size should be expanded to find significant patterns. Growth plate surface morphology should be further explored due to its potential palaeobiological applications for the interpretation of ecology and evolution of extinct taxa.