The echidna is one of the last survivors of the most basal clade of living mammals and as such, it is an important species to understand character transformation in the dawn of mammalian evolution. Based on description of several embryological stages and consideration of characters taken from the classical drawings of Richard Semon (1859–1918), we study the early development of this species using the Standard Event System. We describe and analyse developmental timing with a phylogenetic approach (Parsimov). A mosaic of plesiomorphic and derived features – reportedly found in the echidna adult anatomy and genome, when compared to other living mammals – is also found when examining its embryonic development. In contrast to sauropsids, we found mammals to be characterized by late eye and ear development, possibly related to their contrasting early postnatal life history. The differentiation of the jaw bones may be reflected in the developmental timing of the mandibular process among mammals. Whereas in therians the mandibular process shows an accelerated development, in echidna a delayed timing of mandibular process characters is observable. We highlight the limitation of interpreting heterochronic data and expose the plasticity in the timing of developmental characters, examined under alternative phylogenetic frameworks and character definitions.