Flexure angles of the cranial endocast are defined as cephalic flexure (the angle between the olfactory bulbs, cerebrum, and oblique axis of the midbrain) and pontine flexure (the angle between the medulla oblongata, oblique axis of the midbrain, and cerebrum) and can be summarised as the angles between the forebrain and midbrain, and hindbrain and midbrain, respectively, within archosaurs. Flexure angles in the avian endocast remain approximately consistent through ontogeny, while flexure angles in crocodilian endocasts increase through ontogeny. Considering the relationship of non-avian dinosaurs to both birds and crocodilians, endocranial flexures of non-avian dinosaurs are typically described as either “avian”-like or “crocodilian/basal archosaur”-like. Here we present a study of six (ostriches, chickens, alligators, caiman, Tyrannosaurus, Psittacosaurus, and Dysalotosaurus) different archosaur ontogenetic series (n=29) demonstrating that while flexure angles from individual specimens can be compared to the angles found in birds or crocodilians, ontogenetic trends in flexure change are not so simple. All clades of non-avian dinosaur sampled in this study diverged from the developmental patterns assumed from modern analogues. While the fossil record of dinosaur ontogenetic endocrania is currently too fragmentary to claim that trends in flexure change are phylogenetic, it is clear that changes in cephalic and pontine flexure angles of non-avian dinosaurs though ontogeny are neither constant nor always follows a constant increase throughout postnatal development. This indicates that ontogeny is just as important as phylogeny in terms of endocranial morphology within Archosauria. Our understanding of endocranial evolution within a species can be skewed – or even incorrect – without knowing the age of a specimen from which an endocast is made.