Abstract The skulls of two perinate paravians from Ukhaa Tolgod, Djadoktha Formation, Mongolia, are described here. The skulls are nearly unique in their combination of ontogenetic age and preservational quality and provide us with the first look at the morphology of such important anatomical regions as the rostrum, palate, and braincase at or near the onset of postnatal development in a nonavian paravian coelurosaur. Based on a number of derived characters, the skulls are allocated to a derived position within Troodontidae that is outside the clade consisting of Saurornithoides mongoliensis, Saurornithoides junior, Troodon formosus, and probably Sinornithoides youngi. A single synapomorphy, presence of a lateral maxillary groove, supports the Ukhaa perinates as Byronosaurus. The comparative morphology of the Ukhaa perinates with adult troodontids indicates a number of significant postnatal transformations (e.g., elongation and flattening of the rostrum, increase in the number of maxillary and dentary tee...