Despite megaherbivore ontogeny being relatively well-studied, little research has been conducted on the ecological implications of growth stages. Using ecomorphological dietary correlates, investigations into potential ecological differences between mature and immature ceratopsids and hadrosaurids were undertaken. Results suggest that juvenile megaherbivores selectively consumed softer, lower-growing vegetation than their adult counterparts, which likely reduced intra-specific competition. Ecomorphological investigations into the potential for competition between juvenile megaherbivores and small ornithischians, and investigations into relative abundances, were conducted to test the competition and taphonomic hypotheses. An overlap in results indicated a potential for competition between juvenile megaherbivores and leptoceratopsids, and as predicted by the competition hypothesis small ornithischians were generally less abundant than megaherbivores. Other groups showed separation in the ecomorphospace. Some distributions showed similar abundances between groups as predicted by the taphonomic hypothesis. Thus, size differences were important for resource partitioning, and competition and taphonomy both influenced observed Late Cretaceous size distributions.