Baby Sinclair is the youngest member of the Sinclair family on Dinosaurs. Newly hatched in the premiere episode, "The Mighty Megalosaurus," Baby Sinclair is a precocious if bratty child. His catchphrases include "I'm the baby, gotta love me!" and "Not the mama!" The latter is often uttered to his dad, Earl Sinclair, whom the baby repeatedly batters with a frying pan, and generally disdains, especially in the early episodes, though they form a slightly warmer relationship over time. The baby's relationship with mother Fran Sinclair is more stable and mutually affectionate, but the child's wisecracks are also aimed at siblings Robbie and Charlene. Grandmother Ethyl often reads to him, the pair both rejoicing in Earl's many blunders. Also, Baby is quite intelligent, being able to speak and spell at a very young age. Never formally named until near the end of the second season, the infant is briefly named "Aaah Aagh I'm Dying You Idiot," the result of the then-Chief Elder keeling over during the official christening. By the episode's end, he has been legally named "Baby Sinclair."
Baby Sinclair celebrated his first birthday in "Switched at Birth". He celebrated his second birthday in "Terrible Twos", where he entered a stage called the "terrible twos" (warned of by Ethyl at the beginning of the episode), causing him to display demonic behavior during the year that he's two. In the end, the rest of the family finally tricked him into thinking he was three years old.