Unlike the other groups of sauropods - Diplodocoids, other Macronarians - titanosaurs evolved late in the Mesozoic, around 150 MYA, in the middle Cretaceous. They were the last and the largest of the sauropods; some of them even made it to the K-T extinction. They were most numerous in the south hemisphere - South America, Africa - but some of the latest discoveries indicate that they may've lived in the north as well.