About a third of the way through the Triassic period, (which predated the Jurassic and Cretaceous), a mass extinction did in most of the crocs, and the dinosaurs and pterosaurs took over. Before that there were crocs living in trees, crocs that ran like hell on two legs, and, yes plant-eating crocs. It was the crocs' world to lose, and they did; like we will, someday -- maybe soon. Then the raccoons or bears can take our place.
But according to the article, many interesting croc lineages survived for another 150 million years after the dinos took over, and succumbed along with the non-avian dinos.