Many animals flooded into the South American continent about 4 million years ago during the Great American Interchange in the Pliocene, when the string of volcanic islands linking with the North American continent, fused to form a land bridge (Isthmus of Panama). The animals quickly evolved and developed to fill the ecological niches they found. In most instances the animals that came south were more successful and began to replace the already indigenous animals that had dominated the continent for the previous 50 million years. The reason for this may be that the already indigenous animals had evolved few variations in that period of time. The continent itself had not shifted substantially and there had been little change in the environment and the climate except for the spread of the grasslands. There was no necessity then, for the animals to evolve, and they had settled into a long period of decadence.The animals from the north, on the other hand, had suffered millions of years of change, with changes in climate and interchanges of animals with the Palaearctic ecozone, and had been evolving vigorously. When they reached South America they were more able to adapt to the conditions found there and, as a result, the original native animals suffered. The most noticeable instance is found among the meat-eaters. The first of the northern animals to reach the continent were the mountain leapers and relatives. Being skilled at traversing high mountain ranges, they were well able to cross the mountainous Andes neck of land that connected the two continents and spread into many of the ecological niches. Large meat-eaters quickly developed from this stock and preyed on the swift-footed titanosaur sauropods of the plains. Even larger types developed, some with their heavy heads and small arms, looking as if they had evolved from the great theropods of the Cretaceous period rather than the small agile, basal-looking coelurosaurs.
One such is the cutlasstooth which has evolved huge cutting teeth that enable it to prey on the large titanosaurs like the lumber. Hunting in packs, cutlasstooths can set upon a lumber and slash it until it bleeds to death.