Actualy already discoverd in 1940 - 1950, when paleontologist Alfred Felix de Lapparent found teeth and armour plates of a gigantic crocodile in the Sahara. But in 2000 when Paul Sereno was in Niger looking for dinosaurs, he found this ancester of the modern crocodiles. Sarcosuchus imperator was 12 meters long, which is twice as long as the biggest present day crocodile. Only 50% of the crocs fossils have been found, but thats enough to make a reconstruction. Its lenght is based on the lenght of the skull, compared to modern crocodiles. The skull was about 1,8 meter long.
Actualy more than one incomplete skeleton has been found, of which two juveniles, these are very important because they had their armour plates in the right order. Paleontologist now can see what the right order and the exact place of the plates were.
Sarcosuchus lived around 110 million years ago in Africa and is not yet a real crocodile, it belongs to a family of which the present day crocodiles are decendents. Although its skull looks very much like a modern croc skull, there are a bit differences. Sarcosuchus for example got a very broad snout, which function is not yet clear.