Iguanodont-Hadrosaur Transition

The larger or more robust iguanodonts, such as I. bernassartensis and Lurdusaurus, apparently evolved to fight predators instead of flee from them.The  thumb spikes of iguanodonts apparently served as weapons for stabbing theropods. When under attack,  Iguanodon may have attempted to gouge out an attacker's eyes or damage its jaw. 

This defense does not appear to have been  effective for very long. Ouranosaurus had small thumb spikes suggesting decreased reliance on such weapons. The spikes may have been vestigial or at the point of being abandoned. The subsequent history of large ornithopods certainly suggests failure of their fighting taxa. Lacking any weapons, hadrosaurs relied on speed to escape, and keen senses to warn of approaching danger.
The iguanodontian defense seems to have disappeared in late Aptian or Albian time. Around then, and at the start of the late Cretaceous (Cenomanian), carcharodontosaurs became very large. Earlier carchs such as Concavenator were considerably smaller. Against them, iguanodonts had a fighting chance. Later carchs, however, such as Tyrannotitan and Acrocanthosaurus, would've overwhelmed the ornithopods and probably did. The shark toothed giants evolved to prey on large sauropods and could easily overcome an iguanodont. Even Eocarcharia appears at the threshold of eclipsing iguanodonts, as Lurdusaurus was supplanted by the less combative, fleeing Ouranosaurus.