Final answer:
A. True The claim that many iguanodontids had a specialized thumb is true; it represents an evolutionary adaptation in this group of dinosaurs.
Step-by-step explanation:
The statement that a spike-like "thumb" was an evolutionary novelty of many iguanodontids is true. Iguanodontids, a group of dinosaurs that lived during the Mesozoic Era, were known for having a unique prehensile 'thumb' that could have been used for defense or foraging. This thumb was distinct from other digits and is considered to be an evolutionary adaptation. In contrast, the provided text about Sphenodontia, a different ancient group which includes today's tuataras, and various other prehistoric species, doesn't directly relate to the iguanodontids' thumbs. However, it is useful for understanding the broader context of the evolution of vertebrates during the Mesozoic Era, including adaptations like the whale flipper for aquatic environments or the feather evidence in Archaeopteryx that supports the theory of birds sharing a common ancestor with reptiles.