Final answer:
The hollow crests on ornithopods likely served two key purposes: attracting mates, possibly through visual displays and courtship rituals, and communication, potentially by enhancing or amplifying sounds made for social interaction or warning others of danger.
Step-by-step explanation:
The two most important functions of the hollow crests on a group of ornithopods are thought to be attracting mates and communication. These crests may have been used in courtship rituals to attract potential mates, similar to how some species of birds use colorful plumage to attract mates and assist with parental care. The hollow crests could also have enhanced the animals' ability to communicate with each other, perhaps by amplifying sounds they made, as seen in some bird species that make various calls to communicate the presence of predators or to defend their young.
In addition to these possible functions, the crests might also have played a role in species recognition, allowing ornithopods to identify members of their own species for social interactions.