Evidence from footprints, nesting grounds, and anatomy all suggest
that at least some dinosaurs were at least as social as modern crocodilians.
For example, sauropods and hadrosaurs appear to have nested in large
groups. Numerous trackways show that sauropods traveled in herds,
and similar mammalian-level social structures may have also characterized
other large herbivores such as hadrosaurs and ceratopsians.
Many dinosaur species display large crests, horns, or frills that
would have been highly visible to other animals. In many living
animals, these structures often serve to attract mates or signal
other members of the same species. The hadrosaur and theropod crests,
along with ceratopsians horns, might have been used in this manner.
Hadrosaurs may also have been able to vocalize by using their crests
as resonance chambers.
Like ungulates, ceratopsians may have used their horns to defend themselves
from predators and to battle members of their own species for mates or territories.
Pachycephalosaurs could have engaged in similar behaviors by using their dome-like skulls. The dermal armor of stegosaurs, ankylosaurs, and titanosaur sauropods probably had some role in defense. Some theropod skulls show healed bites that may have been inflicted by a member of the same species, suggesting violent intraspecific interactions. However, most of these hypotheses come through analogy with living animals; paleontologists have found little direct evidence for these behaviors in dinosaur fossils.