They were fairly small stocky quadrupedal dinosaurs, sort of pig-like, with short deep jaws and powerful beaks adapted for eating fibrous low-level plants like ferns and cycads – and to process such tough food they even evolved a chewing style similar to mammals like rodents.
Some hadrosaur crests were purely for visual display, but in the lambeosaurine lineage that Parasaurolophus belonged to they also incorporated complex looping nasal passages that were probably used as resonating chambers, allowing each species to make a unique-sounding loud bellowing call to communicate with each other.
There are also rumors of a currently-undescribed specimen of Parasaurolpphus that has preserved soft tissue around its crest, possibly a keratinous covering or skin flaps that made it appear even larger and more flamboyant in life than the underlying bone. So I’ve given this reconstruction a speculative structure like that, along with hoof-like claws on its hands similar to those recently revealed for Edmontosaurus.
Their striking-looking dome heads were probably used for combat, headbutting or flank-butting each other, and many fossil skulls show evidence of injuries that would have been caused by that sort of behavior.
The eponymous Pachycephalosaurus wyomingensis lived in North America right at the end of the Cretaceous, about 70-66 million years ago. It was one of the largest of its kind, reaching lengths of around 4.5m (14’9″), and was characterized by a large bony dome-head surrounded by small blunt spikes.
But it turns out that was probably only what it looked like as a fully mature adult.
And while typically depicted as purely herbivorous, ceratopsids’ powerful parrot-like beaks and lack of grinding teeth suggest they may actually have been somewhat more omnivorous – the Cretaceous equivalent of pigs – still feeding mainly on plant matter but also munching on carrion and opportunistically eating smaller animals when they got the chance.
Machairoceratops cronusi here lived during the late Cretaceous of Utah, USA, about 77 million years ago. Only one partial skull has ever been found belonging to an individual about 4.5m long (14’9″), but it wasn’t fully grown and so probably reached slightly larger sizes.
It had two long spikes at the top of its frill, similar to its close relative Diabloceratops but curving dramatically forward and downwards above its face. Whether they were purely for display or used in horn-locking shoving matches is unknown, but either way it was a unique arrangement compared to all other known ceratopsids.
One of the earliest known members of this lineage was Scutellosaurus lawleri. Living in Arizona during the Early Jurassic, about 196-183 million years ago, it was a small lightly-built bipedal herbivore, only about 1.2m long (3′11″) – with over half that length being just its unusually long tail.
Its body was covered in rows of hundreds of small bony osteoderms, helping to protect it against larger predators like Dilophosaurus. And this was obviously an evolutionary strategy that worked very well for Scutellosaurus and other early thyreophorans, because within about 20 million years they’d given rise to the first true ankylosaurs and stegosaurs – with the tank-like ankylosaurs being especially successful, spreading to every continent and lasting all the way up until the end-Cretaceous mass extinction.
It was also the first dinosaur fossil found with a specific type of non-cancerous tumor known as an ameloblastoma on its lower jaw – a surprising discovery, since ameloblastomas were previously only known to occur in mammals and a single snake species. Various other types of abnormal tissue growth have been identified in other hadrosauroids and hadrosaurs, however, suggesting that this particular lineage of dinosaurs may have been unusually susceptible to developing tumors.
And towards the very end of the Cretaceous, about 72-66 million years ago, this Adriatic-Dinaric island was home to the hadrosauroid dinosaur Tethyshadros.
Surprisingly it wasn’t very closely related to earlier European hadrosauroids, and its ancestors seem to have actually originated in Asia, island-hopping their way westward over to the Adriatic-Dinaric.
At around 4m long (~13′) it was much smaller than most of its close relatives and was another example of insular dwarfism. But it had some odd body proportions: its head was relatively large, its neck and tail were fairly short, its limbs were long and gracile, and it had a reduced number of fingers in its hands. It appears to have be specialized for running, sort of like a dinosaur mimicking a horse.
It also had a weird highly serrated edge to its beak, which in life would have been even more pronounced and spiky-looking. The purpose of this is unknown for certain, but it may have been an adaptation for a specific food source – and since some hadrosaurs seem to have occasionally snacked on shellfish for extra protein, it’s possible Tethyshadros was also doing something more omnivorous along the shores of its island home.