In the Late Jurassic they were small bipedal animals with horns on their cheeks; by the end of the Cretaceous, they had given rise to multi-ton animals with enormous ornamented skulls that roamed North America in massive herds. This episode, we discuss the evolution and lifestyles of Ceratopsians.
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The Horned Dinosaurs
Ceratopsians include some the most famous and impressive dinosaurs in history: Triceratops, Styracosaurus, Pachyrhinosaurus, and many more. These dinosaurs are easily recognizable from their huge bodies and enormous heads, adorned with an endless diversity of horns and frills. Their incredibly strong jaws powered a mouth made for tough vegetation, from the toothless beaks up front to the complex batteries of teeth in the back.
But our popular image of this group of dinosaurs is a bit biased. Elephant-sized ceratopsians with multi-horned skulls and frills are mainly a feature of the family Ceratopsidae, which lived almost entirely in North America and only during the last 20 million years or so of the Cretaceous Period. The majority of ceratopsians looked quite different.
The earliest ceratopsians, including dinosaurs like Yinlong, date back to the Late Jurassic Period; these were bipedal herbivores with no frills and no large horns, though they did have small horns on their cheeks behind their beaked mouths. This style of ceratopsian was successful throughout the Cretaceous as well, including animals like the extremely common Psittacosaurus in Asia and the more recently identified Aquilops from North America.
It was during the Late Cretaceous that we see the rise of the more famous features of this group. Animals like the Protoceratops were quadrupeds with frills on their heads; Zuniceratops represents an early evolution of larger body size and horns atop the nose and eyebrows; and finally toward the end of the Cretaceous we see true giants like Triceratops.
Those Fantastic Faces
Although film and TV have spent most of the last century depicting Triceratops using its head as shield and weapon against the likes of Tyrannosaurus, scientists have proposed numerous hypotheses to explain just what those heavily-adorned skulls were for.
Defense against predators is certainly possible, and it might also be that all that surface area was great for temperature control, but the most likely explanation for the sheer diversity of skull shapes is that they were primarily evolved for display. Like horned animals today, ceratopsians probably used their heads to signal to mates or family members and to scare off (or maybe fight off) their competition; sexual selection can be a powerful driver of diversity. Of course, many features of organisms are multi-purpose, so these ideas aren’t mutually exclusive!
Speaking of Social Lives…
Some ceratopsians are among the most commonly preserved dinosaurs ever. Some, like Psittacosaurus and Triceratops, are known from dozens of individual skeletons, while others are found in mass death assemblages that can contain hundreds, and a few, like Protoceratops, are even known from family groups or nesting sites. It seems pretty certain that at least some species lived in groups, possibly rivaling the size of some modern mammal herds.
These incredible assemblages have also given us glimpses into the growth patterns of ceratopsians. In many species, skulls, frills, and horns were re-shaped dramatically throughout the animals’ lives. This has even led to some confusion when identifying species: on the one hand, many of the features we use to differentiate species are frill and horn traits that don’t develop until adulthood, making juvenile difficult to identify; on the other hand, such dramatic differences between young and adult have to led to some arguments about whether certain ceratopsians are truly different species or just different life stages of one species. The most famous of these debates regarded Triceratops and Torosaurus, though they are generally considered separate these days.
Meet Stellasaurus ancellae, a new species of ceratopsian named just before our episode came out!
Learn about the controversial history of Triceratops posture on Extinct Monsters.
Tom Holtz’s Dinosaurs: A Natural History is an excellent source of up-to-date dino information.