What Is a Hadrosaur? The Most Successful Plant Eaters of Their Age
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Duck-billed dinosaurs were the dominant large herbivores across much of the late Cretaceous northern hemisphere, and the reason is in their mouths. They had a chewing apparatus no living animal matches and a dental arrangement that renewed itself continuously from below.
The dental battery
A hadrosaur jaw contains hundreds of teeth packed into columns, with several teeth stacked in each position and each column pressed tightly against its neighbours so that the whole assembly forms a single continuous grinding surface rather than a row of individual teeth. As the working surface wears, the tooth beneath pushes up to replace it, so the battery renews itself indefinitely and never becomes blunt, which removes the constraint that limits the lifespan of most large mammalian herbivores whose teeth wear out. The structure contains several distinct tissue types of differing hardness arranged so that the surface wears unevenly and stays rough, which is exactly what is wanted for grinding, and the arrangement has been shown to be more complex than that of horses or bison. A large individual carried well over a thousand teeth in various stages of eruption. That apparatus allowed them to process tough fibrous vegetation efficiently, which is the foundation of their success.
How the jaw worked
Grinding requires a sideways motion that a simple hinge jaw cannot produce, and hadrosaurs solved it differently from mammals:
- •Mammals achieve sideways movement at the jaw joint, which permits chewing but limits bite force during the motion
- •Hadrosaurs kept a simple hinge and instead had a flexible connection in the upper jaw, so the upper tooth rows rotated outward as the lower jaw closed against them
- •That motion, called pleurokinesis, grinds the food between the two surfaces with a sideways component while keeping the jaw joint simple and strong
- •Microscopic wear scratches on the teeth run in the direction the mechanism predicts, which is direct evidence rather than inference from anatomy alone
- •Cheeks are inferred from the inset tooth rows, since an animal grinding food needs something to hold it in the mouth
- •The beak at the front cropped vegetation, separating the gathering function from the processing function entirely
The crests and the social life
The group splits into two branches, one with solid crests or none, and one with the elaborate hollow crests that have made the group famous. Those hollow structures are enlarged nasal passages looping through the crest, and computer modelling of their internal geometry has reproduced the sounds they could produce, which are deep resonant tones differing by species and by age. Combined with evidence of colonial nesting from sites in Montana and elsewhere, with nests spaced about a body length apart, and with bone beds containing many individuals of one species, the picture is of social animals living in groups with acoustic and visual signalling between them. Growth series show crests developing through life, so an adult would have sounded and looked different from a juvenile, which is exactly the pattern expected of a signal conveying age and status. Their eyes and inner ear structures suggest good hearing in the frequency range those crests produced.
Why they did so well
Hadrosaurs spread across North America, Asia, Europe and reached South America and Antarctica, and they are among the most abundant dinosaur fossils in many formations, which indicates very large populations. Several factors combine to explain it. The chewing apparatus extracted more from low-quality vegetation than competitors could, which mattered as flowering plants spread and vegetation changed. Fast growth, documented in bone microstructure, took them through vulnerable small sizes quickly. Colonial nesting with parental care improved juvenile survival. Their skeletons indicate they could move on two legs or four, which suits both browsing at height and grazing at ground level. And they were generalists rather than specialists, which is the usual pattern among successful widespread groups. Their exceptional preservation record includes several specimens with skin impressions and at least one with preserved soft tissue over much of the body, which is why reconstructions of them are unusually well founded.
The takeaway
Hundreds of teeth packed into self-renewing columns formed a single grinding surface that never wore out, which removed the limit that ends the life of most large mammal herbivores. Sideways grinding came from a flexible upper jaw rotating outward rather than from the jaw joint, and tooth scratches run the way that predicts. Hollow crests were resonating nasal passages, and nesting sites show colonial breeding.