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Hadrosaurs

What is a hadrosaur?

Hadrosaurs (Greek for "bulky lizard") are also known as the "duck-billed dinosaurs" because of their long flattened snouts.  They first appeared during the Cretaceous period, near the end of the Age of Dinosaurs. Hadrosaurs were very common dinosaurs and fossils have been found throughout North America, Europe, and Asia.  Based on their teeth, paleontologists believe that hadrosaurs were herbivores.  Their teeth were mostly small and leaf-shaped, but there were plenty of them.  Some hadrosaurs had almost 900 teeth! The hadrosaurs ranged in size from about 10 feet long (the size of a small car) to 40 feet long (the size of a school bus). They weighed up to 3 ½ tons, about as much as two cars!

The hadrosaurs’ feet had three toes, covered in a hoof-like material.  Hadrosaurs could walk (or run if a Tyrannosaurus rex was near!) on their large muscular hind legs, but may have occasionally used all four legs while grazing for food.  Their long thick tails helped them balance while running. Without spikes, plates, or teeth to defend themselves, the hadrosaurs probably relied on keen senses of sight and smell, as well as their legs, to get out of trouble fast.

Detailed genealogy of the hadrosaurs

Dinosaurs fall into two major groups based on the type of hips they have.  The Saurischia, or lizard-hipped, and the Ornithischia, or bird-hipped.  Despite the names, there are dinosaurs that walk on two legs and ones that walk on four legs in both groups.  The Saurischia species dominated during the Triassic Period, then the Ornithischia dominated later in the Cretaceous Period.

The Saurischia include two suborders:  Sauropoda, or reptile-footed, and the Theropoda, or beast-footed.  Different sources break Ornithischia into 3 to 5 suborders (the three suborders described here are as presented by the University of California, Berkeley, Museum of Paleontology).  The Ornithopoda are bird-footed.  The Thyreophora includes the plated Stegasauria and the armored Ankylosauria.  The Marginocephalia, or fringe-headed, dinosaurs includes the horned Ceratopsia, and the bone-headed  Pachycephalosauria. 

All of the Ornithischia were plant eaters who had evolved cheeks.  Prior to these dinosaurs, the flesh along the side of the head ended at the back of the jaw.  Imagine the profile of a Tyrannosaurus; you can see all of the teeth, not just those in front.  While this is effective for animals that use their jaws for killing, it is not effective for animals that chew plants for food.  A lot of the food simply fell out the sides of their mouths.  The Ornithischia were small to medium-sized dinosaurs (usually less than 30 feet long) but very successful.  It was just at the beginning of the Cretaceous period that the number of species of flowering plants began to multiply dramatically.  Perhaps because of these cheeks and the many new plant food sources, the Ornithischia flourished.

The Ornithopoda or bird-footed dinosaurs were one branch (suborder) of the Ornthischia that lived for some 140 million years, during the better part of both the Jurassic and the Cretaceous periods.  Dinosaur species from other branches of the Ornithischia include the stegosaurus, ankylosaurus, and triceratops.  The Ornithopoda were medium to large in size, by dinosaur standards.  They walked on their back legs with three-toed, bird-like feet for which they were named. Four families make up the Ornithopoda, including the Iguanodons and the Hadrosaurs.  The Iguanodon was one of the first dinosaurs to be discovered.  A Hadrosaur footprint was recently discovered in Aniakchak National Monument and Preserve in southwest Alaska.

The Hadrosaurs are the members of the Hadrosauridae family, or the “duck-billed” dinosaurs that evolved probably in Asia during the Jurassic or early Cretaceous period, and by the late Cretaceous period had spread to Europe and North America as well.  These were some of the later dinosaurs, living 80 million years after the Stegosaurus of the Jurassic period.  They died out during the mass extinction that took place at the end of the Cretaceous period. 

Hadrosaur Adaptations

Hadrosaurs had a wide duckbill like snout with no front teeth.   They did have teeth further back for chewing vegetation.  In fact, they had teeth that were continually being replaced.  Their jaws were able to move up and down as well as side to side.  These two adaptations, along with the cheeks common to all Ornithopoda, helped to make them particularly well adapted to chewing and eating even the toughest of the vegetation flourishing at the time.

The hadrosaur family consists of two subfamilies, the hadrosaurine duckbills and the lambeosaurine duckbills.  The hadrosaurine had flat heads and bony ridges on their snout, whereas the lambeosaurine had high domed heads with hollow air passages in crests atop their heads.  The lambeosaurine evolved in North America and have so far only been found there.  The hadrosaurine were very successful, long-lived and were some of the last dinosaurs before the mass extinction at the end of the Cretaceous period.  Common hadrosaurs include: Hadrosaurus (bulky lizard), Edmontosaurus (Edmonton lizard), Maiasaurus (good mother lizard), Lambeosaurus (Lambe's lizard). 

Hadrosaurs might have roamed the earth in giant herds and so have been called the "cows of the Cretaceous."  One fossil site found the bones of over ten thousand hadrosaurs, clearly a very large group!  Hadrosaurs were likely a favored food of the Tyrannosaurus Rex, which lived at the same time.  Hadrosaurs had powerful back legs and smaller front legs.  They probably grazed on all fours, but ran (or fled) upright on their back two legs. 

Nests of some species of hadrosaurs have been found, including a parent dinosaur sitting on a nest of eggs. Paleontologists discovered a particularly interesting Maiasaura (a dinosaur in the hadrosaurine subfamily) nesting area in Montana in 1978.  The site showed that the mother made a crater-like nest with the eggs carefully arranged in a circular pattern.  Several nests were grouped together showing that they were social animals, and the females were nesting in groups.  Along with the mothers were Maiasaura of various ages, young ones, hatchlings, and intact eggs.   This is fossil evidence that Maiasaura mothers cared for their young even after they hatched, rather than leaving the babies to hatch and grow on their own. 

Some hadrosaurs, called Lambeosaurs, had large crests on the tops of their heads with built-in air passages.  These air passages in Lambeosaurinae crests are a matter of speculation for scientists.  Scientists think that Lambeosaurs could blow air through their crests, making tuba-like noises.  Maybe they used these sounds to warn other members of the herd or as a mating ritual.  Some of these air passages were quite large and connected with the hadrosaur’s nasal passages but did not connect directly to the outside.  The theory that most scientists now ascribe to is that the Lambeosaurinae used these as resonating chambers to make very loud, deep sounds that would carry far.  In other words, they might have used them a bit like megaphones to call to others in their herd, to keep track of one another, or to attract mates or to warn of predators.  The crests themselves are large and unusual looking, and another theory is that they used these crests to attract a mate, just like peacocks use their fancy feathers.

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