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HADROSAURS
HADROSAUR RUNNING
Hadrosaurs are often referred to as “duck bill” dinosaurs because of the wide tip of their snout. A hadrosaur (right) is compared to Iguanodon on the left in the adjacent images. edmontosaurus
iguanodon snout

     Hadrosaurs form a monophyletic clade with Telmatosaurus being the most primitive member (Telmatosaurus is a hadrosaur but it lacks derived characteristics that all other hadrosaurs have).  Another partial specimen is known which is more advanced than Telmatosaurus but more primitive than other genera (Casanovas, 1999; Weishampel, 1993).  There were at least 25 genera and 40 species of hadrosaurs, mostly known from Western North America and Eastern Asia.  They can be divided into 2 groups (either families or subfamilies): Hadrosaurinae (flat headed) and Lambeosaurinae (crests).

     Given the large number of hadrosaur fossils in many areas, it appears that they often composed as much as 75% of the dinosaur biomass in a region.  Hadrosaurs may have evolved in Asia where Probactrosaurus, a possible iguanodont ancestor of the hadrosaurs is known.  By the Mid Cretaceous they had spread throughout both the northern and southern continents (Brett-Surman, 1979).  Hadrosaurs are known from Antarctica providing evidence of a land connection between South America and Antarctica in the Late Cretaceous. (Case, 2000).  Three primitive hadrosaurs possess traits which are retained from their igunaodont ancesestors: two members of Hadrosaurinae Secernosaurus and Gilmoreosaurus, and the most primitive member of Lambeosaurinae, Bactrosaurus  (Brett-Surman, 1979).

kritosaurus kritosaurus hip

     There is a great diversity of hadrosaur species.  A number of species may even be known from the same locality, suggesting that many species probably coexisted by specializing for different habitats.  Most hadrosaur fossils have been found near coasts and deltas while most lambeosaur fossils have been found more inland.  Maiasaurus was found even deeper inland than the lambeosaurs.

     The smaller species were primarily bidepal and the larger ones were capable of both bipedal and quadrupedal locomotion.  It is possible that they depended more on quadrupedal locomotion as they aged.  The hindlimbs were large, twice the length of the forelimbs.  The large species could achieve perhaps 15-20 km/hr in an extended run but probably couldn’t gallop.  The smaller species may have achieved up to 60 km/hr.   Their forelimbs were clawed and they could dig up plants and bring plants to their mouths.  All had 4 fingers on their hands and some had hooves on their feet (Fastovsky, 1996). The hadrosaur Shatungosaurus measured almost 15 meters and may have weighed more than 12 tons. The hadrosaur Saurolophus and the lambeosaur Barsboldia each measured 12 meters and would have weighed 10 tons (Russell, 1989).

     All had ossified tendons in their tails that held their tails horizontally and the tails were laterally compressed.  The tails were too stiff to be used in swimming as was once thought.  The brains of hadrosaurs were well developed and their large optic lobes suggest that perhaps good senses were needed to detect predators.  Kritosaurus had a nasal horn.

     Several mummified hadrosaurs have been discovered that dried before burial.  Twigs, berries, conifer needles, and plant material were found in the digestive systems. (An Edmontosaurus specimen even preserved the skin showing that it was covered with both large and small bumps.)  Small species could forage 1-2 meters off the ground and the large species could forage up to 4 meters off the ground.  Hadrosaurs had a beak to crop vegetation and were well prepared for chewing with a dental battery, a deep set tooth row (indicating cheeks) and strong coronoid process on the lower jaw.  They could chew back to front and some feel they could also chew side to side.  In hadrosaurs, the premaxillary and predentary regions of the jaw are even more expanded than in the iguanodonts and there are much higher numbers of cheek teeth (Norman, 1984).

     All of the Euornithopoda also had some pleurokinesis of skull–the upper jaw could move a bit relative to nose and skull roof.  The ability to chew may indicate that they depended on lower quality fibrous plant material. There were 45-60 tooth positions per jaw plus several rows of replacement teeth and an individual could have 700 exposed teeth at once.  Their large gut indicates some fermentation of food material (Fastovsky, 1996).

hadrosaurus

    Gryposaurus and Lambeosaurus had depressions around the nasal area, perhaps indicating the presence of skin flaps.  Modern elephant seals possess similar areas which are used in sound production. 

     Several species probably traveled in herds which included young and juveniles and some nested in large colonies.  Since there are many hadrosaur remains near rivers but very few juvenile specimens, suggesting that hadrosaurs went inland to nest.  Nests have been found in the foothills of the Rockies.  Orodromeus nests contain 12 eggs in an upright position.  The intact eggshells suggest that the young did not stay at the nest after hatching.  Hypacosaurus and Maiasaurus laid up to 17 eggs in a nest.  The nests were located in colonies were separated by about an adult body length, perhaps indicating that an adult sat on the eggs.  The eggshells are recovered in pieces suggesting young stayed for a while at nest.  Seeds found associated with the nest may indicate that adults were regurgitating food for the young.  It is estimated that young grew very quickly.  Maiasauran nests have been found with juveniles of different ages. Some of the Maiasaura hatchlings found at nest sites were three times the size that they would have been at hatching, indicated that they were cared for at the nest after hatching. One Montana fossil site contains a group of Maiasaura remains which may represent 10,000 individuals. The assemblage includes 3 meter long juveniles and 7 meter long adults. The group was perhaps killed by volcanic gases released in the formation of the Rocky Mountains (Czerkas, 1990; Horner, 1979; Fastovsky, 1996). 

maiasaur

Below are some members of the Hadrosaurinae:

gryposaurus edmontosaurus skull
maiasaurua prosaurolophus
saurolophus anatosaurus
Lambeosaurinae
corythosaurus parasaurolophus

corythosaurs

     Each of the lambeosaur species had a distinctive crest.  It was once thought that their skulls indicated that they were aquatic or that crests were used in a sense of smell.  Today, the most common conclusions are that crests were used for vocalizations and visual display.   If these crests had roles in visual display or in vocalization, lambeosaurs would have to have good vision and/or hearing.  Lambeosaurs (and hadrosaurs in general) had both large eyes and good hearing (known from preserved middle and inner ears). Pararhabdodon is  a primitive lambeosaur (Casanovas, 1999b).

hypacrosaurus vertebrae
In Hypacrosaurus the vertebrae were lengthened, producing a low sail.
nasal cavity 2
nasal cavity 1
Hadrosaur crests enclosed an enlarged nasal cavity.
     There was sexual dimorphism in crests: each lambeosaur species had sexually dimorphic crests and the differences between genders became pronounced during maturity.
corythosaurus parasaurolophus
gender differences