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Hadrosaurs were a group of ornithischian dinosaurs that lived in the Late Cretaceous period 13. This group is also known as the duck-billed dinosaurs, for the flat, duck-bill appearance of their mouths. Like the rest of the ornithischians, these animals had a predentary bone and a pubic bone which was positioned backwards in the pelvis. Hadrosauridae inhabited many locations, including Eurasia, North America, South America, and Antarctica 3. The first Hadrosaur remains discovered came from the Late Cretaceous Judith River Formation in Montana 13.

Hadrosaurids (ἁδρός, hadrós, "stout, thick"), or duck-billed dinosaurs, are members of the ornithischian family Hadrosauridae. This group is also known as the duck-billed dinosaurs, for the flat, duck-bill appearance of their mouths. Like the rest of the ornithischians, these animals had a predentary bone and a pubic bone which was positioned backwards in the pelvis. The family, which includes ornithopods such as Edmontosaurus and Parasaurolophus, was a common herbivore in the Upper Cretaceous Period of what is now Asia, Europe, South America, Antarctica and North America. Hadrosaurids are descendants of the Upper Jurassic/Lower Cretaceous iguanodontian dinosaurs and had a similar body layout.

The Hadrosauridae were facultative bipeds, with the young walking mostly on two legs and the adults walking mostly on four6. These herbivores can be further divided into two different groups. These groups are known as the Saurolophinae and the Lambeosaurinae, which are known for their large, hollow crests on their heads 13. Their jaws were designed for grinding plants, with multiple rows of teeth replacing each other as the teeth wore down6. Examples of hadrosaurids include Edmontonosaurus, Maiasaura, and Parasaurolophus.

Anatomy

Duck Bill

The most recognizable aspect of hadrosaur anatomy is the flattened and laterally stretched rostral bones, which gives the distinct duck-bill look 13. Furthermore, some members of the hadrosaurs also had massive crests on their heads, probably for display 13.

Skin

Skin impressions have been found of multiple hadrosaurs have been found2. From these impressions, it has been determined that the hadrosaurs were scaled, and not feathered like some dinosaurs of other groups. Lambeosaurinae versus Saurolophinae There are two groups within the hadrosaurs: the Lambeosaurinae and the Saurolophinae 13. However, the Saurolophinae are often referred to as the Hadrisaurinae in older articles. The major distinction between these two groups is that the Saurolophinae have solid crests or no no crest at all, whereas the Lambeosaurinae have hollow crests 13. In the Lambeosaurinae, the nasal passages travel through the crests, creating this hollow structure9.

Phylogeny

Monophyly

Recent phylogenies support the monophyly of both the Saurolophinae and the Lambeosaurinae 13. The Saurolophinae contain members such as Bactrosaurus, Parasaurolophus, and Saurolophus, while the Lambeosaurinae include Corythosaurus, Hypacrosaurus, Lambeosaurus, and Parasaurolophus 13. Below is the phylogeny determined by Prieto-Marquez’s 2010 study.

Saurolophine Cladogram

Below is a phylogeny of the Saurolophinae from Gates’ and Sampson’s 2007 study8.

Lambeosaurine Cladogram Below is a phylogeny of the Lambeosaurinae from Evans’ and Reisz’s 2007 study.7

Closest Relatives

The closest relatives of the hadrosaurs are the Iguanodontidae 16. The most notable difference between the Hadrosauridae and the Iguanodontidae are in the bones of their jaw 16. Members of Iguanodontidae have a large maxilla bone in the upper jaw, while hadrosaurs have a smaller maxilla, complex teeth organization, and a large premaxilla in the upper jaw 16. hadrosaurs are defined as all of the descendants of the last common ancestor of Telmatosaurus and Parasaurolophus 13.

Lambeosaurinae Origins

The oldest Lambeosaur fossils and the most basal members of the Lambeosaur phylogeny are from Asia 9. As a result, it has been determined that this particular group originated in Asia before spreading to other continents, such as North America 9.

Behavior

Feeding Strategies

Pleurokinesis

The hadrosaurs fed on plants, and therefore had to develop techniques to grind plant material before swallowing their food. As a result, hadrosaurs relied on pleurokinesis to help chew their food 12. During pleurokinesis, the maxilla bones slide laterally outward from the mouth 12. This is accomplished with a hinge made of the joints between the premaxilla and maxilla bones and the lacrimal and prefrontal bones 12. Pleurokinesis allows for the teeth to more precisely grind against one another, for better shearing of plant material, and reduces the stress that chewing puts on the skull 12.

Tooth Rows

Furthermore, multiple rows of teeth in each part of the jaw provided a large surface for chewing. These two factors together made for a jaw that was highly effective at grinding plant material. Analysis of the wear-patterns of the teeth has provided further insight into how these animals may have fed. On the teeth, there can be found four different sets of wear patterns that indicate the lower jaw would have moved directly up and down 15. Furthermore, the wear patterns were parallel, revealing that the rows of teeth were extremely well aligned in the jaw. Diet While evidence may point to the theory that these animals grazed on leaves, there is still a great deal of controversy as to what the hadrosaurs ate 15. One study found fossilized droppings of Maiasaura indicating that the animals may have been eating pieces of rotten wood4. Though the wood itself would not have provided a significant amount of nutrients, there may have been fungus growing on the wood, which would have been an excellent resource.

Facultative Bipedalism

Evidence suggests that young hadrosaurs would have walked on only their two hind legs, while adults would have walked on all four6. As the animal aged, the front limbs became more robust in order to take on weight, while the back legs became less robust as they transitioned to walking on all four legs6. Furthermore, the animals’ front limbs were shorter than their back limbs6.

Hadrosaurs were a group of ornithischian dinosaurs that lived in the Late Cretaceous period 13. This group is also known as the duck-billed dinosaurs, for the flat, duck-bill appearance of their mouths. Like the rest of the ornithischians, these animals had a predentary bone and a pubic bone which was positioned backwards in the pelvis. Hadrosauridae inhabited many locations, including Eurasia, North America, South America, and Antarctica 3. The first Hadrosaur remains discovered came from the Late Cretaceous Judith River Formation in Montana 13. The Hadrosauridae were facultative bipeds, with the young walking mostly on two legs and the adults walking mostly on four6. These herbivores can be further divided into two different groups. These groups are known as the Saurolophinae and the Lambeosaurinae, which are known for their large, hollow crests on their heads 13. Their jaws were designed for grinding plants, with multiple rows of teeth replacing each other as the teeth wore down6. Examples of hadrosaurids include Edmontonosaurus, Maiasaura, and Parasaurolophus.

Table of Contents 1.	Anatomy a.	Duck-bill b.	Skin c.	Lambeosaurinae versus Saurolophinae 2.	Phylogeny a.	Monophyly b.	Saurolophine Cladogram c.	Lambeosaurine Cladogram d.	Closest Relatives e.	Lambeosaur Origins 3.	Behaviors a.	Feeding Strategy i.	Pleurokinesis ii. Tooth rows iii. Diet b.	Facultative Bipedlism Anatomy Duck Bill The most recognizable aspect of hadrosaur anatomy is the flattened and laterally stretched rostral bones, which gives the distinct duck-bill look 13. Furthermore, some members of the hadrosaurs also had massive crests on their heads, probably for display 13. Skin Skin impressions have been found of multiple hadrosaurs have been found2. From these impressions, it has been determined that the hadrosaurs were scaled, and not feathered like some dinosaurs of other groups. Lambeosaurinae versus Saurolophinae There are two groups within the hadrosaurs: the Lambeosaurinae and the Saurolophinae 13. However, the Saurolophinae are often referred to as the Hadrisaurinae in older articles. The major distinction between these two groups is that the Saurolophinae have solid crests or no no crest at all, whereas the Lambeosaurinae have hollow crests 13. In the Lambeosaurinae, the nasal passages travel through the crests, creating this hollow structure9. Phylogeny Monophyly Recent phylogenies support the monophyly of both the Saurolophinae and the Lambeosaurinae 13. The Saurolophinae contain members such as Bactrosaurus, Parasaurolophus, and Saurolophus, while the Lambeosaurinae include Corythosaurus, Hypacrosaurus, Lambeosaurus, and Parasaurolophus 13. Below is the phylogeny determined by Prieto-Marquez’s 2010 study.

Saurolophine Cladogram Below is a phylogeny of the Saurolophinae from Gates’ and Sampson’s 2007 study8. Lambeosaurine Cladogram Below is a phylogeny of the Lambeosaurinae from Evans’ and Reisz’s 2007 study.7

Closest Relatives The closest relatives of the hadrosaurs are the Iguanodontidae 16. The most notable difference between the Hadrosauridae and the Iguanodontidae are in the bones of their jaw 16. Members of Iguanodontidae have a large maxilla bone in the upper jaw, while hadrosaurs have a smaller maxilla, complex teeth organization, and a large premaxilla in the upper jaw 16. hadrosaurs are defined as all of the descendants of the last common ancestor of Telmatosaurus and Parasaurolophus 13. Lambeosaurinae Origins The oldest Lambeosaur fossils and the most basal members of the Lambeosaur phylogeny are from Asia 9. As a result, it has been determined that this particular group originated in Asia before spreading to other continents, such as North America 9. Behavior Feeding Strategies Pleurokinesis The hadrosaurs fed on plants, and therefore had to develop techniques to grind plant material before swallowing their food. As a result, hadrosaurs relied on pleurokinesis to help chew their food 12. During pleurokinesis, the maxilla bones slide laterally outward from the mouth 12. This is accomplished with a hinge made of the joints between the premaxilla and maxilla bones and the lacrimal and prefrontal bones 12. Pleurokinesis allows for the teeth to more precisely grind against one another, for better shearing of plant material, and reduces the stress that chewing puts on the skull 12. Tooth Rows Furthermore, multiple rows of teeth in each part of the jaw provided a large surface for chewing. These two factors together made for a jaw that was highly effective at grinding plant material. Analysis of the wear-patterns of the teeth has provided further insight into how these animals may have fed. On the teeth, there can be found four different sets of wear patterns that indicate the lower jaw would have moved directly up and down 15. Furthermore, the wear patterns were parallel, revealing that the rows of teeth were extremely well aligned in the jaw. Diet While evidence may point to the theory that these animals grazed on leaves, there is still a great deal of controversy as to what the hadrosaurs ate 15. One study found fossilized droppings of Maiasaura indicating that the animals may have been eating pieces of rotten wood4. Though the wood itself would not have provided a significant amount of nutrients, there may have been fungus growing on the wood, which would have been an excellent resource. Facultative Bipedalism Evidence suggests that young hadrosaurs would have walked on only their two hind legs, while adults would have walked on all four6. As the animal aged, the front limbs became more robust in order to take on weight, while the back legs became less robust as they transitioned to walking on all four legs6. Furthermore, the animals’ front limbs were shorter than their back limbs6. Works Cited

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Bell, Phil R. "Standardized terminology and potential taxonomic utility for hadrosaurid skin impressions: a case study for Saurolophus from Canada and Mongolia." PLoS One. 2012: Volume 7 Issue 2.

Case, Judd A., et al. "The first duck-billed dinosaur (Family Hadrosauridae) from Antarctica." Journal of Vertebrate Paleontology. 25 September, 2000: Volume 20, Issue 3. Pps 612-614.

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Gates, Terry A., and Scott D. Sampson. "A new species of Gryposaurus (Dinosauria: Hadrosauridae) from the late Campanian Kaiparowits Formation, southern Utah, USA." Zoological Journal of the Linnean Society 151.2 (2007): 351-376.

Godefroit, Pascal, Yuri Bolotsky, and Vladimir Alifanov. "A remarkable hollow-crested hadrosaur from Russia: an Asian origin for lambeosaurines." Comptes Rendus Palevol. 2003: Volume 2, Issue 2. PPs 143-151.

Heaton, M. J. "The Palatal Structure of some Canadian Hadrosauridae (Reptilia: Ornithischia)." Canadian Journal of Earth Science. 1972: Volume 9, Issue 2. Pps 185-205.

Horner, John R. "Cranial osteology and morphology of the type specimen of Maiasaura peeblesorum (Ornithischia: Hadrosauridae), with a discussion of its phylogenetic position." Journal of Vertebrate Paleontology. 1983: Volume 3, Issue 1. Pps 29-38.

Norman, David B. and Weishampel, David B. “Ornithopod Feeding Mechanisms: Their Bearing on the Evolution of Herbivory.” The American Naturalist. August, 1985: Volume 126, Issue 2. Pps 151-164.

Prieto-Marquez, Albert. "Global phylogeny of Hadrosauridae (Dinosauria: Ornithopoda) using parsimony and Bayesian methods." Zoological Journal of the Linnean Society. 26 May, 2010: Volume 159, Issue 2. Pps 435-503.

Vajda, Vivi, et al. "A snapshot into the terrestrial ecosystem of an exceptionally well-preserved dinosaur (Hadrosauridae) from the Upper Cretaceous of North Dakota, USA." Cretaceous Research. November, 2013: Volume 46. Pps 114-122.

Williams, Vincent S., Paul M. Barrett, and Mark A. Purnell. "Quantitative analysis of dental microwear in hadrosaurid dinosaurs, and the implications for hypotheses of jaw mechanics and feeding." Proceedings of the National Academy of Sciences 106.27 (2009): 11194-11199.

You, Hai-lu, et al. "The earliest-known duck-billed dinosaur from deposits of late Early Cretaceous age in northwest China and hadrosaur evolution." Cretaceous Research. June, 2003: Volume 24, Issue 3. Pps 347-355.