Hadrosauridae

Hadrosaurids (Greek: ἁδρός, hadrós, "stout, thick"), or duck-billed dinosaurs, are members of the ornithischian family Hadrosauridae. This group is known as the duck-billed dinosaurs for the flat duck-bill appearance of the bones in their snouts. The family, which includes ornithopods such as Edmontosaurus and Parasaurolophus, was a common group of herbivores during the Late Cretaceous Period in what is now Asia, Europe, Africa, Antarctica, South America, and North America.[1] Hadrosaurids are descendants of the Upper Jurassic/Lower Cretaceous iguanodontian dinosaurs and had a similar body layout.

Hadrosaurids Temporal range: Late Cretaceous, 86–66 Ma PreꞒ Ꞓ O S D C P T J K Pg N
Mounted skeleton of Edmontosaurus annectens, Oxford University Museum
Scientific classification
Kingdom:	Animalia
Phylum:	Chordata
Clade:	Dinosauria
Order:	†Ornithischia
Suborder:	†Ornithopoda
Clade:	†Hadrosauromorpha
Family:	†Hadrosauridae Cope, 1869
Type species
†Hadrosaurus foulkii Leidy, 1858
Subgroups
†Aquilarhinus †Eotrachodon †Hadrosaurus †Lapampasaurus †Latirhinus? †Euhadrosauria/Saurolophidae †Lambeosaurinae †Saurolophinae
Synonyms
Trachodontidae Lydekker, 1888 Saurolophidae Brown, 1914 Lambeosauridae Parks, 1923 Cheneosauridae Lull & Wright, 1942 Ornithotarsidae Cope, 1871

Like other ornithischians, hadrosaurids had a predentary bone and a pubic bone which was positioned backwards in the pelvis. Hadrosauridae is divided into two principal subfamilies: the lambeosaurines (Lambeosaurinae), which had hollow cranial crests or tubes; and the saurolophines (Saurolophinae), identified as hadrosaurines (Hadrosaurinae) in most pre-2010 works, which lacked hollow cranial crests (solid crests were present in some forms). Saurolophines tended to be bulkier than lambeosaurines. Lambeosaurines included the aralosaurins, tsintaosaurins, lambeosaurins and parasaurolophins, while saurolophines included the brachylophosaurins, kritosaurins, saurolophins and edmontosaurins.

Hadrosaurids were facultative bipeds, with the young of some species walking mostly on two legs and the adults walking mostly on four.[2][3] Their jaws were evolved for grinding plants, with multiple rows of teeth replacing each other as the teeth wore down.[2]

History of discovery

Illustration of Trachodon mirabilis teeth

Ferdinand Vandeveer Hayden, during expeditions near the Judith River in 1854 through 1856, discovered the very first dinosaur fossils recognized from North America. These specimens were obtained by Joseph Leidy, who described and named them in 1856; two of the several species named were Trachodon mirabilis of the Judith River Formation and Thespesius occidentalis of the "Great Lignite Formation". The former was based on a collection of teeth whilst the later on two caudalcentra and a phalanx. Although most of the Trachodon teeth turned out to belong to ceratopsids, the holotype and remains of T. occidentalis would come to be recognized as the first recognized hadrosaur specimens. Around the same time in Philadelphia, on the other side of the continent, geologist William Parker Foulke was informed of numerous large bones accidentally uncovered by farmer John E. Hopkins some twenty years earlier. Foulke obtained permission to investigate the now scattered fossils in 1858, and these specimens as well were given to Leidy. They were described in the same year as Hadrosaurus foulkii, giving a slightly better picture of the form of a hadrosaur. Leidy provided additional description in a 1865 paper.[4] Among his 1858 work Leidy briefly suggested that the animal was likely amphibious in nature; this school of thought about hadrosaurs would come to be dominant for over a century to come.[5]

From the mid 19th century through much of the 20th century, hadrosaurs were considered aquatic animals which subsisted on soft water plants

Further discoveries such as "Hadrosaurus minor" and "Ornithotarsus immanis" would come from the East, and Edward Drinker Cope led an expedition to the Judith River Formation where Trachodon was found. Upon the fragments discovered he named seven new species in two genera, as well as assigning material to Hadrosaurus.[4] Cope had studied the jaws of hadrosaurs and come to the conclusion that the teeth were fragile and could have been dislodged incredibly easily. As such, he supposed the animals must have fed largely on soft water plants; he presented this idea to the Philadelphia Academy in 1883, and this idea would come to be very influential on future study.[4][5] Research would continue in the Judith River area for years to come, but the formation never yielded much more than fragmentary remains, and Cope's species as well as Trachodon itself would be in time be seen as of doubtful validity. The Eastern states, too, would never yield particularly informative specimens. Instead, other sites in the American west would come to provide many very complete specimen that would form the backbone of hadrosaur research. One such specimen was the very complete AMNH 5060 ( belonging to Edmontosaurus annectens), recovered in 1908 by the fossil collector Charles Hazelius Sternberg and his three sons in Converse County, Wyoming. It was described by Henry Osborn in 1912, who dubbed it the "Dinosaur mummy". This specimen's skin was almost completely preserved in the form of impressions. The skin around its hands, thought to represent webbing, was seen as further bolstering the idea that hadrosaurs were very aquatic animals.[4]

Cope had planned to write a monograph about the group Ornithopoda, but never made much progress towards it before his death. This unrealized endeavor would come to be the inspiration for Richard Swann Lull and Nelda Wright to work on a similar project decades later. Eventually they realized the whole of Ornithopoda was too broad of a scope, until eventually it was narrowed down to specifically North American hadrosaurs. Their monograph, Hadrosaurian Dinosaurs of North America, was published in 1942, and looked back at the whole of understanding about the family. It was designed as a definitive work, covering all aspects of their biology and evolution, and as part of it every known species was re-evaluated and many of them redescribed. They agreed with prior authors on the semi-aquatic nature of hadrosaurs, but re-evaluated Cope's idea of weak jaws and found quite the opposite. The teeth were rooted in strong batteries and would be continuously replaced to prevent them getting worn down. Such a system seemed incredibly overbuilt for the job of eating soft Mesozoic plants, and this fact confused the authors. Though they stil proposed a diet of water plants, they considered it likely this would be supplemented by occasional forrays into browsing on land plants.[4]

Skeleton of Maiasaura posed with a nest; the naming of this genus was one of numerous important developments in the Dinosaur Renaissance

Twenty years later, in 1964, another very important work would be published, this time by John H. Ostrom. It challenged the idea that hadrosaurs were semi-aquatic animals, which had been held since the work of Leidy back in the 1850s. This new approach was backed using evidence of the environment and climate they lived in, co-existing flora and fauna, physical anatomy, and preserved stomach context from mummies. Based on evaluation of all this data, Ostrom found the idea that hadrosaurs were adapted for aquatic life incredibly lacking, and instead proposed they were capable terrestrial animals that browsed on plants such as conifers. He remained uncertain, however, as to the purpose of the paddle-like hand Osborn had described, as well as their long and somewhat paddle-like tails. Thus he agreed with the idea that hadrosaurs would have taken refuge from predators in water.[5] Numerous important studies would follow this; Ostrom's student Peter Dodson published a paper about lambeosaur skull anatomy that made enormous changes to hadrosaur taxonomy in 1975, and Michael K. Brett-Surman conducted a full revision of the group as part of his Graduate studies through the 1970s and 1980s. John R. Horner would also begin to leave his impact on the field, including with the naming of Maiasaura in 1979.[6][7][8][9]

Hadrosaur research experienced a surge in the decade of the 2000s, similar to the research of other dinosaurs. In response to this, the Royal Ontario Museum and the Royal Tyrrell Museum collaborated to arrange the International Hadrosaur Symposium, a professional meeting about ongoing hadrosaur research that was held at the latter institution on September 22nd and 23rd in 2011. Over fifty presentations were made at the event, thirty-six of which were later incorporated into a book, titled Hadrosaurs, published in 2015. The volume was brought together primarily by palaeontologists David A. Eberth and David C. Evans, and featured an afterword from John R. Horner, all of whom also contributed to one or more of the studies published therein.[10] The first chapter of the volume was a study by David B. Weishampel about the rate of ornithopod research over history, and the interest in different aspects of it over that history, using the 2004 volume The Dinosauria as the source of data on the amount of works published in each decade. Various periods of high and low activity were found, but the twenty-first century was found to overwhelmingly be the most prolific time, with over two-hundred papers published. The advent of the internet was cited as a likely catalyst for this boom. Hadrosaur research experienced high levels of diversity within the decade, with previously uncommon subjects such as growth, phylogeny, and biogeography experiencing more attention, though the functional morphology of hadrosaurids was found to have declined in study since the Dinosaur Renaissance.[11]

Classification

The family Hadrosauridae was first used by Edward Drinker Cope in 1869, then containing only Hadrosaurus.[12] Since its creation, a major division has been recognized in the group between the hollow-crested subfamily Lambeosaurinae and the subfamily Saurolophinae, historically known as Hadrosaurinae. Both of these have been robustly supported in all recent literature. Phylogenetic analysis has increased the resolution of hadrosaurid relationships considerably, leading to the widespread usage of tribes (a taxonomic unit below subfamily) to describe the finer relationships within each group of hadrosaurids. .[13]

Lambeosaurines have also been traditionally split into Parasaurolophini and Lambeosaurini.[14] These terms entered the formal literature in Evans and Reisz's 2007 redescription of Lambeosaurus magnicristatus. Lambeosaurini is defined as all taxa more closely related Lambeosaurus lambei than to Parasaurolophus walkeri, and Parasaurolophini as all those taxa closer to P. walkeri than to L. lambei. In recent years Tsintaosaurini and Aralosaurini have also emerged.[15]

The use of the term Hadrosaurinae was questioned in a comprehensive study of hadrosaurid relationships by Albert Prieto-Márquez in 2010. Prieto-Márquez noted that, though the name Hadrosaurinae had been used for the clade of mostly crestless hadrosaurids by nearly all previous studies, its type species, Hadrosaurus foulkii, has almost always been excluded from the clade that bears its name, in violation of the rules for naming animals set out by the ICZN. Prieto-Márquez defined Hadrosaurinae as just the lineage containing H. foulkii, and used the name Saurolophinae instead for the traditional grouping.[13]

Phylogeny

Hadrosauridae was first defined as a clade, by Forster, in a 1997 abstract, as simply "Lambeosaurinae plus Hadrosaurinae and their most recent common ancestor". In 1998, Paul Sereno defined the clade Hadrosauridae as the most inclusive possible group containing Saurolophus (a well-known saurolophine) and Parasaurolophus (a well-known lambeosaurine), later emending the definition to include Hadrosaurus, the type genus of the family, which ICZN rules state must be included, despite its status as a nomen dubium. According to Horner et al. (2004), Sereno's definition would place a few other well-known hadrosaurs (such as Telmatosaurus and Bactrosaurus) outside the family, which led them to define the family to include Telmatosaurus by default. Prieto-Marquez reviewed the phylogeny of Hadrosauridae in 2010, including many taxa potentially within the family.[13]

Premaxilla of Eotrachodon, the taxon named by Prieto-Marquez et al. 2016

Below is a cladogram from Prieto-Marquez et al. 2016. This cladogram is a recent modification of the original 2010 analysis, including more characters and taxa. The resulting cladistic tree of their analysis was resolved using Maximum-Parsimony. 61 hadrosauroid species were included, characterized for 273 morphological features: 189 for cranial features and 84 for postcranial features. When characters had multiple states that formed an evolutionary scheme, they were ordered to account for the evolution of one state into the next. The final tree was run through TNT version 1.0.[16]

Skull of Lambeosaurus, the type taxon of Lambeosaurinae

Skull of Saurolophus, the type taxon of Saurolophinae

†Telmatosaurus         †Jintasaurus     †Lophorhothon         †Claosaurus       †Tethyshadros   †Hadrosauridae   †Hadrosaurus       †Eotrachodon   †Saurolophidae †Lambeosaurinae   †Aralosaurus       †Canardia     †Jaxartosaurus         †Tsintaosaurus     †Pararhabdodon           †Charonosaurus     †Parasaurolophus         †Lambeosaurus       †Corythosaurus       †Hypacrosaurus stebingeri         †Magnapaulia       †Velafrons     †Sahaliyania           †Hypacrosaurus     †Olorotitan       †Blasisaurus     †Amurosaurus                       Saurolophinae     †Acristavus     †Maiasaura     †Brachylophosaurus           †Naashoibitosaurus       †Kritosaurus       †Gryposaurus       †Aquilarhinus       †Willinakaqe     †Secernosaurus                 "†Sabinosaur" PASAC-1         †Prosaurolophus       †Augustynolophus     †Saurolophus           †Kerberosaurus       †Kundurosaurus       †Shantungosaurus     †Edmontosaurus

Anatomy

Edmontosaurus skull, Oxford University Museum of Natural History

The most recognizable aspect of hadrosaur anatomy is the flattened and laterally stretched rostral bones, which gives the distinct duck-bill look. Some members of the hadrosaurs also had massive crests on their heads, probably for display.[13] In some genera, including Edmontosaurus, the whole front of the skull was flat and broadened out to form a beak, which was ideal for clipping leaves and twigs from the forests of Asia, Europe and North America. However, the back of the mouth contained thousands of teeth suitable for grinding food before it was swallowed. This has been hypothesized to have been a crucial factor in the success of this group in the Cretaceous compared to the sauropods.

Skin impressions of multiple hadrosaurs have been found.[17] From these impressions, the hadrosaurs were determined to be scaled, and not feathered like some dinosaurs of other groups.

Hadrosaurs, much like sauropods, are noted for having their manus united in a fleshy, often nail-less pad.[18]

The two major divisions of hadrosaurids are differentiated by their cranial ornamentation. While members of the Lambeosaurinae subfamily have hollow crests that differ depending on species, members of the Saurolophinae (Hadrosaurinae) subfamily have solid crests or none at all. Lambeosaurine crests had air chambers that may have produced a distinct sound and meant that their crests could have been used for both an audio and visual display.

Paleobiology

Diet

Early restoration by Charles R. Knight of hadrosaurs as semi-aquatic animals that could only chew soft water plants, a popular idea at the time.

While studying the chewing methods of hadrosaurids in 2009, the paleontologists Vincent Williams, Paul Barrett, and Mark Purnell found that hadrosaurs likely grazed on horsetails and vegetation close to the ground, rather than browsing higher-growing leaves and twigs. This conclusion was based on the evenness of scratches on hadrosaur teeth, which suggested the hadrosaur used the same series of jaw motions over and over again.[19] As a result, the study determined that the hadrosaur diet was probably made of leaves and lacked the bulkier items, such as twigs or stems, that might have required a different chewing method and created different wear patterns.[20] However, Purnell said these conclusions were less secure than the more conclusive evidence regarding the motion of teeth while chewing.[21]

The hypothesis that hadrosaurs were likely grazers rather than browsers appears to contradict previous findings from preserved stomach contents found in the fossilized guts in previous hadrosaur studies.[21] The most recent such finding before the publication of the Purnell study was conducted in 2008, when a team led by University of Colorado at Boulder graduate student Justin S. Tweet found a homogeneous accumulation of millimeter-scale leaf fragments in the gut region of a well-preserved partially grown Brachylophosaurus.[22][23] As a result of that finding, Tweet concluded in September 2008 that the animal was likely a browser, not a grazer.[23] In response to such findings, Purnell said that preserved stomach contents are questionable because they do not necessarily represent the usual diet of the animal. The issue remains a subject of debate.[24]

Edmontosaurus dentary with teeth, typical of hadrosauridae

Mallon et al. (2013) examined herbivore coexistence on the island continent of Laramidia, during the Late Cretaceous. It was concluded that hadrosaurids could reach low-growing trees and shrubs that were out of the reach of ceratopsids, ankylosaurs, and other small herbivores. Hadrosaurids were capable of feeding up to 2 m when standing quadrupedally, and up to 5 m bipedally.[25]

Coprolites (fossilized droppings) of some Late Cretaceous hadrosaurs show that the animals sometimes deliberately ate rotting wood. Wood itself is not nutritious, but decomposing wood would have contained fungi, decomposed wood material and detritus-eating invertebrates, all of which would have been nutritious.[26] Examination of hadrosaur coprolites from the Grand Staircase-Escalante indicates that shellfish such as crustaceans were also an important component of the hadrosaur diet.[27]

Reproduction

Neonate sized hadrosaur fossils have been documented in the scientific literature.[28] Tiny hadrosaur footprints have been discovered in the Blackhawk Formation of Utah.[28]

The head of Gryposaurus notabilis.

In a 2001 review of hadrosaur eggshell and hatchling material from Alberta's Dinosaur Park Formation, Darren Tanke and M. K. Brett-Surman concluded that hadrosaurs nested in both the ancient upland and lowlands of the formation's depositional environment. The upland nesting grounds may have been preferred by the less common hadrosaurs, like Brachylophosaurus and Parasaurolophus. However, the authors were unable to determine what specific factors shaped nesting ground choice in the formation's hadrosaurs. They suggested that behavior, diet, soil condition, and competition between dinosaur species all potentially influenced where hadrosaurs nested.[28]

Sub-centimeter fragments of pebbly-textured hadrosaur eggshell have been reported from the Dinosaur Park Formation. This eggshell is similar to the hadrosaur eggshell of Devil's Coulee in southern Alberta as well as that of the Two Medicine and Judith River Formations in Montana, United States. While present, dinosaur eggshell is very rare in the Dinosaur Park Formation and is only found in two different microfossil sites. These sites are distinguished by large numbers of pisidiid clams and other less common shelled invertebrates, like unionid clams and snails. This association is not a coincidence, as the invertebrate shells would have slowly dissolved and released enough basic calcium carbonate to protect the eggshells from naturally occurring acids that otherwise would have dissolved them and prevented fossilization.[28]

In contrast with eggshell fossils, the remains of very young hadrosaurs are somewhat common. Tanke has observed that an experienced collector could discover multiple juvenile hadrosaur specimens in a single day. The most common remains of young hadrosaurs in the Dinosaur Park Formation are dentaries, bones from limbs and feet, as well as vertebral centra. The material showed little or none of the abrasion that would have resulted from transport, meaning the fossils were buried near their point of origin. Bonebeds 23, 28, 47, and 50 are productive sources of young hadrosaur remains in the formation, especially bonebed 50. The bones of juvenile hadrosaurs and fossil eggshell fragments are not known to have been preserved in association with each other, despite both being present in the formation.[28]

Development

The limbs of the juvenile hadrosaurs are anatomically and proportionally similar to those of adult animals.[28] However, the joints often show "predepositional erosion or concave articular surfaces",[28] which was probably due to the cartilaginous cap covering the ends of the bones.[28] The pelvis of a young hadrosaur was similar to that of an older individual.[28]

Evidence suggests that young hadrosaurs would have walked on only their two hind legs, while adults would have walked on all four.[2] 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 legs.[2] Furthermore, the animals’ front limbs were shorter than their back limbs.[2]

Daily activity patterns

Comparisons between the scleral rings of several hadrosaur genera (Corythosaurus, Prosaurolophus, and Saurolophus) and modern birds and reptiles suggest that they may have been cathemeral, active throughout the day at short intervals.[29]

Pathology

Spondyloarthropathy has been documented in the spine of a 78-million year old hadrosaurid. Other examples of pathologies in hadrosaurs include healed wounds from predators, such as those found in Edmontosaurus annectens, and tumors such as hemangiomas, desmoplastic fibroma, metastatic cancer, and osteoblastomas, found in genera such as Brachylophosaurus and Edmontosaurus.[30][31] Osteochondrosis is also commonly found in hadrosaurs.[32]

Ichnology

Tiny hadrosaur footprints have been discovered in the Blackhawk Formation of Utah.[28]

References

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2. Dilkes, David W. (2001). "An ontogenetic perspective on locomotion in the Late Cretaceous dinosaur Maiasaura peeblesorum (Ornithischia: Hadrosauridae)". Canadian Journal of Earth Sciences. 38 (8): 1205–1227. doi:10.1139/e01-016.
3. Fiorillo, A.R.; Tykoski, R.S. (2016). "Small hadrosaur manus and pes tracks from the Lower Cantwell Formation (Upper Cretaceous) Denali National Park, Alaska: implications for locomotion in juvenile hadrosaurs". PALAIOS. 31 (10): 479–482. Bibcode:2016Palai..31..479F. doi:10.2110/palo.2016.049. S2CID 132975678.
4. Lull, Richard Swann; and Wright, Nelda E. (1942). Hadrosaurian Dinosaurs of North America
5. Ostrom, John H. (1964). "A reconsideration of the paleoecology of the hadrosaurian dinosaurs". American Journal of Science. 262 (8): 975–997. Bibcode:1964AmJS..262..975O. doi:10.2475/ajs.262.8.975.
6. Brett-Surman, Michael K. (1975). The appendicular anatomy of hadrosaurian dinosaurs. M.A. thesis. Berkeley: University of California.
7. Brett-Surman, Michael K. (1979). "Phylogeny and paleobiogeography of hadrosaurian dinosaurs". Nature. 277 (5697): 560–562. Bibcode:1979Natur.277..560B. doi:10.1038/277560a0. S2CID 4332144.
8. Brett-Surman, Michael K. (1989). A revision of the Hadrosauridae (Reptilia: Ornithischia) and their evolution during the Campanian and Maastrichtian. Ph.D. dissertation. Washington, D.C.: George Washington University.
9. Horner, J.R.; Makela, R. (1979). "Nest of juveniles provides evidence of family structure among dinosaurs". Nature. 282 (5736): 296–298. Bibcode:1979Natur.282..296H. doi:10.1038/282296a0. S2CID 4370793.
10. Eberth, David A.; Evans, David C. (2015). "Preface". In Eberth, David A.; Evans, David C. (eds.). Hadrosaurs. Bloomington, Indiana: Indina University Press. pp. xiii–xiv. ISBN 978-0-253-01385-9.
11. Weishampel, David B. (2015). "A history of the study of ornithopods: Where have we been? Where are we now? Where are we going?". In Eberth, David A.; Evans, David C. (eds.). Hadrosaurs. Bloomington, Indiana: Indiana University Press. pp. 2–7. ISBN 978-0-253-01385-9.
12. Cope, Edward D. (1969). "Synopsis of the extinct batrachia, reptilia and aves of North America". American Philosophical Society. 14.
13. Prieto-Márquez, A (2010). "Global phylogeny of Hadrosauridae (Dinosauria: Ornithopoda) using parsimony and Bayesian methods". Zoological Journal of the Linnean Society. 159 (2): 435–502. doi:10.1111/j.1096-3642.2009.00617.x.
14. Glut, Donald F. (1997). Dinosaurs: The Encyclopedia. Jefferson, North Carolina: McFarland & Co. p. 69. ISBN 0-89950-917-7.
15. Prieto-Márquez, A.; Dalla Vecchia, F. M.; Gaete, R.; Galobart, À. (2013). "Diversity, relationships, and biogeography of the Lambeosaurine dinosaurs from the European archipelago, with description of the new aralosaurin Canardia garonnensis". PLOS ONE. 8 (7): e69835. Bibcode:2013PLoSO...869835P. doi:10.1371/journal.pone.0069835. PMC 3724916. PMID 23922815.
16. Prieto-Marquez, A.; Erickson, G.M.; Ebersole, J.A. (2016). "A primitive hadrosaurid from southeastern North America and the origin and early evolution of 'duck-billed' dinosaurs". Journal of Vertebrate Paleontology. 36 (2): e1054495. doi:10.1080/02724634.2015.1054495. S2CID 86032549.
17. Bell, P. R. (2012). Farke, Andrew A (ed.). "Standardized Terminology and Potential Taxonomic Utility for Hadrosaurid Skin Impressions: A Case Study for Saurolophus from Canada and Mongolia". PLOS ONE. 7 (2): e31295. Bibcode:2012PLoSO...731295B. doi:10.1371/journal.pone.0031295. PMC 3272031. PMID 22319623.
18. "Hadrosaur Forelimb Study". Palaeo-electronica.org. Retrieved 2013-07-23.
19. Williams, Vincent S.; Barrett, Paul M.; Purnell, Mark A. (2009). "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): 11194–11199. Bibcode:2009PNAS..10611194W. doi:10.1073/pnas.0812631106. PMC 2708679. PMID 19564603.
20. Bryner, Jeanna (2009-06-29). "Study hints at what and how dinosaurs ate". LiveScience. Retrieved 2009-06-03.
21. Boyle, Alan (2009-06-29). "How dinosaurs chewed". MSNBC. Archived from the original on 2009-07-02. Retrieved 2009-06-03.
22. Tweet, Justin S.; Chin, Karen; Braman, Dennis R.; Murphy, Nate L. (2008). "Probable gut contents within a specimen of Brachylophosaurus canadensis (Dinosauria: Hadrosauridae) from the Upper Cretaceous Judith River Formation of Montana". PALAIOS. 23 (9): 624–635. Bibcode:2008Palai..23..624T. doi:10.2110/palo.2007.p07-044r. S2CID 131393649.
23. Lloyd, Robin (2008-09-25). "Plant-eating dinosaur spills his guts: Fossil suggests hadrosaur's last meal included lots of well-chewed leaves". NBC News. Retrieved 2009-06-03.
24. This information comes from the aforementioned Alan Boyle source from June 29, 2009. However, this specific information is not included in the body of the article, but rather a response by Boyle to comments in the article. Since the comments were written by Boyle himself, and since they cite information he received specifically from Purnell, they are as legitimate a source of information as the article itself.
25. Mallon, Jordan C.; Evans, David C.; Ryan, Michael J.; Anderson, Jason S. (2013). "Feeding height stratification among the herbivorous dinosaurs from the Dinosaur Park Formation (upper Campanian) of Alberta, Canada". BMC Ecology. 13: 14. doi:10.1186/1472-6785-13-14. PMC 3637170. PMID 23557203.
26. Chin, K. (September 2007). "The Paleobiological Implications of Herbivorous Dinosaur Coprolites from the Upper Cretaceous Two Medicine Formation of Montana: Why Eat Wood?". PALAIOS. 22 (5): 554. Bibcode:2007Palai..22..554C. doi:10.2110/palo.2006.p06-087r. S2CID 86197149.
27. Chin, Karen; Feldmann, Rodney M.; Tashman, Jessica N. (2017). "Consumption of crustaceans by megaherbivorous dinosaurs: dietary flexibility and dinosaur life history strategies". Scientific Reports. 7 (1): 11163. Bibcode:2017NatSR...711163C. doi:10.1038/s41598-017-11538-w. PMC 5608751. PMID 28935986.
28. Tanke, D.H. and Brett-Surman, M.K. 2001. Evidence of Hatchling and Nestling-Size Hadrosaurs (Reptilia:Ornithischia) from Dinosaur Provincial Park (Dinosaur Park Formation: Campanian), Alberta, Canada. pp. 206-218. In: Mesozoic Vertebrate Life—New Research Inspired by the Paleontology of Philip J. Currie. Edited by D.H. Tanke and K. Carpenter. Indiana University Press: Bloomington. xviii + 577 pp.
29. Schmitz, L.; Motani, R. (2011). "Nocturnality in Dinosaurs Inferred from Scleral Ring and Orbit Morphology". Science. 332 (6030): 705–708. Bibcode:2011Sci...332..705S. doi:10.1126/science.1200043. PMID 21493820. S2CID 33253407.
30. Rothschild, B.M.; Tanke, D.H.; Helbling II, M.; Martin, L.D. (2003). "Epidemiologic study of tumors in dinosaurs". Naturwissenschaften. 90 (11): 495–500. Bibcode:2003NW.....90..495R. doi:10.1007/s00114-003-0473-9. PMID 14610645. S2CID 13247222.
31. Carpenter, Kenneth (1998). "Evidence of predatory behavior by theropod dinosaurs". Gaia. 15: 135–144. Archived from the original on 2007-11-17. Retrieved 2009-03-08. [not printed until 2000]
32. Rothschild, Bruce; Tanke, Darren H. (2007). "Osteochondrosis is Late Cretaceous Hadrosauria". In Carpenter Kenneth (ed.). Horns and Beaks: Ceratopsian and Ornithopod Dinosaurs. Bloomington and Indianapolis: Indiana University Press. pp. 171–183. ISBN 978-0-253-34817-3.

External links

• Timeline of hadrosaur research
• UCMP Tree of Life
• Trachodon
• "Mummified" Dinosaur Discovered In Montana, National Geographic 2002