| Hadrosaurids Temporal range: Late Cretaceous, 100–65.5 Ma |
|
|---|---|
| Mounted skeleton of Parasaurolophus cyrtocristatus, Field Museum of Natural History | |
| Scientific classification | |
| Phylum: | Chordata |
| Class: | Reptilia |
| Superorder: | Dinosauria |
| Order: | †Ornithischia |
| Suborder: | †Ornithopoda |
| Superfamily: | †Hadrosauroidea |
| Family: | †Hadrosauridae Cope, 1869 |
| Type species | |
| †Hadrosaurus foulkii Leidy, 1858 |
|
| Synonyms | |
|
|
Hadrosaurids or duck-billed dinosaurs are members of the family Hadrosauridae, and include ornithopods such as Edmontosaurus and Parasaurolophus. They were common herbivores in the Upper Cretaceous Period of what are now Asia, Europe and North America. They are descendants of the Upper Jurassic/Lower Cretaceous iguanodontian dinosaurs and had similar body layout. They were ornithischians.
Hadrosaurids are divided into two principal subfamilies. The lambeosaurines (Lambeosaurinae) had hollow cranial crests or tubes, and were generally less bulky. The saurolophines, identified as hadrosaurines in most pre-2010 works (Saurolophinae or Hadrosaurinae), lacked hollow cranial crests (solid crests were present in some forms) and were generally larger.
Contents |
The hadrosaurs are known as the duck-billed dinosaurs due to the similarity of their head to that of modern ducks. In some genera, most notably Anatotitan, the whole front of the skull was flat and broadened out to form a beak, ideal for clipping leaves and twigs from the forests of Asia, Europe and North America. However, the back of the mouth contained literally 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 which were still largely dependent on gastroliths for grinding their food.
In 2009, paleontologist Mark Purnell conducted a study into the chewing methods and diet of hadrosaurids from the Late Cretaceous period. By analyzing hundreds of microscopic scratches on the teeth of a fossilized Edmontosaurus jaw, the team determined hadrosaurs had a unique way of eating unlike any creature living today. In contrast to a flexible lower jaw joint prevalent in today's mammals, hadrosaurs had a unique hinge between the upper jaws and the rest of its skull. The team found the dinosaur's upper jaws pushed outwards and sideways while chewing, as the lower jaw slid against the upper teeth.[1]
Hadrosaurids were the first dinosaur family to be identified in North America, the first traces being found in 1855-1856 with the discovery of fossil teeth. Joseph Leidy examined the teeth, and erected the genera Trachodon and Thespesius (others included Troodon, Deinodon and Palaeoscincus). One species was named Trachodon mirabilis. Now it seems that the teeth genus Trachodon is a mixture of all sorts of cerapod dinosaurs, including ceratopsids. In 1858 the teeth were associated with Leidy's eponymous Hadrosaurus foulkii, named after the fossil hobbyist William Parker Foulke. More and more teeth were found, resulting in even more (now obsolete) genera.
A second duck-bill skeleton was unearthed, and was named Diclonius mirabilis in 1883 by Edward Drinker Cope, which he incorrectly used in favor of Trachodon mirabilis. But Trachodon, together with other poorly typed genera, was used more widely and, when Cope's famous "Diclonius mirabilis" skeleton was mounted at the American Museum of Natural History, it was labeled as "Trachodont dinosaur". The duck-billed dinosaur family was then named Trachodontidae.
A very well-preserved complete hadrosaurid specimen (Edmontosaurus annectens) was recovered in 1908 by the fossil collector Charles Hazelius Sternberg and his three sons, in Converse County, Wyoming. Analyzed by Henry Osborn in 1912, it has come to be known as the "Trachodon mummy". This specimen's skin was almost completely preserved in the form of impressions.
Lawrence Lambe erected the genus Edmontosaurus ("lizard from Edmonton") in 1917 from a find in the lower Edmonton Formation (now Horseshoe Canyon Formation), Alberta. Hadrosaurid systematics were addressed in a 1942 monograph by Richard Swann Lull and Nelda Wright. They proposed the genus Anatosaurus for several species of dubious genera. Cope's famous mount at the AMNH became Anatosaurus copei. In 1990, Anatosaurus was moved to Edmontosaurus. One former Anatosaurus species was distinct enough from Edmontosaurus to be placed in a separate genus, named Anatotitan, so in 1990 the AMNH mount was re-labelled Anatotitan copei.
Paleontologists have found a hadrosaurid leg bone in Paleocene rocks, but it was probably reworked from a Cretaceous source.[2]
One of the most complete fossilized specimens was found in 1999 in Hell Creek Formation of North Dakota and now is nicknamed "Dakota". The hadrosaur fossil is so well preserved that scientists have been able to calculate its muscle mass and learn that it was more muscular than thought, probably giving it the ability to outrun predators such as Tyrannosaurus rex. Unlike the collections of bones found in museums, this mummified hadrosaur fossil comes complete with skin (not merely skin impressions), ligaments, tendons and possibly some internal organs. It is being analyzed in the world's largest CT scanner, operated by the Boeing Co.[3] The machine usually is used for detecting flaws in space shuttle engines and other large objects, but previously none as large as this. Researchers hope the technology will help them learn more about the fossilized insides of the creature. They also found a gap of about a centimeter between each vertebra, indicating that there may have been a disk or other material between them, allowing more flexibility and meaning the animal was actually longer than what is shown in a museum.[4]
The family Hadrosauridae was first used by Edward Drinker Cope in 1869. Since its creation, a major division has been recognized in the group, between the (generally crested) subfamily Lambeosaurinae and (generally crestless) subfamily Saurolophinae (or Hadrosaurinae). Phylogenetic analysis has increased the resolution of hadrosaurid relationships considerably (see Phylogeny below), leading to the widespread usage of tribes (a taxonomic unit below subfamily) to describe the finer relationships within each group of hadrosaurids. However, many hadrosaurid tribes commonly recognized in online sources have not yet been formally defined or seen wide use in the literature. Several were briefly mentioned but not named as such in the first edition of The Dinosauria, under informal names. In this 1990 reference, "gryposaurs" included Aralosaurus, Gryposaurus, Hadrosaurus, and Kritosaurus; "brachylophosaurs" included Brachylophosaurus and Maiasaura; "saurolophs" included Lophorhothon, Prosaurolophus, and Saurolophus; and "edmontosaurs" included Anatotitan, Edmontosaurus, and Shantungosaurus.[5]
Lambeosaurines have also been split into Parasaurolophini (Parasaurolophus) and Corythosaurini (Corythosaurus, Hypacrosaurus, and Lambeosaurus).[6] Corythosaurini and Parasaurolophini as terms entered the formal literature in Evans and Reisz's 2007 redescription of Lambeosaurus magnicristatus. Corythosaurini is defined as all taxa more closely related Corythosaurus casuarius than to Parasaurolophus walkeri, and Parasaurolophini as all those taxa closer to P. walkeri than to C. casuarius. In this study, Charonosaurus and Parasaurolophus are parasaurolophins, and Corythosaurus, Hypacrosaurus, Lambeosaurus, Nipponosaurus, and Olorotitan are corythosaurins.[7] The enigmatic genus Tsintaosaurus may form a clade in Lambeosaurine with Pararhabdodon and its probable synonym Koutalisaurus.[8]
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 only the lineage containing H. foulkii, and used the name Saurolophinae instead for the traditional grouping.[9]
The following taxonomy includes dinosaurs currently referred to the Hadrosauridae and its subfamilies. Hadrosaurids that were accepted as valid but were not placed in a cladogram at the time of Prieto-Márquez's 2010 study[9] are included at the highest level to which they were placed (either then, or in their description if they postdate the papers used here).
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. The following cladogram was recovered in a 2010 phylogenetic analysis by Prieto-Márquez.[9]
| Hadrosauridae |
|
||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||
Hadrosauridae has not been subjected to as many phylogenetic analyses as other dinosaur groups, so other workers may find quite different phylogenies. Gates and Sampson (2007) published the following alternate cladogram of Saurolophinae (identified as "Hadrosaurinae" in the study) in their description of Gryposaurus monumentensis:[10]
| unnamed |
|
||||||||||||||||||||||||||||||||||||||||||||||||||||||
The following cladogram is after the 2007 redescription of Lambeosaurus magnicristatus (Evans and Reisz, 2007):[7]
| Hadrosauridae |
|
|||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||
While studying into 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 upon the evenness of scratches on hadrosaur teeth, which suggested the hadrosaur used the same series of jaw motions over and over again.[11] 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, which might have required a different chewing method and created different wear patterns.[12] However, Purnell said these conclusions were less secure than the more conclusive evidence regarding the motion of teeth while chewing.[1]
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 hadrosaurs studies.[1] 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.[13][14] As a result of that finding, Tweet concluded in September 2008 that the animal was likely a browser, not a grazer.[14] In response to such findings, Purnell said preserved stomach contents are questionable because they do not necessarily represent the usual diet of the animal. The issue remains a subject of debate.[15]
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.[16]
Neonate sized hadrosaur fossils have been documented in the scientific literature.[17] Tiny hadrosaur footprints have been discovered in the Blackhawk Formation of Utah.[17]
In 2001 Darren H. Tanke and M. K. Brett-Surman reviewed and described eggshell and hatchling material from the Dinosaur Park Formation of Dinosaur Provincial Park.[17] Eggshell is rare in the Park, being present in only two microfossil sites, both of which are predominated by the preserved shells of invertebrate life.[17] The survival of hadrosaur eggshell fragments in the presence of these invertebrate shells may be result of calcium in the invertebrate shells buffering contemporary acidic water which would have dissolved them.[17] The hadrosaur eggshell fragments "show little to no stream abrasion" suggesting that the material did not originate far from their final burial place in the Park.[17] The authors felt that their newly reported material corroborated the then-recent suggestion that hadrosaurs did not nest exclusively in upland areas, but also areas of lower elevation.[17] Some recovered hadrosaur fossils might actually be from embryos.[17] Hatchling and nestling-sized hadrosaur remains had been falsely considered rare in Dinosaur Park Formation due to bias on the part of collectors seeking larger specimens and sometimes not recognizing what was encountered.[17] Hadrosaurs had been speculated to be upland breeders due to the lack of preserved egg and hatchling material.[17] However young hadrosaur remains had been previously reported from lowland deposits.[17] Darren Tanke observed that an experienced collector could actually discover multiple juvenile hadrosaur specimens a day.[17] During the 1992 field season a concerted effort was undertaken by the Royal Tyrell Museum to recover the remains of young hadrosaurs.[17] The researchers describe the acquisition that season of 43 specimens as being a success.[17] Most of the recovered fossils were of dentaries missing their teeth, bones from limbs and feet, as well as vertebral centra.[17] 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.[17] The researchers conclude that this meant that hadrosaurs were nesting in the lowlands of the area represented by the strata containing the fossils and that previous workers hypotheses of lowland hadrosaur breeding were "confirmed."[17] It was the slow dissolution of shells left by clams and snails releasing calcium carbonate into the water that raised the water's pH high enough to prevent the eggshells from dissolving.[17] The no fragment's greatest dimension exceeds one cm.[17] The eggshells' surface has a pebbly texture.[17] Dinosaur Provincial Park eggshell is similar to the eggshells from the Two Medicine and Judith River Formations of Montana as well as eggshell from the Devil's Coulee in southern Alberta.[17] Most hadrosaur neonate bones are incomplete due to their small size and vulnerability to the high erosion rates in the Dinosaur Provincial Park.[17] Dentaries are common hadrosaur neonate fossils.[17] Most specimens don't preserve all of the tooth replacement gooves.[17] Most preserve "only about [ten] tooth files."[17] No neural arches are represented among the vertebrae fossils.[17] Although some limb bones show signs of transport wear, the distances traveled before burial were probably not far as such small bones would be unlikely to survive the great diversity of scavengers and acidic water conditions.[17] The breakdown of tannins from coniferous vegetation would have caused the pH of the waters in the park to be acidic.[17] Dinosaur eggshell is lacking from the bone beds producing the hadrosaur juvenile bones.[17]
The authors concluded that hadrosaurs nested in both upland or lowland area, although described factors influencing the division of breeding locations as unknown.[17] They suggested that "diet, soil conditions, habits, [and] competition" between dinosaur genera might have played roles.[17] Some of the less common hadrosaurs in the Dinosaur Park Formation of Dinosaur Provincial Park like Brachylophosaurus or Parasaurolophus may have had a more upland habitat where they may have nested or fed.[17]
The limbs of the juvenile hadrosaurs are anatomically and proportionally similar to those of adult animals.[17] However, the joints often show "predepositional erosion or concave articular surfaces."[17] Probably due to the cartilaginous cap covering the ends of the bones.[17] The pelvis of a young hadrosaur was similar to that of an older individual.[17]
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.[18]
Tiny hadrosaur footprints have been discovered in the Blackhawk Formation of Utah.[17]