| Prosaurolophus | |
|---|---|
 | |
| P. maximus specimen collected 1921,Royal Ontario Museum | |
Scientific classification | |
| Kingdom: | Animalia |
| Phylum: | Chordata |
| Class: | Reptilia |
| Clade: | Dinosauria |
| Clade: | †Ornithischia |
| Clade: | †Ornithopoda |
| Family: | †Hadrosauridae |
| Subfamily: | †Saurolophinae |
| Genus: | †Prosaurolophus Brown,1916 |
| Species: | †P. maximus |
| Binomial name | |
| †Prosaurolophus maximus Brown, 1916 | |
| Synonyms[1][2][3] | |
| |
Prosaurolophus (/ˌproʊsɔːˈrɒləfəs/; meaning "beforeSaurolophus", in comparison to the later dinosaur with a similar head crest) is agenus ofhadrosaurid (or duck-billed)dinosaur from theLate Cretaceous ofNorth America. It is known from the remains of at least 25 individuals, including skulls and skeletons, but this remains obscure. Its fossils have been found in the lateCampanian-agedDinosaur Park Formation inAlberta, and the roughly contemporaneousTwo Medicine Formation inMontana, dating to around 75.7 to 74.1 million years ago. Its most recognizable feature is a small solid crest formed by thenasal bones, sticking up in front of the eyes. Thetype species isP. maximus, described by American paleontologistBarnum Brown of theAmerican Museum of Natural History in 1916. A second species,P. blackfeetensis, was described byJack Horner of theMuseum of the Rockies in 1992. However, subsequent research has foundP. blackfeetensis to be synonymous withP. maximus making the genus monotypic.
Well-knownpaleontologist Barnum Brown recovered a duckbill skull in 1915 for theAmerican Museum of Natural History (AMNH 5836) from theRed Deer River of Alberta, nearSteveville. He described the specimen in 1916 as a new genus,Prosaurolophus. Brown's choice of name comes from a comparison to the genusSaurolophus, which he had described in 1912.Saurolophus had a similar but longer and more spike-like head crest.[4] The skull had a damagedmuzzle and was inadvertently reconstructed too long,[5] but better remains were soon found that showed the true shape; one is a nearly complete skeleton and skull, described byWilliam Parks in 1924.[6] Twenty to twenty-five individuals are known for this species, including seven skulls with at least some of the rest of the skeleton.[7]
The second species,P. blackfeetensis, is based on a specimen in the Museum of the Rockies (MOR 454), which was described by another notable paleontologist, Jack Horner. This specimen, and the remains of three or four other individuals, were found inGlacier County, Montana.[8] In this case, the fossils were found in abonebed ofProsaurolophus remains, which indicates that the animals lived together for at least some time. The bonebed is interpreted as reflecting a group of animals that congregated near a water source during a drought.[9]
Horner differentiated the two species by details of the crest. He interpretedP. blackfeetensis as having a steeper, taller face thanP. maximus, with the crest migrating backward toward the eyes during growth.[8] More recent studies have regarded the differences as insufficient to support two species.[1][2][3]
Prosaurolophus was a large-headed duckbill; the most complete described specimen has a skull around 0.9 metres (3.0 ft) long with its body size measuring 8.5 metres (28 ft) in length and 3 metric tons (3.3 short tons) in body mass.[10][11] It had a small, stout, triangular crest in front of the eyes; the sides of this crest were concave, forming depressions.[12] This crest grew isometrically (i.e. without changing in proportion) throughout the lifetime of the individual, leading to speculation that the species may have had a soft tissue display structure, such inflatable nasal sacs.[13]
When originally described by Brown,Prosaurolophus maximus was known from a skull and jaw. Half of the skull was badly weathered at the time of examination, and the level of theparietal was distortedly crushed upwards to the side. The different bones of the skull could be easily defined, with the exception of the parietals andnasal bones. Brown found that the skull of the already described genusSaurolophus is very similar overall to, but also smaller than the skull ofP. maximus.[4] The unique feature of a shortened frontal inlambeosaurines is also found inProsaurolophus, and the other horned hadrosaurinesBrachylophosaurus,Maiasaura, andSaurolophus. Although they lack a shorter frontal, the generaEdmontosaurus andShantungosaurus share with saurolophins an elongateddentary.[7]
Patches of preserved skin are known from two juvenile specimens,TMP 1998.50.1 and TMP 2016.37.1; these pertain to the ventral extremity of the ninth through fourteenth dorsal ribs, the caudal margin of the scapular blade, and the pelvic region. Small basement scales (scales which makeup the majority of the skin surface[14]), 3–7 millimetres (0.12–0.28 in) in diameter, are preserved on these patches - this is similar to the condition seen in other saurolophine hadrosaurs. More uniquely, feature scales (larger, less numerous scales which are interspersed within the basement scales[14]) around 5 millimetres (0.20 in) wide and 29 millimetres (1.1 in) long are found interspersed in the smaller scales in the patches from the ribs and scapula (they are absent from the pelvic patches). Similar scales are known from the tail of the relatedSaurolophus angustirostris (on which they have been speculated to indicate pattern[14]), and it is considered likely adultProsaurolophus would've retained the feature scales on their flanks like the juveniles.[13]
Because of its name,Prosaurolophus is often associated withSaurolophus. However, this is contentious; some authors have found the animals to be closely related,[15][16][17] whereas others have not, instead finding it closer toBrachylophosaurus,Edmontosaurus,Gryposaurus, andMaiasaura.[7]
In 1918,Lawrence Lambe revised the classifications of Hadrosauridae (then Trachodontidae). He invalidated the family name and Trachodontinae, replacing them with Hadrosauridae and Hadrosaurinae. The other subfamily in Hadrosauridae then was Saurolophinae, which includedStephanosaurus (=Lambeosaurus),Cheneosaurus,Corythosaurus,Prosaurolophus, andSaurolophus. Lambe, in 1920, split Saurolophinae and found only two genera remaining in it,Prosaurolophus, and the type genus. The previous genera were then reclassified into Stephanosaurinae or Hadrosaurinae. In 1928,Prosaurolophus was assigned toSaurolophinae byFranz Nopcsa. The group contained hadrosaurids with a "males with median horn-like protuberance on the skull" and "very numerous teeth", found by Nopsca to beParasaurolophus,Saurolophus, andProsaurolophus.[3]
In 1954,Charles Sternberg reevaluated the genera in Hadrosauridae, invoking the probability that Saurolophinae should be sunk into Hadrosaurinae. This greatly changed the classifications of the family, as the "saurolophines" were kept separate because of their supposedly "footed"ischium. Sternberg identified that the "footed" ischium assigned toSaurolophus was not found with the holotype, and was only assigned to it because of the location of the find. Also, he noted thatWilliam Parks (1924) found a complete skeleton ofProsaurolophus clearly showing an "unfooted" ischium, which Sternberg realized meant that it was unlikely thatSaurolophus possessed a "footed" ischium. Sternberg's reevaluation led to the abandonment of Saurolophinae.[3]
Young (1958) found that the subfamily Saurolophinae, however, was not to be abandoned, and in it placed his new genusTsintaosaurus, as well asProsaurolophus andSaurolophus, and alsoKritosaurus (which includedGryposaurus and excludedK. navajovius). Two years previous,Friedrich von Huene separated Saurolophinae from Hadrosauridae, naming Saurolophidae. Saurolophidae was a family in Huene's Hadrosauria, including the generaProsaurolophus,Saurolophus, and the probably unrelatedBactrosaurus. Another author to support the separation of Saurolophinae wasJohn Ostrom (1961). Ostrom found that the saurolophinesBrachylophosaurus,Prosaurolophus, andSaurolophus all possessed a "pseudonarial crest", a feature which united them, while distinguishing them from hollow-crested lambeosaurines.[3]
Hopson (1975) supported the division of Hadrosauridae into two subfamilies, Hadrosaurinae and Lambeosaurinae, and was first to suspect what modern analyses find. Hopson found that Hadrosaurinae could clearly be divided into groups, the "kritosaurs", the "edmontosaurs", and the "saurolophines", includingProsaurolophus,Saurolophus,Tsintaosaurus andLophorhothon, and intermediate between the "kritosaurs" and "saurolophines". Brett-Surman (1975) also followed Sternberg with sinking Saurolophinae into Hadrosaurinae, and like Hopson, he recognized three groups within the subfamily. Like Hopson, one group was called theEdmontosaurus lineage, the second theKritosaurus group, and the third unitingProsaurolophus andSaurolophus. Over a decade later in 1989, Brett-Surman scientifically named the groups of hadrosaurines, the first becomingEdmontosaurini, the secondKritosaurini, and the thirdSaurolophini.[3]
The first cladistic analysis to encompass the interrelationships of Hadrosauridae was conducted by Weishampel and Horner (1990). They found Saurolophinae synonymous with Hadrosaurinae, but only separated the subfamily into two groups. The first group includedGryposaurus,Aralosaurus,Maiasaura, andBrachylophosaurus. The other containedEdmontosaurus,Saurolophus,Prosaurolophus,Lophorhothon, andShantungosaurus.[3]
A detailed cladgram of hadrosaurid relationships was published in 2013 byActa Palaeontologica Polonica. The study was led byAlberto Prieto-Márquez, and recoveredProsaurolophus in a similar position as suggested by Brown in 1916. The below cladogram was the one recovered by their analysis:[17]
| Saurolophinae |
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In 2001,Prosaurolophus was studied with other hadrosaurids by Wagner. The genus, along withCorythosaurus andMaiasaura, were considered by Wagner to be synonymous withSaurolophus,Hypacrosaurus andBrachylophosaurus respectively.Prosaurolophus maximus was reassigned toSaurolophus asS. maximus. The same year however,Prosaurolophus was found to be distinct fromSaurolophus, in an analysis of Huet al.. Their analysis was unique from any of the time, and they recoveredProsaurolophus in Saurolophinae, withSaurolophus,Lophorhothon,Tsintaosaurus,Jaxartosaurus, andKritosaurus. No other analysis has recovered this group of dinosaurs.[3]
Horneret al. (2004) also recovered a different phylogeny of Saurolophinae.Prosaurolophus was, for the first time, recovered separate fromSaurolophus, in fact not even closely related.Prosaurolophus was found in a group withBrachylophosaurus,Maiasaura,Grpyosaurus, andEdmontosaurus, whileSaurolophus was grouped withNaashoibitosaurus (=Kritosaurus) and "Kritosaurus"australis.[3]
TheProsaurolophus-Saurolophus clade has been a problematic grouping when trying to place among hadrosaurines. Many skull features are similar toEdmontosaurus, while other are closer toGryposaurus, so the group has been classified as close to both. However, the clade might be closer toEdmontosaurus, as the features are more numerous uniting them.[3]
As a hadrosaurid,Prosaurolophus would have been a largeherbivore, eatingplants with a sophisticated skull that permitted a grinding motion analogous tochewing. Itsteeth were continually replaced and packed intodental batteries that contained hundreds of teeth, only a relative handful of which were in use at any time. Plant material would have been cropped by its broad beak, and held in the jaws by acheek-like structure. Feeding would have been from the ground up to around 4 meters (13 ft) above. Like other hadrosaurs, it could have moved bothbipedally andquadrupedally.[7] Comparisons between thescleral rings ofProsaurolophus and modern birds and reptiles suggest that it may have beencathemeral, active throughout the day at short intervals.[18]
As noted, there is bonebed evidence that this genus lived in groups during at least part of the year.[9] Additionally, it had several potential methods for display in a social setting. The bony facial crest is an obvious candidate, and nasal diverticula may also have been present. These postulateddiverticula would have taken the form of inflatable soft-tissue sacs housed in the deep excavations flanking the crest and elongate holes for thenostrils. Such sacs could be used for both visual and auditory signals.[19]
The Dinosaur Park Formation, home toProsaurolophus maximus, is interpreted as a low-relief setting ofrivers andfloodplains that became moreswampy and influenced bymarine conditions over time as theWestern Interior Seawaytransgressed westward.[20] The climate was warmer than present-day Alberta, withoutfrost, but with wetter and drier seasons.Conifers were apparently the dominantcanopy plants, with anunderstory offerns,tree ferns, andflowering plants.[21] In this well-studied formation,P. maximus is only known from the upper part, which had more of a marine influence than the lower section. It was the most common hadrosaurine of this section, which was deposited about 75.5 million years ago.[2] The Dinosaur Park Formation was also home to well-known dinosaurs like thehornedCentrosaurus,Styracosaurus, andChasmosaurus, fellow duckbillsGryposaurus,Corythosaurus,Lambeosaurus, andParasaurolophus,tyrannosauridGorgosaurus, andarmoredEdmontonia andEuoplocephalus.[22]
The roughly contemporaneous Two Medicine Formation, home toP. maximus,[2] is well known for its fossils of dinosaur nests, eggs, and young, produced by the hadrosauridsHypacrosaurus stebingeri andMaiasaura, and thetroodontidTroodon. The tyrannosauridDaspletosaurus,caenagnathidChirostenotes,dromaeosauridsBambiraptor andSaurornitholestes, armored dinosaursEdmontonia andEuoplocephalus,hypsilophodontOrodromeus, and horned dinosaursAchelousaurus,Brachyceratops,Einiosaurus, andStyracosaurus ovatus were also present.[22] This formation was more distant from the Western Interior Seaway, and higher and drier than the Dinosaur Park Formation.[9] The age ofProsaurolophus maximus remains from this formation is from approximately 75.5 to 74.0 million years ago.[2]
Prosaurolophus maximus itself lived in coastalfloodplains fed on theconifer trees and thetree bark in the area, implying that it was more likely a browser rather than a grazer.[23][24][25]
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