Gilmoreosaurus (meaning "Charles Whitney Gilmore's lizard") is a genus of hadrosauromorphan dinosaur from the Late Cretaceous of Asia. The type species is Gilmoreosaurus mongoliensis, known from the Iren Dabasu Formation of Inner Mongolia, dating to 96 million years ago. Additional specimens have been described as distinct species, including G. atavus from the Khodzhakul Formation of Uzbekistan, and G. arkhangelskyi from the Bissekty Formation, also of Uzbekistan. However, these are based on very fragmentary remains, and their classification is dubious. An additional species, G. kysylkumense, also from the Bissekty Formation, is sometimes included, though it has also been referred to the related genus Bactrosaurus.
History of study Asian expeditions and Gilmore's "Mandschurosaurus mongoliensis"
In the early twentieth century, the American Museum of Natural History (AMNH) conducted a series of palaeontological expeditions to the deserts of Asia. One geologic formation where work was conducted during these expeditions was the Iren Dabasu Formation in Inner Mongolia, China. The fieldwork here was done by a field team led by Walter Granger. Nobody among them were specialists in dinosaurs; instead the AMNH had sent mostly geologists and mammal palaeontologists. Many dinosaur remains were discovered, but their nature would remain unknown to the discovers until experts back in America examined the material. The initial excavations of dinosaur fossils at Iren Dabasu took place on three days in April and May 1922. They conducted more extensive digs a year later from April 23 to June 1, 1923. Several new genera and species were discovered from the formation; among them were two new types of hadrosaur. It was these two that would later prove to be the most significant of the finds. They were among the first remains of the group from the continent, preceded only by Mandschurosaurus amurensis, Tanius sinensis, some remains from Turkestan and Saurolophus krischtofovici, with the validity of the latter species already doubted. The material that would eventually come to be known as Gilmoreosaurus was discovered by George Olsen (a member of Granger's team) in two different quarries (145 and 149, about apart), around northeast of the town of Erenhot and not far from the border with Mongolia. Several individuals were represented and between them most of the skeleton found, but the material was disarticulated and so what pieces belonged to each individual is not possible to determine for certain.
Once it had arrived back to New York, the Mongolian material was given to Barnum Brown to study. Brown was renowned for his field collection of dinosaurs, but he was too busy to attend the Asian expeditions. He did extensive work on the material and had prepared names for the new species, but for unknown reasons he ceased work on the project before its completion. Instead, the fossils were sent to the Smithsonian and would be described and named as new species by Charles W. Gilmore. The Smithsonian had little in terms of a dinosaur fossil collection and Gilmore as such was more reliant on the literature than first hand comparisons in his studies. Despite this, his work in the field is considered to be of high quality. He published his findings in 1933, describing both hadrosaurs as well as several other dinosaurs. One of the hadrosaurs, then thought to be a lambeosaur, was the novel genus Bactrosaurus. The second, known from at least four individuals and thought to be a hadrosaurine, was named as a new species of Mandschurosaurus, M. mongoliensis. This choice was made tentatively; Gilmore was mindful of the possibility his species may later require its own genus, but did not currently feel there was sufficient difference from M. amurensis for this. Gilmore's description of the species' anatomy was rather brief and would prove inadequate to understanding the taxon decades later; he provided no clear diagnosis (a list of what traits establish a species as different from its relatives). Another issue caused by Gilmore's work was that all the material, from all the individuals, was catalogued under a singular specimen number. Later palaeontologists thus had to recatalogue all of the elements under more modern standards.
Following its original naming, Gilmore's species would remain poorly understood and received limited attention from other palaeontologists. Despite this, the occasions it was discussed often brought major taxonomic changes. One persistent issue surrounding the taxonomy of the Iren Dabasu hadrosaurs was the nature of cranial material found at the Bactrosaurus quarry. Gilmore assumed, based on its association with the rest of the Bactrosaurus material, that it must belong to that genus; no other elements at the site were inconsistent with just one species being present. He was perplexed, however, by its lack of a hollow-crest, something thought universal among lambeosaurs. He did consider the possibility it belonged to M. mongoliensis, and also noted its resemblance to specimens of the genus Tanius, but eventually discarded both ideas. C. C. Young revived the latter idea, and assigned the material to Tanius. Regarding Mandschurosaurus he agreed with Gilmore's referral of his species to the genus, and went further to say it's possible it actually represents the same species as M. amurensis. Anatoly K. Rozhdestvensky, in the 1960s, considered a different solution; that Bactrosaurus was in fact a hadrosaurine, not a lambeosaurine. Furthermore, he thought that M. mongoliensis was merely a synonym of his hadrosaurine Bactrosaurus, a position he would continue to hold in future works. Future authors would disagree, reaffirming the two hadrosaurs as distinct and Bactrosaurus as a lambeosaur.
Reclassification as Gilmoreosaurus and subsequent research
The next major change in taxonomy would come from Michael K. Brett-Surman, who was in the process of a revision of the entire hadrosaur family. Firstly, he provided a novel diagnosis for the species in his 1975 master's thesis. However, Brett-Surman considered Mandschurosaurus amurensis to be dubious; he therefore thought it necessary to give Gilmore's species a new generic name. As such, he coined the genus Gilmoreosaurus in 1979, honouring Gilmore, and created the new combination Gilmoreosaurus mongoliensis. He considered Gilmore's two hadrosaurs to be of great importance, something he elaborated on in a short 1980 paper. He described the anatomy of Gilmoreosaurus as "partly iguanodont and partly hadrosaurine" and saw it as the ancestral form of hadrosaurine; Bactrosaurus, likewise, was seen as the ancestral lambeosaur. It was based in part on the two species, as well as on Probactrosaurus, that he concluded that hadrosaurs had originated in Asia. One thing that interested Brett-Surman greatly was the similarity of the Asian fauna to the later fauna of the North American Two Medicine Formation; Gilmore had, in his 1933 paper, made similar comments about the nature of Asian and American faunas. The 1980 paper noted that a lot of additional material from Iren Dabasu was never described by Gilmore, especially material pertaining to juvenile material. Brett-Surman reported that he and John R. Horner were at work investigating this material to form a more complete understanding of the formations' fauna. Referring to this, he concluded the paper by saying: "Although no more expeditions are possible to Mongolia by this time, light will be shed on the natural history of dinosaurs by further scientific expeditions to the basement of the natural history museum".
Halszka Osmólska and Teresa Maryańska published a paper in 1981 which focused on Saurolophus angustirostris, but also provided comments on Asian hadrosaurs as a whole. Among these comments, they returned to the issue of the skull material. They disagreed the otherwise monospecific nature of the quarry (i.e. only one species seeming to be present) was stronger evidence than the discordant anatomy of the material. As such, they revived Young's model and tentatively assigned the material to Tanius, which they felt it fit most closely. They noted the skull may prove to belong to either Gilmoreosaurus or Bactrosaurus after additional research. This would come to be in 1985, when Horner and David B. Weishampel would re-examine the material of both Iren Dabasu genera as well as material Gilmore never described, as had been indicated by Brett-Surman. Osmólska provided feedback on their study. The relevant new material was remains that the authors considered to be "clearly lambeosaurine" in nature, and so referrable to Bactrosaurus. As they could not possibly fit with the skull material, they concluded said material wasn't from the genus. Instead, they decided that it, congruent with its seemingly hadrosaurine nature, belonged to Gilmoreosaurus. At the end of the paper they reaffirmed the importance of both genera as the best record of early hadrosaurs.
The issue would finally be put to rest by Pascal Godefroit and colleagues in 1998, when they published a study describing newly discovered Bactrosaurus material. They returned to the original position of Gilmore, that the possibility of two taxa in the Bactrosaurus type quarry was extremely remote and that the skull material belonged not to Gilmoreosaurus but Bactrosaurus. The reasoning for the paradoxical nature of the anatomy was that Bactrosaurus was not a lambeosaur or even hadrosaurid at all but instead a more primitive form of hadrosauroid merely convergent with its later relatives. They remarked that it would be informative to test the phylogenetic affinities of Gilmoreosaurus as well, but that it was too poorly described to allow this. Modern review of the species was considered to be direly needed. Seeking to finally resolve the longstanding issue, Albert Prieto-Márquez and Mark A. Norrel began work on a thorough redescription of Gilmoreosaurus; this was published in 2010. Resolving its phylogenetic position was a priority and the ultimate motive behind the endeavor. All known material of the genus was examined for the study. One thing allowed by the redescription was a modern diagnosis of G. mongoliensis, as the 1975 one was found to be lacking. They also specifically touched on the idea the two Iren Dabasu hadrosaurs could be synonyms as Rozhdestvensky had thought. This idea was rejected, with numerous differences between the two found. Studies since the redescription have been able to include Gilmoreosaurus in research on hadrosaur phylogeny.
Description
Gilmoreosaurus was a relatively large-sized hadrosauroid which could grow up to long and obtain weights between . Genus List for Holtz 2012 Weight Information The genus can be distinguished from other hadrosauroid taxa in having a very reduced paddle-shaped postacetabular process and an asymmetrical manual phalanx III-1 end. Although the preserved elements are very sparse and probably representing more than one individual, the specimen AMNH FARB 30729 represents a single animal composed by a left ulna and radius.
Skull and dentition