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 1 kilometre (0.62 miles) apart), around 14 kilometres (8.7 miles) 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
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