Гиломозавр: История открытия и таксономические проблемы

Hadrosauroid dinosaur genus from the Late Cretaceous

Gilmoreosaurus (meaning "Charles Whitney Gilmore's lizard") is the name given to a genus of dinosaur from the Cretaceous of Asia. The type species is Gilmoreosaurus mongoliensis. It is believed to be a hadrosaur or iguanodont from the Iren Dabasu Formation of Inner Mongolia, dating to 96 Ma ago. Additional specimens have been described as distinct species, including G. atavus from the Khodzhakul Formation of Uzbekistan (120 Ma ago) and G. arkhangelskyi from the Bissekty Formation (89 Ma ago). 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.

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. 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. 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. 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. He considered Gilmore's two hadrosaurs to be of great importance, something he elaborated on in a short 1980 paper. 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. 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.

Gilmoreosaurus was a relatively large sized hadrosauroid which could grow up to long and obtain weights between 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.

A comprehensive study on the bone microstructure of Asian hadrosauroid dinosaurs, evaluating its implications for the knowledge of growth strategies and evolution of gigantism in hadrosauroids, was published by Slowiak and colleagues in 2020 revealing that several growth strategies facilitated gigantism. Several hadrosaurs were histologically analyzed, including Barsboldia, Gobihadros, Saurolophus and ZPAL MgD III/2 17, an indeterminate second species of hadrosauroid present in the Bayan Shireh Formation. The latter was slightly smaller than Gobihadros as seen on the developed external fundamental system (tightly spaced LAGs that indicate adulthood) on the tibia. This hadrosauroid however, shares several traits with Gilmoreosaurus—most likely due to the proximities of both Bayan Shireh and Iren Dabasu formations—and the phylogenetic analyses performed recovered both hadrosaur species in a polytomy along with Lophorhothon and the indeterminate hadrosauroid. This analysis suggests similar growth size and rates in these species. Slowiak and colleagues noted that the growth trend within the Hadrosauroidea featured two important adaptations of bone microstructure similar to those of Eusauropoda. The first important adaptation was the decrease of secondary remodelling in the tibia, allowing the bone to bear higher tensions. This feature is thought to represent a biomechanical response to their increasing body size evolution since the center of mass was concentrated on the pelvic area and therefore supported by well developed hindlimbs. This explains the slender forelimbs in most hadrosaurs. A second adaptation was the disappearance of lines of arrested growth (LAGs). This feature, also reported in giant sauropods, allowed uninterrupted growth during ontogeny.

In 2003, Rothschild and colleagues performed analyses in search for pathologies in several dinosaur specimens. Evidence of tumors, including hemangiomas, desmoplastic fibroma, metastatic cancer, and osteoblastoma were discovered in numerous specimens of Gilmoreosaurus by analyzing them through computerized tomography and fluoroscope screening. Several other hadrosaurids, including Brachylophosaurus, Edmontosaurus, and Bactrosaurus also tested positive. Although more than 10000 fossils were examined in this manner, the tumors were limited to Gilmoreosaurus and other hadrosaurs. The tumors were only found on caudal vertebrae and they may have been caused by environmental factors, such as the way in which these hadrosaurs interacted with their environments, making them more susceptible to get cancer, or, through genetic inheritance. Meaning that they carried cancer promoting genes. In 2007, Rothschild and Tanke reported the increased presence of osteochondrosis in hadrosaurs. Though they examined several other dinosaur groups, this disease appears to be particularly common in hadrosaurs, which were rapidly growing animals. The analysis was focused in pedal bones since osteochondrosis has not been reported from manual elements. Specimens of Edmontosaurus and Hypacrosaurus were more affected by osteochondrosis (5% and 2% of occurrence, respectively) but in hadrosauroids like Gilmoreosaurus and Bactrosaurus the disease was rather absent, likely due to the relatively low sample and therefore the presence of osteochondrosis in all hadrosaurs can not be discarded. These findings indicate that the skeleton of dinosaurs was primarily formed by cartilage.

Gilmoreosaurus is known from the Late Cretaceous Iren Dabasu Formation, which has been dated back to the Cenomanian stage around 95.8 ± 6.2 million years ago. During its times, the formation was dominated by braided floodplains and large meanders with some connections to the oceans. Extensive vegetation that maintained a great variety of herbivorous dinosaurs was also present on the formation as seen on the multiple skeletons of hadrosauroids, the prominent paleosol and the numerous palynological occurrences. Hadrosauroid remains are very common across the formation, which reflect these animals as wide ranging in both time and space. Bactrosaurus is another hadrosauroid from the formation, known from multiple specimens like Gilmoreosaurus. Ponderous herbivorous animals like Gilmoreosaurus and Bactrosaurus may have been medium to low level browsers. Fellow herbivores include the therizinosaurids Erliansaurus and Neimongosaurus. Gigantoraptor, a large oviraptorosaur from the formation, may have been an omnivore as suggested by the mandibular morphology or at least, a partial high browser. The geographic range of Gilmoreosaurus overlapped with the fast running tyrannosauroid Alectrosaurus, likely being the prey item for this theropod.