Xiongguanlong (/ɕiʊŋˌkwaːnˈlʊŋ/) is an extinct genus of tyrannosauroid theropod from the Early Cretaceous period of what is now China. The type and only species is X. baimoensis. The generic name comes from Jiayuguan City (formerly called "Xiong Guan" or "grand pass") and the Mandarin word "long" which means dragon. The specific epithet, "baimoensis" is a latinization of the Mandarin word for "white ghost" in reference to one of the geological features of the type locality (nicknamed the "White Ghost Castle").
Discovery
Xiongguanlong was discovered in the upper member of the Xiagou Formation of the Xinminbao Group at a locality nicknamed the "White Ghost Castle" which is in the Yujingzi Basin of Gansu, China. This is the same locality from which the holotype of the ornithomimosaur Beishanlong was recovered. It was discovered in and prepared in 2006 and 2007 by Mark Norell, Peter Makovicky, and a team of scientists from Beijing University. In their original description, the authors hypothesize that it is from either the Aptian or Albian stages, but subsequent authors have constrained the upper member to the Aptian stage specifically. The holotype, and only specimen, of Xiongguanlong was given the designation FRDC-GS JB16-2-1 and was deposited at the Fossil Research and Development Center of the Gansu Provincial Bureau of Geo-Exploration and Mineral Development in Lanzhou, China. It consists of a complete skull (lacking the lower jaws), a complete series of cervical and dorsal vertebrae, a partial right ilium, and the right femur. It was initially discovered in the early 2000s, but it was not described until 2009 when a paper was published in the Proceedings of the Royal Society by the paleontologists Daqing Li, Mark Norell, Ke-Qin Gao, Nathan D. Smith, and Peter J. Makovicky. In its initial description, and in numerous publications since, it has been remarked as a noteworthy transitional species between the basal tyrannosauroids of the Jurassic period and the derived tyrannosaurids of the Late Cretaceous.
Description
The authors of its original description estimated that Xiongguanlong was about 5 metres (16 ft) long. They used a regression analysis based on the work of P. Christiansen and R.A. Fariña to estimate its mass at approximately 270 kilograms (600 lb). However, Gregory S. Paul estimated that it only weighed about 200 kilograms (440 lb). The holotype is believed to have been completely or almost completely, fully-grown due to the observed closure of the neurocentral sutures. Its estimated size is much greater than earlier tyrannosauroids such as Dilong and Guanlong, which led the authors to suggest that body size increase in tyrannosauroids was a continuous process throughout the Cretaceous period. Although most of the legs and tail are not preserved, it is believed to have had body proportions similar to Alioramus. Autapomorphies of the genus given by the authors in its description include the following: a very elongate pre-orbital region, a basicranium that is wider than it is long, the absence of pneumatic foramina on the lateral edge of the nasal bone, a ridge on the middle of the premaxillary teeth, and lateral processes in the axial neural spines. It is also distinguished from later tyrannosauroids by the lack of rugosities on the nasal bone and a lack of pneumatic elements of the quadrate bone.
Skull The skull of the holotype is preserved completely including the palate and the braincase, but lacks the mandible. It is very elongated in comparison to most other tyrannosauroids, with significant elongation of the maxillary, lacrimal, and the postorbital bones. Despite being relatively complete, the skull is not very well-preserved. It was fossilized in a mineralized nodule which led to difficulty in differentiating the bone from the matrix in which it was preserved. This led to difficulty in determining the overall topology of the skull bones and the obscuring of the existence of the maxillary fenestrae. However, numerous distinctive features are evident in spite of the preservation quality. One of the distinctive features of most tyrannosauroid skulls are the nasal bones, which are fused. Xiongguanlong also exhibits this trait, but it lacks the sagittal rugosities present in derived tyrannosaurids. The nasal also exhibits the advanced feature of lacking the pneumatic elements on the lateral sides of the nasal, which is seen in less-derived genera like Dilong and Eotyrannus. The rest of the skull is also characteristically compact. The prefrontal bone is small, triangular, and does not contact the orbit. The parietal bones are fused and possess a short and concave sagittal crest, which served as attachment sites for the bite muscles. The quadrate bone is also large and has expanded lateral condyles, which are suggested to be related to the jaw musculature as well. The postorbital bone is much anteroposteriorally elongated and vertical in relation to the infratemporal fenestra, which resembles advanced tyrannosaurids. The palate is also very robust due to a deep, blade-like vomer bone and large, blocky ectopterygoid bones. Some of the teeth are not very well-preserved, but the dental alveoli are visible. This led the authors of its description to determine that Xiongguanlong had four premaxillary teeth and fifteen maxillary teeth. The premaxillary teeth are characteristically d-shaped in cross-section and serrated, like most other tyrannosauroids. However, the maxillary teeth differ from more derived tyrannosauroids in being lateromedially narrow and more blade-like.
Post-cranial skeleton
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