| Sinotyrannus | |
|---|---|
 | |
| Reconstructed skeleton | |
Scientific classification | |
| Kingdom: | Animalia |
| Phylum: | Chordata |
| Class: | Reptilia |
| Clade: | Dinosauria |
| Clade: | Saurischia |
| Clade: | Theropoda |
| Superfamily: | †Tyrannosauroidea |
| Family: | †Proceratosauridae |
| Genus: | †Sinotyrannus Ji et al., 2009 |
| Type species | |
| †Sinotyrannus kazuoensis Ji et al., 2009 | |
Sinotyrannus (meaning "Chinese tyrant") is an extinctgenus of largetyrannosauroid theropod dinosaurs known from theEarly CretaceousJiufotang Formation ofLiaoning, China. This genus contains asingle species,Sinotyrannus kazuoensis, known from a single mature specimen including a partial skull, some vertebrae, and part of the pelvic girdle.
While it exhibited greater body size that would put it on par with the latertyrannosaurids such asTyrannosaurus (a group thatSinotyrannus was initially suspected of being in),Sinotyrannus was more likely a member of the familyProceratosauridae, basal within or closely related to tyrannosauroids. This family originated in theJurassic, and its members are known from Europe and Asia.Sinotyrannus and its coeval relativeYutyrannus appear to have been surprisingly large compared to most of their Early Cretaceous relatives, such asDilong. Most of the world during the Early Cretaceous was dominated by more basaltetanurans, such as themegalosauroids andallosaurs, with tyrannosaurids themselves only taking over after both groups started to decline. However,Sinotyrannus andYutyrannus appear to be exceptions to this.
Sinotyrannus was the largest theropod in the Jiufotang Formation, reaching up to 10 metres (33 ft) in length. The smaller theropodHuaxiagnathus from the olderYixian Formation, traditionally regarded as a "compsognathid" and known only from immature specimens, may be an immature form ofSinotyrannus. If this hypothesis is supported,Sinotyrannus would be ajunior synonym ofHuaxiagnathus.
Sinotyrannus was a large tyrannosaur, measuring approximately 9–10 metres (30–33 ft) long and weighing 2.5 metric tons (2.8 short tons).[1][2][3] It is among the largest basaltyrannosauroids known, repudiating the previously presumed trend that tyrannosauroids gradually increased in size throughout the Cretaceous period from small basal forms likeDilong to advancedapex predators such asTyrannosaurus. The holotype, KZV-001, consists of a disarticulated partial skeleton including the front portion of the skull, three dorsal vertebrae, incompleteilia, three articulated manual phalanges (including an ungual), and other fragmentary bones.[1]
The described cranial elements include thepremaxillae,dentary, and anterior portions of themaxillae andnasals. The dorsal margin of the maxilla is unusually concave unlike the convex condition in tyrannosaurids. The nares are large and elliptical, supporting its relation to proceratosauridae. The dentary gradually curves upwards as it approaches its front edge. Many teeth are preserved attached to the maxillae, with a roughly equal number ofdenticles on each side, similarly to those of tyrannosaurids.Sinotyrannus could perceivably have had a tall nasal crest like other proceratosaurids, although not enough of its nasals are preserved to be certain.[1][4]
The three preserved vertebrae have very tall neural spines. The proportions of the preserved manual phalanges support the idea that they belong to the second finger, and the ungual has a deep groove on each side. The ilia are mainly present as molds, with the mold of the external side of the left ilium being the most complete. The preacetabular blade is short and wide, with a massive pubic peduncle, while the postacetabular blade is longer and thinner, with a triangular ischial peduncle. These traits of the ilia differentiate it from more advanced tyrannosauroids such as the tyrannosaurids.[1]
Sinotyrannus was likely an active predator, as has been speculated for most tyrannosaurs, and may have been covered in a simple feathery coat as was the case with most basalcoelurosaurs, including many early tyrannosaurs.[5]
The original description ofSinotyrannus proposed that it could have been the earliesttyrannosaurid due to its large size,[1] but subsequent analyses place it as a proceratosaurid tyrannosauroid. A 2013 study by Loewen and colleagues considered it to be part of a clade containingJuratyrant andStokesosaurus within theProceratosauridae, as they all reputedly share a narrow preacetabular notch. The results of theirphylogenetic analyses are shown in thecladogram below:[4]
However, Brusatte et al. (2016) instead placedJuratyrant andStokesosaurus outside of the Proceratosauridae, and proposed thatSinotyrannus represents thesister taxon ofYutyrannus within Proceratosauridae. The results of their phylogenetic analyses are shown in the cladogram below:[6]
Naish & Cau in 2022 foundSinotyrannus within Proceratosauridae but placedYutyrannus among the more advanced Tyrannosauroidea.[7]
In 2024,Andrea Cau published a comprehensive theropodphylogenetic framework that could be used to identify immature specimens of other taxa.[8] The following year, Cau and Paterna used an updated version of this dataset to deduce thatSinotyrannus—known from a specimen of an adult individual—was a mature form of the "compsognathid"Huaxiagnathus, which is known only from two immature specimens. Both species share a dorsally convexilium with a short preacetabular process without an anteroventral hook. Other differences observed between the two species are likely the result of differences in body size and ontogenetic stage, consistent with the more established growth series of other tyrannosauroids likeTyrannosaurus. SinceHuaxiagnathus was named beforeSinotyrannus, this genus name holds priority, makingSinotyrannus ajunior synonym of the former. In their phylogenetic tree, the combinedSinotyrannus+Huaxiagnathusoperational taxonomic unit (OTU) was recovered as the sister taxon toEotyrannus in a clade with other traditional proceratosaurids.[9]
Sinotyrannus lived in a rather wet environment with well-watered forests and large lakes. The area experienced great diversity in terms of vertebrate fauna, with many taxa, such asMicroraptor, living alongsideSinotyrannus. Despite this, the area was quite prone to high volcanism.[10]
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