Sinotyrannus Temporal range: Early Cretaceous, 120 Ma PreꞒ Ꞓ O S D C P T J K Pg N ↓
Skeletal diagram of Sinotyrannus based on Yutyrannus and other proceratosaurids
Scientific classification
Domain:	Eukaryota
Kingdom:	Animalia
Phylum:	Chordata
Clade:	Dinosauria
Clade:	Saurischia
Clade:	Theropoda
Family:	†Proceratosauridae
Genus:	†Sinotyrannus Ji et al., 2009
Species:	†S. kazuoensis
Binomial name
†Sinotyrannus kazuoensis Ji et al., 2009

Sinotyrannus (meaning "Chinese tyrant") is a genus of large basal tyrannosauroid^[1] dinosaur, known from a single incomplete fossil specimen including a partial skull, from the Early Cretaceous Jiufotang Formation of Liaoning, China. Specifically, it is a member of the Proceratosauridae, a family that originated in the Jurassic whose members are known from Europe and Asia. Though it is not significantly younger than primitive tyrannosauroids such as Dilong, it is similar in size to later forms such as Tyrannosaurus. It was much larger than contemporary tyrannosauroids, reaching a total estimated length of 9–10 m (30–33 ft), and was the largest known theropod from the Jiufotang Formation. The type species, S. kazuoensis, was described by Ji et al. in 2009.^[2]

Description

Sinotyrannus was a large tyrannosaur, measuring approximately 9–10 metres (30–33 ft) long and weighing 2.5 metric tons (2.8 short tons).^[2][3] It was among the largest basal tyrannosauroids known, repudiating the previously presumed trend that tyrannosauroids gradually increased in size throughout the Cretaceous period from small basal forms like Dilong to advanced apex predators such as Tyrannosaurus. The holotype, KZV-001, consists of a disarticulated partial skeleton including the front portion of the skull, three dorsal vertebrae, incomplete ilia, three articulated manual phalanges (including an ungual), and other fragmentary bones.^[2]

The described cranial elements include the premaxillae, dentary, and anterior portions of the maxillae and nasals. 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 of denticles 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.^[2][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.^[2]

Classification

The original description of Sinotyrannus proposed that it could have been the earliest tyrannosaurid due to its large size,^[2] but subsequent analyses place it as a proceratosaurid tyrannosauroid. A 2013 study by Loewen et al. considered it to be part of a clade containing Juratyrant and Stokesosaurus, as they all reputedly share a narrow preacetabular notch. The results of their phylogenetic analyses are shown in the cladogram below:^[4]

However, Brusatte et al. (2016) instead placed Juratyrant and Stokesosaurus outside of the Proceratosauridae, and proposed that Sinotyrannus represents the sister taxon of Yutyrannus within proceratosauridae. The results of their phylogenetic analyses are shown in the cladogram below:^[5]