Yutyrannus

Credit: Gabriel Ugueto.

*Yutyrannus* is an extinct genus of proceratosaurid theropod dinosaur that lived in China during the Early Cretaceous.

## History

*Y. hauli* was named by Xu Xing et al. (2012), derived from the Mandarin Chinese yǔ (羽), "feather" and the Latinized Greek tyrannos (τύραννος), "tyrant". This references how it was covered in feathers and its placement in tyrannosauroidea. The species epithet is from the Mandarin huáli (华丽 simplified, 華麗 traditional), "beautiful", which is due to the perceived beauty of the feathers.

*Y. hauli* consists of three fossils; an adult, a subadult and a juvenile. The fossil dealer who provided the remains claimed they were from a quarry at Batu Yingzi in Liaoning, China. However, *Yutyranus* was likely from a layer in the Yixian Formation, an Aptian formation dating 125 million years old. The specimens had been cut into slabs around the size of a bath mat, which were carried by two people.

ZCDM V5000 is the holotype and largest specimen. It is a near-complete skeleton with a skull, which had been compressed into the slab. It was an adult. The paratypes are ZCDM V5001 and ELDM V1001. The former is a smaller individual consisting of a skeleton found on the same slab as the holotype. The latter is a juvenile estimated to have died 8 years younger than the holotype.

These are part of the Zhucheng Dinosaur Museum and the Erlianhaote Dinosaur Museum collections, prepared by the Institute of Vertebrate Paleontology and Paleoanthropology, guided by Xu.

## Description

*Yutyrannus* were large and bipedal, the holotype growing 9 meters (29.5 feet) long and weighing 1414 kilograms (3117 pounds). Gregory S. Paul (2016) estimated Y. hauli was 7.5 meters (24.6 feet) long and 1.1 ton (1.2 short tons) heavy. Its skull is 905 millimeters (35.6 inches) long, with the paratypes being 80 centimeters (31 inches) and 63 centimeters (25 inches) long and their weights 596 kilograms (1314 pounds) and 493 kilograms (1087 pounds) respectively.

The original describes outlines diagnostic traits in *Yutyrannus*, which differ from related genera. The snout bears a high crest at the midline, which is formed by the nasals and premaxillae, and is covered in large pneumatic recesses. The postorbital has a small secondary process, which juts into the upper hind region of the eye socket. The postorbital's main body is hollowed at the outer side.

The external mandibular fenestra is located in the surangular. A 2018 study showed *Yutyrannus* had a simple hyoid, which suggests a flat tongue (similar to crocodiles). Based on comparisons of extinct and extant archosaurs, they determined all archosaurs would have had fixed, except for birds, pterosaurs and some ornithischians.

*Yutyrannus* contains direct evidence of feathers in the form of imprints. They grew to to 20 centimeters (7.8 inches) long and were filamentous. Due to the low-quality preservation, it could not be determined if they were compound, broad or narrow. They blanketed several areas of the body. The holotype contains plumage on the pelvis and near the feet. ZCDM V5000 had tail feathers that were angled backwards (under 30° of the tail axis). The smallest individual had 20-centimeter (7.9-inch)-long feathers at the base of the neck and 16-centimeter (6.3-inch)-long feathers on the upper arm. It was known since 2004, but the description of *Dilong* showed some tyrannosauroids bore filamentous stage 1 feathers. According to feather typology of Richard Prum, *Y. hauli* represents the largest dinosaur with confirmed plumage, up to 40x heavier than the previous record holder, *Beipiaosaurus*.

Study on the plumage distribution show they may have covered the entire body and regulated temperature, since the Yixian was relatively cold, having an average annual temperature of 10 °Celsius (50 °Fahrenheit). However, if they were restricted to where evidence was found, they may have been for display. Additionally, two adults have distinct "wavy" crests on their snouts with a high, central crest on both sides. These were likely used for display.

Plumage in this genus suggests later tyrannosaurids could have been feathered, despite their size, into adulthood. However, scaly skin impressions from various tyrannosaurids (*Gorgosaurus*, *Tarbosaurus*, *Tyrannosaurus*...etc.) have been reported where *Yutyrannus* would have had feathers. Since there is no good evidence for tyrannosaurids, it was suggested they may have secondarily evolved scales. If many later genera had scaly integument, then the extent and nature of integument may have changed over time after responding to climate, size or other factors.

*Yutyrannus* is made of many different-aged individuals, which allows experts to determine ontogeny. The feet, lower legs, ilia and forelimbs all shrink, and the skull grows more robust and deep. Andrea Cau (2021) speculates *Dilong* could be a juvenile of it or *Sinotyrannus*.

## Classification

All phylogenetic studies place *Yutyrannus* into Tyrannosauroidea. An initial analysis placed it as more primitive than *Eotyrannus*, but more advanced then *Dilong*, *Guanlong* and *Sinotyrannus*. Primitive traits include long forelimbs with three digits and a short foot unspecialized for running. Advanced traits include a large, deep skull, a large cuneiform horn at the lacrimal, a postorbital process at the back rim of the sockets, the squamosal and quadratojugal making a large process at the back rim of the infratemporal fenestra, short dorsals, an ilium with a straight upper rim and an appending lobe, a pubic foot that is large, an ischium that is slender and the outer surface of the premaxilla rotating upwards. Thomas Carr and Stephen Brusatte (2016) reexamined relationships in Tyrannosauroidea, finding *Yutyrannus* was more basal than *Dilong* and was a proceratosaurid:

## Paleoecology

Some paleontologists, including Xu Xing, think Yutyrannus hunted in packs because all three individuals were found closeby, with Xu speculating it hunted sauropods because sauropod material is known from the quarry, with all dying during a hunt. Other sauropod-hunters, such as *Mapusaurus*, hunted in packs. Their true cause of death is unknown, however. If *Yutyrannus* hunted sauropods, it would be 1/2 predators to have done so, the other being an unnamed theropod, who is known from a tooth embedded in *Dongbeititan*, catalogued as LDRCv2.

Because the locality of *Yutyrannus* is truly unknown, what fauna it coexisted with is uncertain. Yutyrannus has been estimated to be from the Lujiatun or Jianshangou beds. If it were, it would be contemporaneous with *Psittacosaurus*, *Dongbeititan*, *Sinosauropteryx* and *Caudipteryx*. *Lycoptera* would have been prevalent in the water. Volcanic eruptions and forest fires may have been common occurrences in the Yixian, being most comparable to the modern temperate rainforests of British Columbia. It experienced prominent seasonal change, specifically in temperature.

## Gallery

### Reconstructions

**Credit: Zhao Chuang.**ELDM V1001.
**Credit: Tom Parker.**
## References

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