Yutyrannus

Fossils on display

Yutyrannus huali was named and scientifically described in 2012 by Xu Xing et al. The name is derived from the Mandarin Chinese word yǔ (羽, "feather") and the Latinised Greek word tyrannos (τύραννος, "tyrant"), which is a reference to its classification as a feathered member ofTyrannosauroidea. The specific name consists of the Mandarin word huáli (华丽 simplified, 華麗 traditional, "beautiful"), in reference to the perceived beauty of its plumage.[1]

Yutyrannus is known from three nearly complete fossil specimens, those being an adult, a subadult, and a juvenile. They were acquired from a fossil dealer who claimed that all three had their provenance in a single quarry at Batu Yingzi in Liaoning Province, China. They were thus probably found in a layer of the Yixian Formation, dating from the Aptian, approximately 125 million years old.[1] The specimens had been cut into several pieces about the size of bath mats so that they could be carried by two people.[3]

The holotype, ZCDM V5000, is the largest specimen and consists of a nearly complete skeleton with a skull of an adult individual. The paratypes are the two other specimens. ZCDM V5001 consists of the skeleton of a smaller individual and part of the same slab as the holotype. ELDM V1001 is a juvenile estimated to have been eight years younger than the holotype. The fossils are part of the collections of the Zhucheng Dinosaur Museum and the Erlianhaote Dinosaur Museum, but have been prepared by the Institute of Vertebrate Paleontology and Paleoanthropology under the guidance of Xu.[1]

A size comparison to a human

Yutyrannus was a fairly large bipedal predator. The holotype, and oldest known specimen, has an estimated length of 29.5 feet / 9 meters and an estimated weight of about 3,117 lbs / 1.4 tons.[1] In 2016, Gregory S. Paul gave lower estimations of 24.6 feet / 7.5 meters long and 2645 lbs / 1.2 tons.[4] Its skull has an estimated length of 35.6 inches / 0.9 meters. The skulls of the paratypes are 31 meters / 0.79 meters and 25 inches / 0.64 meters long and their weights have been estimated at 1,314 lbs / 0.6 tons and 1,087 lbs / 0.46 tons, respectively.[1]

The describers established some diagnostic traits of Yutyrannus, in which it differs from its direct relatives. The snout features a high midline crest, formed by the nasals and the premaxillae and which is covered by large pneumatic recesses. The postorbital bone has a small secondary process, jutting into the upper hind corner of the eye socket. The outer side of the main body of the postorbital is hollowed out. In the lower jaw, the external mandibular fenestra, which is the main opening in the outer side, is mainly located in the surangular.[1]

Feathers

An artist's restoration based on ELDM V1001

The described specimens of Yutyrannus contain direct evidence of feathers in the form of fossil imprints. The feathers were long, up to 7.9 inches / 20 cm, and filamentous. Because the quality of the preservation was low, it could not be established whether the filaments were simple or compound, broad or narrow. The feathers covered various parts of the body and the holotype’s were present on the pelvis and near the foot. Specimen ZCDM V5000 had tail feathers pointing backwards under an angle of 30 degrees with the tail axis. The smallest specimen showed 7.9 inch / 0.2 cm filaments on the neck and 6.3 inch / 16 cm feathers at the upper arm.[1] While it has been known since 2004, thanks to the description of Dilong, that some tyrannosauroids possessed filamentous "stage 1" feathers,[5] according to the feather typology of Richard Prum, Y. huali is currently the largest known species of dinosaur with direct evidence of feathers and is forty times heavier than the previous record holder, Beipiaosaurus.[1][6]

A tail fragment with feather traces

Based on the distribution of the feathers, they may have covered the entire body and served in regulating temperature, given the rather cold climate of the Yixian with an average annual temperature of 50° F. Alternatively, if they were restricted to the regions in which they were found, they may have served as display structures too. In addition, the two adult specimens had distinctive, "wavy" crests on their snouts, on both sides of a high central crest, which were also probably used for display. The presence of feathers on a large basal tyrannosauroid suggests the possibility that later tyrannosaurids were also feathered, even when adults, despite their size.[1] However, scaly skin impressions have been reported from various Late Cretaceous tyrannosaurids (such as Gorgosaurus, Tarbosaurus and Tyrannosaurus) on parts of the body where Yutyrannus was feathered. Since there is no positive evidence for plumage in tyrannosaurids, some researchers have suggested they may have evolved scales secondarily.[7] If scaly skin was the dominant epidermal trait of later genera, then the extent and nature of the integumentary covering may have changed over time in response to an increased body size, a warmer climate, or other factors.[1]

It's possible that the integumentary structures of Yutyrannus might not represent true feathers, instead representing filamentous structures that are the ancestral state to feathers.[8] However, this hypothesis has been overturned with subsequent research that verified that the structures on various dinosaurs and pterosaurs were true feathers.[1][6]

The discovery of feathers may shed light on what colour Yutyrannus was, as the same has been done with other fossilised dinosaurs with feathers such as Microraptor and Archaeopteryx, however this study has not been completed as of recently.[9]

To this day, all phylogenetic analyses of Yutyrannus relationships have classified Yutyrannus within Tyrannosauroidea. An initial analysis of its relationship to other tyrannosauroids showed that it was more primitive than Eotyrannus in the evolutionary tree, but more advanced than earlier tyrannosauroids, like Dilong, Guanlong and Sinotyrannus. Primitive traits relative to advanced tyrannosaurs included long arms with three fingers and a short foot which was not specialized for running. Advanced traits included a large and deep skull, the outer side of the premaxilla having rotated upwards, a large cuneiform horn on the lacrimal in front of the eye socket, a postorbital process on the back rim of the eye socket, the squamosal and the quadratojugal forming a large process on the back rim of the infratemporal fenestra, short dorsal vertebrae, an ilium with a straight upper rim and an appending lobe, a large pubic boot, and a slender ischium.[1]

A close up of a snout

In 2016, a phylogenetic analysis conducted by Thomas Carr and Stephen Brusatte re-examined the evolutionary relationships of Tyrannosauroidea. Their analysis found Yutyrannus to be more basal than Dilong, placing it within the family Proceratosauridae.[10] Their cladogram is shown below:

Because the locality of Yutyrannus is uncertain, it is unknown what fauna it coexisted with. Age estimates point towards Yutyrannus originating from the Lujiatun or the Jianshangou beds of the Yixian, meaning it would have been contemporaneous of dinosaurs like Psittacosaurus, Dongbeititan, Sinosauropteryx, and Caudipteryx. Fish, such as Lycoptera, would also have been prevalent here. Volcanic eruptions and forest fires appear to have been common in the Yixian and the environment would have been littered with bodies of water and coniferous plants. The environment would have been comparable to the modern day temperate rainforests of British Columbia and would have experienced significant seasonal changes in temperature.[15]

• Timeline of tyrannosaur research

• T. rex relative is biggest-ever feathered animal, from BBC News

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