Cryolophosaurus

Cryolophosaurus (/ˌkraɪ.ɵˌlɒfɵˈsɔrəs/ or /kraɪˌɒlɵfəˈsɔrəs/; "CRY-oh-loaf-oh-SAWR-us") is a genus of large theropods known from only a single species Cryolophosaurus ellioti, known from the early Jurassic period of Antarctica. It was about 6.5 metres (21 ft) long and 460 kilograms (1,010 lb) in weight, making it one of the largest theropods of its time. Individuals of this species may have grown even larger, because the only known specimen probably represents a sub-adult.

C. ellioti possessed a distinctive crest on its head that spanned the head from side to side, similar to a Spanish comb. The shape of the crest has also been compared to a Pompadour hairstyle like the one famously worn by Elvis Presley, earning the species the nickname "Elvisaurus" before it was officially named.[1] Based on evidence from related species and studies of bone texture, it is thought that this bizarre crest was used for intra-species recognition.

Cryolophosaurus is known from a skull, a femur and other material, the skull and femur of which have caused its classification to vary greatly. The femur possesses many primitive characteristics that have classified Cryolophosaurus as a dilophosaurid or a neotheropod outside of Dilophosauridae and Averostra, where as the skull has many advanced features, leading the genus to be considered a tetanuran, an abelisaurid, a ceratosaur and even an allosaurid. Since its original description, the consensus is that Cryolophosaurus is either a primitive member of the Tetanurae or a close relative of that group.

Cryolophosaurus was first excavated from Antarctica's Early Jurassic, Sinemurian to Pliensbachian aged Hanson Formation, formerly the upper Falla Formation, by paleontologist Dr. William Hammer in 1991. It was the first carnivorous dinosaur to be discovered in Antarctica and the first non-avian dinosaur from the continent to be officially named. The sediments in which its fossils were found have been dated at ~194 to 188 million years ago, representing the Early Jurassic Period.

Description
The holotype FMNH PR1821 is the only fully described specimen of Cryolophosaurus. The specimen consists of an incomplete skull and mandibles lacking most of their front half; nine maxillary teeth; a fragmentary sixth cervical centrum; cervical vertebrae 7-10; several posterior cervical ribs; several anterior dorsal vertebrae; most mid and posterior dorsal vertebrae; several dorsal ribs; the fifth sacral vertebrae; three chevrons; many partial and complete caudal vertebrae and centra; two partial humeri; a proximal radius; a proximal ulna; a partial ilium; a proximal pubis; both ischia, but only one distal; two incomplete femora; the distal end of a tibia; the distal end of a fibula, and the astragalus and calcaneum.[2] In 2013, new material of Cryolophosaurus was unearthed in Antarctica. The description of this material has not yet been published in a non-abstract form.[3]

Cryolophosaurus was a large, well-built theropod, one of the largest of its time. The genus has been described by Roger Benson and colleagues (2012) as a top predator in Antarctica. It had slender proportions.[4] Cryolophosaurus was estimated as being 6 to 7 m (19.7 to 23.0 ft) in length by William R. Hammer & William J. Hickerson (1998).[5] A 2007 study by Nathan Smith et al. revised the length to 6.5 m (21.3 ft).[2] Its weight estimated at 465 kilograms (1,025 lb).[5][2] Based on these length and weight estimates, Cryolophosaurus is currently the largest known Early Jurassic theropod.[4][6] Smith et al. (2007b) and Benson et al. (2012) noted that the holotype individual probably represents a sub-adult, so adults could have been larger.

Skull
The holotype of Cryolophosaurus consists of a high, narrow skull, which was discovered articulated with the rest of the skeleton.[5] The skull is an estimated 65 centimetres (26 in) long. It has a peculiar nasal crest that runs just over the eyes, where it rises up perpendicular to the skull and fans out. It is thin and highly furrowed, giving it a Spanish comb-like appearance. The crest is an extension of the skull bones, near the tear ducts, fused on either side to orbital horns which rise from the eye sockets. While other theropods like the Monolophosaurus have crests, they usually run along the skull instead of across it.[7]

An unpublished study conducted by Vernon Meidlinger-Chin in 2013 suggested that previous studies lacked focus on endocranial details. The study found that the Cryolophosaurus fossil has a nearly complete, undistorted cranial cavity which is complete enough to give an approximate shape and size of the living brain. The endocast features clarified the dissimilarity of the skull with those of Allosauroids and Coelurosaurs giving Cryolophosaurus a basal position in Theropoda.

Classification
Classification of Cryolophosaurus is difficult because it has a mix of primitive and advanced characteristics. The femur has traits of early theropods, while the skull resembles much later species of the clade Tetanurae, like China's Sinraptor and Yangchuanosaurus. This led Paul Sereno et al. (1994) to place Cryolophosaurus in the taxon Allosauridae.[8] Originally, Hammer and colleagues suspected that Cryolophosaurus might be a ceratosaur or even an early abelisaur, with some traits convergent with those of more advanced tetanurans, but ultimately concluded that it was itself the earliest known member of the tetanuran group.[9] While a subsequent study by Hammer (along with Smith and Currie) again recovered Cryolophosaurus as a tetanuran, a later (2007) study by the same authors found that it was more closely related to Dilophosaurus and Dracovenator.[7][10] Sterling Nesbitt et al. (2009), using the characters of Tawa found Cryolophosaurus to be a neither dilophosaurid nor averostran neotheropod but instead the sister group of a clade composed of dilophosaurids and averostrans.[11] However, in 2012, Matthew Carrano found that Cryolophosaurus was a tetanuran, related to Sinosaurus, but unrelated to Dilophosaurus.

The following cladogram is based on the phylogenetic analysis conducted by Carrano in 2012, showing the relationships of Cryolophosaurus:

Distinguishing anatomical features
The following distinguishing characters were identified by Smith et al. (2007b) in a description on the osteology of Cryolophosaurus: the presence of a large, anterodorsally curving midline crest on the top of the skull, with fluted rostral and caudal surfaces formed by dorsal expansions of the lacrimal bones; a complete constriction across the infratemporal fenestra formed by the squamosal and jugal; and the presence of extremely elongate cranial processes on the cervical ribs.[2]

In addition, Smith et al. (2007) found that Cryolophosaurus can be distinguished based on the following cranial and postcranial features: an antorbital fossa extending onto the lateroventral nasal side; nasolacrimal crests expanding onto the anterior end of the jugal; a deep surangular; a pendant medial process on the articular; amphicoelous cervical centra; a sigmoidal femur with an anteromedially directed head; and a low triangular astragalar ascending process.