Cryolophosaurus

The unrestored holotype of Cryolophosaurus, FMNH PR1821.

Cryolophosaurus originally was collected during the 1990–91 austral summer on Mount Kirkpatrick in the Beardmore Glacier region of the Transantarctic Mountains. The discovery was made by Hammer, a professor at Augustana College, and his team. The fossils were found in the siliceous siltstone of the Hanson Formation, formerly the upper Falla Formation, and dated to the Pliensbachian Stage of the early Jurassic. Cryolophosaurus was the second dinosaur, and first theropod, to be discovered in Antarctica. It was discovered after Antarctopelta, but named earlier.[2]

In 1991, both Hammer and the Ohio State University geologist David Elliot excavated separate outcrops near Beardmore Glacier, sharing logistical expenses. Elliot's team first came across the remains of Cryolophosaurus in a rock formation around the altitude of 4,000 m (13,000 ft) high and about 640 km (400 mi) from the South Pole. When the discovery was made, they soon notified Hammer. Over the next three weeks, Hammer excavated 2,300 kg (5,100 lb) of fossil-bearing rock. The team recovered over 100 fossil bones, including those of Cryolophosaurus.[2] The specimens were formally named and described in 1994 by Hammer and Hickerson, in the journal Science.[2]

During the 2003 season, a field team returned and collected more material from the original site. A second locality was discovered about 30 metres (98 ft) higher in the section on Mt. Kirkpatrick.[3]

The name Cryolophosaurus ellioti is derived from the Greek words κρυος (meaning 'cold' or 'frozen', in reference to its discovery in Antarctica), λοφος (meaning 'crest') and σαυρος (meaning 'lizard'), thus "cold crest lizard". Hammer and Hickerson named the species C. ellioti, after David Elliot, who had made the initial discovery of the fossils.[2]

Cryolophosaurus restored with scales

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.[4] 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.[5]

Size comparison

Cryolophosaurus was a large, well-built theropod, one of the largest of its time. The holotype specimen is estimated to have reached 6–7 m (20–23 ft) long and weighed 350–465 kg (772–1,025 lb).[6][4][7] Some researchers noted that the holotype individual probably represents a sub-adult, so adults could have been larger.[4][8] Despite having slender proportions, Cryolophosaurus is one of the largest known Early Jurassic theropods.[8][9]

Skull

Reconstructed skull

The holotype of Cryolophosaurus consists of a high, narrow skull, which was discovered articulated with the rest of the skeleton.[6] 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 unique "pompadour" appearance and earned it the nickname "Elvisaurus."[10] 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.[11]

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.[9] Closer examination of how the skull bones fused together reviewed details in the snout and forehead that are especially similar to Dilophosaurus.[12]

Fossil pelvis of Cryolophosaurus. The loop at the widest part of the pubis is large compared to later theropods.

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.[13] 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.[2] 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.[11][14] Sterling Nesbitt et al. (2009), using the characters of Tawa found Cryolophosaurus to be neither a dilophosaurid nor averostran neotheropod but instead the sister group of a clade composed of dilophosaurids and averostrans.[15] However, in 2012, Matthew Carrano found that Cryolophosaurus was a tetanuran, related to Sinosaurus, but unrelated to Dilophosaurus.[16] In 2020, a monograph of Dilophosaurus found Cryolophosaurus to be a derived neotheropod, close to Averostra, in a more derived position than Zupaysaurus, but less than Dilophosaurus.[17]

The following cladogram illustrates a synthesis of the relationships of the early theropod groups compiled by Hendrickx et al. in 2015.[18]

Restoration of a Cryolophosaurus in its environment

However, a 2020, a study conducted Adam Marsh and Timothy Rowe found Cryolophosaurus to be a basal Neotheropod. While it was still closer to Averostra than Coelophysoidea was, it was still more basal than Dilophosaurus.[19]

Some sediments in the Hanson Formation are of volcanic origin, suggesting Plinian eruptions during the deposition

All known specimens of Cryolophosaurus have been recovered in the Hanson Formation, which is one of only two major dinosaur-bearing rock formations found on the continent of Antarctica. It was discovered in "tuffaceous" siltstone deposited in the Sinemurian to Pliensbachian stage of the Early Jurassic,[6] approximately 194 to 188 million years ago.[5][27] This geological formation is part of the Victoria Group of the Transantarctic Mountains, which is approximately 4,000 metres (13,000 ft) above sea level.[6] The high altitude of this site supports the idea that early Jurassic Antarctica had forests populated by a diverse range of species, at least along the coast.[28][29] The Hanson Formation was deposited in an active volcano−tectonic rift system formed during the breakup of Gondwana.[14]

In the Early Jurassic, Antarctica was closer to the equator and the world was considerably warmer than today, but the climate was still cool temperate.[30] Models of Jurassic air flow indicate that coastal areas probably never dropped much below freezing, although more extreme conditions existed inland.[31] Cryolophosaurus was found about 650 kilometres (400 mi) from the South Pole but,[6] at the time it lived, this was about 1,000 km (621 mi) or so farther north.[28] This formation has produced the remains of Glacialisaurus[32] (a large basal sauropodomorph), a crow-sized pterosaur (a dimorphodontid), a synapsid (a tritylodont, which is a type of synapsid about the size of a rat), herbivorous synapsid, and two small unnamed sauropodomorphs.[2] There are also the remains of many plant genera recovered from the Early Jurassic Camp Hill Formation, around the same age as fossils of Cryolophosaurus, proving that dense plant matter had once grown on Antarctica's surface before it drifted southward.[33]

Wikispecies has information related to Cryolophosaurus.

Wikimedia Commons has media related to Cryolophosaurus.

• Transantarctic Vertebrate Paleontology Project, official website for the NSF project conducting research on Cryolophosaurus and related fauna (photos, research information, publication list, geology, project members ...)
• Fryxell Geology Museum (photos, information)
• Field Museum of Natural History (video, information)