| Cryolophosaurus | |
|---|---|
 | |
| Reconstructed skeleton,Field Museum of Natural History | |
Scientific classification | |
| Kingdom: | Animalia |
| Phylum: | Chordata |
| Class: | Reptilia |
| Clade: | Dinosauria |
| Clade: | Saurischia |
| Clade: | Theropoda |
| Clade: | Neotheropoda |
| Genus: | †Cryolophosaurus Hammer &Hickerson,1994 |
| Type species | |
| †Cryolophosaurus ellioti Hammer & Hickerson, 1994 | |
Cryolophosaurus (/ˌkraɪəˌloʊfəˈsɔːrəs/ or/kraɪˌɒləfəˈsɔːrəs/;KRY-ə-LOH-fə-SAWR-əs) is agenus of largetheropod dinosaur known from only a singlespecies,Cryolophosaurus ellioti, from theEarly Jurassic ofAntarctica. It was one of the largest theropods of the Early Jurassic, with the subadult, being estimated to have reached 6–7 metres (20–23 ft) long and weighed 350–465 kilograms (772–1,025 lb).
Cryolophosaurus was first excavated from Antarctica's Early Jurassic,Pliensbachian agedHanson Formation, formerly the upper Falla Formation, bypaleontologistDr. William Hammer in 1991. It was the firstcarnivorous 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 to about 196 to 188 million years ago, representing the Early JurassicPeriod.
Cryolophosaurus is known from a skull, a femur and other material, all of which have caused its classification to vary greatly. The femur possesses many primitive characteristics that have classifiedCryolophosaurus as adilophosaurid, or a neotheropod outside of Dilophosauridae and Averostra, whereas the skull has many advanced features, leading the genus to be considered a tetanuran, an abelisaurid, a ceratosaur and even anallosaurid.[2][3]Cryolophosaurus is currently considered to be a derivedneotheropod, close toAverostra. Additionally,Cryolophosaurus possessed a distinctive "pompadour" crest that spanned the head from side to side. Based on evidence from related species and studies of bone texture, it is thought that this bizarre crest was used forintra-species recognition. Thebrain ofCryolophosaurus was also more primitive than those of other theropods.
Cryolophosaurus originally was collected during the 1990–91 austral summer onMount Kirkpatrick in theBeardmore Glacier region of theTransantarctic Mountains. The discovery was made by Hammer, a professor atAugustana College, and his team. The fossils were found in thesiliceoussiltstone of theHanson Formation, formerly the upper Falla Formation, and dated back to thePliensbachian Stage of the earlyJurassic.Cryolophosaurus was the second dinosaur, and first theropod to be discovered in Antarctica. It was discovered afterAntarctopelta, but named earlier.[4]
In 1991, both Hammer and theOhio State University geologist,David Elliot excavated separate outcrops near Beardmore Glacier, sharing logistical expenses. Elliot's team first came across the remains ofCryolophosaurus 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 ofCryolophosaurus.[4] The specimens were formally named and described in 1994 by Hammer and Hickerson, in the journalScience.[4]
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.[5]
The nameCryolophosaurus ellioti is derived from theGreek words κρυος (meaning 'cold' or 'frozen', in reference to its discovery in Antarctica), λοφος (meaning 'crest') andσαυρος (meaning 'lizard'), thus "cold crest lizard". Hammer and Hickerson named the speciesC. ellioti, after David Elliot, who had made the initial discovery of the fossils.[4]
Cryolophosaurus was a large, well-builttheropod, 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][7][8] In 2016 Molina-Pérez and Larramendi gave a larger estimation of 7.7 meters (25.3 ft) and 780 kg (1,720 lb).[9] Some researchers noted that the holotype individual probably represents a sub-adult, so adults could have been larger.[7][10] Despite having slender proportions,Cryolophosaurus is one of the largest knownEarly Jurassic theropods.[10][11]
Theholotype FMNH PR1821 is the only fully described specimen ofCryolophosaurus. The specimen consists of an incompleteskull andmandibles, lacking most of the front half; nine maxillaryteeth; a fragmentary sixth cervical centrum; cervicalvertebrae 7–10; several posterior cervicalribs; several anterior dorsal vertebrae; most mid and posterior dorsal vertebrae; several dorsal ribs; the fifth sacral vertebrae; threechevrons; many partial and complete caudal vertebrae and centra; two partialhumeri; a proximalradius; a proximalulna; a partialilium; a proximalpubis; bothischia, but only onedistal; two incompletefemora; the distal end of atibia; the distal end of afibula, and the astragalus and calcaneum.[7] In 2013, new material ofCryolophosaurus was unearthed in Antarctica. The description of this material has not yet been published in a non-abstract form.[12]
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The holotype ofC. ellioti 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 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."[13] 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 theMonolophosaurus have crests, they usually run along the skull instead of across it.[14]
An unpublished study conducted by Vernon Meidlinger-Chin in 2013 suggested that previous studies lacked focus on endocranial details. The study found that theCryolophosaurus 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 ofAllosauroids andCoelurosaurs givingCryolophosaurus abasal position in Theropoda.[11] Closer examination of how the skull bones fused reviewed details in the snout and forehead that are exceptionally similar toDilophosaurus.[15]
Classification ofCryolophosaurus is difficult because it has a mix of primitive and advanced characteristics.[16] The femur has traits of early theropods, while the skull resembles much later species of thecladeTetanurae, like China'sSinraptor andYangchuanosaurus. This ledPaul Serenoet al. (1994) to placeCryolophosaurus in the taxon Allosauridae.[17] Originally, Hammer and colleagues suspected thatC. ellioti might be aceratosaur, or even an earlyabelisaur, with some traitsconvergent with those of more advanced tetanurans, but ultimately concluded that it was itself the earliest known member of the tetanuran group.[4] While a subsequent study by Hammer (along with Smith and Currie) again recoveredCryolophosaurus as a tetanuran, a later (2007) study by the same authors found that it was more closely related toDilophosaurus andDracovenator.[14][18]Sterling Nesbittet al. (2009), using the characters ofTawa foundCryolophosaurus to be neither a dilophosaurid nor averostran neotheropod, but instead the sister group of a clade composed ofdilophosaurids andaverostrans.[19] However, in 2012, Matthew Carrano found thatCryolophosaurus was atetanuran, related toSinosaurus, but unrelated toDilophosaurus.[20] In 2020, a monograph ofDilophosaurus foundCryolophosaurus to be a derived neotheropod, close to Averostra, in a more derived position thanZupaysaurus, but less thanDilophosaurus.[21]
The following cladogram illustrates a synthesis of the relationships of the early theropod groups compiled by Hendrickxet al. in 2015.[22]
However, a 2020 study conducted by Adam Marsh and Timothy Rowe foundC. ellioti to be a basal Neotheropod. While it was still closer toAverostra thanCoephysoidea was, it was still more basal thanDilophosaurus.[23]
| Neotheropoda |
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Cranial display features, such as the one possessed byCryolophosaurus, make sense in social, gregarious animals, where other members of the species are available to observe and interpret messages of sexual status.[24]Kevin Padianet al. (2004) challenged conventional hypotheses that the purpose of bizarre cranial structures and post-cranial armor in dinosaurs, was either for attracting mates, intimidating/fighting rivals in the group, or intimidating potential predators of other species. Padianet al. noted that based on phylogenetic, histological, and functional evidence these bizarre structures can be explained by the phenomenon ofintra-species recognition, which is supported by the fossil evidence.[25][26]Thomas R. Holtz Jr. (2010) found that the bizarre crest ofC. llioti was primarily for intra-species recognition, based on evidence from related species and studies of bone texture.[27] According to Thomas Rich and his colleagues, the crest would have been ineffective as a weapon and may have possibly functioned as a display feature during certain types of social behavior such as mating.[28] In 2019, a species recognition function was disputed but a socio-sexual display structure model was suggested.[29]
When the type specimen was discovered, several long cervical ribs, of a supposed prosauropod dinosaur were found in the mouth ofCryolophosaurus, which led Hammer (1998) to conclude that it was feeding on the prosauropod when it died. Hammer further noted that since the ribs were found extending all the way back to the theropod's neck region, this individual may have choked to death on these ribs.[6] However, Smithet al. concluded that these remains belonged to theCryolophosaurus specimen itself, and not to Hammer's "prosauropod".[18] Hammer also concluded that a post-canine tooth belonging to atritylodont (an early mammal relative), found with the remains, was part of its stomach contents when it died.[30]
SomeCryolophosaurus bones have pathologies that show evidence of scavenging. Broken teeth from a juvenileCryolophosaurus were found nearby.[28] These teeth have no roots and likely shed naturally while scavenging the adultCryolophosaurus carcass.
Another possible pathology is found in theastragalus (ankle bone) ofCryolophosaurus. This bone was preserved with a small splint from thefibula located just above the ankle. The splint, however, may also be just a unique morphological feature ofCryolophosaurus.[7]
All known specimens ofCryolophosaurus have been recovered in the Hanson Formation, which is one of only two major dinosaur-bearing rock formations found on the continent of Antarctica.Cryolophosaurus was found about 650 kilometres (400 mi) from theSouth Pole but,[6] at the time it lived, this was about 1,000 km (621 mi) or so farther north.[31] The Hanson Formation accumulated in a rift environment located between c. 60 and 70S, fringing theEast Antarctic Craton behind the activePanthalassan margin of southernGondwana, being dominated by two types of facies: "tuffaceous" siltstone of fluvial and volcanic strata, deposited maybe more than 10 million years based on the thickness.[32] In the Early Jurassic, Antarctica was closer to theequator and the world was considerably warmer than today, but theclimate was still cooltemperate similar to that of modern southernChile, and humid, with a temperature interval of 17–18 degrees. Models of Jurassic air flow indicate that coastal areas probably never dropped much below freezing, although more extreme conditions existed inland.[33]
This formation has produced the remains of two smaller theropods, thesauropodomorphGlacialisaurus,[34] a crow-sizedpterosaur (adimorphodontid), atritylodont, herbivoroussynapsid, and two small unnamed sauropodomorphs.[35][36] Beyond vertebrates, Insects (Blattodea,Coleoptera),Ostracodans,Conchostracans and arthropod ichnofossils (Diplichnites,Planolites,Scoyenia) are known from other coeval localities, likeGair Mesa,Mount Carson orShafer Peak.[37] Plant remains are also very common, including large tree trunks (+50 cm) from Mount Carson to Palynomorphs at Shafer Peak.[38] Macrofoliar and cuticle remains have also been recovered from several localities, including Conifers (Araucariaceae,Cheirolepidiaceae,Cupressaceae,Pinaceae orVoltziales), Cycadophytes (Bennettitales), Pteridosperms (Corystospermaceae), Ferns (Dipteridaceae,Matoniaceae,Osmundaceae andPolypodiales),Equisetaceae,Isoetaceae andLiverworts (Marchantiales).[39] Some of the plant remains are relictual genera, like the youngest record ofDicroidium.[40] Common presence of the invertebrate ichnogenusPlanolites indicates the local fluvial, alluvial or lacustrine waters where likely continuous all year, as well the presence of abundantOtozamites trends to suggest high humidity.[39] Overall points to a setting with strong seasonality in day-length, given the high latitude, perhaps similar to warm-temperate, frost-free forest and open woodland as in North Island ofNew Zealand. Despite the proper conditions, peat accumulation was rare, mostly due to the influence of local volcanism, along with common wildfire activity as shown by charred coalified plant remains.[39]
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