| Proceratosaurus | |
|---|---|
 | |
| Right side view of theholotype skull (NHMUK PV R 4860) | |
Scientific classification | |
| Kingdom: | Animalia |
| Phylum: | Chordata |
| Class: | Reptilia |
| Clade: | Dinosauria |
| Clade: | Saurischia |
| Clade: | Theropoda |
| Superfamily: | †Tyrannosauroidea |
| Family: | †Proceratosauridae |
| Genus: | †Proceratosaurus von Huene,1926 |
| Species: | †P. bradleyi |
| Binomial name | |
| †Proceratosaurus bradleyi | |
| Synonyms | |
| |
Proceratosaurus (/proʊsəˌrætoʊˈsɔːrəs/proh-sə-RAT-oh-SAW-rəs[2]) is agenus oftheropod dinosaur that lived during theMiddle Jurassic in what is now England. Theholotype and only known specimen consists of a mostly complete skull with an accompanying lower jaw and ahyoid bone, found nearMinchinhampton, a town inGloucestershire. It was originally described in 1910 as a species ofMegalosaurus,M. bradleyi, but was moved to its own genus,Proceratosaurus, in 1926. The genus was named for its supposed close relationship withCeratosaurus, later shown to be erroneous, due to perceived resemblance ofProceratosaurus's incomplete cranial crest toCeratosaurus's nasal horn.
A small to medium-sized dinosaur, the skull ofProceratosaurus is 26.9 cm (10.6 in) long as preserved, and the dinosaur is estimated to have measured around 3 metres (10 ft) in length. The skull is characterised by a number of distinguishing features, including a cranial crest that begins at the junction between thepremaxilla and thenasal bone. The teeth areheterodont, having D-shaped teeth at the front of the upper jaw and flattenedserrated teeth in the sides of the jaw.Proceratosaurus is considered acoelurosaur, specifically a member of the familyProceratosauridae, and is among the earliest known members of both Coelurosauria andTyrannosauroidea (the broader group which includes thetyrannosaurids, including the famousTyrannosaurus), with its complete crest probably being larger than that ofCeratosaurus and more similar to its close relativeGuanlong.
Proceratosaurus is thought to have been acarnivore, with its diet probably consisting of relatively small prey. The crest was probably used fordisplay. The dinosaur is known from theGreat Oolite Group of England, having been found in either theWhite Limestone Formation or theForest Marble Formation. During theBathonian age whenProceratosaurus lived, Britain along with the rest of Western Europe formed asubtropical islandarchipelago, with southern Britain having a seasonally dry climate. Other dinosaurs known from the Bathonian of Britain include the large theropodMegalosaurus bucklandii, the largesauropodCetiosaurus, as well as indeterminatestegosaurs,ankylosaurs andheterodontosaurids.
In 1910, the British palaeontologistArthur Smith Woodward reported a partial skull of atheropod dinosaur, discovered some time prior by F. Lewis Bradley during excavation for a reservoir in the vicinity ofMinchinhampton, a town inSouth West England.[1] Bradley hadprepared the skull so that the left side was exposed, and submitted it to theGeological Society of London. Woodward made the skull theholotype specimen (the originaltype specimen with which a species is defined) of a new species of thegenusMegalosaurus, naming itM. bradleyi in honour of its discoverer.[1][3]Megalosaurus, the first named non-bird dinosaur, described in 1824 also based on English fossils,[4] was historically used for any fragmentary remains of large theropods from around the world (wastebasket taxon).[5]
At the time it was discovered,M. bradleyi was one of the most complete theropod skulls known from Europe, possibly with the exception of the crushed and hard to interpret skulls ofCompsognathus andArchaeopteryx. Since 1942, the skull has been housed at theNatural History Museum in London, where it is catalogued as specimen NHMUK PV R 4860. The upper part of the skull is missing due to afissure that had eroded the rock and was partially filled withcalcite. While overall well preserved, the skull is somewhat compressed from side-to side compared to what it would have been in life.[3][6]
In a 1923 publication, the German palaeontologistFriedrich von Huene placed the species in the newly coined genusProceratosaurus, assuming it was an early member of theCeratosaurus lineage.[7][3] The name derives from theGreekpro'before' and the genus nameCeratosaurus.[2] However, the name wasinvalidly published as it was only used in a diagram in the paper and not mentioned at all in the main text. Von Huene validated the name three years later in two 1926 articles, which contained the requiredtechnical description in order for the name to be considered valid.[3] Von Huene considered the crest, as well as the shape of themaxilla (the main tooth bearing upper jaw bone),squamosal (a bone towards the back of the skull), theexternal nares (bony nostril openings) and theinfratemporal fenestra (the skull opening behind the eye socket) as distinctive.[8][9] Huene regarded the crest, which he thought to represent the base of a nasal horn, as a feature supporting its relationship withCeratosaurus.[10]
While remaining one of the best preserved theropod skulls in Europe, and globally one of the best preservedMiddle Jurassic theropod skulls, it subsequently received little scientific attention, mainly being mentioned in studies about general aspects of theropod anatomy and evolution. The skull was re-described by the German palaeontologist Oliver Rauhut and colleagues in 2010, undergoing further mechanical preparation using tools to remove the rock encasing the fossil to reveal additional details of the skull, jaw, and teeth, as well as beingCT scanned at theUniversity of Texas during the same year.[3][11]
In 1988, American paleontologistGregory S. Paul considered the much larger theropod speciesPiveteausaurus divesensis from the Middle-Late Jurassic of France to belong toProceratosaurus, coining thenew combinationProceratosaurus divesensis.[12] However, later researchers, including Rahut and colleagues in their 2010 redescription, rejected this suggestion, finding the two species to be unrelated.[3][13]
The only known skull ofProceratosaurus is 26.9 cm (10.6 in) long as preserved. The 2010 redescription considered it a small to medium-sized dinosaur, and estimated a total body length of 2.98–3.16 m (9.8–10.4 ft) and a body mass of 28–36 kg (62–79 lb) for the holotype individual, which was likely at least a subadult.[3] Other sources gave estimates of 3–4 m (9.8–13.1 ft) in length and 50–100 kg (110–220 lb) in body mass.[14][15] Well preserved fossils of the relatedtyrannosauroidsYutyrannus andDilong indicate that they were covered in relatively simple feathers in life, similar to thedown feathers of modern birds, and according to British paleontologist Dave Hone "it is reasonable to infer" that all tyrannosauroids had similar feathers.[16]
When complete, the skull ofProceratosaurus appears to have been relatively long but not particularly deep, being more than three times longer than high. Theexternal naris (the opening at the front of the skull from which the nostril originates), makes up around 20% of the skull length, measured to be around 7 cm (2.8 in) in maximum length, relatively large compared to the size of the skull. The maximum length of the nares is inclined upwards towards the back of the skull (posterodorsally) at an angle of approximately 30 degrees, differing from that of the closely relatedGuanlong, where the maximum length of the nares is roughly horizontal relative to the skull length. Theantorbital fenestra (the large opening in front of the eye) is roughly triangular in shape, with a maximum length of 6.9–7.1 cm (2.7–2.8 in), and as is found in other theropods, this fenestra is also surrounded by a largefossa (depression) extending onto the surrounding skull bones. On the maxilla, the forward edge of the fossa extends considerably further forward (anterior) and below (ventral) the promaxillaryforamen (a hole in the skull), a unique and distinctive characteristic (autapomorphy) of this species. The partially preservedorbit (eye socket) has an inverted egg-shape, and was probably marginally taller than long when complete, with a maximum estimated complete height of 6 cm (2.4 in), and a measured length of 5.55 cm (2.19 in). The infratemporal fenestra is narrow and elongate, being around 5.4 cm (2.1 in) tall and kidney-shaped, and slightly constricted at its midpoint.[3]
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Thepremaxilla (the frontmost bone of the upper jaw) is relatively small, forming a rounded end to the snout. Thenasal bones, as well as the contacting upper back edge of the premaxillae to their front, bear the partially preserved base of a crest. The preserved part of the crest overhangs the internarial bar that forms the upper part of the bony nostril, a distinguishing feature of this genus. Like other proceratosaurids,Proceratosaurus probably had a largepneumatic (hollow) crest that ran along the midline of the skull, which may have been covered bykeratin.[3][16][17] The shape of the complete crest is unknown and was previously thought to be similar to that ofCeratosaurus,[9] but after the discovery of the close, crested relativeGuanlong, that genus has since been considered a likely model.[1][3][18] The maxilla (the main tooth bearing bone of the upper jaw) is long, around 16.3 cm (6.4 in) in length, probably over 50% of the total complete skull length. The front portion of the maxilla forward of the antorbital fenestra differs considerably in shape from its counterpart inGuanlong.[3] In contrast toGuanlong, thejugal bone of the skull makes little contribution to the border (outer edge) of the antorbital fenestra inProceratosaurus.[3]
Themandible (lower jaw) ofProceratosaurus is 26 cm (10 in) long, somewhat shorter than the skull, which is unusual for theropods. The retroarticular process at the posterior end of the mandible where the lower jaw articulates with the skull is relatively short. Thedentary bone (the tooth-bearing front portion of the mandible) is slender, though it becomes considerably wider towards the rear, which bears a large, elongatemandibular fenestra (opening), with a length of 2.6 cm (1.0 in). The dentary bone tapers to a blunt point towards the front. Although not all teeth are preserved, thetooth sockets show that each premaxilla had around 4 teeth, each maxilla had around 22 teeth, and each dentary had around 20 teeth.[3] The teeth areheterodont, showing differences in morphology depending on their position in the jaw. The premaxillary teeth are D-shaped in cross-section, with the front facing surface of the teeth being arched.[16] The maxillary teeth, like those of many other theropods, areziphodont, that is they are narrow from side-to-side andserrated, as is typical ofcarnivorous members ofArchosauria. The three frontmost pairs of teeth of the lower jaw are procumbent, that is they protrude forwards. Thedenticles are rounded and chisel-like in shape. The density of denticles is highest in the frontmost teeth in both the upper and lower jaws. In the teeth further back, denticle density increases in the posteriormost teeth of the maxilla but remains constant in the dentary.[3]
The preserved lefthyoid (a bone that supported the tongue) is around 12 cm (4.7 in) long along its curved length. The central part of the shaft is relatively straight, while the posterior and front ends are flexed upwards.[3]
Woodward classifiedProceratosaurus as a species ofMegalosaurus in his 1910 description, because both had four premaxillary teeth.[1] This trait was later shown to be theancestral condition found in the common ancestor of all theropod dinosaurs, and thus not a distinguishing characteristic.[3] Studies during the 1930s by von Huene suggested a closer relationship withCeratosaurus, and he thought both dinosaurs represented members of the groupCoelurosauria.[19]
It was not until the late 1980s, afterCeratosaurus had been shown to be a much morebasal (early diverging) theropod and not a coelurosaur, that the classification ofProceratosaurus was re-examined. Paul suggested in 1988 that it was a close relative ofOrnitholestes, again mainly because of the crest on the nose (though the idea thatOrnitholestes bore a nasal crest was later disproved).[20] Paul considered bothProceratosaurus andOrnitholestes to be neither ceratosaurs nor coelurosaurs, but instead primitiveallosauroids.[12]
A 1998phylogenetic analysis by American paleontologistThomas R. Holtz Jr. foundProceratosaurus to be a basal coelurosaur.[21] Several subsequent studies confirmed this, findingProceratosaurus andOrnitholestes only distantly related to ceratosaurids and allosauroids, though one opinion published in 2000 consideredProceratosaurus a ceratosaurid without presenting supporting evidence.[22][3] A 2004 study by Holtz and colleagues also placedProceratosaurus among the coelurosaurs, though with only weak support, and again found an (also weakly supported) close relationship withOrnitholestes.[23][3]
The first major re-evaluation ofProceratosaurus and its relationships was published in 2010 by Oliver Rauhut and colleagues. Their study concluded thatProceratosaurus was in fact a coelurosaur, and moreover atyrannosauroid, an early diverging member of the lineage culminating in the largetyrannosaurids of theLate Cretaceous. Furthermore, they found thatProceratosaurus was most closely related to the tyrannosauroidGuanlong from the Late Jurassic of China. They named theclade containing these two dinosaursProceratosauridae, defined as all theropods closer toProceratosaurus than toTyrannosaurus,Allosaurus,Compsognathus,Coelurus,Ornithomimus, orDeinonychus.[3][6]Proceratosaurus is currently the oldest known tyrannosauroid,[3] along withKileskus, which is known from equivalently aged rocks in Western Siberia.[24] The fossil record of early coelurosaurians and their initial diversification is sparse, and it has previously been disputed whether coelurosaurian dinosaurs had suddenly radiated at the Middle–Late Jurassic boundary, or gradually during the late Early Jurassic – Middle Jurassic. The presence of tyrannosauroids such asProceratosaurus in theBathonian implies that the initial diversification of Coelurosauria had already considerably progressed by the early Middle Jurassic.[3]
Subsequent published analyses have consistently recoveredProceratosaurus in a close relationship withGuanlong, as well as the generaKileskus andSinotyrannus. Other genera which may be close relatives includeYutyrannus,Dilong, andStokesosaurus, but the exact affinities of these taxa as they relate toProceratosaurus remain uncertain.[25][26] Below is acladogram from a 2022 study by British paleontologistDarren Naish and Italian paleontologistAndrea Cau on the genusEotyrannus, which recovered similar relationships to previous studies.[27]
The study of the general biology ofProceratosaurus is limited by the lack ofpostcranial remains. However, the better-understood anatomy of the relatedGuanlong allows for general inferences about the biology of Proceratosauridae as a whole.[3] As is ancestral for theropods, all tyrannosauroids, including proceratosaurids likeProceratosaurus, are thought to have been carnivores.[28] Proceratosaurids such asGuanlong differ from tyrannosaurids likeTyrannosaurus in the possession of relatively large and powerfully built forelimbs, suggesting that they were used in capturing and holding prey, unlike in Late Cretaceous tyrannosaurids which have relatively small forelimbs, who are thought to have relied on their heads in combination with a muscular neck to capture and kill prey. In spite of this, proceratosaurids possessed many of the key adaptations of Cretaceous tyrannosaurids. In particular, proceratosaurids already possessed the fusednasal bones that were inherited by their successors. In later forms, the fusion of the left and right nasal bones is believed to have been an adaptation for withstanding higherbite forces.Proceratosaurus also possessed the characteristic "D-shaped" premaxillary teeth that are unique to tyrannosauroids. According to Rauhut and colleagues in 2010, this suite of adaptations indicates that the "puncture-pull" feeding strategy of tyrannosaurids was already present in proceratosaurids.[3] In the "puncture-pull" strategy, which was likely exhibited by many coelurosaurian theropods, following closure of the jaws around prey, the head was pulled back, causing the theropod's teeth to slice through the held flesh to rip it away from the body of the prey.[29]
A 2023 study by the American palaeontologist Evan Johnson-Ransom and colleagues used data from the skulls ofProceratosaurus andGuanlong to create a virtual composite model of a hypothetical, completeProceratosaurus skull, and created other virtual skull models for other tyrannosauroids. They added simulated muscles to these model skulls to estimate the highest possiblebite force. Their model forProceratosaurus exhibited an estimated bite force of 390 N (88 lbf), comparable toDilong, but much lower than those of adult tyrannosaurids. Their results suggested that the skull ofProceratosaurus had a proportionately lower ability to withstand stresses than those of tyrannosaurids, contributing to their low bite force. The crest may have helped to redistribute stress and buttress the skull when biting. The low bite force in combination with the morphology of the skull suggests thatProceratosaurus most likely fed on small-bodied prey.[18] Some of the teeth on the lower jaw of theProceratosaurus holotype display damage likely caused during feeding.[3]
According to Rauhut and colleagues, the prominent head crest ofProceratosaurus was also likely to have been used as adisplay feature.[3] Paul agreed in a 2016 popular book, pointing out that the crest would have been too delicate for head-butting.[15]
The only knownProceratosaurus specimen was found in rocks of theGreat Oolite Group, which date to thelate Bathonian age of the Middle Jurassic.[3] The exactstratigraphic layer in whichProceratosaurus was discovered is unknown, but it is probably part of either theWhite Limestone Formation[30] or the overlyingForest Marble Formation.[31] Theoolitic limestones in whichProceratosaurus was discovered are thought to have formed in veryshallow marine conditions on thecontinental shelf.[30][32]
During the Middle Jurassic, Britain was located in thesubtropics,[33] and along with the rest of Western Europe formed a part of an island archipelago, in a seaway narrowly separated fromLaurentia (the landmass consisting of North America and Greenland) to the west and theFennoscandian Shield to the northeast.[34] Britain was divided into a number of islands separated by shallow seas,[34] including one formed by theLondon–Brabant Massif to the east, the Welsh Massif to the west,[32] theCornubian Massif to the southwest, and the Pennine-Scottish Massif to the north.[35] The Great Oolite Group was deposited in conditions varying from shallow marine to paralic (coastal).[35] The coastlines of these islands fluctuated throughout the Bathonian, with areas of shallow marine deposition being sometimes temporarily transformed intolagoonal or terrestrial environments with lakes and ponds,[32] and it has been suggested that animals were able to disperse between them and possibly the Fennoscandian Shield,[34] with many terrestrial vertebrate species found in Bathonian deposits of the Great Oolite Group of England also found in the equivalently aged rocks of theKilmaluag Formation of theIsle of Skye in northern Scotland.[36]
The flora from the Bathonian-agedTaynton Limestone Formation inOxfordshire (which is immediately east ofGloucestershire whereProceratosaurus was found) was dominated byaraucarian andcheirolepidiacean conifers, the probable coniferPelourdea, as well asbennettitaleans, with other plants includingcycads (Ctenis), ferns (Phlebopteris,Coniopteris),Caytoniales, the living genusGinkgo, and theseed fernsPachypteris andKomlopteris, probably representing a seasonally dry coastal environment.[37] In the White Limestone Formation, evidence ofwildfires indicates the periodic occurrence of long periods of drought.[32]
Other dinosaurs known from the Bathonian age in Britain include the large theropodMegalosaurus[4] and thesauropodCetiosaurus.[38] Ornithischian remains have also been discovered, but none of these remains have been given scientific names. Bones and teeth ofstegosaurs, as well as teeth ofankylosaurs, basalthyreophorans, andheterodontosaurids have been found, alongside remains that have not been confidently assigned to a single group.[39][32]Maniraptoran theropods, possibly includingdromaeosaurs, were also present in the environment, also only known from indeterminate teeth.[40]Pterosaurs from the Great Oolite Group includedrhamphorhynchids such as the genusKlobiodon, as well as probablemonofenestratans.[41] Large rhamphorhynchoids likeDearc and monofenestratans likeCeoptera are also known from other Bathonian aged localities in the British Isles.[42]Crocodyliformes were also present in the environment, includingatoposaurids andgoniopholids.[36]
The Great Oolite Group is also host to a diverse assemblage of small terrestrial vertebrates (microvertebrates), known from over a dozen localities across England.[36][32] The most important locality, theKirtlington Mammal Bed in Oxfordshire,[32][36] deposited inswampy, coastal conditions[43] at the boundary between the White Limestone and Forest Marble,[32] preserves remains from large animals as well, but the majority are microvertebrates. These include primitive mammals and their close relatives, such astritylodontid cynodonts,morganucodonts,docodonts,allotherians,haramiyidans,shuotheriids,eutriconodonts, and early-divergingcladotherians.[43] Remains of amphibians, including salamanders, frogs andalbanerpetontids as well as reptiles like turtles, lizards (among the world's oldest[44]),choristoderes, andsphenodontians have also been discovered in the Kirtlington Mammal Bed.[43]
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