Proceratosaurus ( ) is a genus of theropod dinosaur that lived during the Middle Jurassic in what is now England. The holotype and only known specimen consists of a mostly complete skull with an accompanying lower jaw and a bone, found near Minchinhampton, a town in Gloucestershire. It was originally described in 1910 as a species of Megalosaurus, M. bradleyi, but was moved to its own genus, Proceratosaurus, in 1926. The genus was named for its supposed close relationship with Ceratosaurus, later shown to be erroneous, due to perceived resemblance of Proceratosaurus incomplete cranial crest to Ceratosaurus nasal horn.
A small to medium-sized dinosaur, the skull of Proceratosaurus is long as preserved, and the dinosaur is estimated to have measured around in length. The skull is characterised by a number of distinguishing features, including a cranial crest that begins at the junction between the and the nasal bone. The teeth are heterodont, having D-shaped teeth at the front of the upper jaw and flattened serrated teeth in the sides of the jaw. Proceratosaurus is considered a coelurosaur, specifically a member of the family Proceratosauridae, and is among the earliest known members of both Coelurosauria and Tyrannosauroidea (the broader group which includes the tyrannosaurids, including the famous Tyrannosaurus), with its complete crest probably being larger than that of Ceratosaurus and more similar to its close relative Guanlong.
Proceratosaurus is thought to have been a carnivore, with its diet probably consisting of relatively small prey. The crest was probably used for display. The dinosaur is known from the Great Oolite Group of England, having been found in either the White Limestone Formation or the Forest Marble Formation. During the Bathonian age when Proceratosaurus lived, Britain along with the rest of Western Europe formed a subtropical island archipelago, with southern Britain having a seasonally dry climate. Other dinosaurs known from the Bathonian of Britain include the large theropod Megalosaurus bucklandii, the large sauropod Cetiosaurus, as well as indeterminate stegosaurs, ankylosaurs and heterodontosaurids.
History of discovery
In 1910, the British palaeontologist Arthur Smith Woodward reported a partial skull of a theropod dinosaur, discovered some time prior by F. Lewis Bradley during excavation for a reservoir in the vicinity of Minchinhampton, a town in South West England. Bradley had prepared the skull so that the left side was exposed, and submitted it to the Geological Society of London. Woodward made the skull the holotype specimen (the original type specimen with which a species is defined) of a new species of the genus Megalosaurus, naming it M. bradleyi in honour of its discoverer. Megalosaurus, the first named non-bird dinosaur, described in 1824 also based on English fossils, was historically used for any fragmentary remains of large theropods from around the world (wastebasket taxon).
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 of Compsognathus and Archaeopteryx. Since 1942, the skull has been housed at the Natural History Museum in London, where it is catalogued as specimen NHMUK PV R 4860. The upper part of the skull is missing due to a fissure that had eroded the rock and was partially filled with calcite. While overall well preserved, the skull is somewhat compressed from side-to side compared to what it would have been in life.
In a 1923 publication, the German palaeontologist Friedrich von Huene placed the species in the newly coined genus Proceratosaurus, assuming it was an early member of the Ceratosaurus lineage. The name derives from the Greek and the genus name Ceratosaurus. However, the name was invalidly 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 required technical description in order for the name to be considered valid. Von Huene considered the crest, as well as the shape of the maxilla (the main tooth bearing upper jaw bone), squamosal (a bone towards the back of the skull), the (bony nostril openings) and the (the skull opening behind the eye socket) as distinctive. Huene regarded the crest, which he thought to represent the base of a nasal horn, as a feature supporting its relationship with Ceratosaurus.
While remaining one of the best preserved theropod skulls in Europe, and globally one of the best preserved Middle 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 being CT scanned at the University of Texas during the same year.
In 1988, American paleontologist Gregory S. Paul considered the much larger theropod species Piveteausaurus divesensis from the Middle-Late Jurassic of France to belong to Proceratosaurus, coining the new combination Proceratosaurus divesensis. However, later researchers, including Rahut and colleagues in their 2010 redescription, rejected this suggestion, finding the two species to be unrelated.
Description
The only known skull of Proceratosaurus is long as preserved. The 2010 redescription considered it a small to medium-sized dinosaur, and estimated a total body length of and a body mass of for the holotype individual, which was likely at least a subadult. Other sources gave estimates of in length and in body mass. Well preserved fossils of the related tyrannosauroids Yutyrannus and Dilong indicate that they were covered in relatively simple feathers in life, similar to the down feathers of modern birds, and according to British paleontologist Dave Hone "it is reasonable to infer" that all tyrannosauroids had similar feathers.
When complete, the skull of Proceratosaurus appears to have been relatively long but not particularly deep, being more than three times longer than high. The (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 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 related Guanlong, where the maximum length of the nares is roughly horizontal relative to the skull length. The (the large opening in front of the eye) is roughly triangular in shape, with a maximum length of , and as is found in other theropods, this fenestra is also surrounded by a large (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 promaxillary foramen (a hole in the skull), a unique and distinctive characteristic (autapomorphy) of this species. The partially preserved (eye socket) has an inverted egg-shape, and was probably marginally taller than long when complete, with a maximum estimated complete height of , and a measured length of . The infratemporal fenestra is narrow and elongate, being around tall and kidney-shaped, and slightly constricted at its midpoint.
The premaxilla (the frontmost bone of the upper jaw) is relatively small, forming a rounded end to the snout. The 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 large pneumatic (hollow) crest that ran along the midline of the skull, which may have been covered by keratin. The shape of the complete crest is unknown and was previously thought to be similar to that of Ceratosaurus, but after the discovery of the close, crested relative Guanlong, that genus has since been considered a likely model. The maxilla (the main tooth bearing bone of the upper jaw) is long, around 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 in Guanlong. In contrast to Guanlong, the jugal bone of the skull makes little contribution to the border (outer edge) of the antorbital fenestra in Proceratosaurus.
The mandible (lower jaw) of Proceratosaurus is 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. The bone (the tooth-bearing front portion of the mandible) is slender, though it becomes considerably wider towards the rear, which bears a large, elongate (opening), with a length of . The dentary bone tapers to a blunt point towards the front. Although not all teeth are preserved, the tooth sockets show that each premaxilla had around 4 teeth, each maxilla had around 22 teeth, and each dentary had around 20 teeth. The teeth are heterodont, 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. The maxillary teeth, like those of many other theropods, are ziphodont, that is they are narrow from side-to-side and serrated, as is typical of carnivorous members of Archosauria. The three frontmost pairs of teeth of the lower jaw are procumbent, that is they protrude forwards. The denticles 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.
The preserved left (a bone that supported the tongue) is around long along its curved length. The central part of the shaft is relatively straight, while the posterior and front ends are flexed upwards.
Classification
Woodward classified Proceratosaurus as a species of Megalosaurus in his 1910 description, because both had four premaxillary teeth. This trait was later shown to be the ancestral condition found in the common ancestor of all theropod dinosaurs, and thus not a distinguishing characteristic. Studies during the 1930s by von Huene suggested a closer relationship with Ceratosaurus, and he thought both dinosaurs represented members of the group Coelurosauria.
It was not until the late 1980s, after Ceratosaurus had been shown to be a much more basal (early diverging) theropod and not a coelurosaur, that the classification of Proceratosaurus was re-examined. Paul suggested in 1988 that it was a close relative of Ornitholestes, again mainly because of the crest on the nose (though the idea that Ornitholestes bore a nasal crest was later disproved). Paul considered both Proceratosaurus and Ornitholestes to be neither ceratosaurs nor coelurosaurs, but instead primitive allosauroids.