Proceratosaurus

Proceratosaurus Temporal range: Bathonian, 166 Ma PreꞒ Ꞓ O S D C P T J K Pg N ↓
Holotype skull (NHMUK PV R 4860)
Scientific classification
Domain:	Eukaryota
Kingdom:	Animalia
Phylum:	Chordata
Clade:	Dinosauria
Clade:	Saurischia
Clade:	Theropoda
Superfamily:	†Tyrannosauroidea
Family:	†Proceratosauridae
Genus:	†Proceratosaurus von Huene, 1926
Species:	†P. bradleyi
Binomial name
†Proceratosaurus bradleyi (Woodward, 1910 [originally Megalosaurus])
Synonyms
Megalosaurus bradleyi Woodward, 1910

Proceratosaurus (/proʊ̯sɛrətoʊˈsɔːrəs/ from Greek πρό/πρότερος pro/protero 'before', κέρας/κέρατος keras/keratos 'horn' and σαῦρος sauros 'lizard') is a genus of carnivorous theropod dinosaur from the Middle Jurassic (Bathonian) of England. It contains a single species, P. bradleyi, known from a mostly complete skull and lower jaws. Proceratosaurus was a small dinosaur, estimated to measure around 3 m (9.8 ft) in length.^[1][2][3] Its name refers to how it was originally thought to be an ancestor of Ceratosaurus, due to the partially preserved portion of the crest of Proceratosaurus superficially resembling the small crest of Ceratosaurus.^[4] Now, however, it is considered a coelurosaur, specifically a member of the family Proceratosauridae, and is among the earliest known members of both Coelurosauria and Tyrannosauroidea.^[5]

The type specimen is held in the Natural History Museum in London and was described in 1910 from oolitic limestone of the Great Oolite Group near Minchinhampton while excavating for a reservoir.^[6]

In 1910, the British paleontologist Arthur Smith Woodward reported a partial theropod skull discovered some time prior by F. Lewis Bradley during excavation for a reservoir at Minchinhampton, a town in the Cotswolds in Gloucestershire, 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 of a new species of Megalosaurus (a genus named in 1824), M. bradleyi, in honour of its discoverer. At the time it was discovered, it 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. Currently the skull is housed at the Natural History Museum, 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.^[7][1]

In 1923, the German palaeontologist Friedrich von Huene moved the species to the new genus Proceratosaurus, assuming it was the ancestor of the Jurassic genus Ceratosaurus, but since the name was only used in a schematic, the name has been considered a nomen nudum, an invalidly published name. He validated the name three years later in two 1926 articles by providing a diagnosis of the genus. While remaining one of the best preserved theropod skulls in Europe, and globally one of the best preserved Middle Jurassic theropod skulls, it since received little scientific attention, mainly being mentioned in studies about general aspects of theropod anatomy and evolution. The holotype skull was since CT scanned at the University of Texas, further mechanically prepared to reveal additional details of the skull, jaw, and teeth, and was re-described by the German palaeontologist Oliver W. M. Rauhut and colleagues in 2010.^[1][8]

The only known skull of Proceratosaurus, while overall well preserved, is somewhat compressed from side-to side (laterally). As currently preserved, it is 26.9 centimetres (10.6 in) long. In the 2010 restudy, based on the size of the skull, the total length was estimated at 2.98–3.16 m (9.8–10.4 ft) and the body mass estimated at 28–36 kg (62–79 lb).^[1] Other sources have given estimates of 3–4 m (9.8–13.1 ft) in length and 50–100 kg (110–220 lb) in body mass.^[2][3] Well preserved fossils of the related tyrannosauroids Yutyrannus and Dilong indicate that in life they were covered in relatively simple feathers, similar to the down feathers of modern birds.^[9]

The skull when complete appears to have been relatively long but not particularly deep, being more than three times longer than it is high. The nasal fenestra (a hole in the skull) makes up around 20% of the skull length, being around 7 centimetres (2.8 in) long. The antorbital fenestra is roughly triangular in shape, with a maximum dimension of 6.9–7.1 centimetres (2.7–2.8 in), with the antorbital fenestra also being surrounded by a large fossa (depression) extending onto the surrounding skull bones. The partially preserved orbital fenestra has an "inverted egg-shape" and marginally taller, at maximum 6 centimetres (2.4 in) than its length, around 5.55 centimetres (2.19 in). The infratemporal fenestra is narrow and elongate, being around 5.4 centimetres (2.1 in) tall and kidney shaped, being slightly constricted at its midpoint. The premaxilla is relatively small, forming a rounded end to the snout. The nasal bones, as well as the contacting upper back edge of the premaxillae to their front, bear the partially preserved base of a crest,^[1] which when complete, like other proceratosaurids, likely formed a large pneumatic crest that ran across the midline of the skull, which in life may have been covered by keratin.^[9][10]

The mandible has a length of 26 centimetres (10 in), somewhat shorter than the length of the skull, which is relatively unusual among theropods. The retroarticular process of the mandible is relatively short. The tooth bearing portion of the mandible is rather slender, though the mandible becomes considerably wider towards the rear, which bears a large, elongate mandibular fenestra (hole). The front of the dentary bone of the mandible tapers to a blunt point towards the front. Each premaxilla had around 4 teeth, while each maxilla had around 22 teeth, with 20 teeth in each dentary.^[1] The teeth of Proceratosaurus are heterodont (showing differences in morphology in different positions in the jaw). The premaxillary teeth are relatively rounded in cross section.^[9] The maxillary teeth, like those of many other theropods, are ziphodont, being laterally (side-to-side) compressed and serrated.^[1]

The preserved left hyoid bone is around 12 centimetres (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 (dorsally).^[1]

Arthur Smith Woodward, who initially studied Proceratosaurus, placed it as a species of Megalosaurus, due to same number of premaxillary teeth (4).^[7] Later study during the 1930s by Friedrich von Huene suggested a closer relationship with Ceratosaurus, and Huene thought both dinosaurs represented members of the group Coelurosauria.^[11]

It was not until the late 1980s, after Ceratosaurus had been shown to be a much more primitive theropod and not a coelurosaur, that the classification of Proceratosaurus was again re-examined. Gregory S. Paul suggested that it was a close relative of Ornitholestes, again mainly due to 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. Furthermore, Paul considered the much larger dinosaur Piveteausaurus to be the same genus as Proceratosaurus, making Piveteausaurus a junior synonym.^[12] However, no overlapping bones between the two had yet been exposed from the rock around their fossils, and future study showed that they were indeed distinct.^[1]

Several phylogenetic studies in the early 21st century finally found Proceratosaurus (as well as Ornitholestes) to be a coelurosaur, only distantly related to the ceratosaurids and allosauroids, though one opinion published in 2000 considered Proceratosaurus a ceratosaurid without presenting supporting evidence. Phylogenetic analyses by Thomas R. Holtz Jr. in 2004 also placed Proceratosaurus among the coelurosaurs, though with only weak support, and again found an (also weakly supported) close relationship with Ornitholestes.^[1]

The first major re-evaluation of Proceratosaurus and its relationships was published in 2010 by Oliver Rauhut and colleagues. Their study concluded that Proceratosaurus was in fact a coelurosaur, and moreover a tyrannosauroid, a member of the lineage leading to the giant tyrannosaurs of the Late Cretaceous. Furthermore, they found that Proceratosaurus was most closely related to the Chinese tyrannosauroid Guanlong. They named the clade containing these two dinosaurs the Proceratosauridae, defined as all theropods closer to Proceratosaurus than to Tyrannosaurus, Allosaurus, Compsognathus, Coelurus, Ornithomimus, or Deinonychus.^[1][13]

Subsequent published analyses have consistently recovered Proceratosaurus in a close relationship with Guanlong, as well as the genera Kileskus and Sinotyrannus. Other genera which may be close relatives include Yutyrannus, Dilong, and Stokesosaurus, but the exact affinities of these taxa as they relate to Proceratosaurus remain uncertain.^[14][15] Below is a cladogram from a 2022 study by Darren Naish and Andrea Cau on the genus Eotyrannus, which recovered similar relationships to previous studies.^[16]

Tyrannosauroidea	Juratyrant Stokesosaurus Coeluridae Proceratosauridae Dilong Guanlong Proceratosaurus Sinotyrannus Pantyrannosauria Yutyrannus Eotyrannus Xiongguanlong Megaraptora Eutyrannosauria

The study of the general biology of Proceratosaurus is somewhat limited by the lack of any post-cranial remains attributable to the genus. However, the more well-understood anatomy of the related genus Guanlong allows scientists to make some general inferences about the biology of Proceratosauridae as a whole. Proceratosaurus was a small-bodied animal, in sharp contrast to the tyrannosauroids of the Late Cretaceous. In spite of this, they possessed many of the key adaptations which would come to define the large tyrannosaurids of later periods. In particular, proceratosaurids had already possessed the fused nasal bones that were inherited by large tyrannosaurids. In later forms, the fusion of the dual nasal bones is believed to have been an adaptation to allow the skull to withstand higher bite forces. Proceratosaurus also possessed the characteristic "D-shaped" premaxillary teeth that are unique to tyrannosauroids. This suite of adaptations indicates that the "puncture-pull" feeding strategy of derived tyrannosaurids was already present in proceratosaurids.^[1]

A 2023 study by Evan Johnson-Ransom and colleagues used data from the skulls of Proceratosaurus and Guanlong to create a composite model of a hypothetical, complete proceratosaurid skull. They added virtual muscles to these model skulls and used them to estimate the highest possible bite force. Their model for Proceratosaurus exhibited an estimated bite force of 390 newtons (88 pounds-force), comparable to Dilong or a juvenile Tarbosaurus. These results confirmed the hypothesis that proceratosaurids and other early-diverging tyrannosauroids had a proportionately lower ability to withstand stresses than their derived tyrannosaurid cousins. This is believed to have implications for the dietary ecology and behavior of proceratosaurids. This lower proportional force distribution would have precluded proceratosaurids from attacking large prey, and therefore they most likely fed on small-bodied prey such as small ornithischians and primitive mammals.^[17]

Proceratosaurus and its relatives also possessed highly pneumatized skull bones. Cranial sinus tissue excavated the maxillae, jugals, and lacrimal bones, and the jugals themselves were very rugose, which suggests that the area was heavily irrigated by blood vessels.^[1] The jugal bone is generally not part of the cranial musculature for most dinosaurs, and Xu Xing suggested in a 2016 research paper that they may have been important display areas for many groups of dinosaur, including proceratosaurids.^[18] The prominent head crest of Proceratosaurus, consisting of the fused nasal bones, was also likely to have been used as a display feature.^[1]

During the Middle Jurassic, Britain was located in the subtropics,^[19] and along with the rest of Europe formed a part of an island archipelago.^[20] The flora from the also Bathonian aged Taynton Limestone Formation in Oxfordshire is dominated by araucarian and cheirolepidiacean conifers as well as bennettitaleans and Pelourdea, representing a probably seasonally dry coastal environment.^[21] The Bathonian flora of the Scalby Formation in Yorkshire to the north, representing a floodplain environment, consists of Ginkgoales (including the living genus Ginkgo), Czekanowskiales, bennettitaleans, and conifers (again including araucarian and cheirolepidiacean forms), ferns, lycophytes, and seed ferns including Caytoniales,^[22][23] with this formation also interpreted as representing a seasonally dry environment.^[24]

Other dinosaurs known from the Bathonian age of Britain include the large theropod Megalosaurus,^[25] the sauropod Cetiosaurus,^[26] as well as indeterminate ornithischians,^[27] including stegosaurs, ankylosaurs, and heterodontosaurids,^[28] and maniraptorans, likely including dromaeosaurs.^[29] A diverse microvertebrate fauna is known from the equivalently aged Forest Marble Formation, including frogs, salamanders, turtles, choristoderes, lizards, rhynchocephalians, crocodyliformes, and mammaliamorphs including tritylodontids, morganucodonts, docodonts, allotherians and eutriconodont mammals.^[30]

• Timeline of tyrannosaur research

• 3D scan of the holotype skull on Sketchfab