Duriavenator is a genus of theropod dinosaur that lived in what is now England during the Middle Jurassic, about 168 million years ago. In 1882, upper and lower jaw bones of a dinosaur were collected near Sherborne in Dorset, and Richard Owen considered the fossils to belong to the species Megalosaurus bucklandii, the first named non-bird dinosaur. By 1964, the specimen was recognised as belonging to a different species, and in 1974 it was described as a new species of Megalosaurus, M. hesperis; the specific name means 'the West' or 'western'. Later researchers questioned whether the species belonged to Megalosaurus, in which many fragmentary theropods from around the world had historically been placed. After examining the taxonomic issues surrounding Megalosaurus, Roger B. J. Benson moved M. hesperis to its own genus in 2008, Duriavenator; this name means "Dorset hunter".
Estimated to have been long and weighed , Duriavenator has been described as a medium-sized theropod. Duriavenator is distinct in features of the , the main bone of the upper jaw; it has a deep groove on the upper surface of the process with numerous pneumatic (air-filled) (openings), and small foramina on the lower part of the surface that connected with the , the front bone of the upper jaw. It had about four teeth in the premaxilla, about 14–16 in the maxilla behind, and about 14–15 in the of the lower jaw. While its classification was long uncertain, phylogenetic analyses in 2010 and onwards have shown it to be among the oldest tetanuran theropods (a diverse group that includes modern birds), and to belong in the family Megalosauridae. The long teeth at the front of the lower jaw may have been used for plucking and grasping during feeding. Duriavenator is known from the Upper Inferior Oolite Formation which is Bajocian in age, a stage in the Middle Jurassic.
Discovery
In 1882, the British scientist and businessman Edward Cleminshaw obtained skull bones and teeth of a dinosaur from the freestone of the Inferior Oolite in Greenhill near Sherborne in Dorset, England. Blocks of the quarry-stone were being prepared for a building when Cleminshaw detected fossils embedded in them on cracked surfaces of the stones, and all such stones were withdrawn from the building yard. Cleminshaw presented the fossils at an 1882 meeting of the Dorset Natural History and Antiquarian Field Club, where he expressed hope that more remains would be found when the quarry had been worked, and that he would send them to the British naturalist Richard Owen for examination at the British Museum.
In 1883, Owen considered these fossils to have belonged to the species Megalosaurus bucklandii (using the now obsolete spelling M. bucklandi), which had been scientifically described and named in 1824, making it the first named non-bird dinosaur. Owen found the jaws of the then known Megalosaurus specimens to only differ in size, and due to the similarity of the teeth, thought there was no reason to divide them into different species. The skull of Megalosaurus was only known from fragments, so these new fossils aided Owen in producing a more complete reconstruction of it, incorporating the various specimens, guided by the skull of a monitor lizard. Owen believed the large opening behind the (the main bone of the upper jaw) to be the , while this is now known to be the , a large opening in front of the eye socket of many dinosaurs and their relatives.
The specimen (today catalogued as NHM R332 at the Natural History Museum in London, formerly BMNH) mainly consists of the front third of the skull, including the (first bones of the upper jaw), the right maxilla, the front part of a (a bone at the middle of the palate), both (the tooth-bearing part of the lower jaw), the fragmentary right (a bone at the back of the lower jaw), fragmentary unidentified elements, and associated teeth. The maxilla is split into two slabs, with the front and hind margins broken, and the junction between the premaxilla and maxilla is covered in plaster. The right dentary is the most complete, while the left is missing much of the front and hind regions.
New species and genus
The German palaeontologist Friedrich von Huene used the specimen as basis for his skull reconstruction of Megalosaurus bucklandii (guided by the skull of Allosaurus) in 1926, while noting that many of the forms assigned to the genus Megalosaurus probably did not belong there. In 1964, the British palaeontologist Alick D. Walker stated that while the specimen was commonly accepted as belonging to Megalosaurus bucklandii, it had more teeth in the maxilla, was older, and at least distinct at the species level. The British palaeontologist Michael Waldman further prepared the specimen in 1964–1965 and made it the holotype (on which the scientific name is based) of a new species of Megalosaurus, M. hesperis in 1974. The distinction was based on its higher tooth count in both the upper and lower jaws, but he cautioned no further comparisons could be made due to lack of material. The specific name is derived from the Greek hesperos (Ἑσπερίς), meaning "the West" or "western". He found its dentary similar to that of Zanclodon cambrensis, but noted taxonomic issues with that species, and refrained from classifying it further.
In the 1970s, the American palaeontologists Samuel P. Welles and Jaime Emilio Powell prepared a study on European theropods, planning to move the species to the new genus "Walkersaurus". The name remained unpublished while the American amateur palaeontologist Stephan Pickering sent copies of a revised version of the manuscript to some colleagues in the 1990s, but this does not conform to the requirements of the International Code of Zoological Nomenclature, and the name is therefore considered a nomen nudum, and therefore invalid.
The British palaeontologist David B. Norman noted in 1985 that the genus Megalosaurus had historically been used as a "dustbin" for any fragmentary remains of large theropods from around the world, which probably did not belong in the genus. In 1988, the American palaeontologist Gregory S. Paul listed the species M. hesperis under fragmentary "theropod odds and ends" as Megalosaurus? hesperis (the question mark indicating incertae sedis, uncertain placement), stating the material was not sufficient to tell if it really belonged to Megalosaurus. The Australian palaeontologist Ralph E. Molnar and colleagues stated in 1990 that M. hesperis could plausibly be retained in Megalosaurus, as it resembled M. bucklandii in several characters. The American palaeontologist Thomas R. Holtz Jr. found various positions for the species possible in a 2000 phylogenetic analysis, referring to it as "Megalosaurus" hesperis. Holtz and colleagues cautioned in 2004 that while plausible, there was no diagnostic, derived feature that could support the assignment of the species to Megalosaurus, and listed it as an unnamed tetanuran (a diverse group of theropod dinosaurs that includes modern birds). In 2007, the British palaeontologists Darren Naish and David M. Martill stated that while a valid species, 'M.' hesperis was probably not assignable to Megalosaurus, and did not preserve enough information to reliably classify it within Tetanurae.
In 2008, the British palaeontologist Roger B. J. Benson and colleagues concluded that only the lectotype dentary of Megalosaurus bucklandii could be reliably attributed to the species, thereby stripping the name from all other species and specimens that had previously been assigned to Megalosaurus, as part of an effort to disentangle the historical confusion surrounding the genus. Benson later examined British fossils that had earlier assigned to M. bucklandii and found that some of them belonged to the species after all. Also in 2008, Benson redescribed the 'M.' hesperis holotype (aided by casts of the block made prior to preparation) and identified several features distinguishing it from Megalosaurus and other related theropods, and moved it to its own genus, Duriavenator, creating the new combination D. hesperis. The generic name is a combination of the Latin words Duria, "Dorset", and venator, "hunter", in full "Dorset hunter" or "hunter of Dorset".
Description
Duriavenator is estimated to have been long and weighed , and has been described as a medium-sized theropod, comparable to Megalosaurus in size (size estimates of incompletely known dinosaurs are extrapolated from better known relatives). As a megalosaurid, it would have been massively built, with a large, low head, a short and stout lower arm, and a broad and shallow pelvis.
Upper jaw
The front surface of the holotype premaxilla is inclined hindward and upwards at an angle of about 50 degrees relative to the lower margin, an angle comparable to the megalosaurid Torvosaurus, but lower than in Dubreuillosaurus and Eustreptospondylus. Though the outer side surface of the premaxilla is poorly preserved, randomly distributed nutrient foramina (openings for blood-vessels) are visible towards its front. The bone surface at the middle front of the premaxilla is flat and marked by hind and upwards oriented striations where it would have connected with the other premaxilla. A (opening) is present at the front middle surface, right below the front end of the . Three prominent processes (outgrowths) extend backwards from the upper part of the premaxilla. The lowest of these is the palatal process, which was prominent in Duriavenator compared to other basal (early diverging) tetanurans, and the two processes above together form a subnarial (below the nostril) process. A foramen opens hind-wards between the palatal and subnarial processes, with another foramen opening just behind it. One of these foramina probably opened into a canal for the nerve or blood vessel that also continued into the foramen on the surface of the maxilla that connected with the premaxilla, a feature also seen in Megalosaurus.
The holotype specimen only preserves two tooth sockets in the left premaxilla (the right premaxilla does not preserve any), while four premaxillary teeth were reported prior to the preparation of the specimen; two preserving the tooth crowns, one preserved as a cross-section of its original position, and one as an imprint of a juvenile tooth. Today, this number of teeth cannot be observed directly in either the specimen or casts of it made prior to preparation. Waldman suggested there were five tooth sockets in the premaxilla, and if this is true, Duriavenator would have been unique among megalosauroids, which have either lower or higher numbers of premaxillary teeth. The surface of the maxilla that connects with the premaxilla is slightly concave, and there are eleven foramina about in diameter each as well as smaller foramina present on the lower part of this surface. Such foramina are absent in most basal tetanurans, but are present in Carcharodontosaurus where they extend further upwards than in Duriavenator, and the condition in Duriavenator is considered an autapomorphy, a distinguishing feature.