Duriavenator

''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building when Cleminshaw detected fossils embedded in them on cracked surfaces of the stones, and all such stones were withdrawn from the building yard.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.

The frontwards directed anterior process of the maxilla is about as long as it is high, which is long compared to some theropods, like non-tetanurans, carcharodontosaurines and sinraptorids, which either lack such a process or have one that is short. A tab-like flange projects frontwards from the side margin of the anterior process, which must have overlapped the side surface of the premaxilla, which is similar to structures seen in ''Torvosaurus'' but absent in ''Dubreuillosaurus''. Because part of the side surface of the maxilla adheres to the counterpart slab, details such as texture and foramina cannot be observed properly there. Two pneumatic (air-filled) chambers can be seen through a break in the maxilla in a corner of the antorbital fossa (a depression in front of the antorbital fenestra). The frontmost of these chambers is the maxillary fenestra (though it forms a fossa rather than a true fenestra), and the hindmost chamber is the frontwards extension of a pneumatic excavation on the upper surface of the jugal process (a process that connects with the ), which forms a deep, longitudinal groove with many foramina. While such pneumatic excavations are present in other tetanurans, in those they form two almost oval excavations that do not extend far hind-wards, and the condition seen in ''Duriavenator'' is therefore a distinguishing feature. The inwards surface of the maxilla is smooth, and an anteromedial process extends from this surface. The preserved part of the vomer has a similar morphology to that of ''Allosaurus''.

All 11 tooth sockets of the maxilla contain emerging or fully-erupted teeth; based on comparison with the similar maxilla of ''Dubreuillosaurus'', 14–16 teeth may have been present in the complete bone, whereas comparison based on ''Megalosaurus'' would indicate fewer than 14 teeth. On the inwards side of the maxilla, replacement teeth at varying development stages can be seen between the interdental plates (bony plates between the teeth). The teeth are curved and , like most theropod teeth, with short (grooves) between the serrations. There are 8–9 serrations per on the front and hind edges at the mid-height of the frontmost tooth crowns, and 8–13 on the crowns further back. The teeth have weak, band-like enamel wrinkles, extending between the front and hind cutting edges of the teeth, along their outer and inner surfaces. The hind cutting edge continues to the base of the tooth crown and the front cutting edge extends about halfway to the crown's base. The interdental plates are roughly pentagonal and unfused, and extend to the bottom level of the side wall of the maxilla. The tallest plate is between the second and third tooth sockets, and the plates decrease in height in front of and behind this. The inwards surfaces of the plates are lightly pitted and have weak striations.

Lower jaw

The dentary of the lower jaw has tall, roughly pentagonal interdental plates on the inwards side of the tooth row, and the inwards sides of these plates have a weakly pitted texture. The plates are separated from the inner wall of the dentary by a paradental groove, which is wide at the front, but narrow hind-wards by the fourth tooth socket, where the inner wall of the dentary contacts the interdental plates. The symphyseal region at the front of the dentary (where the two halves of the lower jaw connected) appears to have been much less distinct than that of ''Allosaurus''. There are three front and upwards curving ridges on the dentary, oriented parallel to the lower surface at the front of the dentary, similar to ''Magnosaurus'' and other megalosauroids.

The Meckelian groove

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