Megalosaurus

 Megalosaurus (meaning "Great Lizard", from Greek μέγας/megas meaning 'big', 'tall' or 'great' and σαυρος/sauros meaning 'lizard') is a genus of large meat-eating theropod dinosaurs of the Middle Jurassic period (Bathonian stage, 166 million years ago) of Southern England. Although fossils from other areas have been assigned to the genus, the only certain remains of Megalosaurus come from the late Middle Jurassic of the Oxfordshire.

The earliest possible fossils of the genus came from the Taynton Limestone Formation. One of these was the lower part of a femur, discovered in the 17th century. It was originally described by Robert Plot as a thighbone of a Roman war elephant, and then as a biblical giant. The first scientific name given for it, in the 18th century, was Scrotum humanum, created by Richard Brookes as a caption; today, this is not considered valid, however.

Megalosaurus was, in 1824, the first genus of dinosaur to be validly named, apart from birds. The type species is Megalosaurus bucklandii, named in 1827. In 1842, Megalosaurus was one of three genera on which Richard Owen based his Dinosauria. On Owen's directions a model was made as one of the Crystal Palace Dinosaurs, which greatly increased the public interest for prehistoric reptiles. Subsequently, over fifty other species would be classified under the genus, originally because dinosaurs were not well known, but even during the 20th century after many dinosaurs had been discovered. Today it is understood these additional species were not directly related to M. bucklandii, which is the only true Megalosaurus species. Because a complete skeleton of it has never been found, much is still unclear about its build.

The first naturalists who investigated Megalosaurus mistook it for a gigantic lizard of twenty metres length. In 1842, Owen concluded that it was no longer than nine metres, standing on upright legs. He still thought it was a quadruped, though. Modern scientists, by comparing Megalosaurus with his direct relatives in the Megalosauridae, were able to obtain a more accurate picture. Megalosaurus was about seven metres long, weighing roughly 1.5 tonnes. It was bipedal, walking on stout hindlimbs, its horizontal torso balanced by a horizontal tail. Its forelimbs were short, though very robust. Megalosaurus had a rather large head, equipped with long curved teeth. It was generally a robust and heavily muscled animal.

"Scrotum humanum"
Megalosaurus may have been the first dinosaur, apart from modern birds, to be described in the scientific literature. Part of a bone was recovered from the Taynton Limestone Formation of Stonesfield limestone quarry, Oxfordshire in 1676. Sir Thomas Pennyson gave the fragment to Robert Plot, Professor of Chemistry at the University of Oxford and first curator of the Ashmolean Museum, who published a description and illustration in his Natural History of Oxfordshire in 1676.[2] It was the first illustration of a dinosaur bone published.[3] Plot correctly identified the bone as the lower extremity of the thighbone or femur of a large animal and he recognized that it was too large to belong to any species known to be living in England. He therefore at first concluded it to be the thighbone of a Roman war elephant and later that of a giant human, such as those mentioned in the Bible.[4] The bone has since been lost, but the illustration is detailed enough that some have since identified it as that of Megalosaurus.[5]

It has also been argued that this possible Megalosaurus bone was given the very first species name ever applied to an extinct dinosaur. Plot's engraving of the Cornwell bone was again used in a book by Richard Brookes in 1763. Brookes, in a caption, called it "Scrotum Humanum," apparently comparing its appearance to a pair of "human testicles".[6] In 1970, paleontologist Lambert Beverly Halstead pointed out that the similarity of Scrotum humanum to a modern species name, a so-called Linnaean "binomen" that has two parts, was not a coincidence. Linnaeus, the founder of modern taxonomy, had in the eighteenth century not merely devised a system for naming living creatures, but also for classifying geological objects. The book by Brookes was all about applying this latter system to curious stones found in England. According to Halstead, Brookes thus had deliberately used binomial nomenclature, and had in fact indicated the possible type specimen of a new biological genus. According to the rules of the International Code of Zoological Nomenclature (ICZN), the name Scrotum humanum in principle had priority over Megalosaurus because it was published first. That Brookes understood that the stone did not actually represent of pair of petrified testicles, was irrelevant. Merely the fact that the name had not been used in subsequent literature meant that it could be removed from competition for priority, because the ICZN states that if a name has never been considered valid after 1899, it can be made a nomen oblitum, an invalid "forgotten name".[7]

In 1993, after the death of Halstead, his friend William A.S. Sarjeant submitted a petition to the International Commission on Zoological Nomenclature to formally suppress the name Scrotum in favour of Megalosaurus. He wrote that the supposed junior synonym Megalosaurus bucklandii should be made a conserved name to ensure its priority. However, the Executive Secretary of the ICZN at the time, Philip K. Tubbs, did not consider the petition to be admissible, concluding that the term "Scrotum humanum", published merely as a label for an illustration, did not constitute the valid creation of a new name, and stated that there was no evidence it was ever intended as such. Furthermore, the partial femur was too incomplete to definitely be referred to Megalosaurus and not a different, contemporary theropod.

Buckland's research
During the last part of the eighteenth century, the number of fossils in British collections quickly increased. According to an hypothesis published by science historian Robert William Theodor Gunther in 1925, among them was a partial lower jaw of Megalosaurus, acquired in October 1797 by Christopher Pegge for 10s.6d. and added to the collection of the Anatomy School of Christ Church College.[9]

In the early nineteenth century, more discoveries were made. In 1815, John Kidd reported the find of bones of giant tetrapods, again at the Stonesfield quarry. The layers there are currently considered part of the Taynton Limestone Formation, dating to the mid-Bathonian stage of the Jurassic Period.[10] The bones were apparently acquired by William Buckland, Professor of Geology at the University of Oxford and dean of Christ Church. Buckland also studied a lower jaw, according to Gunther the one bought by Pegge. Buckland did not know to what animal the bones belonged but, in 1818, after the Napoleonic Wars, the French comparative anatomist Georges Cuvier visited Buckland in Oxford and realised that they were those of a giant lizard-like creature. Buckland further studied the remains with his friend William Conybeare who in 1821 referred to them as the "Huge Lizard". In 1822 Buckland and Conybeare, in a joint article to be included in Cuvier's Ossemens, intended to provide scientific names for both gigantic lizard-like creatures known at the time: the remains found near Maastricht would be named Mosasaurus – then seen as a land-dwelling animal – while for the British lizard Conybeare had devised the name Megalosaurus, from the Greek μέγας, megas, "large". That year a publication failed to occur, but the physician James Parkinson already in 1822 announced the name Megalosaurus, illustrating one of the teeth and revealing the creature was forty feet long and eight feet high.[11] It is generally considered the name in 1822 was still a nomen nudum ("naked name").[12] Buckland, urged on by an impatient Cuvier, continued to work on the subject during 1823, letting his later wife Mary Morland provide drawings of the bones, that were to be the basis of illustrating lithographies. Finally, on 20 February 1824, during the same meeting of the Geological Society of London in which Conybeare described a very complete specimen of Plesiosaurus, Buckland formally announced Megalosaurus. The descriptions of the bones in the Transactions of the Geological Society, in 1824, constitute a valid publication of the name.[5][13] Megalosaurus was the first non-avian dinosaur genus named; the first of which the remains had with certainty been scientifically described was Streptospondylus, in 1808 by Cuvier.

By 1824, the material available to Buckland consisted of specimen OUM J13505, a piece of a right lower jaw with a single erupted tooth; OUM J13577, a posterior dorsal vertebra; OUM J13579, an anterior caudal vertebra; OUM J13576, a sacrum of five sacral vertebrae; OUM J13585, a cervical rib; OUM J13580, a rib; OUM J29881, an ilium of the pelvis, OUM J13563, a piece of the pubic bone; OUM J13565, a part of the ischium; OUM J13561, a thighbone and OUM J13572, the lower part of a second metatarsal. As he himself was aware, these did not all belong to the same individual; only the sacrum was articulated. Because they represented several individuals, the described fossils formed a syntype series. By modern standards, from these a single specimen has to be selected to serve as the type specimen on which the name is based. In 1990, Ralph Molnar chose the famous dentary (front part of the lower jaw), OUM J13505, as such a lectotype.[15] Because he was unaccustomed to the deep dinosaurian pelvis, much taller than with typical reptiles, Buckland misidentified several bones, interpreting the pubic bone as a fibula and mistaking the ischium for a clavicle. Buckland identified the organism as being a giant animal belonging to the Sauria – the Lizards, at the time seen as including the crocodiles – and he placed it in the new genus Megalosaurus, repeating an estimate by Cuvier that the largest pieces he described, indicated an animal twelve metres long in life.

Etymology
Buckland had not provided a specific name, as was not uncommon in the early nineteenth century, when the genus was still seen as the more essential concept. In 1826, Ferdinand von Ritgen gave this dinosaur a complete binomial, Megalosaurus conybeari,[16] which however was not much used by later authors and is now considered a nomen oblitum. A year later, in 1827, Gideon Mantell included Megalosaurus in his geological survey of southeastern England, and assigned the species its current valid binomial name, Megalosaurus bucklandii.[17] Until recently, the form Megalosaurus bucklandi was often used, a variant first published in 1832 by Christian Erich Hermann von Meyer[18] – and sometimes erroneously ascribed to von Ritgen – but the more original M. bucklandii has priority.

Later finds of Megalosaurus bucklandii
The quarries at Stonesfield, which were worked until 1911, continued to produce Megalosaurus bucklandii fossils, mostly single bones from the pelvis and hindlimbs. Vertebrae and skull bones are rare. In 2010, Roger Benson counted a total of 103 specimens from the Stonesfield Slate, from a minimum of seven individuals.[19] It has been contentious whether this material represents just a single taxon. In 2004, Julia Day and Paul Barrett claimed that there were two morphotypes present, based on small differences in the thighbones.[20] In 2008 Benson favoured this idea,[21] but in 2010 concluded the differences were illusory.[19] A maxilla fragment, specimen OUM J13506, was, in 1869 assigned, by Thomas Huxley, to M. bucklandii.[22] In 1992 Robert Thomas Bakker claimed it represented a member of the Sinraptoridae;[23] in 2007, Darren Naish thought it was a separate species belonging to the Abelisauroidea.[24] In 2010, Benson pointed out that the fragment was basically indistinguishable from other known M. bucklandii maxillae, to which it had in fact not been compared by the other authors.[19]

Apart from the finds in the Taynton Limestone Formation, in 1939 Sidney Hugh Reynolds referred remains to Megalosaurus that had been found in the older Chipping Norton Limestone Formation dating from the early Bathonian, about thirty single teeth and bones.[25][26] Though the age disparity makes it problematic to assume an identity with Megalosaurus bucklandii, in 2009 Benson could not establish any relevant anatomical differences with M. bucklandii among the remains found at one site, the New Park Quarry, and therefore affirmed the reference to that species. However in another site, the Oakham Quarry, the material contained one bone, an ilium, that was clearly dissimilar.[10]

Sometimes trace fossils have been referred to Megalosaurus or to the ichnogenus Megalosauripus. In 1997, a famous group of fossilised footprints (ichnites) was found in a limestone quarry at Ardley, twenty kilometres northeast of Oxford. They were thought to have been made by Megalosaurus and possibly also some left by Cetiosaurus. There are replicas of some of these footprints, set across the lawn of the Oxford University Museum of Natural History. One track was of a theropod accelerating from walking to running.[20] According to Benson, such referrals are unprovable, as the tracks show no traits unique to Megalosaurus. Certainly they should be limited to finds that are of the same age as Megalosaurus bucklandii.[19]

Finds from sites outside of England, especially in France, have in the nineteenth and twentieth century been referred to M. bucklandii. In 2010m Benson considered these as either clearly different or too fragmentary to establish an identity.

Early reconstructions
The first reconstruction was given by Buckland himself. He considered Megalosaurus to be a quadruped. He thought it was an "amphibian", i.e. an animal capable of both swimming in the sea and walking on land. Generally, in his mind Megalosaurus resembled a gigantic lizard, but Buckland already understood from the form of the thighbone head that the legs were not so much sprawling as held rather upright. In the original description of 1824, Buckland repeated Cuvier's size estimate that Megalosaurus would have been forty feet long with the weight of a seven foot tall elephant. However, this had been based on the remains present at Oxford. Buckland had also been hurried into naming his new reptile by a visit he had made to the fossil collection of Mantell, who during the lecture announced to have acquired a fossil thighbone of enormous magnitude, twice as long as that just described. Today, this is known to have belonged to Iguanodon, or at least some iguanodontid, but at the time both men assumed this bone belonged to Megalosaurus also. Even taking into account the effects of allometry, heavier animals having relatively stouter bones, Buckland was forced in the printed version of his lecture to estimate the maximum length of Megalosaurus at sixty to seventy feet.[13] The existence of Megalosaurus posed some problems for christian orthodoxy, which typically held that suffering and death had only come into the world through Original Sin, which seemed irreconcilable with the presence of a gigantic devouring reptile during a pre-Adamitic phase of history. Buckland rejected the usual solution, that such carnivores would originally have been peaceful vegetarians, as infantile and claimed in one of the Bridgewater Treatises that Megalosaurus had played a beneficial rôle in creation by ending the lives of old and ill animals, "to diminish the aggregate amount of animal suffering".

Around 1840, it became fashionable in England to espouse the concept of the transmutation of species as part of a general progressive development through time, as expressed in the work of Robert Chambers. In reaction, on 2 August 1841 Richard Owen during a lecture to the British Association for the Advancement of Science claimed that certain prehistoric reptilian groups had already attained the organisational level of present mammals, implying there had been no progress. Owen presented three examples of such higher level reptiles: Iguanodon, Hylaeosaurus and Megalosaurus. For these, the "lizard model" was entirely abandoned: they would have had an upright stance and a high metabolism. This also meant that earlier size estimates had been exaggerated. By simply adding the known length of the vertebrae, instead of extrapolating from a lizard, Owen arrived at a total body length for Megalosaurus of thirty feet. In the printed version of the lecture published in 1842, Owen united the three reptiles into a separate group: the Dinosauria. Megalosaurus was thus one of the three original dinosaurs.

The presumption that carnivorous dinosaurs, like Megalosaurus, were quadrupeds was first challenged by the find of Compsognathus in 1859. That, however, was a very small animal, the significance of which for gigantic forms could be denied. In 1870, near Oxford, the type specimen of Eustreptospondylus was discovered - the first reasonably intact skeleton of a large theropod. It was clearly bipedal. Shortly afterwards, John Phillips created the first public display of a theropod skeleton in Oxford, arranging the known Megalosaurus bones, held by recesses in cardboard sheets, in a more or less natural position.[24] During the 1870s, North American discoveries of large theropods, like Allosaurus, confirmed that they were bipedal. The Oxford University Museum of Natural History display contains most of the specimens from the original description by Buckland.

Diagnosis
For decades after its discovery, Megalosaurus was seen by researchers as the definitive or typical large carnivorous dinosaur. As a result, it began to function as a wastebasket taxon, and many large or small carnivorous dinosaurs from Europe and elsewhere were assigned to the genus. This slowly changed during the twentieth century, when it became common to restrict the genus to fossils found in the middle Jurassic of England. Further restriction occurred in the late twentieth and early twenty-first centuries, researchers such as Ronan Allain and Dan Chure suggesting that the Stonesfield Slate fossils perhaps belonged to several, possibly not directly related, species of theropod dinosaur. Subsequent research seemed to confirm this hypothesis, and the genus Megalosaurus and species M. bucklandii became generally regarded as limited to the taxon having produced the lectotype, the dentary of the lower jaw. Furthermore, several researchers failed to find any characteristics in that jaw that could be used to distinguish Megalosaurus from its relatives, which would mean the genus were a nomen dubium.[21] However, a comprehensive study by Roger Benson and colleagues in 2008, and several related analyses published in subsequent years, overturned the previous consensus by identifying several autapomorphies, or unique distinguishing characteristics, in the lower jaw that could be used to separate Megalosaurus from other megalosaurids.[21]

Various distinguishing traits of the lower jaw have been established. The longitudinal groove on the outer surface of the dentary is wide. The third tooth socket of the dentary is not enlarged. Seen from above, the dentary is straight without an expanded jaw tip. The interdental plates, reinforcing the teeth from behind, of the lower jaw are tall. Benson also concluded it would be most parsimonious to assume that the Stonesfield Slate material represents a single species. If so, several additional distinctive traits can be observed in other parts of the skeleton. The low vertical ridge on the outer side of the ilium, above the hip joint, shows parallel vertical grooves. The bony skirts between the shafts of the ischia are thick and touch each other forming an almost flat surface. There is a boss present on the lower outer side of the ischium shaft with a rough surface. The underside of the second sacral vertebra has an angular longitudinal keel. A ridge on the upper side of the third metatarsal connected to a groove in the side of the second metatarsal. The middle of the front edge of the scapula forms a thin crest.

Skeleton
Since the first finds, many other Megalosaurus bones have been recovered; however, no complete skeleton has yet been found. Therefore, the details of its physical appearance cannot be certain. However, a full osteology of all known material was published in 2010 by Benson.

Size
Traditionally, most texts, following Owen's estimate of 1841, give a body length of thirty feet or nine metres for Megalosaurus.[33] The lack of an articulated dorsal vertebral series makes it difficult to determine an exact size. David Bruce Norman in 1984 thought Megalosaurus was seven to eight metres long.[34] Gregory S. Paul in 1988 estimated the weight tentatively at 1.1 tonnes, given a thighbone seventy-six centimetres long.[35] The trend in the early twenty-first century to limit the material to the lectotype, inspired even lower estimates, disregarding outliers of uncertain identity. Paul in 2010 stated Megalosaurus was six metres long and seven hundred kilogrammes heavy.[36] However, the same year Benson claimed that Megalosaurus, though medium-sized, was still among the largest of Middle Jurassic theropods. Specimen BMNH 31806, a thighbone 803 millimetres long, would indicate a body weight of 943 kilogrammes, using the extrapolation method of J.F. Anderson — which method, optimised for mammals, tends to underestimate theropod masses by at least a third. Furthermore, thighbone specimen OUM J13561 has a length of about eighty-six centimetres.

General build
In general, Megalosaurus had the typical build of a large theropod. It was bipedal, the horizontal torso being balanced by a long horizontal tail. The hindlimbs were long and strong with three forward-facing weight-bearing toes, the forelimbs relatively short but exceptionally robust and probably carrying three digits. Being a carnivore, its large elongated head bore long dagger-like teeth to slice the flesh of its prey.[33] The skeleton of Megalosaurus is highly ossified, indicating a robust and muscular animal, though the lower leg was not as heavily built as that of Torvosaurus, a close relative.

Skull and lower jaws
The skull of Megalosaurus is poorly known. The discovered skull elements are generally rather large in relation to the rest of the material. This can either be coincidental or indicate that Megalosaurus had an uncommonly large head. The praemaxilla is not known, making it impossible to determine whether the snout profile was curved or rectangular. A rather stubby snout is suggested by the fact that the front branch of the maxilla was short. In the depression around the antorbital fenestra to the front, a smaller non-piercing hollowing can be seen that is probably homologous to the fenestra maxillaris. The maxilla bears thirteen teeth. The teeth are relatively large, with a crown length up to seven centimetres. The teeth are supported from behind by tall, triangular, unfused interdental plates. The cutting edges bear eighteen to twenty denticula per centimetre. The tooth formula is probably 4, 13–14/13–14. The jugal bone is pneumatised, pierced by a large foramen from the direction of the antorbital fenestra. It was probably hollowed out by an outgrowth of an air sac in the nasal bone. Such a level of pneumatisation of the jugal is not known from other megalosaurids and might represent a separate autapomorphy.

The lower jaw is rather robust. It is also straight in top view, without much expansion at the jaw tip, suggesting the lower jaws as a pair, the mandibula, were narrow. Several traits in 2008 identified as autapomorphies, later transpired to have been the result of damage. However, a unique combination of traits is present in the wide longitudinal groove on the outer side (shared with Torvosaurus), the small third dentary tooth and a vascular channel, below the row of interdental plates, that only is closed from the fifth tooth position onwards. The number of dentary teeth was probably thirteen or fourteen, though the preserved damaged specimens show at most eleven tooth sockets. The interdental plates have smooth inner sides, whereas those of the maxilla are vertically grooved; the same combination is shown by Piatnitzkysaurus. The surangular has no bony shelf, or even ridge, on its outer side. There is laterally an oval opening present in front of the jaw joint, a foramen surangulare posterior, but a second foramen surangulare anterior to the front of it is lacking.

Vertebral column
Although the exact numbers are unknown, the vertebral column of Megalosaurus was probably divided into ten neck vertebrae, thirteen dorsal vertebrae, five sacral vertebrae and fifty to sixty tail vertebrae, as is common for basal Tetanurae.[37]

The Stonesfield Slate material contains no neck vertebrae; but a single broken anterior cervical vertebra is known from the New Park Quarry, specimen BMNH R9674. The breakage reveals large internal air chambers. The vertebra is also otherwise heavily pneumatised, with large pleurocoels, pneumatic excavations, on its sides. The rear facet of the centrum is strongly concave. The neck ribs are short. The front dorsal vertebrae are slightly opisthocoelous, with convex front centrum facets and concave rear centrum facets. They are also deeply keeled, with the ridge on the underside representing about 50% of the total centrum height. The front dorsals perhaps have a pleurocoel above the diapophysis, the lower rib joint process. The rear dorsal vertebrae, according to Benson, are not pneumatised. They are slightly amphicoelous, with hollow centrum facets. They have secondary joint processes, forming a hyposphene–hypantrum complex, the hyposphene having a triangular transverse cross-section. The height of the dorsal spines of the rear dorsals is unknown, but a high spine on a tail vertebra of the New Park Quarry material, specimen BMNH R9677, suggests the presence of a crest on the hip area. The spines of the five vertebrae of the sacrum form a supraneural plate, fused at the top. The undersides of the sacral vertebrae are rounded but the second sacral is keeled; normally it is the third or fourth sacral having a ridge. The sacral vertebrae seem not to be pneumatised but have excavations at their sides. The tail vertebrae are slightly amphicoelous, with hollow centrum facets on both the front and rear side. They have excavations at their sides and a longitudinal groove on the underside. The neural spines of the tail basis are transversely thin and tall, representing more than half of the total vertebral height

Appendicular skeleton
The shoulderblade or scapula is short and wide, its length about 6.8 times the minimum width; this is a rare and basal trait within Tetanurae. Its top curves slightly to the rear in side view. On the lower outer side of the blade a broad ridge is present, running from just below the shoulder joint to about midlength where it gradually merges with the blade surface. The middle front edge over about 30% of its length is thinned forming a slightly protruding crest. The scapula constitutes about half of the shoulder joint, which is oriented obliquely sideways and to below. The coracoid is in all known specimens fused with the scapula into a scapulocoracoid, lacking a visible suture. The coracoid as such is an oval bone plate, with its longest side attached to the scapula. It is pierced by a large oval foramen but the usual boss for the attachment of the upper arm muscles is lacking.[19]

The humerus is very robust with strongly expanded upper and lower ends. Humerus specimen OUMNH J.13575 has a length of 388 millimetres. Its shaft circumference equals about half of the total humerus length. The humerus head continues to the front and the rear into large bosses, together forming a massive bone plate. On the front outer side of the shaft a large triangular deltopectoral crest is present, the attachment for the Musculus pectoralis major and the Musculus deltoideus. It covers about the upper half of the shaft length, its apex positioned rather low. The ulna is extremely robust, for its absolute size more heavily built than with any other known member of the Tetanurae. The only known specimen, BMNH 36585, has a length of 232 millimetres and a minimal shaft circumference of 142 millimetres. The ulna is straight in front view and has a large olecranon, the attachment process for the Musculus triceps brachii. Radius, wrist and hand are unknown.

The thighbone is straight in front view. Seen from the same direction its head is perpendicular to the shaft, seen from above it is orientated 20° to the front. The greater trochanter is relatively wide and separated from the robust lesser trochanter in front of it, by a fissure. At the front base of the lesser trochanter a low accessory trochanter is present. At the lower end of the thighbone a distinct front, extensor, groove separates the condyles. At the upper inner side of this groove a rough area is present continuing inwards into a longitudinal ridge, a typical megalosauroid trait. The shinbone, or tibia, is relatively straight, slightly curving inwards. To below, its shaft progressively flattens from front to rear, resulting in a generally oval cross-section. For about an eighth of its length the front lower end of the shaft is covered by a vertical branch of the astragalus. Of the foot, only the second, third and fourth metatarsals are known, the bone elements that were connected to the three weight-bearing toes. They are straight and robust, showing ligament pits at their lower sides. The third metatarsal has no clear condyles at its lower end, resulting in a more flexible joint, allowing for a modicum of horizontal movement. The top inner side of the third metatarsal carries a unique ridge that fits into a groove along the top outer side of the second metatarsal, causing a tighter connection.