|Reconstruction of the holotype skeleton, Natural History Museum, London|
Charig & Milner, 1986
Charig & Milner, 1986
Baryonyx (//) is a genus of theropod dinosaur which lived in the Barremian stage of the early Cretaceous Period, about 130–125 million years ago. The holotype specimen was discovered in 1983 in Surrey, England, and the animal was named B. walkeri in 1986. The generic name, Baryonyx, means "heavy claw" and alludes to the animal's very large claw on the first finger; the specific name (walkeri) refers to its discoverer, amateur fossil hunter William J. Walker. Fragmentary specimens were later discovered in other parts of the United Kingdom and Iberia. The holotype specimen is one of the most complete theropod skeletons from the UK, and its discovery attracted media attention.
The holotype specimen, which may not have pertained to a fully grown individual, was estimated to be between 7.5 and 10 m (25 and 33 ft) long and to weigh between 1.2 and 1.7 t (1.3 and 1.9 short tons). Baryonyx had a long, low, and narrow snout, which has been compared to that of a gharial. The tip of the snout expanded to the sides in the shape of a rosette. Behind this, the upper jaw had a notch which fitted into the lower jaw (which curved upwards in the same area). It had a triangular crest on the top of its nasal bones. Baryonyx had many finely serrated, conical teeth, with the largest teeth in front. The neck formed an S-shape, and the neural spines of its dorsal vertebrae increased in height from front to back. It had robust forelimbs, with the eponymous first-finger claw measuring about 31 cm (12 in) long.
Now recognised as a member of the family Spinosauridae, Baryonyx's affinities were obscure when it was discovered. Apart from the type species (B. walkeri), some researchers have suggested that Suchomimus tenerensis belongs in the same genus and that Suchosaurus cultridens is a senior synonym; subsequent authors have kept them separate. Baryonyx was the first theropod dinosaur demonstrated to have been piscivorous (fish-eating), as evidenced by fish scales in the stomach region of the holotype specimen. It may also have been an active predator of larger prey and a scavenger, since it also contained bones of a juvenile Iguanodon. The creature would have caught and processed its prey primarily with its forelimbs and large claws. Baryonyx may have had aquatic habits, and coexisted with other theropod, ornithopod, and sauropod dinosaurs, as well as pterosaurs, crocodiles, turtles and fishes, in a fluvial environment.
Baryonyx is estimated to have been between 7.5 and 10 m (25 and 33 ft) long, 2.5 m (8.2 ft) in hip height, and to have weighed between 1.2 and 1.7 t (1.3 and 1.9 short tons). The fact that elements of the skull and vertebral column of the B. walkeri holotype specimen (NHM R9951) do not appear to have co-ossified (fused) suggests that the individual was not fully grown, and the mature animal may have been much larger. On the other hand, the specimen's fused sternum indicates that it may have been fairly mature.
The skull of Baryonyx is incompletely known, and much of the middle and hind portions are not preserved. The full length of the skull has been estimated to be 950 mm (37.4 in), based on comparison with that of the related genus Suchomimus (which is 20% larger). It was elongated, and the front 170 mm (6.6 in) of the premaxillae formed a long, low snout (rostrum) with a rounded upper surface. The nostrils, far back from the tip, passed horizontally from one side of the skull to the other. The front 130 mm (5.1 in) of the snout expanded into a spatulate (flared outwards to the sides), "terminal rosette" shape similar to the modern gharial, and the front 70 mm (2.7 in) of the lower margin was downturned (or hooked). The snout was very narrow just behind the rosette. The creature's maxilla and premaxilla fit together in a complex articulation, resulting in a strongly curved tooth row. The gap in the row is comparable to that of Dilophosaurus. The front 140 mm (5.5 in) of the dentary in the mandible curved upwards towards this area, and the gap between the upper and lower jaw is known as the subrostral notch. The snout had extensive pits (which would have been exits for blood vessels and nerves), and the maxilla appears to have housed sinuses.
Baryonyx had a rudimentary secondary palate, similar to crocodiles but unlike most theropod dinosaurs. A rugose (roughly wrinkled) surface suggests the presence of a horny pad in the roof of the mouth. It had a sagittal crest above the eyes, on the upper mid-line of the nasal bones, which was triangular, narrow, and sharp in front. The lacrimal bone in front of the eye appears to have formed a horn core similar to those seen, for example, on Allosaurus. The dentary was very long and shallow, with a prominent Meckelian groove. The rest of the lower jaw was fragile; the hind third was much thinner than the front, with a blade-like appearance. The front part of the dentary curved outwards to accommodate the large front teeth, and this area formed the mandibular part of the rosette. The dentary had many foramina (openings), which were passages for nerves and blood vessels. It has been suggested that some of Baryonyx's cranial bones had been misidentified (resulting in the occiput's too-deep reconstruction), and the skull was probably as low, long and narrow as that of the closely related Suchomimus.
Most of the teeth found with the holotype specimen were not attached to the skull; a few remained in the upper jaw, and only small replacement teeth were in the lower jaw. The teeth had the shape of recurved cones, flattened somewhat sideways. The larger teeth were less recurved than the smaller ones, but were otherwise uniform. The roots were very long, and the teeth slender. The carinae (edges) of the teeth were finely serrated with denticles on the front and back. There were seven narrow, uniform denticles per millimetre (0.039 in), more than in most theropods. Some of the teeth were fluted, with six to eight ridges along the length of their inner sides and fine-grained enamel. The inner side of each tooth row had a bony wall. The number of teeth was large, with seven teeth in the right premaxilla (other theropods have three to five) and thirty-two in the dentary, where sixteen is typical. The lower jaw would have had sixty-four teeth, and the difference between the number of teeth in the upper and lower jaws is more pronounced than in other theropods. The teeth in the dentary were more densely packed than those in the maxilla, and probably smaller. The terminal rosette in the upper jaw had thirteen dental alveoli (tooth sockets), six on the left and seven on the right side; the first four were large (with the second and third the largest), while the fourth and fifth progressively decreased in size. The diameter of the largest was twice that of the smallest. The first four alveoli of the dentary (corresponding to the tip of the upper jaw) were the largest, with the rest more regular in size. Interdental plates were between the alveoli.
The neck formed a straighter S shape (a sigmoid curve typical of theropods) than that seen in other theropods; in fact, the neck was initially thought to lack the S curve. The shape of the cervical vertebrae indicate that they tapered towards the head and were progressively longer front to back. The neural spines of the cervical vertebrae were low, thin, and were not always sutured to the centra (the bodies of the vertebrae). The axis vertebra, small relative to the size of the skull, had a well-developed hyposphene. The centra of the dorsal vertebrae were similar in size. Like other dinosaurs, Baryonyx reduced its weight (skeletal pneumaticity) with fenestrae (openings) in the neural arches and with pleurocoels (hollow depressions) in the centra (primarily near the transverse processes). From front to back, the neural spines of the dorsal vertebrae changed from short and stout to tall and broad.
The scapulae (shoulder blades) were robust; the bones of the forelimb were short in relation to the animal's size, but broad and sturdy. The humerus was short and stout, with its ends broadly expanded and flattened—the upper side for the deltopectoral crest and muscle attachment and the lower for articulation with the radius and ulna. The radius was short, stout and straight, and the olecranon of the ulna apparently very powerful. The lower part of the ulna had a broad expansion. The hands had three fingers; the first finger bore a large claw (ungual bone) measuring about 31 cm (12 in) along its curve, which would have been lengthened by a keratin sheath in life. Apart from its size, the claw's proportions were fairly typical of a theropod; it was bilaterally symmetric, slightly compressed, smoothly rounded, and sharply pointed. A groove for the sheath ran along the length of the claw. The pubic foot of the pelvis was not expanded.
History of discovery
In January of 1983 the amateur fossil hunter William J. Walker discovered a large claw, a phalanx bone, and part of a rib in Smokejacks Pit, a clay pit near Ockley in Surrey, England. The tip of the claw was missing, but Walker found it a week later. The British palaeontologists Alan J. Charig and Angela C. Milner examined the finds at the Natural History Museum of London and found more bones at the site in February, but the entire skeleton could not be collected until May and June due to conditions at the pit. A team of eight museum staff members and several volunteers excavated two tonnes of matrix. Walker donated the claw to the museum, and the Ockley Brick Company (owners of the pit) donated the rest of the skeleton and provided equipment. The area had been explored for 200 years, but no similar remains had been found before.
Most of the bones collected were encased in siltstone nodules surrounded by fine sand and silt, with the rest lying in clay. The bones were disarticulated and scattered over a 5 x 2 m (17 x 8 ft) area, but most were not far from their natural positions. The position of some bones was disturbed by a bulldozer, and some were broken by mechanical equipment before they were collected. Preparing the specimen was difficult, due to the hardness of the siltstone matrix and the presence of siderite; acid preparation was attempted, but most of the matrix was removed mechanically. The specimen represents about 65 percent of the skeleton, and consists of partial skull bones; teeth; cervical, dorsal and caudal vertebrae; ribs; a sternum; coracoids; arm and hand bones; claws; hip bones, and leg bones. The original specimen number was BMNH R9951, but it was later re-catalogued as NHMUK VP R9951.
In 1986 Charig and Milner made the skeleton the holotype specimen of a new genus and species: Baryonyx walkeri. The genus name derives from ancient Greek; βαρύς (barys) means "heavy" or "strong", and ὄνυξ (onyx) means "claw" or "talon". The specific name honours Walker, for discovering the specimen. At that time, the authors did not know if the large claw belonged to the hand or the foot (as in dromaeosaurs, which it was then assumed to be). Due to ongoing work on the bones (70 percent had been prepared at the time), they called their article preliminary (a "Letter to Nature") and promised a more detailed description at a later date. Baryonyx was the first large Early Cretaceous theropod found anywhere in the world by that time. Before the discovery of Baryonyx the last significant theropod find in the United Kingdom was Eustreptospondylus in 1871, and in a 1986 interview Charig called Baryonyx "the best find of the century" in Europe. It was widely featured in international media, and its discovery was the subject of a 1987 BBC documentary. Baryonyx was nicknamed "Claws" by journalists punning on the title of the film Jaws. A cast of the skeleton is mounted at the Natural History Museum in London, and in 1997 Charig and Milner published a monograph describing the holotype skeleton in detail. The holotype specimen remains the most completely known spinosaurid skeleton.
Fossils from other parts of the UK and Iberia, mostly isolated teeth, have subsequently been attributed to Baryonyx or similar animals. Isolated teeth and bones from the Isle of Wight, including hand bones reported in 1998 and a vertebra reported by the British palaeontologists Steve Hutt and Penny Newbery in 2004, have been attributed to this genus. In 2017 the British palaeontologist Martin C. Munt and colleagues reported cranial remains of two Baryonyx individuals from the Isle of Wight, and stated they would be examined and described in the future. A maxilla fragment from La Rioja, Spain, was attributed by the Spanish palaeontologists Luis I. Viera and José Angel Torres in 1995 (although the American palaeontologist Thomas R. Holtz and colleagues raised the possibility that it could have belonged to Suchomimus in 2004). In 1999 a postorbital bone, a squamosal bone, a tooth, vertebra remains, metacarpals, and a phalanx from the Sala de los Infantes deposit in Burgos Province, Spain, were attributed to an immature Baryonyx (though some of these elements are unknown in the holotype) by the Spanish palaeontologist Carolina Fuentes Vidarte, and colleagues, and dinosaur tracks near Burgos have been identified as those of Baryonyx or a similar theropod. In 2011 a specimen (ML1190) from the Papo Seco Formation in Boca do Chapim, Portugal, with a fragmentary dentary, teeth, vertebrae, ribs, hip bones, a scapula, and a phalanx bone, was attributed to Baryonyx by the Portuguese palaeontologist Octávio Mateus and colleagues, the most complete Iberian remains of the animal. The skeletal elements of this specimen are also represented in the more complete holotype (which was of similar size), except for the mid-neck vertebrae. In 2018, the British palaeontologist Thomas M. S. Arden and colleagues found that the Portuguese skeleton did not belong to Baryonyx, since the front of its dentary bone was not strongly upturned. Some additional spinosaurid remains from Iberia appear to belong to taxa other than Baryonyx.
In 1997, Charig and Milner noted that two fragmentary spinosaurid snouts from the Elrhaz Formation of Niger reported by the French palaeontologist Philippe Taquet in 1984 were similar enough to Baryonyx that they considered them to belong to an indeterminate species of Baryonyx, despite their much younger geological age. In 1998, these fossils became the basis of the genus and species Cristatusaurus lapparenti, named by Taquet and the American palaeontologist Dale Russell. The American palaeontologist Paul Sereno and colleagues named the new genus and species Suchomimus tenerensis later in 1998, based on more complete fossils from the Elrhaz Formation. In 2002 the German palaeontologist Hans-Dieter Sues and colleagues proposed that Suchomimus tenerensis was similar enough to Baryonyx walkeri to be considered a species within the same genus (as B. tenerensis), and that Suchomimus was identical to Cristatusaurus. Milner concurred about this synonymy in 2003. At about 9.5 m (30 ft) and 2.5 tonnes (5,511 lb), Suchomimus was 30 percent larger than Baryonyx. In a 2004 conference abstract, Hutt and Newberry supported the synonymy based on a large theropod vertebra from the Isle of Wight which they attributed to an animal closely related to Baryonyx and Suchomimus, which they used to reconfigure the spinal column of Baryonyx. Later studies have kept Baryonyx and Suchomimus separate, whereas Cristatusaurus has been proposed to be either a nomen dubium (dubious name) or possibly distinct from both. A 2017 review paper by the Brazilian palaeontologist Carlos Roberto A. Candeiro and collages stated that this debate was more in the realm of semantics than science, as it is generally agreed that B. walkeri and S. tenerensis are distinct, related species.
In 2003, Milner noted that teeth at the Natural History Museum previously identified as belonging to the genus Suchosaurus and Megalosaurus probably belonged to Baryonyx. The type species of Suchosaurus, S. cultridens, was named by the British biologist Richard Owen in 1841 based on teeth from Tilgate Forest in Sussex, discovered by the British geologist Gideon Mantell. Owen originally thought the teeth to have belonged to a crocodile; he was yet to name the group Dinosauria, which happened only the following year. A second species, S. girardi, was named by the French palaeontologist Henri Émile Sauvage in 1897 based on jaw fragments and a tooth from Boca do Chapim. In 2007 the French palaeontologist Éric Buffetaut considered the teeth of S. girardi very similar to those of Baryonyx (and S. cultridens) except for the stronger development of the crown flutes (or "ribs"), suggesting that the remains belonged to the same genus. Buffetaut agreed with Milner that the teeth of S. cultridens were almost identical to those of B. walkeri, but with a ribbier surface. The former taxon might be a senior synonym of the latter (since it was published first), depending on whether the differences were within a taxon or between different ones. According to Buffetaut, since the holotype specimen of S. cultridens is a single tooth and that of B. walkeri is a skeleton, it would be more practical to retain the newer name. In 2011 Mateus and colleagues agreed that Suchosaurus was closely related to Baryonyx, but considered both species in the former genus nomina dubia since their holotype specimens were not considered diagnostic (lacking distinguishing features) and could not be definitely equated with other taxa.
In their original description, Charig and Milner found Baryonyx unique enough to warrant a new family of theropod dinosaurs: Baryonychidae. They found Baryonyx to be unlike any other theropod group (and considered the possibility that it was a thecodont, due to apparently primitive features), but noted that the articulation of the maxilla and premaxilla was similar to that in Dilophosaurus. They also noted that the two snouts from Niger (which later became the basis of Cristatusaurus), assigned to the family Spinosauridae by Taquet in 1984, appeared almost identical to that of Baryonyx and they referred them to Baryonychidae instead. In 1988, the American palaeontologist Gregory S. Paul agreed with Taquet that Spinosaurus, described in 1915 based on fragmentary remains from Egypt which were destroyed in World War II, and Baryonyx were similar and (due to their kinked snouts) possibly late-surviving dilophosaurs. Buffetaut also supported this relationship in 1989. In 1990 Charig and Milner dismissed the spinosaurid affinities of Baryonyx, since they did not find their remains similar enough. In 1997, they agreed that Baryonychidae and Spinosauridae were related, but disagreed that the former name should become a synonym of the latter, because the completeness of Baryonyx compared to Spinosaurus made it a better type genus for a family, and because they did not find the similarities between the two significant enough. Holtz and colleagues listed Baryonychidae as a synonym of Spinosauridae in 2004.
Discoveries in the 1990s shed more light on the relationships of Baryonyx and its relatives. In 1996, a snout from Morocco was referred to Spinosaurus, and Irritator and Angaturama from Brazil (the two are possible synonyms) were named. Cristatusaurus and Suchomimus were named based on fossils from Niger in 1998. In their description of Suchomimus, Sereno and colleagues placed it and Baryonyx in the new subfamily Baryonychinae within Spinosauridae; Spinosaurus and Irritator were placed in the subfamily Spinosaurinae. Baryonychinae was distinguished by the small size and larger number of teeth in the dentary behind the terminal rosette, the deeply keeled front dorsal vertebrae, and by having serrated teeth. Spinosaurinae was distinguished by their straight tooth crowns without serrations, small first tooth in the premaxilla, increased spacing of teeth in the jaws, and possibly by having their nostrils placed further back and the presence of a deep neural spine sail. They also united the spinosaurids and their closest relatives in the superfamily Spinosauroidea, but in 2010 Roger Benson considered this a junior synonym of Megalosauroidea (an older name). In 2017 study by the Brazilian palaeontologists Marcos A. F. Sales and Cesar L. Schultz found that the clade Baryonychinae was not well supported, since serrated teeth may be an ancestral trait among spinosaurids.
Spinosaurids appear to have been widespread from the Barremian to the Cenomanian ages of the Cretaceous, about 130 to 95 million years ago, while the oldest known spinosaurid remains date to the Middle Jurassic. They shared features such as long, narrow, crocodile-like skulls; sub-circular teeth, with fine to no serrations; the terminal rosette of the snout, and a secondary palate which made them more resistant to torsion. In contrast, the primitive and typical condition for theropods was a tall, narrow snout with blade-like (ziphodont) teeth with serrated carinae (front and back edges). The skull adaptations of spinosaurids converged with those of crocodilians; early members of the latter group had skulls similar to typical theropods, later developing elongated snouts, conical teeth, and secondary palates. These adaptations may have been the result of a dietary change from terrestrial prey to fish. Unlike crocodiles, the post-cranial skeletons of baryonychine spinosaurids do not appear to have aquatic adaptations. Sereno and colleagues proposed in 1998 that the large thumb-claw and robust forelimbs of spinosaurids evolved in the Middle Jurassic, before the elongation of the skull and other adaptations related to fish-eating, since the former features are shared with their megalosaurid relatives. They also suggested that the spinosaurines and baryonychines diverged before the Barremian age of the Early Cretaceous.
Several theories have been proposed about the biogeography of the spinosaurids. Since Suchomimus was more closely related to Baryonyx (from Europe) than to Spinosaurus—although that genus also lived in Africa—the distribution of spinosaurids cannot be explained as vicariance resulting from continental rifting. Sereno and colleagues proposed that spinosaurids were initially distributed across the supercontinent Pangea, but split with the opening of the Tethys Sea. Spinosaurines would then have evolved in the south (Africa and South America: in Gondwana) and baryonychines in the north (Europe: in Laurasia), with Suchomimus the result of a single north-to-south dispersal event. Buffetaut and the Tunisian palaeontologist Mohamed Ouaja also suggested in 2002 that baryonychines could be the ancestors of spinosaurines, which appear to have replaced the former in Africa. Milner suggested in 2003 that spinosaurids originated in Laurasia during the Jurassic, and dispersed via the Iberian land bridge into Gondwana, where they radiated. In 2007, Buffetaut pointed out that palaeogeographical studies had demonstrated that Iberia was near northern Africa during the Early Cretaceous, which he found to confirm Milner's idea that the Iberian region was a stepping stone between Europe and Africa, which is supported by the presence of baryonychines in Iberia. The direction of the dispersal between Europe and Africa is still unknown, and subsequent discoveries of spinosaurid remains in Asia and Australia indicate that it may have been complex.
In 2016 the Spanish palaeontologist Alejandro Serrano-Martínez and colleagues reported the oldest known spinosaurid fossil, a tooth from the Middle Jurassic of Niger, which they found to suggest that spinosaurids originated in Gondwana, since other known Jurassic spinosaurid teeth are also from Africa, but they found the subsequent dispersal routes unclear. Candeiro and colleagues suggested in 2017 that spinosaurids of northern Gondwana were replaced by other predators, such as abelisauroids, since no definite spinosaurid fossils are known that date from after the Cenomanian anywhere in the world. They attributed the disappearance of spinosaurids and other shifts in the fauna of Gondwana to changes in the environment, perhaps caused by transgressions in sea level.
Diet and feeding
In 1986 Charig and Milner suggested that its elongated snout with many finely serrated teeth indicated that Baryonyx was piscivorous (a fish-eater), speculating that it crouched on a riverbank and used its claw to gaff fish out of the water (similar to the modern grizzly bear). Two years earlier, Taquet pointed out that the spinosaurid snouts from Niger were similar to those of the modern gharial and suggested a behaviour similar to herons or storks. In 1997 Charig and Milner rejected their initial proposal that the articulation between the premaxilla and maxilla was mobile. In 1987 the Scottish biologist Andrew Kitchener disputed the piscivorous behaviour of Baryonyx and suggested that it would have been a scavenger, using its long neck to feed on the ground, its claws to break into a carcass, and its long snout (with nostrils far back for breathing) for investigating the body cavity. Kitchener argued that Baryonyx's jaws and teeth were too weak to kill other dinosaurs and too heavy to catch fish, with too many adaptations for piscivory. According to the Irish palaeontologist Robin E. H. Reid, a scavenged carcass would have been broken up by its predator and large animals capable of doing so—such as grizzly bears—are also capable of catching fish (at least in shallow water).
In 1997, Charig and Milner demonstrated direct dietary evidence in the stomach region of the B. walkeri holotype. It contained the first evidence of piscivory in a theropod dinosaur, acid-etched scales and teeth of the common fish Scheenstia mantelli (then classified in the genus Lepidotes), and abraded bones of a young Iguanodon. An apparent gastrolith (gizzard stone) was also found. They also presented circumstantial evidence for piscivory, such as crocodile-like adaptations for catching and swallowing prey: long, narrow jaws with their "terminal rosette", similar to those of a gharial, and the downturned tip and notch of the snout. In their view, these adaptations suggested that Baryonyx would have caught small to medium-sized fish in the manner of a crocodilian: gripping them with the notch of the snout (giving the teeth a "stabbing function"), tilting the head backwards, and swallowing them headfirst. Larger fish would be broken up with the claws. That the teeth in the lower jaw were smaller, more crowded and numerous than those in the upper jaw may have helped the animal grip food. Charig and Milner maintained that Baryonyx would primarily have eaten fish (although it would also have been an active predator and opportunistic scavenger), but it was not equipped to be a macro-predator like Allosaurus. They suggested that Baryonyx mainly used its forelimbs and large claws to catch, kill and tear apart larger prey. In 2004, a pterosaur neck vertebra from Brazil with a spinosaurid tooth embedded in it reported by Buffetaut and colleagues confirmed that the latter were not exclusively piscivorous.
A 2007 finite element analysis of CT scanned snouts by the British palaeontologist Emily J. Rayfield and colleagues indicated that the biomechanics of Baryonyx were most similar to those of the gharial and unlike those of the American alligator and more-conventional theropods, supporting a piscivorous diet for spinosaurids. Their secondary palate helped them resist bending and torsion of their tubular snouts. A 2013 beam-theory study by the British palaeontologists Andrew R. Cuff and Rayfield compared the biomechanics of CT-scanned spinosaurid snouts with those of extant crocodilians, and found the snouts of Baryonyx and Spinosaurus similar in their resistance to bending and torsion. Baryonyx was found to have relatively high resistance in the snout to dorsoventral bending compared with Spinosaurus and the gharial. The authors concluded (in contrast to the 2007 study) that Baryonyx performed differently than the gharial; spinosaurids were not exclusive piscivores, and their diet was determined by their individual size. A preceding 2005 beam-theory study by the Canadian palaeontologist François Therrien and colleagues was unable to reconstruct force profiles of Baryonyx, but found that the related Suchomimus would have used the front part of its jaws to capture prey, and suggested that the jaws of spinosaurids were adapted for hunting smaller terrestrial prey in addition to fish. They envisaged that spinosaurids could have captured smaller prey with the rosette of teeth at the front of the jaws, and finished it by shaking it. Larger prey would instead have been captured and killed with their forelimbs instead of their bite, since their skulls would not be able to resist the bending stress. They also agreed that the conical teeth of spinosaurids were well-developed for impaling and holding prey, with their shape enabling them to withstand bending loads from all directions.
A 2016 study by the Belgian palaeontologist Christophe Hendrickx and colleagues found that adult spinosaurs could displace their mandibular rami (halves of the lower jaw) sideways when the jaw was depressed, which allowed the pharynx (opening that leads from the throat to the stomach) to be widened. This jaw-articulation is similar to that seen in pterosaurs and living pelicans, and would likewise have allowed spinosaurids to swallow large prey such as fish and other animals. They also reported that the possible Portuguese Baryonyx fossils were found associated with isolated Iguanodon teeth, and listed it along with other such associations as support for opportunistic feeding behaviour in spinosaurs. Another 2016 study by the French palaeontologist Romain Vullo and colleagues found that the jaws of spinosaurids were convergent with those of pike conger eels; these fish also have sideways compressed jaws (whereas the jaws of crocodilians are compressed from top to bottom), an elongated snout with a "terminal rosette" that bears enlarged teeth, and a notch behind the rosette with smaller teeth. This type of jaws were likely evolved for grabbing prey in aquatic environments with low light, and may have helped in prey detection.
Motion and aquatic habits
In their original description, Charig and Milner did not consider Baryonyx to be aquatic (due to its nostrils being on the sides of its snout—far from the tip—and the form of the post-cranial skeleton), but thought it was capable of swimming, like most land vertebrates. They speculated that the elongated skull, long neck, and strong humerus of Baryonyx indicated that the animal was a facultative quadruped, unique among theropods. In their 1997 article they found no skeletal support for this, but maintained that the forelimbs would have been strong enough for a quadrupedal posture and it would probably have caught aquatic prey while crouching—or on all fours—near (or in) water. A 2014 re-description of Spinosaurus by the German palaeontologist Nizar Ibrahim and colleagues based on new remains suggested that it was a quadruped, based on its anterior centre of body mass. The authors found quadrupedality unlikely for Baryonyx, since the better-known legs of the closely related Suchomimus did not support this posture.
A 2010 study by the French palaeontologist Auguste Hassler and colleagues proposed that spinosaurids were semiaquatic, based on the oxygen isotope composition of spinosaurid teeth from around the world compared with that of other theropods and extant animals. Spinosaurids probably spent much of the day in water, like crocodiles and hippopotamuses, and had a diet similar to the former; both were opportunistic predators. Since most spinosaurids do not appear to have anatomical adaptations for an aquatic lifestyle, the authors proposed that submersion in water was a means of thermoregulation similar to that of crocodiles and hippopotamuses. Spinosaurids may also have turned to aquatic habitats and piscivory to avoid competition with large, more-terrestrial theropods. A 2018 study by Hassler and colleagues of calcium isotopes in the teeth of North African theropods found that spinosaurids had a mixed diet of fish and herbivorous dinosaurs, whereas the other theropods examined (abelisaurids and carcharodontosaurids) mainly fed on herbivorous dinosaurs. This indicates ecological partitioning between these theropods, and that spinosaurids were semi-aquatic predators.
Various theories have been proposed for the tall neural spines that formed the "sail" of Spinosaurus, such as use in thermoregulation, fat-storage in a hump, or display, and in 2015, the German German biophysicist Jan Gimsa and colleagues suggested that such a sail could also have aided aquatic movement by improving manoeuvrability when submerged. In 2017, the British palaeontologist David E. Hone and Holtz pointed out that (like other theropods) there was no reason to believe that the forelimbs of Baryonyx were able to pronate (crossing the radius and ulna bones of the lower arm to turn the hand), and thereby make it able to rest or walk on its palms. Resting on or using the forelimbs for locomotion may have been possible (as indicated by tracks of a resting theropod), but if this was the norm, the forelimbs would probably have showed adaptations for this. Hone and Holtz furthermore suggested that the forelimbs of spinosaurids do not seem optimal for trapping prey, but instead appear similar to the forelimbs of digging animals. They suggested that the ability to dig would have been useful when excavating nests, digging for water, or to reach some kinds of prey. Hone and Holtz also believed that spinosaurids would have waded and dipped in water rather than submerging themselves, due to their sparsity of aquatic adaptations.
A 2017 histological study of growth lines by the German palaeontologist Katja Waskow and Mateus found that the possible Portuguese Baryonyx specimen had died between the age of 23 and 25 years old, and was close to its maximum size and skeletal maturity. This contradicted a younger age indicated by the neurocentral sutures not being fused, and the presence of both mature and sub-adult traits may be due to paedomorphosis (where juvenile traits are retained into adulthood). Paedomorphic traits may be related to swimming locomotion, as they have been suggested in other extinct animals thought to have been aquatic (such as plesiosaurs and temnospondyls). The study also found that the animal had reached sexual maturity at the age of 13 to 15 years, due to a decrease in growth rate at this point.
The Weald Clay formation consists of sediments of Hauterivian (Lower Weald Clay) to Barremian (Upper Weald Clay) in age, about 130–125 million years old. The B. walkeri holotype was found in the latter, in clay representing non-marine still water, which has been interpreted as a fluvial or mudflat environment with shallow water, lagoons, and marshes. During the Early Cretaceous, the Weald area of Surrey, Sussex, and Kent was partly covered by the large, fresh-to-brackish water Wealden Lake. Two large rivers drained the northern area (where London now stands), flowing into the lake through a river delta; the Anglo-Paris Basin was in the south. Its climate was sub-tropical, similar to the present Mediterranean region. Since the Smokejacks Pit consists of different stratigraphic levels, fossil taxa found there are not necessarily contemporaneous. Dinosaurs from the locality include the ornithopods Mantellisaurus, Iguanodon, and small sauropods. Other vertebrates include sharks (such as Hybodus), bony fishes (including Scheenstia), crocodiles, and pterosaurs. Members of ten orders of insects have been identified, including Valditermes, Archisphex, and Pterinoblattina. Other invertebrates include ostracods, isopods, conchostracans, and bivalves. The plants Weichselia and the aquatic, herbaceous Bevhalstia were common. Other plants found include ferns, horsetails, club mosses, and conifers.
Other dinosaurs from the Wessex Formation of the Isle of Wight include the theropods Neovenator, Aristosuchus, Thecocoelurus, Calamospondylus, and Ornithodesmus; the ornithopods Iguanodon, Hypsilophodon, and Valdosaurus; the sauropods Ornithopsis, Eucamerotus, and Chondrosteosaurus, and the ankylosaur Polacanthus. The Papo Seco Formation of Portugal where Baryonyx has possibly been identified is composed of marl, representing a lagoon environment. Other dinosaur remains from the area include fragments tentatively assigned to Mantellisaurus, a macronarian sauropod, and Megalosaurus.
Charig and Milner presented a possible scenario explaining the taphonomy (changes during fossilisation) of the B. walkeri holotype specimen. The fine-grained sediments around the skeleton, and the fact that the bones were found close together (skull and forelimb elements at one end of the excavation area and the pelvis and hind-limb elements at the other), indicates that the environment was quiet at the time of fossilisation and water currents did not carry the carcass far—possibly because the water was shallow. The area where the specimen died seems to have been suitable for a piscivorous animal. It may have caught fish and scavenged on the mud plain, becoming mired before it died and was buried. Since the bones are well-preserved and had no gnaw marks, the carcass appears to have been undisturbed by scavengers (suggesting that it was quickly covered by sediment). The disarticulation of the bones may have been the result of soft-tissue decomposition. Parts of the skeleton seem to have weathered to different degrees, perhaps because water levels changed or the sediments shifted (exposing parts of the skeleton). The girdle and limb bones, the dentary, and a rib were broken before fossilisation, perhaps from trampling by large animals while buried. The orientation of the bones indicates that the carcass lay on its back, which may explain why all the lower teeth had fallen out of their sockets and some upper teeth were still in place.
Most of the bones of Portuguese specimen ML1190 were damaged, and some scratches may be marks from small scavengers. The specimen's disarticulation indicates it was transported from a more-terrestrial environment (since many bones are missing), but those found were close together.
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- Natural History Museum – "Baryonyx: the discovery of an amazing fish-eating dinosaur" – four minute video presented by Angela Milner
- Media related to Baryonyx at Wikimedia Commons
- Data related to Baryonyx at Wikispecies