Spinosauridae

Size comparison of various spinosaurids with a human

Although reliable size and weight estimates for most known spinosaurids are hindered by the lack of good material, all known spinosaurids were large animals.[2] The smallest genus known from good material is Irritator, which was 6 and 8 meters (20 and 26 ft) long.[3][4] While Ichthyovenator, Baryonyx, and Suchomimus ranged from 7.5 to 11 meters (25 to 36 ft), and weighing between 1 and 5.2 metric tons (1.1 and 5.7 short tons).[5][4][6] Oxalaia may have reached 12 to 14 metres (39 to 46 ft).[7] The largest known genus is Spinosaurus, which was capable of reaching lengths of around 15 meters (49 ft) and weighing around 6.4–7.2 tonnes (7.1–7.9 short tons), making it the largest known terrestrial predator.[8][9] The closely allied Sigilmassasaurus may have grown even larger.[10] This consistency in large body size among spinosaurids could have evolved as a byproduct of their preference for semiaquatic lifestyles, as without the need to compete with other large theropod dinosaurs for food, they would have been able to grow to massive lengths.[11]

Spinosaurid skull anatomy is similar in many respects to that of crocodilians. Spinosaurid skulls were long, low and narrow.[2] As in other theropods, various fenestrae (openings) in the skull aided in reducing its weight. In spinosaurs however, the antorbital fenestrae were greatly reduced, akin to those of crocodilians.[12] The tips of the premaxillae (frontmost snout bones) and dentaries (tooth bearing bones of the mandible) were expanded, forming what has been called a "terminal rosette" of enlarged teeth. Behind this expansion, the upper and lower jaws were notched.[2] The maxillae (main upper jaw bones) were long and formed a low branch under the nostrils that connected to the rear of the premaxillae.[13] Spinosaurid teeth were conical, with an oval to circular cross section and either absent or very fine serrations. Their teeth ranged from straight to slightly recurved, and the crown was often ornamented with longitudinal grooves or ridges.[14][15]

Annotated skull diagram of Spinosaurus

Lengthwise atop their skulls ran a thin and shallow sagittal crest that was usually tallest near or above the eyes, either becoming shorter or disappearing entirely towards the front of the head.[2][9][14] Spinosaurus's head crest was comb shaped and bore distinct vertical grooves,[9] while those of Baryonyx and Suchomimus looked like small triangular bumps.[16][17] Irritator's median crest stopped above the eyes in a bulbous, flattened shape. However, given that no fully preserved skulls are known for the genus, the complete shape of Irritator's crest is unknown.[13] Angaturama (a possible synonym of Irritator) had an unusually tall crest on its premaxillae that nearly overhung the tip of the snout with a small forward protrusion.[14]

Spinosaurid nostrils were set far back on the skull, at least behind the teeth of the premaxillae, instead of at the front of the snout as in most theropods.[2] Those of Baryonyx and Suchomimus were large and started between the first and fourth maxillary teeth, while Spinosaurus's nostrils were far smaller and more retracted. Irritator's nostrils were positioned similarly to those of Baryonyx and Suchomimus, and were between those of Spinosaurus and Suchomimus in size.[14] Spinosaurids had long secondary palates, bony and rugose structures on the roof of their mouths that are also found in extant crocodilians, but not in most theropod dinosaurs.[12] Oxalaia had a particularly elaborate secondary palate, while most spinosaurs had smoother ones.[7]

Neural spine sail variation in three spinosaurid species

Spinosaurids had relatively large and well-built forelimbs, the radius (forearm bone) was stout and usually only half as long as the humerus (upper arm bone). Each hand wielded an enlarged, strongly recurved claw on the first finger (or "thumb"), and were otherwise typical of tetanurans, bearing three fingers.[2][5] The coracoid bones of the shoulders were hook shaped.[17] Spinosaurid hindlimbs were somewhat short and mostly conventional of other megalosauroid theropods.[2][5] Spinosaurus however, had a particularly small pelvic (hip) region and hindlimbs proportional to its body size.[18]

The upwards projecting neural spines of spinosaurid vertebrae (backbones) were very tall, more so than in most theropods. In life, these spines would have been covered in skin or fat tissue and formed a sail down the animal's back, a condition that has also been observed in some carcharodontosaurid and ornithopod dinosaurs.[2][19] The eponymous neural spines of Spinosaurus were extremely tall, measuring over 1 meter (3 ft 3 in) in height on some of the dorsal (back) vertebrae.[20] Suchomimus had a lower, ridge-like sail across the majority of its back, hip, and tail region.[17] Baryonyx showed a reduced sail, with only few of the rearmost vertebrae being elongated.[16] Ichthyovenator had a sinusoidal (wave-like) sail that dipped down and then up again over the hips, with the upper ends of some neural spines being broad and fan-shaped.[21] One partial skeleton possibly referable to Angaturama also had elongated neural spines on its hip region.[22][23] The presence of a sail in fragmentary genera like Sigilmassasaurus is unknown.[24]

Traditionally, Spinosauridae is divided into two subfamilies: Spinosaurinae, which contains the genera Icthyovenator, Irritator, Oxalaia, Sigilmassasaurus and Spinosaurus, is marked by unserrated, straight teeth, and external nares which are further back on the skull than in Baryonychinae.[33][34] And Baryonychinae, which contains the genera Baryonyx and Suchomimus,[35] is marked by serrated, slightly curved teeth, smaller size, and more teeth in the lower jaw behind the terminal rosette than in spinosaurines.[33][34] Others, such as Siamosaurus, may belong to either Baryonychinae or Spinosaurinae, but are too incompletely known to be assigned with confidence.[35] Siamosaurus was classified as a spinosaurine in 2018, but the results are provisional and not entirely conclusive.[36]

The subfamily Spinosaurinae was named by Sereno in 1998, and defined by Holtz and colleagues in 2004 as all taxa closer to Spinosaurus aegyptiacus than to Baryonyx walkeri. The subfamily Baryonychinae was named by Charig & Milner in 1986. They erected both the subfamily and the family Baryonychidae for the newly discovered Baryonyx, before it was referred to the Spinosauridae. Their subfamily was defined by Holtz and colleagues in 2004, as the complementary clade of all taxa closer to Baryonyx walkeri than to Spinosaurus aegyptiacus. Examinations in 2017 by Marcos Sales and Cesar Schultz indicate that the South American spinosaurids Angaturama and Irritator were intermediate between Baronychinae and Spinosaurinae based on their craniodental features and cladistic analysis. This indicates that Baryonychinae may in fact be non-monophyletic. Their cladogram can be seen below.[14]

Skeletons of Suchomimus (above) and Baryonyx (below) to scale

The next cladogram displays an analysis of Tetanurae simplified to show only Spinosauridae from Allain colleagues in 2012:[21]

The 2018 phylogenetic analysis by Arden and colleagues, which included many unnamed taxa, resolved Baryonychinae as monophyletic, and also coined the new term Spinosaurini for the clade of Sigilmassasaurus and Spinosaurus.[36]

Comparison of a spinosaurid skull with that of Dubreuillosaurus and two extant pike conger eels

Spinosaurid teeth resemble those of crocodiles, which are used for piercing and holding prey. Therefore, teeth with small or no serrations, such as in spinosaurids, were not good for cutting or ripping into flesh but instead helped to ensure a strong grip on a struggling prey animal.[37] Spinosaur jaws were likened by Vullo and colleagues to those of the pike conger eel, in what they hypothesized was convergent evolution for aquatic feeding.[38] Both kinds of animals have some teeth in the end of the upper and lower jaws that are larger than the others and an area of the upper jaw with smaller teeth, creating a gap into which the enlarged teeth of the lower jaw fit, with the full structure called a terminal rosette.[38]

Life restoration of Baryonyx with a fish in its jaws

In the past, spinosaurids have often been considered piscivores (fish-eaters) in the main, based on comparisons of their jaws with those of modern crocodilians.[34] British paleontologist Emily J. Rayfield and colleagues, in 2007, conducted biomechanical studies on the skull of Baryonyx, which had a long, laterally compressed skull, comparing it to gharial (long, narrow, tubular) and alligator (flat and wide) skulls. They found that the structure of baryonychine jaws converged on that of gharials, in that the two taxa showed similar response patterns to stress from simulated feeding loads, and did so with and without the presence of a (simulated) secondary palate. The gharial, exemplar of a long, narrow, and tubular snout, is a fish specialist. However, this snout anatomy does not preclude other options for the spinosaurids. The gharial is the most extreme example and a fish specialist; Australian freshwater crocodiles (Crocodylus johnstoni), which have similarly shaped skulls to gharials, also specialize more on fish than sympatric, broad snouted crocodiles and are opportunistic feeders which eat all manner of small aquatic prey, including insects and crustaceans. Thus, spinosaurids' snouts correlate with piscivory; this is consistent with hypotheses of this diet for spinosaurids, in particular baryonychines, but it does not indicate that they were solely piscivorous.[39]

Life restoration of the head of Spinosaurus

Further study by Andrew R. Cuff and Rayfield in 2013 on the skulls of Spinosaurus and Baryonyx did not recover similarities in the skulls of Baryonyx and the gharial that the previous study did. Baryonyx had, in models where the size difference of the skulls was corrected for, greater resistance to torsion and dorsoventral bending than both Spinosaurus and the gharial, while both spinosaurids were inferior to the gharial, alligator, and slender-snouted crocodile in resisting torsion and medio-lateral bending. When the results from the modeling were not scaled according to size, then both spinosaurids performed better than all the crocodilians in resistance to bending and torsion, due to their larger size. Thus, Cuff and Rayfield suggest that the skulls are not efficiently built to deal well with relatively large, struggling prey, but that spinosaurids may overcome prey simply by their size advantage, and not skull build.[40] Sues and colleagues studied the construction of the spinosaurid skull, and concluded that their mode of feeding was to use extremely quick, powerful strikes to seize small prey items using their jaws, whilst employing the powerful neck muscles in rapid up-and-down motion. Due to the narrow snout, vigorous side-to-side motion of the skull during prey capture is unlikely.[37] Based the size and positions of their nostrils, Sales & Schultz in 2017 suggested that Spinosaurus possessed a greater reliance on its sense of smell and had a more piscivorous lifestyle than Irritator and baryonychines.[14]

A fossil of the fish Scheenstia, prey of Baryonyx

Direct fossil evidence shows that spinosaurids fed on fish as well as a variety of other small to medium-sized animals, including dinosaurs. Baryonyx was found with scales of the prehistoric fish Scheenstia in its body cavity, and these were abraded, hypothetically by gastric juices. Bones of a young Iguanodon, also abraded, were found alongside this specimen. If these represent Baryonyx’s meal, the animal was, whether in this case a hunter, or a scavenger, an eater of more diverse fare than fish.[34][37][16] Moreover, there is a documented example of a spinosaurid having eaten a pterosaur, as one Irritator tooth was found embedded within the fossil vertebrae of an ornithocheirid pterosaur found in the Romualdo Formation of Brazil. This may represent a predation or a scavenging event.[41][42] In the Sao Khua Formation of Thailand, isolated tooth crowns from Siamosaurus have been found in association with sauropod remains, indicating possible predation or scavenging.[43]

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 might indicate ecological partitioning between these theropods.[44]

Reconstructed arm of Suchomimus, Museum of Ancient Life, Utah

The use of the robust forelimbs and giant recurved claws of spinosaurs remains a debated topic. Charig and Milner speculated in 1986 that Baryonyx may have crouched by the riverbank and used its claws to gaff fish out of the water, similarly to grizzly bears.[45] In 1987, British biologist Andrew Kitchener argued that with both its crocodile-like snout and enlarged claws, Baryonyx seemed to have too many adaptations for piscivory when one would have been enough. Kitchener instead postulated that Baryonyx more likely used its arms to scavenge the corpses of large dinosaurs, such as Iguanodon, by breaking into the carcass with the large claws, and subsequently probing for viscera with its long snout.[46] In their 1997 article, Charig and Milner rejected this hypothesis, pointing out that in most cases, a carcass would have already been largely emptied out by its initial predators.[16] Later research has also ruled out this sort of specialized scavenging.[2] Charig and Milner further suggested that the robust forelimbs and giant thumb claws would have been Baryonyx's primary method of capturing, killing, and tearing apart large prey; whereas its long snout would have been used mostly for fishing.[16] A 2005 study by Canadian paleontologist the François Therrien and colleagues agreed that spinosaur forelimbs were probably used for hunting larger prey items, given that their snouts could not resist the bending stress.[47] In a 2017 review of the family, David Hone and Holtz considered a possible function in digging for water sources or hard to reach prey, as well as burrowing into soil to construct nests.[2]

Theropod heads were often decorated with some form of crest, horn, or rugose structure.[48] Although there has been little discussion on the head crests of spinosaurs, Hone and Holtz in 2017 considered that their most likely use was for displaying to potential mates or as a means of threatening rivals and predators.[2] Such has been suggested for theropod cranial structures before, which may have been aided by unusual or bright coloration to provide further visual cues.[48] Many theories have been proposed over the years for the use of spinosaurid dorsal sails, such as thermoregulation;[49] to aid in swimming;[50] to store energy or insulate the animal; or for display purposes, such as intimidating rivals and predators, or attracting mates.[51][52]

Juvenile spinosaurid fossils are exceedingly rare and almost unknown. However, an ungual phalanx measuring 21 mm (0.83 in) belonging to a very young Spinosaurus indicates that Spinosaurus and probably by extent other spinosaurids, may have developed their semiaquatic adaptations at birth or while at a very young age and maintained these adaptations throughout their lives. The specimen, found in 1999 and described by Simone Maganuco and Cristiano Dal Sasso and colleagues in 2018, is believed to have come from a very small juvenile measuring 1.78 m (5.8 ft), making said specimen the smallest known example of a spinosaurid currently described.[53][54]

Restoration of Spinosaurus wading in a floodplain

A 2010 publication by Romain Amiot and colleagues found that oxygen isotope ratios of spinosaurid bones indicates semiaquatic lifestyles. Isotope ratios from teeth from Baryonyx, Irritator, Siamosaurus, and Spinosaurus were compared with isotopic compositions from contemporaneous theropods, turtles, and crocodilians. The study found that, among theropods, spinosaurid isotope ratios were closer to those of turtles and crocodilians. Siamosaurus specimens tended to have the largest difference from the ratios of other theropods, and Spinosaurus tended to have the least difference. The authors concluded that spinosaurids, like modern crocodilians and hippopotamuses, spent much of their daily lives in water. The authors also suggested that semiaquatic habits and piscivory in spinosaurids can explain how spinosaurids coexisted with other large theropods: by feeding on different prey items and living in different habitats, the different types of theropods would have been out of direct competition.[55]

In 2018, an analysis was conducted on the partial tibia of an indeterminate spinosaurine from the early Albian, the bone was from a sub-adult between 7–13 m (22–42 ft) in length still growing moderately fast before its death. This specimen (LPP-PV-0042) was found in the Araripe Basin of Brazil and taken to the University of San Carlos for a CT Scan, where it revealed osteosclerosis (high bone density).[11] This condition had previously only been observed in Spinosaurus, as a possible way of controlling its buoyancy.[18] The presence of this condition on the leg fragment showed that semi-aquatic adaptations in spinosaurids were already present at least 10 million years before Spinosaurus aegyptiacus appeared. According to the Phylogenetic bracketing method, this high bone density might have been present in all spinosaurines.[11]

Map illustrating spinosaurid fossil discoveries

Spinosaurids are known to exist from as early as the Late Jurassic, through characteristic teeth which were found in Tendaguru, Tanzania, and attributed to Ostafrikasaurus.[56] Baryonychines were common, as represented by Baryonyx, which lived during the Barremian of England and Spain. Baryonyx-like teeth are found from the earlier Hauterivian and later Aptian sediments of Spain, as well as the Hauterivian of England, and the Aptian of Niger. The earliest record of spinosaurines is from Africa; they are present in Albian sediments of Tunisia and Algeria, and in Cenomanian sediments of Egypt and Morocco. Spinosaurines are also found in Hauterivian and Aptian-Albian sediments of Thailand, and southern China. In Africa, baronychines were common in the Aptian, and then replaced by spinosaurines in the Albian and Cenomanian.[35]

Some intermediate specimens extend the known range of spinosaurids past the youngest dates of named taxa. A single baryonychine tooth was found from the mid-Santonian, in the Majiacun Formation of Henan, China.[57]

NMV P221081, cervical vertebra of a possible Australian spinosaurid from various angles, compared with that of Baryonyx (bottom right)

Confirmed spinosaurids have been found on every continent except for North America and Antarctica, the first of which was discovered in 1912 at the Bahariya Formation in Egypt and described in 1915 as Spinosaurus aegyptiacus.[52] Africa has shown a great abundance in spinosaurid discoveries,[58] such as in the Kem Kem beds of Morocco, which housed an ecosystem containing many large coexisting predators.[59][55] A fragment of a spinosaurine lower jaw from the Early Cretaceous was also reported from Tunisia, and referred to Spinosaurus.[35] Spinosaurinae's range has also extended to South America, particularly Brazil, with the discoveries of Irritator, Angaturama, and Oxalaia.[42][7] There was also a fossil tooth in Argentina which has been referred to spinosauridae by Leonardo Salgado and colleagues.[60] This referral is doubted by Gengo Tanaka, who offers Hamadasuchus, a crocodilian, as the most likely animal of origin for these teeth.[61]

Baryonychines have been found in Africa, with Suchomimus and Cristatusaurus,[35][62][63] as well as in Europe, with Baryonyx and Suchosaurus.[64] Baryonyx-like teeth are also reported from the Ashdown Sands of Sussex, in England, and the Burgos Province, in Spain. A partial skeleton and many fossil teeth indicate spinosaurids were widespread in Asia. As of 2012, three have been named: Ichthyovenator, Siamosaurus and "Sinopliosaurus" fusuiensis.[21][35][57] In 2014, a spinosaurid tooth was discovered in Malaysia; the first dinosaur remains discovered in the country.[65] At la Cantalera-1, a site in the Early Barremanian Blesa Formation in Treul, Spain, two types of spinosaurid teeth were found, and they were assigned, tentatively, as indeterminate spinosaurine and baryonychine taxa.[66] An intermediate spinosaurid was discovered in the Early Cretaceous Eumeralla Formation, Australia.[67] It is known from a single 4 cm long partial cervical vertebra, designated NMV P221081. It is missing most of the neural arch. The specimen is from a juvenile estimated to be about 2 to 3 meters long (6–9 ft). Out of all spinosaurs it most closely resembles Baryonyx.[68] In 2019, it was suggested that the vertebra instead belonged to a megaraptorid theropod, as opposed to a spinosaur.[69]