Carcharodontosaurus

Outdated skull reconstruction and endocast of IPHG 1922 X46 by Stromer (1936)

In 1924, two teeth of Carcharodontosaurus were unearthed from wall cuts in different foggaras near Timimoun, French Algeria. These sedimens came from the Cretaceous-aged[1] Continental intercalaire Formation.[2] The fossils were taken to the governor of Timimoun, Captain Burté, who gave them to French geologist Charles Depéret later that year. In 1925, Depéret and his colleague Justin Savornin described the teeth as syntypes (name-bearing specimens) of a new species of theropod dinosaur, Megalosaurus saharicus. These were the first fossils of theropods to be described from the region.[3] The name saharicus refers to the Sahara Desert where the teeth had been found.[4] The genus Megalosaurus was a wastebasket taxon, with many new species referred to it without justification, including M. saharicus.[5] It was later considered to be a species of Dryptosaurus in 1927,[2] though this is unjustified.[6][7] By accident, another species of Megalosaurus, M. africanus, was named by German paleontologist Friedrich von Huene based on the teeth.[6] It is therefore considered a junior synonym of M. saharicus. Both syntypic teeth of M. saharicus have since been lost, possibly being kept in a collection in Algeria, Paris, or Lyon, and lack distinguishing characteristics from other carcharodontosaurids.[8] In 1960, French paleontologist Albert-Félix de Lapparent reported the discovery of more teeth and several caudal vertebrae from sites in Algeria belonging to Carcharodontosaurus,[9] though some of these fossils might belong to other genera.[10] Later authors have mentioned finds of teeth and isolated fossils from other provinces of Algeria.[11][1]

However, fossils of C. saharicus were first found in marls near Ain Gedid, Egypt in early April 1914 by Austro-Hungarian paleontologist Richard Markgraf. Marls from this region derive from the Cenomanian-aged Bahariya Formation, one of many Cretaceous-aged sites of North Africa.[7][12][13] In Bahariya, Markgraf did extensive collecting of dinosaur skeletons for his employer, German paleontologist Ernst Stromer of the Paläontologisches Museum München (Bavarian State Collection of Paleontology). The skeleton of Carcharodontosaurus (IPHG 1922 X46) consisted of: a partial skull, including much of the braincase, teeth, three cervical and a caudal vertebra, incomplete pelvis, a manual ungual, femora, and the left fibula. An isolated ilium was also considered to be from C. saharicus,[14] but it is likely from a ceratosaur.[10]

Due to political tensions between the German Empire and then British-owned Egypt, the Carcharodontosaurus skeleton took years to get to Germany. It was not until 1922 that they were transported overseas to Munich where they were described by Stromer in 1931.[15] Stromer recognized that IPHG 1922 X46’s teeth matched the characteristic dentition of those described by Depéret and Savornin, which led to Stromer conserving the species name saharicus. However, he found it necessary to erect a new genus for this species, Carcharodontosaurus, for their similarities, in sharpness and serrations, to the teeth of the great white shark (Carcharodon carcharias).[13] World War II would break out in 1939, leading IPHG 1922 X46 and other material from Bahariya to be destroyed during a British bombing raid on Munich during the night of April 24/25, 1944.[16][17] An endocast was made and survived the war, being the only remaining relic of the specimen.[12]

Diagram of the skull of C. saharicus, showing known material in white

Few discoveries of Carcharodontosaurus material were made until 1995, when American paleontologist Paul Sereno found an incomplete skull during an expedition embarked on by the University of Chicago. This skull (UCRC PV12) was found in the Cenomanian-aged rocks of the Lower Douira Formation, Kem Kem Beds in Errachidia, southeastern Morocco. The specimen was taken to the University of Chicago and described in 1996 by Sereno and colleagues. In a later paper, UCRC PV12 was designated as the neotype of C. saharicus due to the loss of other specimens and the similar age and geographic location to previously noted material. The taxonomy of Carcharodontosaurus was discussed by Chiarenza and Cau (2016),[18] who suggested that the neotype of C. saharicus was similar but distinct from the holotype in the morphology of the maxillary interdental plates. However, paleontologist Mickey Mortimer put forward that the suggested difference between the C. saharicus neotype and holotype was actually due to damage to the neotype.[10]

Several other fossils of C. saharicus have been unearthed from the Kem Kem Beds, such as dentary fragments, a cervical vertebra, and many teeth.[19][20][12] Sereno et al also referred a multitude of cervical vertebrae described as the spinosaurids Sigilmassasaurus and "Spinosaurus B" to C. saharicus reasoning that stout cervicals would be needed to carry the skulls of carcharodontosaurids.[8][7] Later research proved otherwise, with the vertebrae being placed in Spinosaurus aegyptiacus by Ibrahim et al (2020).[12] French paleontologist René Lavocat was the first to note the possible presence of Carcharodontosaurus in Morocco as early as 1954.[21]

In 2007, a novel species of Carcharodontosaurus, C. iguidensis, was dubbed by paleontologists Steve Brusatte and Paul Sereno. Fossils of C. iguidensis had been uncovered during an expedition to the Echkar Formation of Iguidi, Niger, a partial maxilla (MNN IGU2) being designated the holotype. The species name iguidensis is after Iguidi, where the fossils were unearthed. Several other remains such as a braincase, a lacrimal, a dentary, a cervical vertebra, and a collection of teeth were referred to C. iguidensis based on size and supposed similarities to other Carcharodontosaurus bones. Chiarenza and Cau (2016) identified the referred material of C. iguidensis as belonging to Sigilmassasaurus (later referred to Spinosaurus sp.)[22] and a non-carcharodontosaurine, and therefore chose to limit C. iguidensis to the holotype pending future research.[18] Another carcharodontosaurid from the Kem Kem Beds, Sauroniops pachytholus, was dubbed in 2012 based on a single frontal.[23] This species has been proposed to be synonymous with C. saharicus,[12] though this has seen resistance and the validity maintained in most literature.[23][24][18][25] The South American genus Giganotosaurus was synonymized with Carcharodontosaurus by Figueiredo (1998) and Paul (2010),[26][27] however no authors have since followed this assessment.[18]

• Lapparent (1951, 1960) described several Carcharodontosaurus teeth from the Continental intercalaire Formation of Guermessa, Tunisia.[3]
• A postorbital bone and several postcranial remains assigned to Carcharodontosaurus were found in the Elrhaz Formation of northern Niger. Taquet (1976) noted that the postorbital was similar to that of Acrocanthosaurus, a relative of Carcharodontosaurus,[28] while the postcranial fossils could belong to other theropods.[10]
• Two braincase fragments, 137 teeth, two caudal vertebrae, and a manual phalanx from the Echkar Formation were referred to as Carcharodontosaurus by Lapparent (1960).[9] A pedal phalanx had also been described as Carcharodontosaurus but it likely is from a spinosaurid instead.[29]

Fossil localities of Carcharodontosaurus Legend: C. saharicus C. iguidensis Possible specimens

• Many vertebrae, including two associated dorsals, were found in the Early Cretaceous strata of the Irhazer Group of Agadez, Niger. Lapparent mentioned these fossils as C. saharicus in 1960,[9] though they may belong to other theropod genera.[10]
• Caudal vertebrae from the Tefidet and teeth from Akarazeras sites of the Continental intercalaire Formation of Agadez, Niger were recorded by Lapparent (1960)[9] and Taquet (1976) respectively.[28] The vertebrae could be from other theropods.[10]
• From an unknown locale in the Continental intercalaire of the Sahara Desert, Lapparent (1960) documented eight vertebrae, a humerus, and a manual phalanx as coming from C. saharicus.[9] These elements could be from other theropods.[10]
• Two papers, the first in 1978, have described teeth and a caudal vertebra of Carcharodontosaurus from the Chenini Formation of southern Tunisia.[30] However, the caudal vertebra is now labeled Carcharodontosauridae indet.[31]
• Fossils from the Quseir Formation of western Egypt were assigned to Carcharodontosaurus in 1999.[32]
• A 1999 paper described several vertebrae and limb elements of a carcharodontosaurid which were unearthed from the Wadi Milk Formation of Sudan. These were placed only as carcharodontosaurid, but researcher Mickey Mortimer proposed that they belong to Carcharodontosaurus itself based on the presence of pleurocoels (shallow openings) in the caudal vertebrae.[10]
• In 2015, a large neural arch of a dorsal vertebra from the Kem Kem Beds was informally described as belonging to a new genus and species of megaraptoran dubbed "Osteoporosia gigantea". This specimen is owned head of a Polish theme park chain who described it as belonging to a 15 metres (49 ft) long carnosaur similar to Mapusaurus and Carcharodontosaurus.[33] However, it was much smaller than proposed and may belong to C. saharicus or Sauroniops based on its carcharodontosaurid traits and origin.[34]

• A maxillary tooth recovered from the Villar del Arzobispo Formation of Spain was referred to Carcharodontosaurus in 1966.[35] However, it lacks the traits of carcharodontosaurid teeth and instead is more similar to that of other allosauroids.[36][37]
• Bond and Bromley (1970) described teeth deriving from the Gokwe Formation of Zimbabwe as being similar to Carcharodontosaurus, with Mickey Mortimer assigning them to the genus tentatively.[38] However, later studies have found them to be indeterminate.[39]
• Teeth from the Alcantara Formation of Brazil were placed in Carcharodontosaurus in 2002,[40] but this has been disputed based on its geographic origin.[10]

Stromer hypothesized that C. saharicus was around the same size as the tyrannosaurid Gorgosaurus, which would place it at around 8–9 metres (26–30 ft) long, based on his specimen IPHG 1922 X46. This individual was around 15% smaller than the neotype,[41] the latter was estimated to be 12–12.5 metres (39–41 ft) in length and approximately 6–6.2 metric tons (6.6–6.8 short tons) in body mass.[42][27][43] This makes Carcharodontosaurus saharicus one of the largest known theropod dinosaurs and terrestrial carnivores known, being the 3rd biggest carcharodontosaurid and 5th largest theropod overall according to most estimates.[44][27] C. iguidensis was much smaller, only reaching 10 metres (33 ft) in length and 4 metric tons (4.4 short tons) in body mass.[27]

Life restoration of C. saharicus

The largest and most complete skull of C. saharicus would measure 1.6 metres (5.2 ft) when complete, around the same size as the largest Tyrannosaurus skulls. No skulls of the genus preserve premaxillae, complete posterior skull regions, or mandibles. Skulls of carcharodontosaurids tend to be more slender and lightly built than those of later tyrannosaurids, which have robust builds and adaptations for crushing. The neotype cranium tapers towards the front in side view creating a triangular outline. This is similar to that of other carcharodontosaurids like Mapusaurus and Giganotosaurus. Its skull was lighter than that of tyrannosaurids, with the antorbital fenestra composing over 30% of the total skull length as well as being surrounded by fossae in the maxillae (upper jaw bone), nasals (nose bone), jugals (cheekbone), and lacrimals (front orbit bone). Akin to other genera, its nasal is elongated and bears an anterior face covered in a rugose surface. These bumps were likely extended by keratin sheaths, creating a horn-like structure as in Ceratosaurus. A similar rugosity is found on the lacrimal which would also be lengthened by keratin, forming a similar element.[12][7] The most distinctive trait of Carcharodontosaurus' skull is the sculpted exterior of the maxillae , which is unique to the genus. However, C. iguidensis has antorbital fossae limited to the proximity of the antorbital fenestra, a crest running along the medial face of the maxilla, and a process along its midline. These traits are missing in C. saharicus, differentiating the two species.[8]

Braincase of specimen UCRC PV12 (above), with endocasts of same (A–D) and MB. R. 2056 (E–F) below

The maxilla of IPHG 1922 X46 would have been 70 centimetres (28 in) long when complete, whereas the neotype's complete maxilla is much larger. 14 teeth sockets are present in each maxilla. Parts of the braincase are known though much of their morphology is the same as Giganotosaurus'. However, C. saharicus has a much more prominent nuchal crest, which overhangs the skull roof. The frontal bones are firmly fused, a characteristic evident in most theropods.[45] The jugals are broad and triangle-shaped. The lower jaw articulation was placed farther back behind the occipital condyle (where the neck is attached to the skull) compared to other theropods.[7] Two dentary (lower jaw bone) fragments which were referred to C. saharicus by Ibrahim et al (2020) have deep and expanded alveoli (tooth sockets), traits found in other large theropods.[19][12] If like Tyrannotitan and Giganotosaurus, the dentary would have 16 alveoli (tooth sockets).[46]

Skull bones of specimen UCRC PV12; right maxilla (upper left), nasal and lacrimal (upper right), jugals (lower left), and postorbitals (lower right)

Estimations of the tooth count of Carcharodontosaurus vary, but a recent estimate of 30 dentary, 8 premaxillary, and 24 maxillary teeth for a total of 62 teeth was made.[47] Carcharodontosaurid teeth are some of the largest of any dinosaur group, with a maxillary tooth from IPHG 1922 X46 being 6.8 centimetres (2.7 in) tall and 3.5 centimetres (1.4 in) wide.[13][48] However, they are extremely thin, with most being under a centimeter thick. Serrations are numerous on the anterior and posterior margins, with over 18 to 20 serrations per centimeter of edge in C. saharicus and up to 32 per centimeter in C. iguidensis.[8][13] Its teeth are straight, laterally flattened, and spindle-shaped in cross-section. However, dentition towards the back of the mouth became more recurved than those in the maxilla. The posterior margin of these crowns are recurved and convex at its termination. Bowed enamel wrinkles are present on both dorsoventral sides of the crowns. These wrinkles curve towards the marginal serrations, composing a band-shape along the ends.[7][49][46]

In 2001, Hans C. E. Larsson published a description of the inner ear and endocranium of Carcharodontosaurus saharicus. Starting from the portion of the brain closest to the tip of the animal's snout is the forebrain, which is followed by the midbrain. The midbrain is angled downwards at a 45-degree angle and towards the rear of the animal. This is followed by the hindbrain, which is roughly parallel to the forebrain and forms a roughly 40-degree angle with the midbrain. Overall, the brain of C. saharicus would have been similar to that of a related dinosaur, Allosaurus fragilis. Larsson found that the ratio of the cerebrum to the volume of the brain overall in Carcharodontosaurus was typical for a non-avian reptile. Carcharodontosaurus also had a large optic nerve.[50]

The three semicircular canals of the inner ear of Carcharodontosaurus saharicus—when viewed from the side—had a subtriangular outline. This subtriangular inner-ear configuration is present in Allosaurus, lizards, and turtles, but not in birds.[50] The semi-"circular" canals themselves were very linear, which explains the pointed silhouette. In life, the floccular lobe of the brain would have projected into the area surrounded by the semicircular canals, just like in other non-avian theropods, birds, and pterosaurs.[50]

Pelvis and left hindlimb of IPHG 1922 X46 (destroyed)

Few postcranial elements are confidently known from Carcharodontosaurus, though many isolated bones from the Sahara have been referred to the genus without detailed study.[8][12][28][9] Like other carcharodontosaurids, it was robust with small forelimbs, an elongated tail, and short neck. The most complete specimen was IPHG 1922 X46, but it was destroyed. This specimen preserved 3 cervical vertebrae, which were weathered severely. One is an axis and the other two are articulating anterior cervicals which are longer and wider than the axis. The cervical vertebrae of Carcharodontosaurus are stout and opisthocoelus (concave posterior ends).[51][46] Cervical vertebrae in this genus, as in Giganotosaurus, are topped by low neural spines joined with sturdy transverse processes which hung over the pleurocoels (shallow depressions on the sides of centra), which would contain pneumatic air sacs to lighten the vertebrae. The centra of these vertebrae are adorned by keels along their ventral sides. An anterior caudal vertebra was also known, which was platycoelous (flat anterior and posterior ends) and short. This caudal was incomplete, missing much of the neural spine, but had diapophyses that would conjugate with the chevrons. The sides of its centrum were pleurocoelus as well. Two blade-like chevrons were preserved in this individual as well.[13]

The pelvis was incomplete, containing both pubes and the left ischium, though complete pelves are known in related genera. The ischium pointed backwards whereas the pubes pointed forwards, a diagnostic trait of saurischians. The pubes were likely nearly 1 metre (3.3 ft) when fully preserved, with shafts that were thin but were transversely expanded at the anterior ends where they connected, creating a V-shape in anterior view. Both femora in addition to the left fibula were recovered, the former element being one of the largest recorded from a theropod at 1.26 metres (4.1 ft) in length. Its femora lacked strong curvature and are mostly straight except for the anterior and posterior ends. The greater trochanter is small but has a notable protrusion, which would attach to the m. caudofemoralis longus muscle of the tail. Its fibula was only 88 centimetres (35 in) long, around 1/3rd the length of the femora. The anterior end was triangular in lateral view with bulging condyles whereas the posterior end is rounded.[13]

Coria and Salgado suggested that the convergent evolution of gigantism in theropods could have been linked to common conditions in their environments or ecosystems.[54] Sereno and colleagues found that the presence of carcharodontosaurids in Africa (Carcharodontosaurus), North America (Acrocanthosaurus), and South America (Giganotosaurus), showed the group had a transcontinental distribution by the Early Cretaceous period. Dispersal routes between the northern and southern continents appear to have been severed by ocean barriers in the Late Cretaceous, which led to more distinct, provincial faunas, by preventing exchange.[56][7] Previously, it was thought that the Cretaceous world was biogeographically separated, with the northern continents being dominated by tyrannosaurids, South America by abelisaurids, and Africa by carcharodontosaurids.[57] The subfamily Carcharodontosaurinae, in which Carcharodontosaurus belongs, appears to have been restricted to the southern continent of Gondwana (formed by South America and Africa), where they were probably the apex predators.[52] The South American tribe Giganotosaurini may have been separated from their African relatives through vicariance, when Gondwana broke up during the Aptian–Albian ages of the Early Cretaceous.[58]

A biomechanical analysis of Carcharodontosaurus' lifting capabilities was conducted by paleontologist Donald Henderson and paleoartist Robert Nicholls in 2015. The authors used 3D models of the animal as well as a subadult sauropod Limaysaurus, which although not found alongside Carcharodontosaurus, is similar to the rebbachisaurids of the Kem Kem Beds. The models included the size of the lungs and other pneumatic structures of the two, fostering an accurate weight simulation of the scenario. Henderson & Nicholls' study found that an adult C. saharicus could hold a maximum of 424 kg (935 lb), half the weight of an adult Limaysaurus. However, two C. saharicus adults could together lift as much as 850 kilograms (1,870 lb).[42]

Tooth of C. saharicus

The dentition of allosauroids is distinct, with carcharodontosaurid teeth bearing distinctly thin and blade-like teeth. However, these teeth are thin and likely could not sustain impact against hard surfaces like bone without potentially bending and snapping. This danger is exacerbated by the straight edges, slightly recurved tips, and sinusoidal shapes observed in their dentition. Despite these traits, the teeth are still much more robust than those of smaller theropods and due to their overall size could take more pressure. Carcharodontosaurus also had a high tooth replacement rate meaning that damaged teeth could be replaced easily in contrast to extant bone-crushing mammals who spend much of their energy maintaining their teeth.[59][60] Evidence of bone-crunching bites is observed in Allosaurus, which would engage in ritual face-biting with other individuals and bite into the pelves of Stegosaurus as shown by bite marks.[61][62][63]

Bite forces of Carcharodontosaurus as well as other giant theropods including Acrocanthosaurus and Tyrannosaurus have been analyzed. Studies reported that carcharodontosaurids had much lower bite forces than Tyrannosaurus despite being in the same size class. The anterior bite force of C. saharicus was estimated in a 2022 paper to be 11,312 newtons while the posterior bite force was 25,449 newtons. This is much lower than that of Tyrannosaurus, implying that it did not eat bones.[64][65] Finite element accounts of the skulls of theropods have also been taken, which further supported the idea that Carcharodontosaurus ate softer food than tyrannosaurids. Great amounts of stress were recovered in the posterior part of the cranium near the quadrate in Carcharodontosaurus, Spinosaurus, and Acrocanthosaurus. The skulls of these theropods had higher relative stress quantities in opposition to that of smaller genera. This indicates that the crania of giant taxa (ex. Carcharodontosaurus) were unstable due to having large pneumatic structures to save weight instead of creating a firm build. However, Spinosaurus and Suchomimus experienced even greater values of stress meaning that they could only consume light, small prey instead of larger items, which the stronger skull of Carcharodontosaurus could bite while sustaining the stress.[66]

Skull of C. saharicus showing its elongated, thin rostrum and lack of binocular vision

Isotopic analyses of the teeth of C. saharicus have found δ18O values that are higher than that of the contemporary Spinosaurus, suggesting the latter pursued semi-aquatic habits whereas Carcharodontosaurus was more terrestrial.[67] This is further supported by the taphonomy of C. saharicus teeth, which are more often found in land terrains than aquatic ones.[47] Carcharodontosaurus was also a homeotherm with an endotherm-like thermophysiology as inferred by these isotopes meaning that most of its oxygen was accumulated by drinking water rather than being in it.[68][69]

Theropods such as Carcharodontosaurus, Allosaurus, and Acrocanthosaurus have enlarged lacrimal crests, whose purpose is unknown. Paleontologist Daniel Chure hypothesized that these crests were used for "head-butting" between individuals, but how durable they are has not been studied.[70]

A 2006 study by biologist Kent Stevens analyzed the binocular vision capabilities of the allosauroids Carcharodontosaurus and Allosaurus as well as several coelurosaurs including Tyrannosaurus and Stenonychosaurus. By applying modified perimetry to models of these dinosaurs' heads, Stevens deduced that the binocular vision of Carcharodontosaurus was limited, a side effect of its large, elongated rostrum. Its greatest degree of binocular vision was at higher elevations, suggesting that Carcharodontosaurus may have habitually held its head at a downward 40° angle with its eyes facing up accordingly to achieve maximum binocular vision. The range of vision seen in these allosauroids is comparable to that of crocodiles, suggesting that they were ambush predators. They likely sensed prey via motion parallax between prey and background, with a narrow binocular field of vision helping predators judge prey distances and time attacks.[71]

The neotype skull of C. saharicus is one of many allosauroid individuals to preserve pathologies, with signs of biting, infection, and breaks observed in Allosaurus and Acrocanthosaurus among others.[72][73] This skull bears a circular puncture wound in the nasal and "an abnormal projection of bone on the antorbital rim".[73] A later study theorized that this was the result of craniofacial bites.[74]