Suchosaurus

The taxonomic history of Suchosaurus begins in 1822, when British palaeontologist Gideon Mantell and his partner Mary Ann mentioned, in their book The Fossils of the South Downs, teeth that had been discovered by Mantell in a quarry in Tilgate Forest, near Cuckfield in Sussex, England. Based on their morphology, characterized by well-defined lateral ridges, Mantell's mentor William Clift, then curator of the Hunterian Museum, London, suggested that these fossils might belong either to a crocodile or to a monitor lizard, the couple favoring the former interpretation.^[1][2][3] In June 1823, Scottish geologist Charles Lyell brought the fossils described by the Mantell couple to Paris, France, so that they could be examined by French naturalist Georges Cuvier.^[2] In his work published the following year, Cuvier endorsed the initial 1822 interpretation, although he described most of the teeth only very briefly. Nevertheless, he illustrated four of the Tilgate fossil teeth discovered by Mantell.^[4] In 1827, Mantell redescribed these fossils in greater detail, distinguishing them as belonging to two types of crocodilians, separating the blunt-crowned teeth from the more slender and recurved ones, which he considered comparable to those of gharials. Although he did not erect any scientific name to formalize this observation, he attributed the teeth of the second category to a crocodilian that he referred to by the common name "gavial of Tilgate Forest".^[5]

In 1841, British palaeontologist Richard Owen established, within the genus Crocodilus, a subgenus and species he named Crocodilus (Suchosaurus) cultridens, based on the fossils discovered by Mantell.^[6] The name Suchosaurus comes from the Ancient Greek σοῦχος (souchos, "crocodile"), and σαῦρος (saûros, "lizard"), reflecting its then-accepted affinities with crocodilians.^[7] The specific name comes from the Latin culter, "dagger", and dens, "tooth", in reference to the taxon's dental morphology.^[8] In his work published the following year (the same work in which he first coined the term Dinosauria), Owen retained Suchosaurus at the rank of subgenus and also referred to it two fossil vertebrae collected from the original locality by Mantell. He further considered the expression "gavial of Tilgate Forest" to be the vernacular name of this taxon.^[9] The holotype, cited in the scientific literature since the Cuvier's work, consists of a single 3 centimetres (1.2 inches) tooth broken at the upper third of the crown. Like many fossils discovered at Tilgate, this tooth is now housed in the palaeontological collections of the Natural History Museum, London. The original specimen number was BMNH R36536,^[2] but it was later re-catalogued as NHMUK PV R36536.^[10] In 1878, Owen elevated Suchosaurus as a distinct genus and illustrated the vertebrae in question for the first time, while noting that they had not been found in direct association with the holotype tooth.^[11] In the second volume of his 1884 work, when he figured again the holotype tooth of S. cultridens, he named it as S. laevidens.^[12] This designation was probably a lapsus calami, as he did not use this specific epithet elsewhere in the same publication.^[2] In 1888, British naturalist Richard Lydekker determined that the two vertebrae previously attributed to the genus actually belonged to ornithischian dinosaurs. He assigned the first to the family Iguanodontidae and suggested that the second might be referred to the ankylosaur Hylaeosaurus. Lydekker also regarded S. laevidens as a junior synonym of S. cultridens,^[13] a position likewise adopted in 1890 by British palaeontologists Arthur Smith Woodward and Charles Davies Sherborn.^[14]

In 1897, French palaeontologist Henri-Émile Sauvage erected the second species, Suchosaurus girardi, on the basis of two jaw fragments and a single tooth discovered by Swiss geologist Paul Choffat at Boca do Chapim, in Portugal. The specific epithet honors the Portuguese naturalist Albert Girard.^[15] The two jaw fragments are currently numbered as MG 324 at the Museu Geológico [pt] in Lisbon.^[16][17] In 2007, French palaeontologist Éric Buffetaut noted that the associated single tooth had not been relocated in the institution and therefore regarded it as lost. However, he identified a third mandibular fragment, probably complementary to one of the first two and previously undescribed.^[18] In a conference abstract published in 2013, Portuguese palaeontologist Elisabete Malafaia and her colleagues reported the rediscovery of the isolated tooth in the palaeontological collections of the National Museum of Natural History and Science, Lisbon. Now catalogued as MNHN/UL.I.F2.176, the tooth is among the fossils that were saved from a fire that destroyed a significant part of the museum in 1978.^[17]

After some fossil teeth collected in Sussex and the Isle of Wight were routinely labelled as Suchosaurus, the taxon was only rarely mentioned in the scientific literature after the end of the nineteenth century, probably due to the very limited nature of the fossil material.^[2] One of the few notable mentions during the following century comes from Spain, where the palaeontologist José Royo y Gómez [es] in 1927 attributed to this genus fossils discovered in the deposits of Morella, in Castellón.^[19] However, these specimens were never described in detail afterwards and appear to have been lost during the Spanish Civil War.^[2] They were nevertheless briefly mentioned in 1960 by the Spanish palaeontologist Josep Ramon Bataller [ca].^[20]

In 1986, British palaeontologists Alan Charig and Angela Milner described the theropod dinosaur Baryonyx walkeri on the basis of a relatively well-preserved partial skeleton discovered near the Ockley, in Surrey.^[21][22] Its initial descriptions highlighted distinctive dental characters, which led some authors to assign several isolated teeth uncovered in the Wealden Supergroup of England to this genus.^[23][2][3] On the basis of these same morphological criteria, Milner suggested in 2003 that some teeth previously referred to Megalosaurus and Suchosaurus might instead belong to Baryonyx.^[24] In his 2007 study, Buffetaut considered the teeth of S. girardi very similar to those of Baryonyx (and S. cultridens) except for the stronger development of the tooth crown flutes (or "ribs"; lengthwise ridges), 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.^[18][25][2]

In 2010, in a review devoted to the taxonomic history of spinosaurids prior to the original description of the type genus Spinosaurus by the German palaeontologist Ernst Stromer in 1915, Buffetaut stated that it is very likely that the fossils attributed to Suchosaurus by Royo y Gómez belong to the baryonychines, in light of the discovery of additional fossil teeth from this subgroup at Morella.^[2] The discovery of these same teeth had previously been reported in a conference abstract by the Spanish palaeontologist José Ignacio Canudo and his colleagues in 2004.^[26] In 2011, Portuguese paleontologist Octávio Mateus and colleagues agreed that Suchosaurus was closely related to Baryonyx, but considered both species in the former genus nomina dubia (dubious names) since their holotype specimens were not considered diagnostic (lacking distinguishing features) and could not be definitely equated with other taxa. Nevertheless, since its formal reidentification in the early twenty-first century, Suchosaurus has been recognized as the very first spinosaurid to have been named, as well as one of the earliest dinosaurs ever described in the history of palaeontology, although it was not initially identified as such.^[16] Nevertheless, since its formal reidentification in the early 21th century, Suchosaurus has been recognized as the very first spinosaurid to have been named, as well as one of the earliest dinosaurs described in the history of palaeontology, although it was not initially identified as such.^{[24][25][2][3][10][8]} Furthermore, the illustration of the holotype tooth published by Cuvier in 1824 is now regarded as the first printed depiction of a spinosaurid fossil, and as one of the oldest dinosaur figures in general, even though these groups had neither been defined nor named at the time of its publication.^[2]

Although known only from very fragmentary fossil remains, Suchosaurus would likely have had an overall morphology broadly similar to that of many other spinosaurids, i. e. large bipedal carnivore with robust forelimbs and an elongated skull resembling that of crocodiles.^[16][3][27] In 2012, American vertebrate palaeontologist Thomas R. Holtz Jr. tentatively estimated Suchosaurus at around 10 metres (33 ft) in length and weighing between 1 and 4 tonnes (1.1 and 4.4 short tons).^[28][29] And in 2016, Spanish palaeontologists Rubén Molina-Pérez and Asier Larramendi estimated S. cultridens at approximately 8.6 m (28 ft) long, 2.15 m (7.1 ft) tall at the hips and weighing 1.4 t (1.5 short tons).^[30] When Mantell described in 1827 the assemblage of teeth of the "gavial of Tilgate Forest", now recognized as chimeric,^[2] he stated that the largest of these teeth would have belonged to an animal of comparable length, estimated between 6–9 metres (20–30 ft) in length.^[5]

The dentition of Suchosaurus shows some notable differences between the two species. However, the teeth display only the basic characteristics typical of baryonychines and do not exhibit any diagnostic traits allowing distinction at the generic level, which has led the taxon to be regarded as a nomen dubium.^[16] Like Baryonyx, the holotype tooth of S. cultridens is round in cross-section, slightly recurved, and bears numerous flutes on the crown. It differs, however, in having more pronounced fluting and an apparent absence of serrations on the carinae (the anterior and posterior cutting edges of a tooth). Because the carina is strongly worn, it remains difficult to determine whether the tooth was truly devoid of serrations or whether their absence simply results from this wear. The teeth of S. girardi are ziphodont, being subcircular in cross-section and bearing seven denticles per millimetre, a number comparable to that observed in Baryonyx. The roots, very long and slender, exceed half the length of the crown. The teeth of S. girardi are just as strongly fluted as the holotype of S. cultridens, showing eight longitudinal ridges on their inner surface and an enamel texture that is microscopically wrinkled. Nevertheless, unlike the type species, at least one tooth of S. girardi appears to possess a serrated anterior carina. Little information has been reported about the rare fossil jaw fragments of the holotype of S. girardi because of their very incomplete preservation, but they are understood to represent the right portion of the dentary (the largest bone of the lower jaw in diapsids).^[18][16]

Though few skull material has been discovered for Suchosaurus,^[18][16] it is known that spinosaurid skulls resembled those of crocodiles; they were long, low, narrow and expanded at their front ends into a terminal rosette-like shape, with a robust secondary palate on the roof of the mouth that made them more resistant to stress and bending. In contrast, the primitive and typical condition for theropods was a tall, broader and wedge-like snout with a less developed secondary palate. The skull adaptations of spinosaurids converged with those of crocodilians; early members of the latter group had skulls similar to typical non-avian (or non-bird) 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.^[37][38] Most theropod dinosaurs have recurved, blade-like teeth with serrated carinae for slicing through flesh, whereas spinosaurid teeth evolved to be straighter, more conical, and have small or nonexistent serrations. Such dentition is seen in living piscivorous predators such as gharials, as it is better suited for piercing and maintaining grip on slippery aquatic prey so it can be swallowed whole, rather than torn apart.^[27][37][39]