Suchomimus

Suchomimus is a genus of large theropod dinosaur that lived in what is now Niger during the Aptian to Albian stages of the Early Cretaceous period, 112 million years ago. The only known species is S. tenerensis, originally described in 1998 by paleontologist Paul Sereno and colleagues from several fossils discovered one year earlier in the Elrhaz Formation. Although these fossils come from multiple specimens, they represent one of the most anatomically well-documented spinosaurids. The animal's generic name, which means "crocodile mimic", alludes to its elongated skull and piscivorous adaptations, while the specific name refers to the Ténéré Desert, where the fossils were discovered.

With an estimated length of and a body mass ranging from , Suchomimus was a particularly large theropod. As its generic name suggests, the animal’s skull was elongated, low, and narrow, resembling that of crocodilians. The tip of the snout flared laterally to form a rosette, and the jaws bore numerous finely serrated conical teeth, with the largest ones positioned near the front. The neck of Suchomimus was relatively short, while its powerfully built forelimbs bore a large, curved claw on each thumb. Along the midline of the animal's back ran a low dorsal sail, built from the long neural spines of its vertebrae. Its cranial and limb features indicate that it was a primarily piscivorous predator adapted for hunting in shallow waters.

Many paleontologists consider Suchomimus to be a probable junior synonym of the contemporaneous spinosaurid Cristatusaurus, although the latter taxon is based on fragmentary remains. Some researchers have also suggested that Suchomimus might represent an African species of the European spinosaurid Baryonyx, and it has occasionally been referred to in the scientific literature as B. tenerensis. However, more recent studies continue to regard the two genera as distinct. According to the fossil record of the Elrhaz Formation, Suchomimus lived and hunted in a fluvial environment of vast floodplains alongside many other dinosaurs, in addition to pterosaurs, crocodylomorphs, bony fishes, turtles, and bivalves.

History of study

Discovery and naming

thumb|left|Outcrops of the [[Erlhaz Formation (Gadoufaoua in lower right)|alt=]]

In late 1997, American paleontologist Paul Sereno and his team conducted an expedition at the Gadoufaoua site in the Ténéré Desert of Niger, where they uncovered various fossils. On 4 December, team member David Varricchio discovered a large thumb claw belonging to a massive theropod dinosaur, remarkably well exposed on the surface. According to Sereno, the sand and wind had gradually revealed this claw, which had remained visible in this state for at least two centuries. Subsequent excavations at the site recovered several fossils from different individuals of this enigmatic theropod, which proved to belong to the spinosaurid family. On 13 November 1998, the Science Magazine published a study led by Sereno, formally naming and describing the new genus and species Suchomimus tenerensis based on the fossils discovered during the expedition. The generic name Suchomimus derives from the Ancient Greek (souchos, "crocodile"), and (mimos, "mimic") literally meaning "crocodile mimic", in reference to its elongated snout and piscivorous adaptations. The specific name tenerensis refers to the desert where the animal’s fossils were found.

thumb|upright=1.2|alt=|Digital [[Paleoart|reconstruction of the Suchomimus skeleton showing known bones based on the holotype (in red), a partial skeleton (in blue), and other referred specimens (in yellow).]]

All known fossil specimens were recovered from the Tegama Beds of the Elrhaz Formation and are now housed in the paleontological collections of the Musée National Boubou Hama in Niamey. The holotype, catalogued as MNN GDF500, consists of a partial skeleton lacking the skull. It contains three neck ribs, parts of fourteen dorsal (back) vertebrae, ten dorsal ribs, gastralia (or "belly ribs"), pieces of three sacral vertebrae, parts of twelve caudal (tail) vertebrae, chevrons (bones that form the underside of the tail), a scapula (shoulder blade), a coracoid, a partial forelimb, most of the pelvis (hip bone), and parts of a hindlimb. Other notable specimens are also mentioned in the paper describing the taxon. Specimens MNN GDF 501 to 508 include a snout, a quadrate from the back of the skull, three dentaries (tooth-bearing bones of the lower jaw), an axis (second neck vertebra), a rear cervical vertebra, and a rear dorsal vertebra. Specimens MNN GDF510 and 511 consist of two caudal vertebrae. In the same article, Sereno and colleagues also reported additional bones and teeth attributed to this dinosaur, though their catalogue numbers were not specified.

Suggested synonymy with Baryonyx and Cristatusaurus

thumb|left|upright=1.08|alt=|Fragmentary jaw elements from the holotype specimen of [[Cristatusaurus, on display at the Muséum national d'Histoire naturelle in Paris]]

More than two months before S. tenerensis was formally described in the scientific literature, The fossils of this taxon were first discovered in 1973 by Taquet at Gadoufaoua, who later reported the find and described the fossil premaxillae in a paper published in 1984. Although he did not assign them a scientific name at the time, the author recognized the fossils as belonging to a large theropod of the Spinosauridae family based on anatomical features shared with the now-destroyed holotype of Spinosaurus aegyptiacus. In a 1986 publication, British paleontologists Alan Charig and Angela Milner noted that the jaw elements described by Taquet were nearly indistinguishable from those of Baryonyx walkeri, which they had just described from a partial skeleton dating to the Barremian stage of the Weald Clay Formation in England. In a 1997 follow-up to their preliminary paper, the same authors referred Taquet’s fossils to an indeterminate species of Baryonyx, despite their younger geological age. In their 1998 paper, Sereno and colleagues agreed with Charig and Milner that there were no significant differences between the fossil skulls of Baryonyx and Cristatusaurus, concluding that the latter should be regarded as a nomen dubium. The same year, American paleontologist Hans-Dieter Sues and colleagues concluded that S. tenerensis was sufficiently similar to B. walkeri to be placed within the same genus, renamed as Baryonyx tenerensis, and that it was likely identical to Cristatusaurus. These interpretations were also concurred by Milner in 2003. In a conference abstract released the following year, American paleontologist Stephen Hutt and British researcher Penny Newbery also supported the synonymy based on a large theropod vertebra discovered on the Isle of Wight, England, which they attributed to an animal closely related to these two taxa. Nevertheless, subsequent studies have continued to treat Suchomimus and Baryonyx as separate genera. A 2017 review paper by the Brazilian palaeontologist Carlos Roberto A. Candeiro and colleagues stated that this debate was more in the realm of semantics than science, as it is generally agreed that Suchomimus and Baryonyx are distinct, related genera. As for the validity of Cristatusaurus, it continues to be disputed in recent studies. This taxon is generally regarded as a likely senior synonym of Suchomimus, as both originate from the same stratigraphic units and show no sufficiently distinct anatomical differences to justify a clear separation between them. Thus, Cristatusaurus is currently considered as a nomen dubium, pending further analyses that could clarify its taxonomic position. Canadian paleontologists François Therrien and Donald M. Henderson proposed that a long Suchomimus would have weighed more than based on their ratio between skull length and body length; however, they noted that they might have overestimated the size of spinosaurids (i.e., Suchomimus and Baryonyx). The holotype specimen of Suchomimus is considerably larger than that of Baryonyx,

thumb|left|alt=|Reconstructed forelimb and hand of Suchomimus, [[Museum of Ancient Life, Utah]]

The scapula had a rectangular acromion, or attachment site for clavicle (collarbone). The (upper arm bone) was very strongly built, only equaled in size among non-spinosaurid theropods by that of Megalosaurus and Torvosaurus, with robust upper corners. The humerus had a boss (bone overgrowth) above the condyle that contacted its hook-shaped (forearm bone). Accordingly, the of the lower arm was well-developed with an enormous olecranon (upper process set-off from the shaft), an exceptional trait shared with Baryonyx. The heavy arm musculature powered sizable hand claws, that of the first digit (or "thumb") being the largest with a length of . Only the third metacarpal (long bone of the hand) is known, showing a robust morphology (form). In the pelvis, the (main hip bone) was high. The (pubic bone) had a front surface that was wider than the side surface, and its forward-facing lower end was flattened and rectangular, with a brief flange along the midline, in contrast to the expanded boot shape it had in other theropods. The ischium (lower and rearmost hip bone) bore a low obturator flange. The (thighbone) was straight and robust, with a length of in the holotype. Its is markedly plate-like. In the ankle, the astragalus had an ascending process taller than that of Allosaurus.

290x290px|alt=Life restoration of the African spinosaurid Suchomimus tenerensis|thumb|[[Paleoart|Life restoration]]

In their original 1998 paper, Sereno and colleagues analyzed the distribution of forty-five traits to produce a cladogram that showed Suchomimus and Baryonyx to be distinct but closely related genera. The following year, however, American palaeontologist Thomas R. Holtz Jr. and his colleagues regarded Baryonychidae as a synonym of Spinosauridae and reassigned these genera to the latter family,

Evolution

alt=Map of Europe and North Africa|thumb|Distribution of spinosaurids in Europe and North Africa during the [[Cretaceous; 9 is Suchomimus]]

Spinosaurids appear to have been widespread from the Barremian to the Cenomanian stages of the Cretaceous period, 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 that 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. 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. and subsequent discoveries of spinosaurid remains in Asia and possibly Australia indicate that it may have been complex. though this has been critiqued by other researchers who pointed out that in most cases, a carcass would have already been largely emptied out by its initial predators. In a 2017 review of the family, American paleontologist David Hone and Holtz Jr. also considered possible functions in digging for water sources or hard-to-reach prey, as well as burrowing into soil to construct nests. Isotopic geochemical lines of evidence also support a semi-aquatic foraging habit for Suchomimus; its δ^44/42Ca values are highly negative, in contrast to contemporary abelisaurids and carcharodontosaurids, indicating a greater reliance on aquatic resources.

Palaeoecology

left|thumb|Restoration of Suchomimus and the [[sauropods Nigersaurus in the environment of the Elrhaz Formation]]

The Elrhaz Formation, part of the Tegama Group, consists mainly of fluvial sandstones with low relief, much of which is obscured by sand dunes. The sediments are coarse- to medium-grained, with almost no fine-grained horizons. Suchomimus lived in what is now Niger, during the late Aptian to early Albian stages of the Early Cretaceous, 112 million years ago. The sediment layers of the formation have been interpreted as an inland habitat of extensive freshwater floodplains and fast-moving rivers, with a tropical climate that likely experienced seasonal dry periods.), and an undescribed noasaurid. Herbivorous dinosaurs of the region included iguanodontians like Ouranosaurus nigeriensis, Elrhazosaurus nigeriensis, Lurdusaurus arenatus, and two sauropods: Nigersaurus taqueti and an unnamed titanosaur. Crocodylomorphs were abundant, represented by the giant pholidosaur species Sarcosuchus imperator, as well as small notosuchians like Anatosuchus minor, Araripesuchus wegeneri, and Stolokrosuchus lapparenti.