Stratiotosuchus

''Stratiotosuchus'' (from Greek, (stratiōtēs, "soldier") and (suchos, "crocodile")) is an extinct genus of baurusuchid mesoeucrocodylian from the Adamantina Formation in Brazil. It lived during the Late Cretaceous. The first fossils were found in the 1980s, and the type species ''Stratiotosuchus maxhechti'' was named in 2001. A hyperpredator, it and other baurusuchids may have filled niches occupied elsewhere by theropod dinosaurs.

Description

''Stratiotosuchus'' has a deep, laterally compressed skull long. ''Stratiotosuchus'' reached up to in total length (including tail), making it about the same size as ''Baurusuchus''. The teeth are ziphodont, meaning that they are laterally compressed, curved, and serrated. Like other baurusuchids, ''Stratiotosuchus'' has a reduced number of teeth: three in its premaxilla and five in its maxilla. When the jaw is closed, the teeth of the upper jaw overlie those of the lower jaw and shear closely together. ''Stratiotosuchus'' has one large caniniform tooth in its premaxilla, and several large maxillary teeth behind it. An enlarged fourth dentary tooth in the lower jaw also forms a canine, and is visible when the jaw is closed.

Posture

Like all crocodyliforms, ''Stratiotosuchus'' was quadrupedal. Unlike the sprawling gait of crocodilians living today, ''Stratiotosuchus'' is thought to have been a fully erect quadruped. The transition from a sprawling, low to the ground posture in the ancestors of ''Stratiotosuchus'' to an erect, elevated posture involved a significant transformation of the limbs, hips, and shoulders.

Large crests are present over the acetabulum, or hip socket, stabilizing the hip in what is known as a pillar-erect stance (the acetabular crest lies like a shelf on top of the femur to form a pillar). The ischium bone, which extends backward from the hip, has a large projection that in life would serve as a strong attachment site for the puboischiotibialis muscle. The puboischiotibialis is also present in living crocodilians and is used primarily to keep the legs upright in what is known as a high walk, in which they hold their legs underneath them while walking. The puboischiotibialis is very weak in living crocodilians, so they cannot sustain a high walk for very long. ''Stratiotosuchus'' is thought to have had a much stronger puboischiotibialis, allowing it to have a permanent upright stance.

''Stratiotosuchus'' also has a relatively straight femur bone; while the bone is somewhat twisted along its length, the degree of torsion is not as high as that of other crocodyliforms. The shape of the femur is more similar to that of rauisuchids and poposaurids, which were early crocodile relatives that are known to have had erect gaits. The femur even shares similarities with those of early theropod dinosaurs, which were fully bipedal. When compared to crocodilians, the top of the femur of ''Stratiotosuchus'' is rotated toward the front, so that the femoral head faces backward rather than medially inward. This position restricts the movement of the hindlimbs along a forward-backward or parasagittal axis. Muscles that attach to the side of the leg in crocodilians would have attached to the back of the leg in ''Stratiotosuchus'', enabling a powerful backward extension of the hind leg.

The arm socket faces backwards and downwards from a bone in the shoulder girdle called the coracoid, suggesting that the arms were held beneath the body. The large articular surface on the head of the humerus implies that the arms had a wide range of movement, but restricted to a parasagittal axis. The deltopectoral crest on the front of the humerus would have anchored large arm muscles to bring the arm forward while walking. Modern crocodilians also have a deltopectoral crest, but it is positioned laterally and anchors to muscles that pull the arms up to the sides, not forward. The muscle thought to have facilitated forward movement in ''Stratiotosuchus'' is called the deltoideus clavicularis; it is also present in modern crocodilians, which use it for high walking.

Other features that suggest an erect posture are tightly clustered metacarpals forming narrow hands well-suited for walking and a backward-projecting calcaneal tuberosity in the ankle, which would have attached to muscles that fixed the lower limb in a parasagittal axis. A backward-projecting calcaneal tuberosity is present in most early crocodilian relatives, including those that are thought have sprawling gaits, yet modern crocodilians have more laterally projected tuberosities impeding a parasagittal orientation of the hind foot.

History

The first known fossil of ''Stratiotosuchus'' was a nearly complete skeleton, cataloged as DGM 1477-R. It was found by paleontologist José Martin Suárez in the town of Irapuru in São Paulo State in 1988. This skeleton and all other specimens of ''Stratiotosuchus'' come from the Adamantina Formation, which is either Turonian-Santonian in age (about 85 million years old) or Campanian-Maastrichtian in age (about 70 million years old). DGM 1477-R includes a nearly complete skull, partial lower jaw, vertebral column, and limb bones. The skeleton was identified as that of a baurusuchid, closely related but distinct from '' Baurusuchus pachechoi'', which had been known since 1945. ''Stratiotosuchus maxhechti'' was named in 2001 on the basis of this skeleton, designated the holotype of the species. The generic name, ''Stratiotosuchus'', means "soldier crocodile" in Greek, and the specific name, ''maxhechti'', honors paleontologist Max Knobler Hetch (1925–2002).Beolens, Bo; Watkins, Michael; Grayson, Michael (2011). ''The Eponym Dictionary of Reptiles''. Baltimore: Johns Hopkins University Press. xiii + 296 pp. . (''Stratiotosuchus maxhechti'', p. 119).

Further preparation of DGM 1477-R revealed that two individuals were present in the same block of sandstone, as indicated by two extra leg bones, an extra fragment of the hip, and extra metatarsals. They are the same size as the other bones, suggesting that the second individual was equal in body size to the first.

Classification

''Stratiotosuchus'' has been recognized as a baurusuchid since it was first described in 1988. In 2004, Baurusuchidae was even defined as the most recent common ancestor of ''Baurusuchus'' and ''Stratiotosuchus'' and all of its descendants; thus, the definition of Baurusuchidae relies on the inclusion of ''Stratiotosuchus''. ''Stratiotosuchus'' and ''Baurusuchus'' both belong to a large clade called Metasuchia, which includes living crocodilians and many extinct relatives extending back into the Jurassic. However, the exact position of ''Stratiotosuchus'' and ''Baurusuchus'' within Metasuchia is still uncertain. Below are several possibilities that have been uncovered in various phylogenetic analyses:
* Baurusuchidae is grouped with the family Sebecidae in a clade called Sebecosuchia.
* Sebecids are closer to Neosuchia (the group including modern crocodilians), while baurusuchids are either basal metasuchians or deeply nested within Notosuchia, a large clade of extinct metasuchians.
* Baurusuchidae is polyphyletic, with ''Stratiotosuchus'' and ''Baurusuchus'' positioned as basal metasuchians while other baurusuchids are placed in the clade Sebecia along with sebecids. This also makes Sebecosuchia polyphyletic.

Montefeltro ''et al.'' (2011) found support for baurusuchids as advanced notosuchians, and divided the family into two subfamilies, Baurusuchinae and Pissarrachampsinae. ''Stratiotosuchus'' belonged to Baurusuchinae along with ''Baurusuchus''. Below is the cladogram from Montefeltro ''et al.'' (2011):

Another phylogenetic analysis of baurusuchids was conducted by Riff and Kellner (2011). Their analysis placed ''Stratiotosuchus'' and ''Baurusuchus'' deep within Notosuchia, as the sister taxon of the family Sphagesauridae. Below is the cladogram from that study:

Paleobiology

Paleoenvironment

Based on the types of deposits in the Adamantina Formation, ''Stratiotosuchus'' most likely lived alongside a river system with many small ephemeral lakes.

Similarities to theropod dinosaurs

With a fully erect stance, ''Stratiotosuchus'' has many features convergent with theropod dinosaurs, which are fully bipedal. In ''Stratiotosuchus'', a roughly surfaced region on the upper part of the femur is analogous to the accessory trochanter common to tetanuran theropods. These projections are thought to have anchored the same muscle, called the puboischiofemoralis internus pars dorsalis. A crest on the forward edge of the tibia is similar to those seen in early theropod dinosaurs. The articular surface of the tibia that attaches to the femur is laterally compressed, which is unlike the more circular surface in living crocodilians and more like that of a theropod dinosaur. On the hip of ''Stratiotosuchus'', a depression on the ilium is convergent with the brevis fossa of dinosaurs, and the small bump anchoring the puboischiotibialis muscle is convergent with the obturator tubercle of maniraptoriform theropods.

Along with anatomical similarities, ''Stratiotosuchus'' and other baurusuchids are thought to have had lifestyles very similar to those of theropod dinosaurs. While many small carnivorous crocodyliforms are known from the Adamantina Formation, ''Stratiotosuchus'' and ''Baurusuchus'' are believed to have been the only large carnivores the Adamantina ecosystem. Decades of paleontological exploration in these deposits have uncovered only a few theropod dinosaur bones, so it appears that baurusuchids like ''Stratiotosuchus'' occupied the niche of top predators in the absence of these dinosaurs. A nearby Cretaceous deposit in Argentina called the Neuquén Group also contains baurusuchids, but they are much smaller than ''Stratiotosuchus'' and were likely out-competed by the wide range of theropod dinosaurs known from these deposits. In the absence of large theropods, carnivores like ''Stratiotosuchus'' may have fed on large herbivorous titanosaurs, including '' Adamantisaurus'', ''Aeolosaurus'', '' Gondwanatitan'', and '' Maxakalisaurus''.

Pseudosuchians superficially resembling ''Stratiotosuchus'' were the top predators of the Triassic period, until they were decimated by the Triassic–Jurassic extinction event and replaced by large theropods. The appearance of ''Stratiotosuchus'' and other baurusuchids marks a brief recovery of this top position during the Late Cretaceous.

References

{{Taxonbar, from=Q2206496

Late Cretaceous crocodylomorphs of South America
Terrestrial crocodylomorphs
Fossil taxa described in 2001
Baurusuchids
Taxa named by Alexander Kellner
Adamantina Formation
Prehistoric pseudosuchian genera