Sarcosuchus


Sarcosuchus is an extinct genus of crocodyliform and distant relative of living crocodylians that lived during the Early Cretaceous, from the late Hauterivian to the early Albian, 133-112 million years ago of what is now Africa and South America. It was one of the largest crocodile-line reptiles, reaching an average estimate of 9 meters and 3.5 tonnes but estimated to grow up to 9.5 m in body length and weigh up to 4.3 tonnes. It is known from two species, S. imperator from the early Albian Elrhaz Formation of Niger and S. hartti from the Late Hauterivian of north-east Brazil, other material is known from Morocco and Tunisia and possibly Libya and Mali.
The first remains were discovered during several expeditions led by the French paleontologist Albert-Félix de Lapparent, spanning from 1946 to 1959, in the Sahara. These remains were fragments of the skull, vertebrae, teeth, and scutes. In 1964, an almost complete skull was found in Niger by the French CEA, but it was not until 1997 and 2000 that most of its anatomy became known to science, when an expedition led by the American paleontologist Paul Sereno discovered six new specimens, including one with about half the skeleton intact and most of the spine.

Description

Sarcosuchus was a giant relative of crocodiles, with fully grown individuals estimated to have reached up to in total length and in weight. It had somewhat telescoped eyes and a long snout comprising 75% of the length of the skull. There were 35 teeth in each side of the upper jaw, while in the lower jaw there were 31 teeth in each side. The upper jaw was also noticeably longer than the lower one leaving a gap between them when the jaws were shut, creating an overbite. In young individuals the shape of the snout resembled that of the living gharial but in fully grown individuals it became considerably broader.

Bulla

At the end of its snout, Sarcosuchus presented an expansion, known as a bulla, which has been compared to the ghara seen in gharials. However, unlike the ghara, which is only found in male gharial, the bulla is present in all Sarcosuchus skulls that have been found so far, suggesting that it was not a sexually dimorphic trait. The purpose of this structure remains enigmatic.

Osteoderms

The osteoderms, also known as dermal scutes, of Sarcosuchus were similar to those goniopholodids like Sunosuchus and Goniopholis; they formed an uninterrupted surface that started in the posterior part of the neck up to the middle of the tail like is seen in Araripesuchus and other basal crocodyliforms, different from the pattern seen in living crocodiles, which present discontinuity between the osteoderms of the neck and body.

Size

A common method to estimate the size of crocodiles and crocodile-like reptiles is the use of the length of the skull measured in the midline from the tip of the snout to the back of the skull table, since in living crocodilians there is a strong correlation between skull length and total body length in subadult and adult individuals irrespective of their sex, this method was used by Sereno et al. for Sarcosuchus due to the absence of a complete enough skeleton. Two regression equations were used to estimate the size of S. imperator, they were created based on measurements gathered from 17 captive gharial individuals from northern India and from 28 wild saltwater crocodile individuals from northern Australia, both datasets supplemented by available measurements of individuals over in length found in the literature. The largest known skull of S. imperator is long, and it was estimated that the individual it belonged to had a total body length of, its snout-vent length of was estimated using linear equations for the saltwater crocodile and in turn this measurement was used to estimate its body weight at. This shows that Sarcosuchus was able to reach a maximum body size not only greater than previously estimated but also greater than that of the Miocene Rhamphosuchus, the Late Cretaceous Deinosuchus and the Miocene Purussaurus according to current estimates at that time.
However, extrapolation from the femur of a subadult individual as well as measurements of the skull width further showed that the largest S. imperator was significantly smaller than was estimated by Sereno et al. based on modern crocodilians. O’Brien et al. estimated the length of the largest S. imperator specimen at 9.5 meters and body weight at 4.3 tonnes based on longirostrine crocodylians skull width to total length ratio. This estimate is very close to the femur based estimate.

Classification

Sarcosuchus is commonly classified as part of the clade Pholidosauridae, a group of crocodile-like reptiles related but outside Crocodylia. Within this group it is most closely related to the North American genus Terminonaris. Most members of Pholidosauridae had long, slender snouts and they all were aquatic, inhabiting several different environments, some forms are interpreted as marine, capable of tolerating saltwater while others, like Sarcosuchus, were freshwater forms, the most primitive members of the clade, however, were found in coastal settings, zones of mixing of freshwater and marine waters. Sarcosuchus stands out among pholidosaurids for being considered a generalist predator, different from most known members of the clade which were specialized piscivores. A 2019 study found it to be in a more derived position in Tethysuchia, being phylogenetically closer to Dyrosauridae.
Simplified cladogram after Fortier et al..

Discovery and naming

Early findings

During the course of several expeditions on the Sahara from 1946 to 1959, led by the French paleontologist Albert-Félix de Lapparent, several fossils of a crocodyliform of large size were unearthed in the region known as the Continental Intercalaire Formation, some of them were found in Foggara Ben Draou, in Mali and near the town of Aoulef, Algeria while others came from the Ain el Guettar Formation of Gara Kamboute, in the south of Tunisia, the fossils found were fragments of the skull, teeth, scutes and vertebrae. In 1957, in the region now known as the Elrhaz Formation in the north of Niger several isolated teeth of great size were found by H. Faure. The study of this material by French paleontologist France De Broin helped identify them as coming from a new long snouted crocodile.
Later, in 1964, the research team of the French CEA discovered an almost complete skull in region of Gadoufaoua, in the north of Niger, said skull was shipped to Paris for study and became the holotype of the then new genus and species Sarcosuchus imperator in 1966. The genus name comes from the Greek σάρξ meaning flesh and σοῦχος meaning crocodile.

Fossils from Brazil

In 1977, a new species of Sarcosuchus was recognised, S. hartti, from remains found in the late 19th century in late Hauterivian pebbly conglomerates and green shales belonging to the Ilhas Group in the Recôncavo Basin of north-eastern Brazil. In 1867, American naturalist Charles Hartt found two isolated teeth and sent them to the American paleontologist O. C. Marsh who erected a new species of Crocodylus for them, C. hartti, this material, along with other remains were assigned in 1907 to the genus Goniopholis as G. hartti. Now residing in the British Museum of Natural History the fragment of the lower jaw, dorsal scute and two teeth compromising the species G. hartti were reexamined and conclusively placed in the genus Sarcosuchus.

Recent findings

The next major findings occurred during the expeditions led by the American paleontologist Paul Sereno in 1995, 1997 and the follow-up trip in 2000. Partial skeletons, numerous skulls and 20 tons of assorted other fossils were recovered from the deposits of the Elrhaz Formation, which has been dated as late Aptian or early Albian stages of the Late Cretaceous. It took about a year to prepare the Sarcosuchus remains.
Fossils were found in 2010 in the Ifezouane Formation of Morocco. Fossil teeth from the area of Nalut in northwestern Libya, possibly Hauterivian to Barremian in age, might be referable to S. imperator. Indeterminate Sarcosuchus material including dorsal osteoderms in anatomical connection, isolated teeth and fragmentary skeletal remains including a left scapula, mandible fragment, dorsal vertebrae, illium and a proximal portion of a femur was described from the Oum Ed Dhiab Member in Tunisia in 2018

Paleobiology

Growth pattern

Sereno took thin sections from trunk osteoderms of an estimated subadult individual. Approximately 40 lines of arrested growth were counted in these thin sections, suggesting that S. imperator took 50 to 60 years to reach adult size. Given that extant wild crocodylians rarely reach these advanced ages, Sereno suggested that S. imperator achieved its large size by extending its period of rapid, juvenile, growth. A similar growth strategy has been suggested for the equally titanic crocodylian Deinosuchus, based on similar criteria.

Diet

Based on the broader snout of fully grown S. imperator when compared to the living gharial and other narrow-snouted crocodiles, along with a lack of interlocking of the smooth and sturdy-crowned teeth when the jaws were closed, Sereno et al. hypothesized that S. imperator had a generalized diet similar to that of the Nile crocodile, which would have included large terrestrial prey such as the abundant dinosaurs that lived in the same region.
However, a 2014 analysis of a biomechanical model of its skull suggested that unlike Deinosuchus, Sarcosuchus may not have been able to perform the "death roll" maneuver used by extant crocodylians to dismember their prey. This suggests that if S. imperator did hunt big game, it probably did not dismember prey in the same fashion as extant crocodylians.

Habitat

The remains of S. imperator were found in a region of the Ténéré Desert named Gadoufaoua, more specifically in the Elrhaz Formation of the Tegama Group, dating from the late Aptian to the early Albian of the Early Cretaceous, approximately 112 million years ago. The stratigraphy of the region and the aquatic fauna that was found therein indicates that it was an inland fluvial environment, entirely freshwater in nature with a humid tropical climate. S. imperator shared the waters with the holostean fish Lepidotus and the coelacanth Mawsonia. The dinosaur fauna was represented by the iguanodontian Lurdusaurus, which was the most common dinosaur in the region, and its relative Ouranosaurus; there were also two sauropods, Nigersaurus and a currently unnamed sauropod while the theropod fauna included the spinosaurid Suchomimus, the carcharodontosaurid Eocarcharia and the abelisaurid Kryptops.
Meanwhile, S. hartti was found in the Recôncavo Basin of Brazil, specifically in the Ilhas Formation of the Bahia series, it was a shallow lacustrine environment dating from the late Aptian, similar in age to the habitat of S. imperator, with similar aquatic fauna, including Lepidotus and two species of Mawsonia. The dinosaur fauna is of a very fragmentary nature and identification does not go beyond indeterminate theropod and iguanodontid remains.