Sarcosuchus

Sarcosuchus (meaning 'flesh crocodile' and commonly called "SuperCroc", is an extinct genus of crocodyliform and distant relative of the crocodile. It dates from the early Cretaceous Period of what is now Africa and is one of the largest giant crocodile-like reptiles that ever lived. It was almost twice as long as the modern saltwater crocodile and weighed approximately 8 to 10 tonnes.

Until recently, all that was known of the genus was a few fossilised teeth and armour scutes, which were discovered in the Sahara Desert by the French paleontologist Albert-Félix de Lapparent, in the 1940s or 1950s. However, in 1997 and 2000, Paul Sereno discovered half a dozen new specimens, including one with about half the skeleton intact and most of the spine. All of the other giant crocodiles are known only from a few partial skulls, so which is actually the biggest is an open question.

Description
When fully mature, Sarcosuchus is believed to have been as long as a city bus (11.2–12.2 meters or 37–40 ft) and weighed up to 8 tonnes (8.75 tons). The largest living crocodilian, the saltwater crocodile, is less than two-thirds of that length (6.3 meters or 20.6 ft is the longest confirmed individual) and a small fraction of the weight (1,200 kg, or 1.3 tons). The very largest Sarcosuchus is believed to have been the oldest. Osteoderm growth rings taken from an 80% grown individual (based on comparison to largest individual found) suggest that Sarcosuchus kept growing throughout its entire 50–60 year average life span (Sereno et al., 2001). Modern crocodiles grow at a rapid rate, reaching their adult size in about a decade, then growing more slowly afterwards.

Its skull alone was as big as a human adult (1.78 m, or 5 ft 10 inches). The upper jaw overlapped the lower jaw, creating an overbite. The jaws were relatively narrow (especially in juveniles). The snout comprises about 75% of the skull's length (Sereno et al., 2001).

The huge jaw contained 132 thick teeth (Larsson said they were like "railroad spikes" ). The teeth were conical, adapted for grabbing and holding; instead of narrow, adapted for slashing (like the teeth of some land-dwelling carnivores), and more like that of true crocodilians. Sarcosuchus could probably exert a force of 80 kN (18,000 lbf) with its jaw, making it very unlikely that prey could escape. It had a row of bony plates or osteoderms, running down its back, the largest of which were 1 m (3 ft) long. The scutes served as armor and may have helped support its great mass, but also restricted its flexibility.

Sarcosuchus also had a strange depression at the end of its snout. Called a bulla, it has been compared to the ghara seen in gharials. Unlike the ghara, though, the bulla is present in all Sarcosuchus skulls that have been found so far. This suggests it was not a sexually selected character (only the male gharial has a ghara). The purpose of this structure remains enigmatic. Sereno and others asked various reptile researchers what their thoughts on this bulla were. Opinions ranged from it being an olfactory enhancer to being connected to a vocalization device (Geology News, 2001).

Behaviour and diet
Like true crocodiles, Sarcosuchus might have made a wide range of vocalisations, from grunts and squeaks to hisses, growls, barks, bellows and roars. These sounds may have been used by Sarcosuchus to stake out territory, attract mates and to communicate with their progeny.

The eye sockets of Sarcosuchus rotated upwards and were somewhat telescoped (Sereno et al., 2001). This suggests that the animal probably spent most of its time with the majority of its body submerged, watching the shore for prey.

It seems likely that it ate the large fish and turtles of the Cretaceous. As the overhanging jaw and stout teeth are designed for grabbing and crushing, its primary prey may have been large animals and smaller dinosaurs, which it ambushed, dragged into the water, crushed, drowned and then tore apart. It may have come into conflict with Suchomimus, an 11 m (36 ft) theropod dinosaur with a gharial-like snout, whose fossils were found in the same geological formation as Sarcosuchus. According to Sereno, "because the ancient animal was so large, it could easily handle huge dinosaurs, including the massive long-necked, small-headed sauropods that were common in that African region".

Other crocodilian biologists are skeptical of the animal's "giant killing" capabilities. The long, thin snout of Sarcosuchus was very similar to the thin snouts of the modern gharial, the false gharial and the slender-snouted crocodile, all of which are nearly exclusive fish-eaters and incapable of tackling large prey. This can be contrasted to both the modern Nile crocodile and the extinct Deinosuchus, both of which exhibit very broad, heavy skulls, suitable for dealing with large prey. This, coupled with the abundance of large, lobe-finned fish in its environment, leads many to suggest that, far from being a dinosaur killer, Sarcosuchus was simply a large piscivore, a scaled-up version of the modern gharial. However, while the snout of juvenile Sarcosuchus strongly resembled modern narrow-snouted crocodiles in width, it expands dramatically in mature individuals (Sereno et al., 2001). While still comparatively narrower than the snout of a Nile crocodile, the snout is still much wider than the snouts of crocodilians like the gharial. In addition, the teeth do not interlock, like those of mostly piscivorous crocodilians. This suggests that, like the Nile crocodile, it may have complemented a primarily fish diet with terrestrial animals, at least upon maturity.

It is pertinent to note, though, that the lobe-finned fish that shared the waters with Sarcosuchus were often in excess of 1.8 m (6 ft) long and 90 kg (200 lb) in weight. This raises the possibility of those adaptations, which seem to indicate large or moderate-sized terrestrial prey, may instead have been adaptations for dealing with exceptionally large fish (many species of which possessed a layer of osteoderms, for protection).

Environment
110 million years ago, in the Early Cretaceous, the Sahara was still a great tropical plain, dotted with lakes and crossed by rivers and streams that were lined with vegetation. Based on the number of fossils discovered, the aquatic Sarcosuchus was probably plentiful in these warm, shallow, freshwater habitats.

Unlike modern true crocodiles, which are very similar in size and shape to one another and tend to live in different areas; Sarcosuchus was just one of many Crocodyliformes, of different sizes and shapes, all living in the same area. Four other species of extinct Crocodyliformes were also discovered in the same rock formation along with the Sarcosuchus, including a dwarf crocodile with a tiny, 8 cm (3 in) long skull. They filled a diverse variety of ecological niches, instead of competing with each other for resources.

Scientific study
The Sarcosuchus remains are from several individuals and include a spine (vertebrae), limb bones, hip bones (a pelvic girdle), the bony armored plates that ran down its back (scutes) and more than a half-dozen skulls. Many crocodyliform skulls are thick and heavy. They tend to be found more frequently than the rest of the body. This is quite a contrast with dinosaurs, whose relatively fragile skulls rarely become part of the fossil record. The osteoderms of ancient reptiles have been used to determine age (Erickson & Brochu, 1999). Since they retain growth rings, like those found in trees (most other bones 'suffer' remodeling with age, which destroys former growth rings), it is theoretically possible to count the age of the individual that the bones belonged to. One 80% grown specimen was discovered with 40 rings, indicating that it had lived for 40 years. This form of growth rate calculation has been somewhat controversial. Others (Schwimmer, 2002) have criticised this form of growth measurement, as annular rings are harder to determine in a creature that lives in an environment that is without extreme seasonality (such as the Mesozoic). No skeleton was complete enough to measure directly, so the maximum length estimate was calculated by measuring the largest skull and comparing it to modern crocodiles. In modern crocodiles, the skull and body are the same proportion regardless of age or sex (Sereno et al., 2001). The primary difference is that species with a long snout have larger heads in proportion to their bodies than species with relatively broad snouts. The length of Sarcosuchus was the average of the expected length of the narrow-snouted gharial and the intermediate-snouted saltwater crocodile, while the mass was the expected mass of the latter. Sereno also measured living crocodilians in India and Costa Rica and used that data in his analysis.

As part of a National Geographic Special, Greg Erickson of Florida State University, Kent Vliet of the University of Florida and Kristopher Lapping of Northern Arizona University provoked American alligators, at the St. Augustine Alligator Farm and Zoological Park in Florida, into biting a bar studded with piezoelectric sensors. The largest alligator they tested was able to exert a force of 9.45 kN (2,125 lbf). By comparing the force exerted by more than 60 animals, they were able to determine that the force exerted was proportional to the size of the animal, which allowed an estimate of the biting power of the Sarcosuchus. This force was calculated to be 80 kN (18,000 lbf).

The giant croc phenomenon

 * "With today's crocs, you basically have a period of fast growth, then a little bit of growth &mdash; this guy wasn't slowing down."
 * &mdash;Paul C. Sereno

Giant crocodiles seem to be a good example of convergence because, according to Schwimmer, "the idea of really big crocs is a repeat theme in evolution". This may in part be due to body design (the armoured plates of the back can provide structural support to a massive body ) and in part due to environment (water can buoy up their massive bodies) (See also: Cope's law).

A study of another giant crocodilian, Deinosuchus, indicated that it grew at about the same rate as modern crocodiles, up to 0.5 m (1.5 ft) per year. It was larger because it kept growing, reaching full adulthood in 35 years, instead of 10. While there is a genetic component, growing that large also requires a rich diet. All the different giant crocodiles must have lived in near-perfect environments, with vast areas of warm, shallow water and abundant prey.

Deinosuchus, which was from the Late Cretaceous of what is now North America, is also a good example of a giant crocodile that is only distantly related to Sarcosuchus. Deinosuchus, which is only known from skulls, had a smaller skull than Sarcosuchus, but a broad snout, like an alligator. This means that the skull of the Deinosuchus is probably a smaller portion of its total body length than the skull of the long-snouted Sarcosuchus, so its total size may be as large, or even larger. Other rivals in size of Sarcosuchus are Purussaurus from the Miocene of what is now Peru and Brazil, and Rhamphosuchus from the Miocene and Pliocene of what is now India. However, their fossils are less complete.

The relationship between the giant crocodilians can be seen in the following simplified evolutionary tree:
 * Neosuchia
 * Sarcosuchus
 * Order Crocodylia
 * Superfamily Alligatoroidea
 * Purussaurus
 * Deinosuchus
 * Mourasuchus
 * Superfamily Gavialoidea
 * Rhamphosuchus

Classification
Sarcosuchus is not an ancestor of modern crocodiles. Also, it is not a crocodilian in the phylogenetic definition of the term. A crocodilian is any member of the clade Crocodilia. Crocodilia includes all modern forms (such as crocodiles proper, alligators etc) and their immediate prehistoric relatives. Sarcosuchus is a member of the family Pholidosauridae, more distantly related to today's crocodilians.

"Crocodile" is a term commonly used in a much broader sense. The first 'crocodile-like reptiles' (the Crocodylomorpha), which split from the bird-line of archosaurs (the group of reptiles that include dinosaurs, pterosaurs and birds) about 230 million years ago (in the Late Triassic), looked somewhat like modern crocodilians. They had long legs, long bodies covered with armour.

Until the 1980s, the pholidosaurids were classified as part of the presumed suborder "Mesosuchia", within the order Crocodylia. However, Benton and Clark determined in 1988 that Mesosuchia was a paraphyletic group, containing the ancestor of all modern crocodiles. A simplified evolutionary tree (Sereno, 1998):


 * Crocodylomorpha
 * Mesoeucrocodylia
 * Metasuchia
 * Neosuchia
 * Pholidosauridae
 * Sarcosuchus
 * Crocodylia (modern crocodiles)

Teeth and osteoderms found in Brazil belong to a close relative of Sarcosuchus. This is possibly evidence that land bridges between Africa and South America existed much later than was previously believed.

On the other hand, based on the structure of the snout, the closest relative of Sarcosuchus is the pholidosauridTerminonaris, with Dyrosaurus and Pholidosaurus as slightly more distant relatives. As a group, they are narrow-snouted fish-eaters from saltwater environments, except for the broader snouted, river-dwelling Sarcosuchus.

Desert discoveries
The fossils were discovered in Gadoufaoua, Niger in the Ténéré Desert, which is part of the Sahara. The first Sarcosuchus teeth and scutes were recovered by the French paleontologist Albert-Félix de Lapparent, in the 1940s or 1950s. It was 1964, however, before a skull was discovered by geologists and brought to the attention of Philippe Taquet. He shipped it to Paris, where it was examined by France de Broin. Together, they formally named and described the species, in 1966, before returning the specimen to Niger.

The crocodile was given the name Sarcosuchus imperator, which is dervived from sarco (meaning 'flesh'), suchus (meaning 'crocodile') and imperator (Latin, meaning 'emperor'). The holotype specimen is MNN 604.

The next major expedition was Paul C. Sereno's trip, in 1997 and the follow-up trip in 2000. He recovered partial skeletons, numerous skulls and 20 tons of assorted other fossils from the deposits of the El Rhaz Formation, which has been dated as Aptian to Albian stages of the late Cretaceous. It took about a year to prepare the Sarcosuchus remains. The discovery was then published on October 25, 2001, in the scientific journal Science by Paul C. Sereno of the University of Chicago and National Geographic's Explorer-in-Residence, Hans C. E. Larsson from Yale University and the University of Toronto (formerly a student at the University of Chicago), Christian Sidor of the New York College of Osteopathic Medicine in Old Westbury, New York and Boubé Gado of the Institut de Recherches en Sciences Humaines in Niamey, Niger.

Further viewing

 * National Geographic Special on SuperCroc. National Geographic Channel, December, 2001.