Sarcosuchus

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Sarcosuchus

Conservation status
Extinct (fossil)
Scientific classification
Kingdom: Animalia
Phylum: Chordata
Class: Sauropsida
(unranked) Mesoeucrocodylia
Suborder: Tethysuchia
Family: Pholidosauridae
Genus: Sarcosuchus
Binomial name
Sarcosuchus imperator
(Broin & Taquet, 1966)

Sarcosuchus (IPA pronunciation: [ˌsα(r)ko'sukəs]) meaning 'flesh crocodile' and commonly called "SuperCroc", is an extinct genus of 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 largest modern crocodile and weighed up to 10 times as much.

Until recently, all that was known of the species 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.

Contents

[edit] Description

Reconstruction of Sarcosuchus imperator
Reconstruction of Sarcosuchus imperator

When fully mature, Sarcosuchus was believed to be as long as a city bus (11–12 m or 37–40 ft) and weighed up to 8,000 kg (8.8 short tons). The largest living crocodilian, the saltwater crocodile, is less than two-thirds of that length (6.3 meters or 20.6 feet is the longest confirmed individual) and a small fraction of the weight (1,200 kg, or 1.3 tons).

The largest Sarcosuchus was believed to be 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 and designed for grabbing and holding, instead of being narrow and designed for slashing (like the teeth of some land-dwelling carnivores) and more like that of true crocodylians. Sarcosuchus could probably exert a force of 18,000 lbf (80 kN) with its jaw, making it very unlikely that prey could escape.

It had a row of overlapping 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).

[edit] 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 jaw, whose fossils were found in the same geological formation as the SuperCroc. 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 crocodylians like the gharial. In addition, the teeth do not interlock, like those of mostly piscivorous crocodylians, which suggests that, like the Nile crocodile, it may have complemented a primarily piscean 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).

[edit] 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 prehistoric crocodyliformes were also discovered in the same rock formation along with the SuperCroc, 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.

[edit] Scientific study

The SuperCroc 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 crocodyliforme 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 criticized 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 crocodylians 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 2,125 lbf (9.45 kN). 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 SuperCroc. This force was calculated to be 18,000 lbf (80 kN).

[edit] The giant croc phenomenon

"With today's crocs, you basically have a period of fast growth, then a little bit of growth — this guy wasn't slowing down."
—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 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 croc, the 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.

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

The relationship between the giant crocs can be seen in the following simplified evolutionary tree:

[edit] Classification

The SuperCroc is not an ancestor of modern crocodiles. Also it is not a crocodile in the phylogenetic definition of the term. A crocodile is any member of a species belonging to the clade Crocodylia. Crocodylia includes all modern species and all extinct species descended from the nearest common ancestor of all the living crocodilians. The "SuperCroc" is a member of the family Pholidosauridae, a more distantly related form from 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 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". The SuperCroc is more recent, but is still a crocodyliform.

Based on the structure of the snout, the closest relative of Sarcosuchus is the pholidosaur Terminonaris, with Dryosaurus 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.

Until the 1980s, the pholidosaurids were classified as part of the suborder Mesosuchia, within the order Crocodilia. However, Benson and Clark determined, in 1988, that Mesosuchia was a paraphyletic group, from which the ancestor of all modern crocodiles is descended. A simplified evolutionary tree, or cladogram (Sereno, 1998):

  • Crocodyliformes
    • Mesoeucrocodylia
      • Metasuchia
        • Neosuchia
          • Pholidosauridae
            • Sarcosuchus
          • Crocodilia (modern crocodiles)

Teeth and osteodrems have also been found in Brazil, belonging to a close relative of the SuperCroc. This is possibly evidence that land bridges between Africa and South America existed much later than was previously believed.

[edit] Desert discoveries

The fossils were discovered in Gadoufaoua, Niger in the Ténéré Desert, which is part of the Sahara. The first SuperCroc 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, which indicates that it is 604th exhibit at the Musee National du Niger.

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 SuperCroc 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.

[edit] References

  • Erickson, G.M. and Brochu, C.A. 1999. How the "Terror Crocodile" Grew so Big. Nature. Vol. 398 pgs 205-206
  • Schwimmer, D. 2002. King of the Crocodylians: The Paleobiology of Deinosuchus. Indiana University Press. ISBN 0-253-34087-X
  • Sloan, C. 2002. SuperCroc and the Origin of Crocodiles. National Geographic. ISBN 0-7922-6691-9. (children's book)

[edit] Gallery




[edit] Further viewing

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

[edit] External links

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