Fossil Record



Cladogram of the Crocodylomorpha
There are many fossils of crocodylomorphs from the Mesozoic and Cenozoic. Crocodilians have been very conservative, changing their body form very little through their history of some 230 million years. There are a couple of exceptions:

  • The most ancient crocodylomorphs, the sphenosuchians, had long limbs and were probably capable of being both quadrupedal and bipedal, a trait inherited from their archosaurian ancestors. Over time, as crocodilians adapted to a more semi-aquatic life style the limb size was reduced, although long-legged crocodilians did survive well in to the Tertiary.
  • The tail became highly modified in certain Jurassic marine forms: these lost their body armour, and the tail became deep and was used for propulsion through the water.

Sphenosuchia

(From Macmillan Illustrated Encyclopedia of Dinosaurs and Prehistoric Animals)

(From Macmillan Illustrated Encyclopedia of Dinosaurs and Prehistoric Animals)

Gracilisuchus

Terrestrisuchus

The Saltoposuchidae and Sphenosuchidae were the most basal crocodylomorphs, and they are known from the Late Triassic and Jurassic.

Gracilisuchus

This small, 30 cm long creature is so unlike a modern crocodile that it was originally classified as a basal archosaur before it was recognised as a primitive sphenosuchid in the 1970s. It had a lightly built body and a disproportionately large head, and it could run erect on its slim hindlegs. Gracilisuchus was a well-adapted land animal, protected by a double row of bony plates that interlocked down the length of its backbone, to the tip of the tail. It probably chased after small lizards on its long hindlegs, snatching them up in its powerful jaws.

Terrestrisuchus

Despite some crocodilian-like features of the skull, most skeletal features are rather primitive. Terrestrisuchus was around 50 cm long, and had a delicately built frame. Its body was short, with extremely long, slim limbs and greatly elongated foot bones. The tail was almost twice the length of the body and head combined. From its light build - almost like that of a greyhound - Terrestrisuchus could have sprinted over the dry landscape of Late Triassic Europe, snapping up insects and small lizards in its elongated jaws. It probably ran mostly on all fours, but could easily have risen upright to move even faster.

Protosuchia

The Protosuchia are close to the base of Crocodylomorpha, but more derived than Sphenosuchia. They are known from the Late Triassic and Early Jurassic of South America, southern Africa, North America and China. Despite resembling modern forms, they retained a number of primitive characters including long, but heavier, limbs, a terrestrial habitat, and a lower canine-like tooth that would have fitted between the premaxilla and maxilla.


Protosuchus (from Benton 2005)

Protosuchus

Protosuchus was approximately 1 metre long and was terrestrial in habit, just like the sphenosuchians. A number of skeletons have been found in close association with dinosaur remains in the Early Jurassic of South Africa. The body outline is very like a modern crocodilian, but Protosuchus retains a minute fifth metatarsal, a shorter coracoid and the pubis is excluded from the acetabulum.

Mesosuchia


Pelagosaurus, photo by Stephanie Pierce.
Pelagosaurus

Pelagosaurus was a small marine crocodile that lived during the late Liassic (Early Jurassic) of Europe. The fossil shown is from Ilminster in Somerset, England, and is currently being re-studied by Stephanie Pierce in Bristol.

(From Macmillan Illustrated Encyclopedia of Dinosaurs and Prehistoric Animals)

Metriorhynchus

Teleosaurus

Teleosaurus

The teleosaurs, from the Early and Mid Jurassic, were marine forms, characterised by a short forelimb (1/2 that of the hind limbs), but the girdles and limbs are only slightly modified from those of terrestrial crocodilians. As with the modern fish-catching gharial, the snout is slender and greatly extended. Unlike other mesosuchians, the teleosaurs retain some dorsal armour.

Metriorhynchus

Metriorhynchus is probably the most specialised of all the Crododylia and was certainly the only archosaur to fully adapt to live in water. Specimens are known from the Middle and Late Jurassic of Europe and South America. In Metriorhynchus the forelimbs have become proper paddles and the caudal vertebrae are bent ventrally to support a large caudal fin. In addition, the heavy dorsal armour has been lost, presumably to save weight and make the body more streamlined. Metriorhynchids would probably have returned to land only to lay their eggs.

Sarcosuchus

Sacrosuchus imperator (flesh-crocodile emperor) was discovered in the Niger desert in 1964. Sarcosuchus lived during the Mid Cretaceous (112 Ma) and estimates from fossil skull fragments suggest it grew to at least 11 - 12 metres long and weighed around 8 metric tons. Unlike its closest relatives which were specialised piscivores, it seems likely that Sarcosuchus fed on large vertebrates, including fishes and dinosaurs. Although the snout is long (~75% of the skull length), it is considerably broader than in the living gharial. Scientists studying growth rings in the bone believe it was able to obtain its vast size by growing constantly throughout its 50-60 year life span.


Sarcosuchus skull in comparison to modern crocodile skull, from National Geographic.

Eusuchia

(From Macmillan Illustrated Encyclopedia of Dinosaurs and Prehistoric Animals)

Deinosuchus

Pristichampsus

Deinosuchus

Deinosuchus is known from remains from the Late Cretaceous of North America. Although fossil remains are poor, there are enough remains to accurately reconstruct the skull at around 2 metres in length. Assuming that Deinosuchus scales with other crocodiles, this would give it a body size of just under 50 ft (around 15 m!!). Some workers have disputed its size, claiming that it was smaller and short-bodied. Comparisons with Sarcosuchus suggest a similar life style as a riverine ambush predator, feeding on fishes, dinosaurs and other large vertebrates. Growth rings in the bone also suggest that it grew constantly throughout its life. Deinosuchus is an alligator, while Sarcosuchus is a 'mesosuchian' (above), so this shows that gigantism evolved independently at least twice among crocodilians.

Pristichampsus

Some heavily armoured eusuchians lived on land and possessed long running legs and hooves instead of claws. A typical example is Pristichampsus from the Eocene of Germany and Noth America. It was 3 m (10 ft) long. Like all carnivores, it had sharp serrated teeth and probably preyed on mammals.