Australia: The Land Where Time Began

A biography of the Australian continent 

The capitosaurs were a large and important group of large to huge flat-headed semi-aquatic or completely aquatic tetrapods. Some forms reached 3 to 4 or even 5 metres in length.  They dominated the freshwater ponds, lakes and rivers of the Triassic, but were pushed to extinction by the carnivorous phytosaurs which appeared at the end of the period. 

This family is characterised by a long, broad snout, with the orbits far back and nostrils near the anterior margin of the snout. A well-developed otic notch situated near the rear corners of the skull, though it sometimes closed up in more advanced forms. The head had the appearance of that of a crocodile.

Very flat skulls, small limbs, ossification reduced (obligate aquatic); some very large forms

Paracyclotosaurus

Paracyclotosaurus davidi (British Museum of Natural History)

Paracyclotosaurus is one of Australia's best known fossil amphibians, it is known from an almost complete skeleton. The specimen is 2.25 m long, its mouth containing many small, sharp teeth. It was recovered from a large ironstone nodule in a brick pit. It was given the name Paracyclotosaurus because of its resemblance to another species, Cyclotosaurus. Reconstructed casts are on display at the Australian Museum, Sydney, and the Natural History Museum in London.

An impression of its skin was also found in the nodule showing that it had dry skin, unlike the condition in modern amphibians that use the skin as a supplementary breathing surface. The rib cage suggests it was better suited to breathing like a reptile. A large part of the skeleton was supported by cartilage and it had very small arms and legs. It probably spent most of its time in the water, only coming to land occasionally. It is thought it could have hunted fish by waiting for them to come within range then opening its large jaws suddenly causing water to rush in taking the fish with it.

The skull was almost 70 cm long with the small orbits being placed in the rear third of the skull. The otic notch was closed. The tabulars are large, rounded bones, larger than in its close relative Parotosuchus.

Parotosuchus

This is one of the best known Australian amphibians because it was widely distributed across Australia. The largest known Australian species was P. gunganj, with a head length of 24 cm, so it was about 1.25 m. The species name if from the Aboriginal word for water dweller.

The small skulls of P. aliciae demonstrate how the skulls of Parotosuchus changed in shape and proportions with growth. Some believe the Queensland species P. gunganj and P rewanensis are more closely related to the South African and European species from the Lystrosaurus zone than to other Australian species. This would suggest rapid dispersal of the species group throughout Gondwana.

It had a broad, blunt, flat head with the eyes set well back on top of the head. Open otic notches define the genus, a single anterior palatal vacuity, and by both the jugal and frontal bones entering the orbits. The various species of Parotosuchus are distinguished by the relative proportions of the snout, the dermal ornamentation and the relative degree of closure of the otic notch.

P. brookvalensis has a semi-closed notch, making it the most advanced Australian species.

The specimen from the Erskine Ranges, a relatively complete skull 20 cm long,  is in vary hard ironstone rock making it difficult to extract without damaging the fossil so the palate hasn't been seen. The species has yet to be determined.

Sources & Further reading

John A Long, Dinosaurs of Australia and New Zealand, University of New South Wales Press

Links

A revision of the Labyrinthodont family Capitosauridae and the description of Parotosaurus peabodyi. n. sp