Australia: The Land Where Time Began

A biography of the Australian continent 

Reptiles - The first amniotes

Reptiles first appear in the fossil record of Nova Scotia, Canada, in the Early Carboniferous. They don't appear in the Australian fossil record until the Triassic.

In Australia the first preserved footprints of reptiles appear to have been in the Permian, but no fossils of reptiles have been found earlier than the Triassic.

Prior to the arrival of the reptiles, all terrestrial vertebrates, whether or not they lived in water, were tied to the water for reproduction, so they had to be close enough to water bodies suitable for breeding to find it in the breeding season. The great innovation of the reptiles that finally cut their ties to water was the development of the amniotic egg.  In effect, this type of egg enclosed the water necessary for reproduction in a protective outer covering, so the young could still develop in water, but the egg didn't have to be in a water body. This allowed the reptiles to range widely, much further from water than previous terrestrial vertebrates, the amphibians. Many amphibians appear to have become adapted to living permanently on land, but still had the requirement to return to water to reproduce.

The amniotic Egg

A number of features evolved by the  reptiles led to their success, a key feature being the amniotic egg. The reptiles and their successors are all members of the Amniota. The evolution of the amniotic egg was one of the turning points of life on Earth. For the first time vertebrates broke free from the necessity to return to water to reproduce, and because their eggs were protected, and provided food in the form of a yolk sack, their larval stages were passed in the egg, the young emerging at a more advanced stage when they had a better chance of avoiding predation. This process was improved further by eggs becoming larger.

The amniotic egg is composed of several layers. The outer layer consists of a tough outer coating, often a shell, to prevent water loss. Inside this are several internal membranes that that help protect the developing young and collect its waste products. The albumen of the egg replicates the pond that amphibian float in, but is enclosed and protected from the drying up of the pond as well as at least some degree of protection from predation at a very vulnerable stage of development of the young animal.

Radiation of the amniotes

In the Late Carboniferous there were 3 major types of reptile -

Protorothyridids such as Hylonomus and Paleothyris from the Late Carboniferous and Early Permian of North America and central Europe, represents the 1st type. Small, agile insectivores that appear to have survived almost unchanged for more than 30 million years. They had relatively solid skulls with no major openings in the cheek or behind the eyes.

Petrolacosaurus from the Later Permian of Kansas represented the 2nd type. It appeared similar to Protorothyridids, a slender long-limbed insectivore. In this form there is a extra opening in the palate beneath the orbit. There are 2 temporal fenestrae (temporal openings) in the cheek region that is absent in the Protorothyridids.

Ophiacodon from the Late Carboniferous and Early Permian of Mexico represents the 3rd type. At about 3 m it was larger than the other 2 types. Compared to the body size the skull is large, has a high sided snout that comprises more than half its length. It was carnivorous and probably ate fish and small tetrapods. Its evolutionary significance lies in the single temporal fenestra that is believed to have been the equivalent of the lower temporal fenestra in Petrolacosaurus.

These reptiles represent the 3 major lineages of amniotes that have dominated life for the past 300 million years. The temporal fenestrae are the keys to the relationships. The fenestrae are openings in the skull bone behind the eye socket associated with jaw muscle attachment.

The Protorothyridids are primitive anapsids ("no windows") reptile, they had no fenestrae, a condition seen in amphibians and reptiles. 

Petrolacosaurus is a diapsid ("2 windows") reptile, with 2 temporal fenestrae on each side of the skull.

Ophiacodon is a synapsid ("joined together windows") reptile.

A 4th type is found in some Mesozoic marine reptiles, euryapsid ("wide window"). The euryapsids had a single upper temporal fenestra that is a modification of the diapsid pattern, and is found several groups of marine reptile. This doesn't seem to represent a single major amniote lineage.

The relationship of these reptile types to each other is uncertain, but it seems likely that the first split of the amniotes occurred between the synapsids on the one hand  and the anaspids and diapsids on the other.

Early Reptile Diversity

The main lineages of the reptiles had already begun in the Carboniferous and Permian that evolution followed for the following 250 million years. The captorhinomorphs, petrolacosaurs and mammal-like reptiles are close to the ancestors of all later significant groups, reptiles, birds and mammals.

In the Middle to Late Carboniferous a rapid diversification of reptiles took place, not long after their appearance in the fossil record. Right from their start the reptiles had key distinguishing features that are seen on the sides of the skull in the posterior part. These are the pattern of the fenestrae mentioned earlier.

In Australia, 3 fossil sites have produced the known Triassic reptiles - the Blina Shale in northwestern Western Australia, the Knocklofty Formation in Tasmania and the Arcadia Formation in Queensland. There are also 2 unnamed sites, one of which is at the tip of Cape York Peninsula.

Anapsida - Captorhinus

Caporhinomorphs from the Late Carboniferous and Early Permian are thought to be line that led to turtles, both having the primitive anapsid fenestral skull pattern. The jaws of Captorhinus had multiple rows of teeth suggesting a diet of tough plants or insects.

Diapsida - Petrolacosaurus

This is the first known diapsid had smaller teeth, but is believed to have been an insectivore.

Synapsida - mammal-like reptiles

The Synapsida were dominant in the Permian. The Early Permain pelycosaurs (sail reptiles) such as Edaphosaurus had a single lower temporal fenestra characteristic of the synapsids. It had a deep, powerful lower jaw set with peg-like teeth.

 In the Late Permian synapsids such as the gorgonopsians (sabre-toothed reptiles), such as Lycaenops, had lithe, active bodies and many advanced skull and post-cranial skeletal features.

Towards the end of the Permian Procynosuchus, a dog-like predator, indicates the direction evolution would take the group. It had the beginnings of a hard palate and its teeth are differentiated into incisors, canines and cheek teeth, the brain case increased in size, the single temporal fenestra has disappeared as a simple skull opening. It showed many features of true mammals.

The Blina Shale

This site has produced what is believed to be an ichthyosaur. These were dolphin-like marine reptiles that possessed very distinctive vertebrae. These vertebrae are in the form bi-concave discs that are very similar along the entire length of the vertebral column, the only differences in different parts of the column are size. Their limbs had been greatly modified to form flippers with greatly increased numbers of individual bones in each digit. If this does turn out to be an ichthyosaur it would be the oldest known ichthyosaur fossil in the world. Triassic ichthyosaurs are known only from China, Japan and Spitsbergen from a slightly younger date.

The Knocklofty Formation

A proterosuchian thecodont, Tasmaniosaurus triassicus, has been found in this deposit.

The Arcadia Formation

Australian Reptiles

Sources & Further reading

Michael J. Benton Ph D, The Reign of the Reptiles, Quarto Publishing, 1990


Dinosaurs and other Mesozoic Reptiles of Australasia

Author: M. H. Monroe
Last Updated 08/11/2012

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