Fossil Deposits Along the Flinders Highway

Australia: The Land Where Time Began

Fossil Deposits Along the Flinders Highway

At the time the fossil deposits were being laid down about 110 million years ago the area between Julia Creek and Hughenden was covered by the Eromanga Sea. Among the animals were large oysters, Inoceramus, squid are known to have been present by the cuttlefish bones found that are more than 1 m long, many large fish. The ichthyosaurs (Platypterygius) were among the last survivors of this group. Other marine reptiles were plesiosaurs (Woolungasaurus) and Kronosaurus, giant marine turtles (Cratochelone) as well as the much smaller Notochelone. There were also primitive birds and pterosaurs.

On the land were herds of the ornithopod dinosaur Muttaburrasaurus and Minmi, an armoured dinosaur, that is believed to have lived either in small groups or as individuals. There were also some large sauropods. The sediments deposited in the Eromanga Sea led to the formation of the aquifers of the Great Artesian Basin that brings water from the eastern ranges west to about 20 % of the continent.

Fauna

Ichthyosaurs

Among the large animals found in these deposits was the ichthyosaur Platypterygius longmani that grew to about 7 m. A distinguishing feature among ichthyosaurs was the broad pectoral fins of this species. It had a large tail that, like other ichthyosaurs, moved from side to side instead of up and down as in the later marine mammals such as whales. It also had a large head, some were about 1.5 m long, most of which was a long thin snout lined with sharp, piercing teeth. As with all ichthyosaurs, as well as some reptiles and birds, the very large eyes were supported internally by a ring of bones, sclerotic ossicles. Even in the skeleton it has the appearance of a staring eye. A number of finds of ichthyosaurs, including at least 1 at this site, has shown an adult ichthyosaur associated with a baby, presumably a mother and baby, indicating that they gave birth at sea and cared for the baby as do the mothers of whales and dolphins.

Ichthyosaurs have been studied for more than 200 hundred years but it is only recently that they could be released from the rock matrix by the method of acid leaching. It had been believed that they fed on ammonites and belemnites or fish. Use of the acid leaching technique has shown that the stomaches of a specimen of P. longmani actually contained a variety of vertebrates that included, very unexpectedly, turtles and a bird. It is unlikely that anyone who studied ichthyosaurs over the last 200 years could have suspected the long thin snouts of ichthyosaurs could possibly have been used to eat hard shelled turtles.

Another surprising feature of the fossils of Platypterygius found in the deposits of the northern area of inland Queensland that was covered by the Eromanga Sea is that many were found with their snouts buried in the then sediment, often with the remains of their bodies scattered around the head, in the same layer as the head, showing that the snout had actually penetrated the sediment, the rest of the body either snapping off at the time of the impact or subsequent to decomposition. As it is unlikely a living ichthyosaur speared its snout into the sediment while chasing prey, it has been suggested that the most likely explanation could be that on death the putrefaction gases caused the body to float to the surface and when the gas was released, either by further decomposition of by the carcass being fed on by scavengers, the body sank, probably rapidly, and because of the large head it almost certainly dropped head first, aided by the streamlined nature of the body, could have reached high speed by the time it reached the bottom. In some cases the body seems to have snapped off behind the skull, the body lying close by on the sediment, while in other cases the bones of the body, mostly vertebrae, were scattered over an area of the sea bed. It has been suggested that where the bones are scattered around the head, the head may have dropped off while the body remained floating and being scavenged or decomposing, the rest of the bones falling as the body disintegrated.

Pliosaurs

Kronosaurus queenslandiscus, a pliosaur, is known to have reached at least 12.8 m long, with a head 2 m long armed with 30 cm teeth, a short, thick neck, a large barrel shaped body and short tail. Most of the propulsion would have been derived from the 4 large, powerful flippers. Some think it is probably the largest marine reptile that ever lived. Specimens have been found around Richmond, Hughenden and Julia Creek in Queensland.

Plesiosaurs

Woolungasaurus is also found in the sediments of the Eromanga Sea in north west Queensland. These animals had the appearance usually ascribed to the Loch Ness monster, with small heads on long necks that were longer than the body and tail combined, and with 4 large flippers for propulsion. It was named after an Aboriginal dreamtime reptile. A relatively complete skeleton has been found in the deposits near Richmond and also in a South Australian deposit. It was an elasmosaurid plesiosaur about 9.5 m long. Most fossils of plesiosaurs that are found are missing the head. It is believed to result from the small head falling off the long neck early in the process of decomposition. In the past reconstructions of plesiosaurs have shown the neck being used to reach out of the water to snatch birds or pterosaurs out of the air. A more recent suggestion is that they swam with their necks outstretched so that the small head would not alarm prey it was approaching as the large body was 4 m behind the head, so less threatening.

Turtles

Parts of a marine turtle, Cratochelone, have been found, the shoulder girdle, parts of the front paddles and bits of the shell. It was a marine turtle that grew to about 2.25 m long. It seems to be among the largest turtles known. It was found on Sylvania Station to the north of Hughenden. Notochelone, a smaller turtle has also been found in the same deposits was more common, being found in areas near Hughenden, Julia Creek and Richmond. It was about 1 m long and seems to have looked like a modern green turtle.

Pterosaurs

2 pterosaurs have been found near Boulia. Only fragmentary remains have been found of them. One appears to be similar to Ornithocheirus from England, that appears to have had a wingspan of about 2-4 m, and the other similar to Pteradnodon from the US, was much larger, possibly having a wingspan of 4-6 m.

Birds

Nanantius, one of the oldest known birds, dating from about 110 million years ago, is only known from fragmentary bones, 1 vertebra and some leg bones, that were found near Boulia on Warra Station and Canary Station in Western Queensland. The bones were discovered when limestone containing other fossils were acid leached in the lab.

Dinosaurs

Muttaburrasaurus

Minmi parvavertebra

Found near Minmi Crossing, this was a herbivorous dinosaur that was heavily armoured. The back was covered with small bony pellets and bony shields that were embedded in the skin. A feature that was unusual for an armoured dinosaur was the way the armour continued around the sides to the belly. Minmi was a stocky animal about 2.5 m long, broad at the shoulders and hips, with all 4 legs short. It had a small box-like head with a bone arrangement that differed from other 2 known armoured dinosaurs. Its relationship to other dinosaurs in uncertain, but it has been suggested that it may have split off from the other armoured dinosaurs at an early stage of their evolution, possibly being in the same relationship to other dinosaurs that monotremes are to other mammals. As both Muttaburrasaurus and Minmi were both definitely land animals it is assumed that they washed into the Eromanga Sea from rivers, the bodies drifting long distances before they sank to the seabed.

Sources & Further reading