Marine reptiles

Overview

In the blistering heat of outback Queensland are remnants of a very different world: a cold, polar sea filled with bizarre extinct creatures. During the Lower Cretaceous period (100.5-145 million years ago) the land that is now western Queensland was flooded by the ocean at least five times. These ‘inland seas’ are collectively called the “Eromanga Sea”. Mud and sand that washed into the Eromanga Sea settled on the sea floor, entombing the remains of animals that lived there. Among these were giant marine reptiles, such as ichthyosaurs, plesiosaurs and pliosaurs. Swimming in the shadows of these giants were a variety of bony fishes, sharks, cephalopods (relatives of squids such as belemnites and ammonites), and sea turtles.

Scientific study

Palaeontology is the study of the animals and plants that lived in the distant past. Fossils of vertebrate animals are predominantly bones, but can also include footprints, bite marks, and coprolites (fossilised dung).

Images

Collection highlights

Outback Queensland boasts Australia’s largest exposures of Cretaceous sedimentary rocks. After more than 160 years of collecting, Queensland Museum has built a substantial collection of fossils from these rocks. Among the highlights of the collection are:

Kronosaurus
Giant pliosaurs were the top predators in the Lower Cretaceous seas. Kronosaurus queenslandicus is among the largest pliosaurs known, with an estimated length of ten metres. Kronosaurus was named by Queensland Museum palaeontologist Heber Longman on the basis of a small piece of jawbone that was sent to the museum in 1899.

The ‘Richmond polycotylid’
Polycotylids are a group of plesiosaurs with long, narrow jaws. A specimen discovered by graziers near the town of Richmond in north Queensland is the most complete plesiosaur from Australia, and one of the most complete polycotylids anywhere in the world. This species is yet to receive a formal scientific name. The ‘Richmond polycotylid’ is currently on loan for display at the Kronosaurus Korner museum in Richmond.

Elasmosaurs
Elasmosaurs are plesiosaurs with exceptionally long necks. Species are usually identified by their skulls, but most fossil skeletons of elasmosaurs are missing the skull, meaning that most specimens can’t be definitively identified. Queensland Museum has the only complete skull of an Australian elasmosaur: Eromangasaurus australis.

One of Australia’s most complete elasmosaur fossils was found by fishermen on the bank of the Walsh River in north Queensland in 1999. Nicknamed ‘Dave’, it is unfortunately missing its skull, making comparison with other elasmosaurs difficult.

Ichthyosaurs
Ichthyosaurs were among the first reptiles to adapt to life in the ocean, and have streamlined, fish-like bodies with long jaws lined with sharp teeth. Platypterygius australis is currently the only species of ichthyosaur known from western Queensland. It was also among the last of the ichthyosaurs; the group became extinct a few million years later, well before the mass extinction at the end of the Cretaceous period that destroyed most of the other marine reptiles.

Marine turtles
Turtles first evolved in the Triassic period, but it wasn’t until the Cretaceous that fully marine forms evolved. These mostly belonged to a now-extinct group called the protostegids. The Eromanga Sea hosted at least three different species of turtles: Notochelone costata, Bouliachelys suteri, and the enormous Cratochelone berneyi.

Bony fishes
A great variety of bony fishes (technically called osteichthians) inhabited the Eromanga Sea. Some, such as Flindersichthys denmeadi and Pachyrhizodus marathonensis resembled fishes familiar to us today. Others, however, were unlike anything now alive. Australopachycormus hurleyi looked somewhat like a swordfish, but with blade-like teeth lining the snout. Cooyoo australis was a short-faced predator that grew to three metres long. Richmondichthys sweeti was an armoured ‘gulp-feeder’ that probably fed on plankton. Dugaldia emmilta had jawbones that could expand sideways, giving it a massive gape. It’s possible that this was intended to intimidate rivals or would-be predators.

Cartilaginous fishes
Cartilaginous fishes (technically called chondrichthians) include modern sharks, rays and chimeras. Shark teeth are abundant and highly varied in the Cretaceous rocks of western Queensland. Sharks lack mineralised bones, so their teeth are commonly the only part of their bodies that fossilise. There are also, however, some fossil vertebrae of very large sharks from western Queensland. These sharks are lamnids, distant relatives of modern Great White Sharks. Another group of cartilaginous fishes that appear in the Cretaceous of Queensland are the chimeroids, bizarre fishes that today are found only in the deep sea. During the Cretaceous they inhabited the shallow waters of the Eromanga Sea, and grew larger than any alive today. Fossils of chimeroids are limited to their tooth-plates, fin spines, and denticles (tooth-like scales).

Cephalopods
The most famous Cretaceous cephalopods are the ammonites. Their coiled, chambered shells are found in great variety in some parts of Queensland, and the intricate fractal patterns in their chamber walls make them fossils of exceptional beauty.

Belemnites are superficially squid-like molluscs. As with all molluscs, the soft parts of their bodies fossilise only rarely, but belemnites have a bullet-shaped guard (a type of internal shell) the fossilises readily. As a result, belemnites are common fossils in marine rocks from the Cretaceous period, and are often found among fossilised stomach contents of marine reptiles.

Large squid-like cephalopods such as Eromangateuthis soniae are, like the belemnites, mostly known from their fossilised internal support structures. Eromangateuthis probably grew to about 1.5 metres long. Its long body resembles a squid, but these cephalopods are thought to be more closely related to octopuses.

Pterosaurs
Soaring above the Eromanga Sea were flying reptiles: pterosaurs. Queensland Museum’s collection includes two named species: Mythunga camara and Aussiedraco molnari. The museum has the largest collection of Australian pterosaur fossils, which are generally rare because of the fragility of their lightweight bones.

Research overview

Queensland Museum palaeontologists have been studying marine fossils from western Queensland since the 1870s. Museum curators including Heber Longman, Mary Wade and Alan Bartholomai made major advances in our understanding of the creatures of the Eromanga Sea, but there is much that we still don’t know.

The rocks left by the Eromanga Sea cover a time span of over thirty million years. How do the different rock units relate to each other? Were there different assemblages of marine reptiles inhabiting the sea over this time span? Were there multiple species of ichthyosaur and pliosaur in the Eromanga Sea? How many different species of elasmosaurs inhabited the Eromanga Sea, and did they have different feeding strategies? Were the marine reptiles permanent inhabitants of the Eromanga Sea, or did they migrate into it seasonally?

These are some of the questions currently under investigation by Queensland Museum palaeontologists. To answer them, we are re-evaluating fossils already in the collection, undertaking field work to acquire more specimens, determining ages of fossils using radiometric methods, and using new geochemical techniques to gain more information from the fossils.

Publications

• In Search of Ancient Queensland (2015)