Tiny Fossils Reveal Australia Had A Late Jurassic Inland Sea

An aquatic reptile from the Jurassic Period. It's unclear whether large creatures like this lived in the ancient inland sea. Nobu Tamura/Wikimedia Commons

European explorers of central Australia were so convinced of the existence of an inland sea they risked their lives to try to find it. In fact, they were 110 million years too late to discover the well-documented Eromanga Sea. However, a new discovery reveals the Cretaceous-era Eromanga was not the only time inland Australia possessed such a feature. Forty million years earlier, at least part of the same region was also a permanent sea, one that may have supported a similarly fascinating ecology.

The existence of a Late Jurassic waterway was revealed through the discovery of tiny dinoflagellate cysts near Roma in Queensland. Dinoflagellates are single-celled organisms that produce the red tides associated with seafood poisoning, as well as the magnificent bioluminescent seas that occasionally visit fortunate shorelines.

This new fossilized species of dinoflagellate, known as Skuadinium fusum, reveals Australia had a fresh or brackish water Jurassic inland sea. Adelaide University

Although dinoflagellates are usually marine inhabitants, some have been found to adapt to slightly saline or even freshwater environments. The fossils' “small size, thin walls and simple proximate shapes are typical of freshwater to brackish dinoflagellate cysts,” their discoverers announce in the journal Palynology.

The ocean at the time was more than 1,000 kilometers (620 miles) away. The team that discovered the fossils conclude that they must have arrived as part of a major incursion of seawater driven by warm conditions producing very high sea levels. With time the connection to the ocean would have been broken, but the single-celled organisms survived, adapting to fresher waters as the era's great rivers emptied into the inland basin.

Moorodinium crispa, the other of the two species of dinoflagellate that dominate the deposits. University of Adelaide

First author Carmine Wainman of the University of Adelaide told IFLScience that we currently only have evidence from cores taken close together, leaving us with little idea of the sea's extent. Similarly, while the rocks that mark the sea's ending are dated precisely to 150 million years ago, the timings of its origins are less clear. Consequently, it could have lasted anything from a few hundred thousand to 2 million years.

The Eromanga Sea produced great marine reptiles, some of which have been spectacularly fossilized as opals. The small segments of Jurassic rocks examined in this study did not reveal any such great beasts, or indeed macroscopic animals of any sort, but Wainman hopes this will be corrected with further study. We know that at the time dinosaurs roamed Australia, then still connected to Antarctica, and the shores of an inland sea, one containing somewhat fresh water would have been a tempting habitat.

Shallow seas with the right rock formations can make for hydrocarbon deposits, but Wainman told IFLScience there are many unanswered questions about whether the region's geology is suitable, particularly for anything sufficiently large enough to be commercially viable.

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