A continent isolated for tens of millions of years, Australia was home to some of the most bizarre mammals the world has ever known. Unlike all other mammals, monotremes laid eggs. Rather than giving birth to live young, they hatched it. Monotremes breed like birds and give birth like mammals. But Australia’s monotremes are not alone. The most familiar of them are the platypus and the echidna — two of the most bizarre of all living mammals. Through the study of fossil remains, we know that Australia was once home to a wide variety of strange egg-laying mammals.

### The Opal Treasure Trove of Lightning Ridge

It was in the opal mines of Australia’s Lightning Ridge in New South Wales that palaeontologists discovered two jawbones of three previously unknown species of monotremes that were alive in the early Cretaceous period, in the Cenomanian age. They fill a gap in our knowledge of the diversity of monotremes and force us to rethink some very traditional ideas about what mammals looked like in this ancient Australia.

### The Echidnapus: A Fusion of FOSSILS

One of the most remarkable species to emerge from this treasure trove is a ‘transitional’ form between echidna and platypus, appropriately named the ‘echidnapus’. This fossil, formally known as *Opalios splendens* (meaning splendid and opal), displays a melding of traits from both of today’s living monotreme families. Its opalised remains preserve its narrow and slightly curved jaw so characteristic of the echidna, as well a dentary that bears a striking resemblance to one from a platypus. This weird combination of features suggests the echidnapus might have been a moment of transition in monotreme evolutionary history.

### A Glimpse into Monotreme Diversity

With the echidnapus, the team revealed the existence of two more new species: *Dharragarra aurora* and *Parvopalus clytiei*, which adds to our knowledge of the diversity of monotremes in the Cretaceous. Coexisting with three known species, the range of these egg-laying mammals suggests that Australia was once home to as many monotremes as it is now inhabited by marsupials – a testament to a time when niches were available for monotremes to fill.

### The Mystery of Monotreme Teeth

In one curious example, these creatures had teeth – a feature lost in modern monotremes – and it’s been a topic of intense debate concerning the modern platypus and the echidna as to why they lost these normally crucial devices for seizing and processing food. Again, theory concerning dietary shift and competition in the environment provides a plausible narrative about what would have enabled these creatures to adapt to survive into the present day.

### Survivors of an Ancient Extinction

The late Cretaceous ended with a true extinction: the moment when the asteroid slammed into Earth’s Yucatan peninsula and doomed the dinosaurs. Some monotremes survived into our new era, bequeathing us a living legacy. The complete absence of monotreme fossils in the thousand years immediately following that cataclysm makes the end of the dinosaurs a fascinating blank space in the monotreme story; active research continues to fill in the details of how the lineage made it through such a tumultuous time.

### The Future of Fossil Research

With every new monotreme fossil to be excavated and studied, the story of monotreme evolution will be enriched a little more, demonstrating just how resilient and adaptable life on Earth has always been, and giving us one more precious thread that links us to a long-ago world. The opal fields will continue to yield up bones from a distant part of Earth history, with more secrets of these strange creatures yet to come.


Fossils – the preserved remains of past life – bear witness to the history of our planet. They are key to understanding the deep process of change that is evolution, showing how species have adapted to their environments over very long periods of time. Fossils take many different forms: they might be bones, teeth, impressions of leaves, footprints. But each is a piece of a larger story, recounting the history of planet Earth. The study and stewardship of fossils provides information that is essential for the modern world. Through careful scrutiny of these ancient histories, we can track changes in biodiversity and learn more about the biological drivers of evolution on Earth.

Jun 08, 2024
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