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| Nessie the Opalized Marine Reptile |
Beneath its shimmering surface lies the story of a powerful marine reptile that ruled the Early Cretaceous oceans roughly 110 million years ago, at a time when much of inland Australia was drowned beneath a warm, shallow epicontinental sea.
The lovely remains you see here are from one of those amazing marine reptiles, a pliosaur, who swam in those ancient seas. So what exactly is a pliosaur?
Pliosaurs are a subgroup within the Plesiosauria, the great marine reptiles (not dinosaurs!) of the Mesozoic.
While long-necked plesiosaurs favored dainty heads and elongated cervical vertebrae for sweeping, panoramic strikes at small fish and cephalopods, pliosaurs evolved in the opposite direction:
- Skulls short and massive
- Necks abbreviated
- Jaws deep and muscular
- Teeth robust and conical
These were the ambush predators, built less like swans and more like crocodilian torpedoes, with four powerful flippers and a muscular body plan that let them sprint through the water column to surprise prey.
Though not an ichthyosaur — those fast, fish-shaped reptiles that converged spectacularly toward the form of modern dolphins — pliosaurs shared the same ecosystems.
Ichthyosaurs hunted squid and fish in speed-based chases, while pliosaurs handled bigger, tougher fare: other marine reptiles, ammonites, and the occasional large fish unlucky enough to cross their path.
The Early Cretaceous seas hosted a diverse guild of reptiles:
- Ichthyosaurs (fish-shaped pursuit predators)
- Long-necked plesiosaurs (precision feeders)
- Pliosaurs (apex ambush predators)
- Crocodyliforms (semi-aquatic opportunists)
- Ammonites & belemnites (cephalopods forming the backbone of the food web)
Nessie sits among a lineage that includes broad-skulled bruisers like Kronosaurus queenslandicus, a fellow Australian celebrity whose skull approached 3 meters in length and whose bite force was probably among the strongest of any Mesozoic reptile.
Pliosaurs didn’t so much swim as fly underwater. Their four hydrofoil flippers generated lift in alternating strokes, allowing bursts of speed followed by graceful pursuit. Streamlined bodies meant low drag, essential for surprise attacks in open water.
Dentition tells the tale:
- Deep-rooted conical teeth resist torsional stress
- Interlocking jaws grip slippery prey
- Short snout adds leverage for skull-crushing force
Ammonites — including opalized forms from the same Australian basins — bear puncture marks suggestive of pliosaur predation. Large fish and other marine reptiles likely rounded out the menu.
Like ichthyosaurs and most plesiosaurs studied from articulated skeletons, pliosaurs were viviparous — they gave birth to live young at sea. No nests, no frantic beach crawls, and no hatchling gauntlet. Babies were miniature versions of adults, already hydrodynamic and hungry.
How do we know this? Well, a few ways. We have fossilized pregnant plesiosaur specimens with embryos and there is always the biomechanical absurdity of hauling such a creature onto land to lay eggs. So, wee ones at sea it is!
Why Opal? Why Here?
Opalization is an Australian specialty, the result of silica-rich groundwater percolating through Cretaceous sediments and replacing bone over geologic time. Fossils from Lightning Ridge and Coober Pedy preserve everything from ammonites to plesiosaurs as shockingly colourful silica pseudomorphs — Earth chemistry as jeweler.
Nessie’s preservation is thus a double marvel for its biological rarity (pliosaur skeletons are uncommon) and mineralogical rarity (precious opal replacement is even rarer)
Pliosaurs survived well into the Late Cretaceous before vanishing in a wave of marine turnover alongside ichthyosaurs, mosasaurs, and ammonites. Their departure marks a reshuffling of oceanic power dynamics — a story of climate, sea levels, and evolutionary competition.
