03-05-2025
94 million years ago sea monsters got wiped out by boiling oceans, high CO2: Study
For millions of years, the Late Jurassic and Early Cretaceous oceans belonged to enormous and scary hunters.
Among them were pliosaurs with giant jaws, toothy crocodile-like reptiles (thalattosuchia), and sleek, fast, fish-shaped predators (ichthyosaurians, or sea monsters) that dominated the seas.
These were apex predators, ruling their underwater kingdoms with unchallenged power.
But then, something shifted. The middle Cretaceous saw a dramatic upheaval in the marine world. The long reign of ichthyosaurs, thalattosuchians, and pliosaurids started to end abruptly.
They vanished from the fossil record. In their place, a new cast of characters emerged: the mosasaurs, plesiosaurs, and even diverse kinds of sharks.
The abrupt disappearance of these long-reigning marine animals and the rise of new ones has long been a mystery for experts.
A new study, presented at the General Assembly of the European Geosciences Union next week, sheds light on this ancient mystery.
According to Valentin Fischer of the Université de Liège and his colleagues, the culprit was likely a period of intense ocean anoxia and dramatic climate volatility. Ocean anoxia is when large parts lose most or all of their oxygen.
This tumultuous era coincided with the Cenomanian/Turonian transition, which estimates suggest occurred around 93.9 million years ago. This was a relatively short but significant period of environmental stress.
It is 'the hottest interval of the last 541 million years.'
During this transition, carbon dioxide levels soared to their highest point in the Cretaceous. The delicate balance of ocean nutrients, like sulfur and iron, was thrown into disarray.
The combination of extreme heat, widespread oxygen loss in the oceans, high CO2 levels, and nutrient imbalances created a challenging environment that likely triggered the die-off of marine life and reshaped marine ecosystems.
The study suggests it triggered a major reshuffling of the oceanic food web, leading to the disappearance of established predators and the rise of new ones.
'This transition is associated with a shift in top predators, creating the unique and somewhat short-lived oceanic food webs of the Late Cretaceous,' the researchers noted.
The research involved a comprehensive analysis of hundreds of marine reptile lineages.
The team examined the evolutionary relationships and compiled the largest 2D and 3D marine reptile data dataset.
Through this extensive analysis, they pinpointed how these extinctions unfolded and impacted the predatory capabilities of Cretaceous marine reptiles.
'Our analyses showed that the Cenomanian-Turonian transition is associated with elevated rates of extinction and that these extinctions disproportionally targeted some groups of large and fast predators, in a stepwise manner,' Fischer noted in the press release.
Dr. Fischer highlights a key piece of evidence for the shift in predatory capabilities: the significant differences in skull shapes of marine reptiles before and after the Cenomanian/Turonian transition.
These altered skull morphologies directly impacted their ability to hunt and consume prey. It led to 'distinct bite force.'
The reign of the ancient marine giants ended not with a bang, but with a shift. This dramatic environmental upheaval paved the way for the iconic marine predators of the Late Cretaceous.
However, the asteroid event led to the extinction of most of these marine creatures, including mosasaurs and plesiosaurs, approximately 66 million years ago.
This new research provides insight into how Earth's forces can cause major evolutionary shifts.
