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Triops Longicaudatus
Triops Longicaudatus Red
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Palaeontology: The silent decline of sharks began 45 million years ago
A new study by the University of Vienna corrects the image of cartilaginous fish as invincible. Sharks and rays experienced their peak diversity in the Eocene epoch and have been battling a steady decline in biodiversity ever since – a trend that is being dramatically accelerated by humans.
Sharks and rays have always been considered the great survivors of Earth’s history. These cartilaginous fish, which have survived several mass extinctions, have been roaming the oceans for around 400 million years. Until now, researchers assumed that they also survived the last major catastrophe 66 million years ago unscathed and were even able to expand their diversity.
However, a Viennese research team led by Manuel Staggl and Jürgen Kriwet from the Institute of Palaeontology at the University of Vienna is now questioning this assumption. Their analysis of over 50,000 fossils, published in the journal Scientific Reports, reveals an alarming trend: the diversity of sharks and rays peaked around 45 million years ago and has been declining steadily ever since.
Peak in the late Eocene
The comprehensive data analysis compared fossil finds from different eras with the environmental conditions at the time – from global temperatures and CO2 concentrations to the availability of habitats.
The evaluation shows that sharks and rays actually coped well with the asteroid impact that wiped out the dinosaurs. In the period that followed, the late Eocene (approx. 45 million years ago), their species diversity reached an all-time high.
The key to this former success lay in favourable conditions: a warmer climate overall and an abundance of extensive, shallow and coastal marine areas. ‘The more different shallow marine areas were available, the more species developed,’ explains co-author Jürgen Kriwet.
The dangers of the present
This finding is particularly worrying because these critical coastal ecosystems are now under serious threat. Over the last few million years, cartilaginous fish have been battling a slow, natural decline. However, with the arrival of humans and the onset of anthropogenic change, this trend has turned into an existential crisis.
Today’s combination of extreme overfishing, increasing development of important coastal zones, massive pollution and rapidly advancing ocean warming is affecting more than a third of modern shark and ray species (Neoselachii) more severely than ever before.
The researchers also investigated the relationship between biodiversity and the most important greenhouse gas. Historically, animals benefited from moderate CO2 levels, as these promoted the growth of algae and seagrass beds, thereby strengthening food chains.
Particularly critical is the extreme speed of current environmental changes, which leave animals that have long inhabited the Earth with little opportunity to adapt. The study therefore sends a clear signal to marine conservationists: ‘Marine conservation is not just about fishing quotas – we need to look at entire habitats and the climate system,’ concludes Staggl.
‘In summary: moderate CO2 levels were beneficial for the biodiversity of sharks and rays in the past, but excessive levels were harmful,’ says lead author Manuel Staggl. This underscores the acute danger posed by current ocean acidification caused by human-induced climate change.
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