Sharks and rays have populated the world's oceans for around 450 million years, but more than a third of the species living today are severely threatened by overfishing and the loss of their habitat. An international research team led by palaeobiologist Manuel A. Staggl from the University of Vienna has now investigated whether and how global warming influences the diversity of sharks based on climate fluctuations between 200 and 66 million years ago. According to the study, higher temperatures and more shallow water areas have a positive effect, while higher CO₂ levels have a clearly negative effect. The study was recently published in the scientific journal Biology.

"The current rapid climate warming could also have a negative impact on this group of animals -- in an international study based on earlier climate changes, we have now looked at how exactly this might impact rays and sharks," explains palaeobiologist Manuel A. Staggl from the University of Vienna. The international team investigated the driving forces behind the biodiversity of sharks and rays during the so-called Jurassic (200-143 million years ago) and Cretaceous (143-66 million years ago), a heyday of shark and ray evolution with a wide range of different environmental conditions. Fossilised shark and ray teeth were used to determine the species diversity for each age and compared with the climate data of the respective age. "We wanted to understand which environmental factors influence the diversity of sharks and rays in order to be able to develop possible future scenarios with regard to current global warming," says Jürgen Kriwet, professor of palaeobiology at the University of Vienna.

The results show that three environmental factors are decisive: Higher temperatures and more shallow water areas have a positive effect; however, a higher carbon dioxide (CO₂) concentration clearly has a negative effect. This is the first time that the negative impact of CO₂ concentrations in the ocean has been so clearly described: "We cannot yet fully explain the exact mechanisms behind the negative effect of CO₂ on the biodiversity of sharks and rays," says Staggl. However, laboratory studies on sharks and rays living today have revealed direct physiological effects of higher CO₂ concentrations on the animals -- from impacts on the animals' senses to changes in the skeleton during embryonic development. In any case, the fossil record shows that a higher CO₂ content contributed to the extinction of individual shark and ray species.