Sharks’ high levels of diversity have declined over the past 10 million years, and they’re at their lowest value in the present day, according to a study published in the scientific publication Global Ecology and Biogeography.
A team of researchers from the University of Swansea and the University of Zurich revealed that sharks managed to retain their diversity for over 60 million years, but this has been decreasing over the past 10 million years to reach its lowest in the present day.
Sharks are some of the most threatened species, yet they have survived multiple environmental changes in their 250-million-year history. Today, more than 500 species have many different ecological roles, from apex predators to nutrient transporters.
Ecological roles depend on the species’ characteristics, including body size and diet. Measuring the diversity associated with these traits helps scientists to assess the range of ecological roles in a community, also known as functional diversity.
Sharks have soft cartilaginous skeletons that don’t fossilise easily, which means these traits are difficult to measure in extinct species. Instead, researchers use measurements from their teeth — which are hard and well-preserved in the fossil record — to quantify functional diversity in the geological past. “Measurements like tooth size, shape, and types of edges broadly reflect a shark’s functional traits such as body size and diet, allowing us to assess their functional diversity through time,” said the study’s author, Jack Cooper, a PhD student at Swansea University.
The team measured over 9000 fossil and living shark teeth collected from over 500 species to measure functional diversity from the Cenozoic era (66 million years ago) to the present day.
The results that sharks kept high levels of functional diversity – meaning a wide range of ecological roles – for most of the Cenozoic era. This diversity reached a maximum about 20 million years ago in the Miocene era. However, after this peak, the number of ecological functions has steadily declined for the last 10 million years, and present-day shark functional diversity is at the lowest point in the last 66 million years.
The authors believe this decline was caused by the loss of ecologically unique and highly specialised species, including the megalodon, the largest shark that ever lived and an apex superpredator, an ecological role not played by any shark living today. “Not only did we see a clear decline in functional diversity, but we also found that extinct sharks as a whole contributed a wider range of ecological roles than living sharks, said Cooper.
The results warn that human threats like overfishing, which is driving today’s sharks towards extinction, are going to further erode the already diminished ecological contributions of sharks to the environment. “By identifying the modern species holding some of the Cenozoic functional space, our study could potentially complement conservation priorities for the preservation of shark functional diversity in our changing world,” concluded Prof. Catalina Pimiento from the University of Swansea.
Cooper J and Pimiento C (2024) The rise and fall of shark functional diversity over the last 66 million years. Global Ecology and Biogeography, https://doi.org/10.1111/geb.13881