New research from Swansea University and the University of Zurich reveals that sharks maintained high functional diversity for most of the last 66 million years. However, over the past 10 million years, this diversity has steadily declined to its lowest levels today.
Despite being among the ocean’s most threatened species, modern sharks have survived numerous environmental changes in their 250-million-year history. With over 500 species, they fulfill various ecological roles, from apex predators to nutrient transporters.
These ecological roles depend on traits like body size and feeding habits, which determine functional diversity. This diversity measures the range of ecological roles in a community.
Since sharks have soft cartilaginous skeletons that do not fossilize well, scientists use their teeth as proxies to measure traits in extinct species. Teeth, complex and well-preserved in fossils, reflect traits like size, shape, and cutting edges, indicating body size and diet. This approach helps quantify shark functional diversity over geological time.
Tooth measurements provide insights into a shark’s functional traits, such as body size and diet, allowing us to track their diversity over millions of years.
The team analyzed over 9,000 fossil and modern shark teeth from about 500 species, sourced from museum collections and literature, to study functional diversity across the Cenozoic era, spanning 66 million years.
Researchers discovered that sharks maintained high functional diversity, meaning they had a wide range of ecological roles, for most of the Cenozoic era. This diversity peaked around 20 million years ago during the Miocene epoch. However, since then, there has been a steady decline in the variety of ecological functions sharks perform. Shark functional diversity is lower than in the past 66 million years.
The decline in diversity is mainly due to the loss of unique and specialized species. For example, the megalodon extinction caused the extinction of the largest shark ever known and a top predator. No modern shark fills this apex superpredator role.
The lead researcher explained, “We not only observed a clear decrease in functional diversity but also found that extinct sharks collectively filled a broader range of ecological roles compared to living sharks.”
The study warns that human activities such as overfishing, which threatens sharks with extinction today, also reduce their essential roles in ecosystems. Dr. Catalina Pimiento, senior author from the University of Zurich and Swansea University, suggested identifying which modern species occupy historical ecological roles could help prioritize shark conservation efforts to preserve their functional diversity in a changing world.
Journal reference:
- Jack A. Cooper, Catalina Pimiento, et al., The rise and fall of shark functional diversity over the last 66 million years. Global Ecology and Biogeography. DOI: 10.1111/geb.13881.