New research by Swansea University and the University of Zurich has revealed that sharks have maintained high levels of functional diversity for most of the last 66 million years, before experiencing a steady decline in the last 10 million years to its lowest point in the present day. Despite being one of the ocean’s most threatened species, sharks have survived multiple environmental changes over their 250-million-year history, with over 500 species playing various ecological roles in marine ecosystems.
Ecological roles in shark communities are determined by species’ traits such as body size and feeding habits, which allow scientists to measure functional diversity. Since sharks have soft skeletons that do not fossilize well, researchers utilized tooth measurements as proxies for traits like body size and diet, which can then be used to assess functional diversity in extinct species. Lead author Jack Cooper explained that tooth characteristics like size, shape, and edge types reflect a shark’s functional traits and can provide insights into their ecological roles throughout history.
The study involved analyzing more than 9,000 fossil and living shark teeth from around 500 species, collected from museum collections and literature, to quantify functional diversity over the Cenozoic era, spanning 66 million years to the present day. The researchers found that sharks maintained high levels of functional diversity until about 20 million years ago during the Miocene epoch, after which there was a steady decline in the range of ecological roles held by sharks. The present-day functional diversity of sharks is now lower than at any point in the last 66 million years, indicating a significant loss in ecological contributions.
By quantifying the ecological contributions of individual shark species, the researchers determined that the decline in functional diversity was driven by the loss of unique and specialized species, including the extinction of the megalodon, the largest shark to ever exist. These extinctions have resulted in a narrower range of ecological roles held by living sharks compared to their extinct counterparts. The researchers also highlighted the impact of human threats like overfishing, which are pushing sharks towards extinction and further eroding their already diminished ecological contributions to ecosystem functioning.
Senior author Dr. Catalina Pimiento emphasized that identifying modern species holding some of the Cenozoic functional space could help prioritize conservation efforts to preserve shark functional diversity in the face of ongoing environmental changes. The study, published in Global Ecology and Biogeography, provides valuable insights into the historical trends in shark functional diversity and the potential consequences of current threats on the ecological roles played by sharks in marine ecosystems. By understanding the past dynamics of shark communities, researchers hope to inform conservation strategies aimed at safeguarding the diversity and resilience of these iconic marine predators.
