Sharks have depleted functional diversity compared to the past 66 million years, research shows

New research from Swansea University and the University of Zurich has found that sharks maintained high levels of functional diversity for most of the past 66 million years, before steadily declining to the lowest value of Today.

The research was published in Global ecology and biogeography.

Modern sharks are among the most endangered species in the ocean, yet have survived numerous environmental changes over their 250 million year history. Today, more than 500 species play many different ecological roles, from top predators to nutrient transporters.

Ecological roles are determined by the characteristics of species, such as body size and what and how they eat. As such, measuring the diversity of these traits allows scientists to quantify the range of ecological roles in a community, known as functional diversity.

Because sharks possess soft cartilaginous skeletons that are unlikely to fossilize, these traits are difficult to measure directly in extinct species. However, measurements of their teeth, which are difficult and therefore well preserved in the fossil record, can act as proxy features, which in turn can be used to quantify functional diversity in the geological past.

Lead author Jack Cooper, a Ph.D. student at Swansea University, said: “Measurements such as tooth size, shape and types of edges broadly reflect a shark’s functional characteristics, such as body size and diet, allowing us to assess their functional diversity over time.”

The researchers measured more than 9,000 fossil and living shark teeth from approximately 500 species, collected from museum collections and literature, and quantified functional diversity throughout the Cenozoic era, from 66 million years ago to the present.

They found that throughout most of the Cenozoic, sharks maintained a high level of functional diversity – meaning a wide range of ecological roles. This diversity peaked in the Miocene about 20 million years ago. However, they also found that after this peak, the extent of sharks’ ecological functions has steadily declined over the past 10 million years, with current shark functional diversity lower than at any time in the past 66 million years.

By quantifying the ecological contributions of individual species, the researchers determined that the observed declines were driven by the loss of ecologically unique and specialized species. Such losses include the extinction of the megalodon, the largest shark that ever lived, which was an apex predator, an ecological role that no shark alive today plays.

He added: “Not only did we see a marked decrease in functional diversity, but we also found that extinct sharks as a whole contributed a broader range of ecological roles than living sharks.”

Ultimately, the results warn that human threats such as overfishing, which is driving today’s sharks toward extinction, are likely to further erode sharks’ already diminished ecological contribution to ecosystem functioning.

Senior author Dr. Catalina Pimiento, professor at the University of Zurich and senior lecturer at Swansea University, said: “By identifying the modern species occupying part of the Cenozoic functional space, our study could potentially complement the priorities for conservation to preserve shark functional diversity in our changing world.”