Scientists have long hypothesized that distinct species such as whales, sea turtles and seals independently evolved similar physical traits to adapt to life in the ocean. This process, known as convergent evolution, has generally been supported using qualitative, visual comparisons of bones and fossils. For the first time, a team of scientists from the Smithsonians National Museum of Natural History
and University of California at Davis used dietary data of living marine species to develop a comprehensive ecological perspective of their evolutionary origins.
The researchers developed a novel, quantitative approach to classify diets of marine mammals and reptiles and compared them with measurements of each species teeth and skulls. They found that distantly related animals with shared diets evolved similar adaptations to successfully live in the sea. Details from this study are published in the Jan. 28 edition of the Royal Society journal Biology Letters.
This new framework for understanding convergent evolution could shed light on the biology of marine species that are a part of museum collections but rarely observed feeding in the wild, like beaked whales and elephant seals, said Neil Kelley, a Peter Buck post-doctoral researcher in the museums department of paleobiology and lead author in the study. It may also provide insights into the diets of extinct creatures that ruled the seas during the age of the dinosaurs.
Many of the worlds most iconic marine species share an evolutionary past that is firmly rooted on land. Todays populations of marine mammals and reptiles all descended from separate groups of terrestrial vertebrates that convergently evolved to thrive in aquatic environments over the course of 350 million years. Each of these transitions from land to sea resulted in major, similar changes in anatomy and ecology as different species of animals adapted to moving, feeding and reproducing in the water.
The research team analyzed dietary data for 69 marine mammals and reptiles and grouped them into categories based on their primary food sources. They then conducted an extensive survey of marine skulls and teeth from the same species using collections in the Smithsonian and other museums to better understand how separate groups of animals have physically adapted to feeding in the ocean. After comparing the two data sets, they found that different species evolved similar solutions to allow feeding on particular types of food, including fish, squid, shellfish and ocean plants. They discovered that long jaws with numerous teeth have evolved in species that feed on fast swimming fish, while shellfish and marine plant-eaters have evolved shorter and deeper skulls with strong jaws and teeth for crushing their food. They also showed that within individual groups, like seals or whales, different species have evolved to feed on different types of food, allowing them to diversify in the oceans. For example, crabeater seals in Antarctica have specialized teeth for straining krill, while closely related leopard seals have evolved massive skulls and sharp teeth allowing them to feed on a wide range of prey including fish, crustaceans and penguins.
The teams research was made possible with the support of museum collections at the National Museum of Natural History. The museums collection of marine mammals is the largest in the world, consisting of more than 8,900 specimens of cetaceans (whales and dolphins), 3,200 specimens of pinnipeds (seals, sea lions and walruses) and 380 specimens of sirenians (sea cows). These fossils and bones provide the foundation for tracing the life histories of species that have adapted to the oceans fluctuating ecosystems over time. Future studies may apply the data from this research to anticipate how marine species may continue to physically adapt as they face ecological pressures from climate change, ocean acidification and overfishing.