Why do so many marine animals have a fish-shaped body? Is it for swimming faster, eating better or for reproduction? Are all fish-shaped animals doing the same things? Through the history of life, a streamlined body propelled by a strong tail has evolved in several animal groups. This phenomenon is called convergent evolution: Two or more organisms evolving similar adaptations in response to the same environment and lifestyle. In most biology books on the topic, you will find two fish-shaped animals as the prime example: One reptile, the extinct ichthyosaur (“fish lizards”), and one mammal, the dolphin. Both of these had ancestors on land, and evolved into fully marine creatures in the wake of a mass extinction. The ichthyosaurs existed for 150 million years ago before they died out. Like an echo of these oceanic top predators, whales evolved a fish-shape and a similar lifestyle millions of years after and share the oceans with humans today.
In the research project ECHO, we will take a closer look at ichthyosaurs and whales, and their evolution. In order to answer whether they really were each other’s echo through deep time, we will study fossil ichthyosaurs, fossil whales and the whales that live today, and we will not only look on the outside, we will use data from the inside of their skeletons, too. This will enable us to answer questions about the important things in life: moving, eating and giving birth. Modern whales are excellent models for understanding how to live under water, and will help us understand ichthyosaurs, which we can never see in the wild. The ichthyosaurs, on the other hand, offer an evolutionary history three times as long as whales. They went through mass extinctions and climate change and might tell us how large, marine vertebrates cope with change.
The most recent 300 million years of evolution of life on Earth has demonstrated the repeated evolution of fish-shaped marine predators. Vertebrate lineages have evolved fish-shaped bodies dozens of times, oftentimes from four-legged terrestrial ancestors. Of these, ichthyosaurs and whales evolved similar adaptations in response to environmental pressures, as one of the most classic examples of convergent evolution. Yet, we have been left to ponder the significance of evolutionary convergences. The ECHO project will address this by systematically quantifying the extent of convergence between the textbook example of whales and ichthyosaurs in feeding, locomotion and birth, on multiple scales.
Drawing on the ECHO team’s expertise, we will deploy a novel methodological approach to study these two groups, leveraging ichthyosaur fossils collected in the Arctic, a large Scandinavian whale collection, and the world’s largest collection of whale fossils at the Smithsonian’s National Museum of Natural History. We will use data from the inside and outside of the skeletons, measuring size and shape of the bones that contribute to the streamlined body shape, to find out whether the evolutionary pathways were similar for the two groups. From the inside of skeletons, we will use CT scanning and actual thin sections to study the microstructure, which records ecology, growth and movements. This will enable us to find out when ichthyosaurs evolved live birth, and whether they grow in the same manner as whales.
Just as society applies a cumulative understanding of history to frame and understand present day events, we need paleontological evidence to better understand present-day ecosystems. As 2/3 of the ocean is negatively impacted by human activity, it is increasingly obvious that more sustainable management is needed, and by using this case of convergent evolution to understand patterns in marine evolution, we aim to contribute to the new field of conservation paleobiology.