Imagine dolphins disappearing from the world’s oceans as a result of prolonged climate change and slower evolution. As shocking and unlikely as such an event might be, it happened in the past to a group of marine animals: the ichthyosaurs.
These “fish-reptiles” were an iconic group of marine predators from the dinosaur era – and the ichthyosaurs underwent the most profound modifications to become fast, efficient swimmers. They evolved a shark-like body shape, their limbs transformed into muscular paddles, and they had some of the largest eyes in the entire animal kingdom, presumably to seek out and hunt prey in deep or turbid marine settings. About a hundred ichthyosaur species are currently known, covering a 157 million-year reign in the ancient oceans that ended around 90 million years ago.
But ichthyosaurs mysteriously met their demise long before the mass extinction of 66 million years ago that claimed the lives of non-avian dinosaurs, ammonites and a series of other ancient creatures.
Two theories have previously been put forward to explain the out-of-the-blue extinction of the ichthyosaurs. First, increased competition from other marine predators, especially new, fast-moving and fast-reproducing fish. Second, that their food disappeared. The last ichthyosaurs were thought to rely only on one type of food – small belemnite cephalopods. And so when these underwent a partial extinction about 94 million years ago, ichthyosaurs soon followed.
These two theories have one point in common. They are based on the idea that there wasn’t enough diversity among ichthyosaurs for the group to respond to minor changes in competition and food. More diverse groups are more varied in their physical characteristics so can more easily survive changing circumstances and environments.
But we now know that the fossil record indicates the last ichthyosaurs actually were very diverse. To close this long-standing enigma, my colleagues and I at the University of Oxford studied ichthyosaur diversity during the last chapter of their history, which occurred during the Cretaceous period (145 million to 66 million years ago). We reconstructed the fluctuation in the number of ichthyosaur species and their feeding capabilities, as well as their evolutionary and extinction rates.
We found that ichthyosaurs were highly diversified in the Cretaceous, and that several species with distinct physical structures and ecological niches (ways of surviving in their environment) existed simultaneously. Some evidence even suggests that they had never been more diverse in the previous 120 million years of their history.
But, at the same time, our analyses indicated that ichthyosaurs had never evolved more slowly. In fact, only a few novel species and body shapes evolved during this lengthy period. This possibly indicates that ichthyosaurs simply were well adapted to their Cretaceous environments and didn’t need to evolve much further. The key point, however, is that the previous theories about ichthyosaur extinction can’t explain this pattern of diversity.
We then looked at what we know was happening to the global environment by reviewing evidence of changes in things such as sea surface temperature and sea levels. And we found that increased ichthyosaur extinction rates coincided with higher sea level and sea temperature volatility (by how much they changed). This gave us the first evidence of global environment change driving ichthyosaur extinction. And this extinction happened in two phases, separated by about 5 million to 6 million years.
Global climate change
These events didn’t happen in a void. A vast series of extinctions and accelerated evolutions in other marine groups took place precisely during the extinction of ichthyosaurs. These changes affected almost all of the marine ecosystems, from coral reefs to the homes of large predators.
On top of that, these events coincided with profound climatic changes: fast-moving continents, intense volcanism, ice-free poles and episodes of anoxia (absence of oxygen) on the sea floor. So the extinction of the ichthyosaurs appears to be part of a much larger event that was probably triggered by global environmental changes.
Our new work supports a growing body of evidence suggesting that a major, global series of events profoundly reorganised marine ecosystems at the beginning of the Late Cretaceous, about 94 million to 100 million years ago. This gave rise to the highly peculiar and geologically brief Late Cretaceous marine world. Not only did the ichthyosaurs disappear in the course of this change, but numerous lineages of bony fishes and sharks also evolved. We are currently expanding our research to many of these other marine groups, in order quantify these changes and their links with ancient global climate change.