Ichthyosaurs, dolphin-shaped reptiles that lived alongside the dinosaurs, were among the most formidable marine predators in Earth's history. For roughly 160 million years, these spectacular animals flourished in the Mesozoic oceans, rapidly diversifying into numerous shapes and sizes ranging from a few feet in length to 50 foot giants.
But despite a long and successful reign as apex hunters, ichthyosaurs fell on hard times during the mid-Cretaceous period, and died out completely around 90 million years ago. The forces that drove them to extinction have been debated among scientists for decades. Unlike the catastrophic impact event that killed off the dinosaurs 66 million years ago, there is no obvious smoking gun that explains why these marine badasses were edged out of existence forever.
Now, a team led by Valentin Fischer, a paleontologist based at the University of Oxford, may have narrowed down the main culprit: climate change. In a study published Tuesday in Nature Communications, Fischer and his colleagues reconstruct the downward spiral of the ichthyosaur clan relative to other species, concluding that their extinction was propelled by their failure to adapt to the volatile conditions of a warming planet.
"In our research, we found that a vast number of marine groups are affected by one or a series of events that are coeval with the extinction of ichthyosaurs," Fischer told me over email. "On top of that, all these events coincide with profound climatic changes: fast-moving continents, intense volcanism, ice-free poles, and episodes of anoxia (absence of oxygen) on the seafloor."
"The extinction of ichthyosaurs thus appears as a facet of a much larger event, which was probably triggered by global environmental changes," he said.
In contrast, many of the ichthyosaur family's main competitors were able to exploit the new niches that opened during this time of profound ecological disruption, which is known as the Cenomanian-Turonian boundary event. For instance, mosasaurs, the giant carnivores recently fictionalized in Jurassic World, soldiered through the upheavals and emerged as the dominant marine predators until they were killed alongside the dinosaurs. Ichthyosaurs, in contrast, could not find a foothold—or rather, a finhold—in the new evolutionary world order.
"Ichthyosaurs did not evolve new species that took advantage of the novel opportunities in this rapidly changing world," Fischer explained. "It probably isn't the rise of temperature and sea level that directly affected them; these changes are what the rocks throughout the world record of these highly changing times."
"Changes in food availability, migratory routes (if any), competitors, or birthing places, etc. are all potential drivers, which probably occurred in conjunction to drive ichthyosaurs to extinction," he continued. "But the sheer number of biotic and abiotic change going on at that time makes it difficult to pinpoint a precise subset of factors driving ichthyosaurs to extinction."
In other words, ichthyosaurs seem to have succumbed to death by a thousand climatic cuts. This may sound familiar given that modern oceans face similar threats from warmer temperatures, and marine ecosystems around the world are indeed crashing with disturbing rapidity.
But though it's tempting to draw parallels between the extinction of ichthyosaurs and modern marine collapse, Fischer points out that the Cenomanian-Turonian event differs in many ways that defy direct analogy.
"In our paper, we are talking about climate change and volatility going on over several million years," he said. "The world was also quite distinct from today: continents were moving much faster, resulting in increased volcanic activity, which also led to higher greenhouse gases concentrations and thus much higher temperatures, disrupted ocean floor oxygenation, and water acidification."
Still, the last stand of the ichthyosaur family does highlight the inherent fragility of life on Earth, which is always a relevant topic for those who live on it. After millions of years of living large in the Mesozoic oceans, these magnificent animals ultimately became yet another reminder that the course of evolutionary history never did run smooth.