Evolution is great at producing novelty. Every organism that has ever lived – from the first cell to the grass on your lawn and the blue whales in the sea – is a testament to that. But evolution can also repeat itself. From disparate starting points, evolution can spur some lineages to serendipitously converge on similar forms or behaviors. Among these replays – arguably among evolution’s greatest hits – are no less than five varieties of marine crocodile.
None of these seabound reptiles exist today. True, saltwater crocodiles and even American alligators can be seen at sea, but they are not as tied to the ocean as the prehistoric crocodylomorphs that spent most – if not all – of their lives in the marine realm. These salty archosaurs all lived between 199 and 34 million years ago, with each of the five groups representing independent oceanic invasions.
But why did seagoing crocodiles keep evolving and going extinct? What regulated their rise and fall? Part of the answer, University of Bristol paleontologist Jeremy Martin and colleagues suggest, may be the constantly-changing temperature of the ocean.
Paleontologists have floated a number of explanations for the recurring rise and crash of marine crocodiles. These range from a lack of complete fossil sampling to major sea changes wherein ocean chemistry drastically restructured marine life. But in their study, Martin and coauthors compared the varying diversity of marine crocs to a pair of possible explanations – shifting sea levels and changes in the sea surface temperature.
The reach of the ocean appears to have little to do with marine croc evolution. That’s probably because these carnivores were able to catch a broad variety of prey and even make forays into freshwater habitats, Martin and colleagues suspect. Changes in surface sea temperature offer a better fit.
Starting with an Early Jurassic group called thalattosuchians, almost every lineage of crocodylomorph to slip into the seas did so during a time of warm surface temperatures. The crocs crashed when sea temperature dipped. With the exception of one lineage, Martin and colleagues found, the marine crocs followed a pattern seen among their close relatives.
Fossil crocodylomorphs are often taken as a rough proxy for warm habitats. During a global heat spike 52 million years ago, for example, alligators lived in the Arctic, and paleontologists can track how gators followed warm temperatures down the latitudes as the global climate cooled. That’s because the prehistoric crocodylomorphs, like their living relatives, had a physiology in which their body temperature was regulated by the outside the environment. They were ectotherms. To survive, they had to follow the heat.
But one group didn’t behave like the rest. Metriorhynchoid crocs became intricately-adapted to ocean life, their limbs modified to flippers and their tails bearing large, vertical flukes. They may not have been able to return to land at all. And, Martin and coauthors found, they proliferated during a time when Jurassic sea temperatures were dropping.
Perhaps the metriorhynchoids were stuck. They were so quickly adapted to an exclusively marine life that the only evolutionary paths open to them laid in the seas. Then again, Martin and colleagues write, perhaps metriorhynchoids like the fearsome Dakosaurus had a way to cope with cooler marine temperatures. Maybe, like some other marine reptiles, the metriorhynchoids were able to keep their body temperatures several degrees above the ambient water temperature. Further study is needed to find out, but metriorhynchoids may have been hot-running crocs.
No one has ever found marine crocodiles from sites that sat near the poles, though. Whether ectothermic or warm-bodied, seagoing crocs were restricted in time and space by the warmth of the seas. Crocs slid into the sea during warm times, and they disappeared when the chill set in.
The last of the truly marine crocs died out over 34 million years ago. Among these holdouts were the dyrosaurids – crocs that survived the fifth mass extinction and enjoyed the global hothouse that came in the disaster’s aftermath, only to disappear as sea temperatures again fell. But thanks to our addiction to fossil fuels, we’re rapidly recreating the hothouse conditions under which the marine dyrosaurids thrived. As we warm the world’s seas, perhaps marine crocs will evolve anew.
Martin, J., Amiot, R., Lécuyer, C. Benton, M. 2014. Sea surface temperature contributes to marine crocodylomorph evolution. Nature Communications. doi: 10.1038/ncomms5658