The gharial-like pelagosaurs and their croc relatives loved the shallow seas of the lengthy Mezozoic and Cenozoic periods of the Earth's history. Their fossils are found in Normandy and in Somerset. Plesiosaurs and ichthyosaurs were warm-blooded reptilians to be found enjoying the same seas in higher latitudes, but the crocs were cold-blooded and needed, it seems now, global warming to continue at that time to persevere. They got no continuous warm climate and therefore declined or became extinct!
This cooperative effort by Jeremy E. Martin, Romain Amiot, Christophe Lecuyer and Michael J. Benton of the Universite de Lyon and the Institut Universitaire de France and the University of Bristol is published in the journal Nature Communications today as: Sea surface temperature contributes to marine crocodylomorph evolution.
Our modern true crocodilians are but a pale shadow of these "good old days" for the reptile, as we only have 2 of the 23 species that go to sea. They all like tropical conditions which is a clue to the major argument for these researchers in the paper. They argue that croc species invaded the ocean 4 times, but have become extinct whenever the temperature cooled. To see how prevalent marine crocodylomorphs were in the ancient Atlantic, read the information on several successes here: - Orcasaurs ruled the Atlantic.
If we take a temperature curve for sea temperatures over the relevant 200 million years, the extinctions of these animals all took place during global freezing periods. 180 mya, the first marine crocodiles colonised the environment. 25 million years after they all became extinct. The next time we warmed up, another salty croc invasion from freshwater took place.
The way in which we can assess exactly how the fossil remains lived is through the isotopic composition of oxygen in nearby fish scales and bones. The temperature of the water can then be accurately assessed.
As always, one group goes against the trend, but the authors are saving up these metriorhynchoids for further research into how their lifestyle may have helped them survive cold spells. Good luck with that! Climate change is such a focus for all science at the the moment , it is a relief to further our knowledge by investigating temperature changes that don't threaten our immediate future but killed off so many species in our past.