I didn't know you could detect "the bends" in fossils, but apparently evidence of such events is preserved as bony deformities (microinfarcts), as reported in The Economist:
That these Mesozoic marine reptiles, contemporaries of the dinosaurs, got the bends can be seen from their bones. What can also be seen is a curious evolutionary tale—for not all ichthyosaurs succumbed...It's the latter sentence that leads to the interesting part:
To this end, [Bruce Rothschild of the University of Kansas] and his colleagues travelled the world’s natural-history museums, looking at a total of 116 ichthyosaurs from the Triassic period... and 190 from the later Jurassic and Cretaceous periods...Fascinating.
When he started, he assumed that signs of the bends would be rarer in younger fossils, reflecting their gradual evolution of measures to deal with decompression, such as the ability found in many whales to store lots of oxygen in their blood. Instead, he was astonished to discover the reverse. More than 15% of Jurassic and Cretaceous ichthyosaurs had suffered the bends before they died, but not a single Triassic specimen showed evidence of that sort of injury...
As he reports in Naturwissenschaften, he suspects it was evolution in other species that caused the change. Whales that suffer the bends often do so because they have surfaced to escape a predator such as a large shark. One of the features of Jurassic oceans was an abundance of large sharks, and also of huge marine crocodiles, both of which were partial to ichthyosaur lunches. Triassic oceans, by contrast, were (from the ichthyosaur’s point of view) mercifully shark- and crocodile-free. In the Triassic, then, ichthyosaurs were top of the food chain. In the Jurassic and Cretaceous, they were prey as well as predator—and often had to make a speedy exit as a result.
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