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What caused all the giant underwater reptiles to die out at the Cretaceous Mass Extinction but not other ocean life?(r/askscience)
Two things. Firstly, it’s important to remember that the Mesozoic Era, the ‘Age of Dinosaurs’ lasted 190 million years. The Cenozoic, or the ‘Age of Mammals’ has only lasted 65 million years. During the Mesozoic many different groups of plants and animals waxed and waned, and they did not all exist at once. Secondly, none of the marine reptiles during the Mesozoic were Dinosaurs; instead they belonged to six main groups:
Sauropterygi (which includes the Plesiosauria)
Squamata (includes modern lizards and snakes, but also the Cretaceous Mosasaurs and Aigialosaurs)
The latter three groups are still around, and all three groups contain marine species such as sea snakes, sea turtles, marine iguanas and salt water crocodiles. In saying that, aside from the Sea turtles, all the modern marine groups are more recently descended from terrestrial organisms. I’m also going to ignore the Thalattosaurs, because they died out at the end of the Triassic and weren’t around for most of the Mesozoic.
The first major group of marine reptiles during the Mesozoic was the Ichthyosaurs. The Ichthyosaurs evolved really early on, at the beginning of the Triassic, and survived a major extinction that happened at the end of the Triassic. The Ichthyosaurs looked remarkably like dolphins, with highly streamlined bodies and gave birth to live young. During the Triassic and early Jurassic they were extremely successful. During the late Jurassic however, they went into decline, and they went extinct during the mid-Cretaceous, some 25 million years before the K/T extinction event.
It is hypothesized that the rise of the ‘ray-finned’ teleost fishes led to the decline of the Ichthyosaurs; this group includes the modern pelagic fishes that outcompeted the ichthyosaurs preferred prey belemnites. Predation by larger marine reptiles, such as the pliosaurs and mosasaurs may have also lead to the extinction of the ichthyosaurs. A third hypothesis is that a major anoxic event in the world’s oceans around 91 mya knocked them out.
The Plesiosaurians were the second major group of Mesozoic marine reptiles. They became successful following a mass extinction event at the end of the Triassic, which wiped out many earlier groups of marine reptiles (with the exception of the Ichthyosaurs). While there were many types of Plesiosaurians that flourished early on, two main groups became established, and stuck around for the rest of the Mesozoic; the pliosaurs and plesiosaurs.
Loosely speaking, the plesiosaurs were the ‘long-necked’ ones such as elasmosaurus, while the pliosaurs were the ‘short-necked’ ones such as liopleurodon (which starred in Walking with Dinosaurs). These two groups thrived during the Jurassic and became increasingly less common during the Cretaceous.
Some of the pliosaurs grew to be quite massive, and probably occupied an ecological niche similar to that of the modern orca. Despite their successes, the pliosaurs were wiped out around the same time as the Ichthyosaurs, possibly due to the same anoxic event around 91mya. The pleisosaurs were probably slow swimmers, perhaps ambush predators and lasted until the K/T mass extinction.
The third main group of marine reptiles during the Mesozoic was the Mosasaurs. The Mosasaurs were descended from lizards (think monitor lizards), and were very much late comers, entering the marine environment only 20 million years before the K/T event. The mosasaurs took advantage of the vacant ecological niche left by the then extinct pliosaurs and ichthyosaurs. Despite their short period of success, they grew to massive sizes- up to 15m long, and were apex predators of the time.
So by the time the asteroid strike that wiped out the last of the dinosaurs came, there were only the pleisosaurs and the mosasaurs left, in addition to the crocodilians and the sea turtles. And when the K/T event actually happened, neither of these groups were doing particularly well, because by the end of the Cretaceous the world’s sea levels had massively regressed, drying up much of the shallow continental shelves which they would have inhabited. So it’s likely that these groups were doing poorly prior to the asteroid impact, and the asteroid impact was the final nail on the coffin for these groups.
Benson, R.B.J., Butler, R.J., Lindgren, J., Smith, A.S. Mesozoic marine tetrapod diversity: Mass extinctions and temporal heterogeneity in geological megabiases affecting vertebrates (2010) Proceedings of the Royal Society B: Biological Sciences, 277 (1683), pp. 829-834.