吴中妍

If you drop a horse in the water， it will swim—as will a wolf， a hedgehog①， and a grizzly bear. Granted， these animals wont swim very elegantly and they may run out of steam after a few minutes， but neither will they immediately plunge to the bottom of a given lake or river and drown. Thats why the issue of whether or not dinosaurs could swim isnt intrinsically② very interesting. Of course， dinosaurs could swim， at least a little bit because otherwise， theyd be unlike every other terrestrial animal in the history of life on the Earth. Also， researchers published a paper concluding that Spinosaurus， at least， was an active swimmer， perhaps even pursuing its prey underwater.    Before we proceed further， its important to define our terms. Many people use the word “dinosaur” to describe giant marine reptiles like Kronosaurus and Liopleurodon. However， these were technically plesiosaurs， pliosaurs， ichthyosaurs， and mosasaurs. They are closely related to dinosaurs， but they are not in the same family by a long shot. And if by “swim” you mean “crossing the English Channel without breaking a sweat”， that would be an unrealistic expectation for a modern polar bear， much less a hundred-million-year-old Iguanodon. For our prehistoric purposes， lets define swimming as “not immediately drowning， and being able to climb out of the water as quickly as possible.”

Wheres the Evidence for Swimming Dinosaurs？

As you can guess， one of the problems with proving that dinosaurs could swim is that the act of swimming， by definition， leaves no fossil evidence. We can tell a lot about how dinosaurs walked by footprints that have been preserved in silt. Since a swimming dinosaur would have been surrounded by water， theres no medium in which it could have left a fossil artifact. Many dinosaurs have drowned and left spectacular fossils， but theres nothing in the posture of these skeletons to indicate whether its owner was actively swimming at the time of death.

It also doesnt make sense to infer that dinosaurs couldnt swim because so many fossil specimens have been discovered in ancient river and lake beds. The smaller dinosaurs of the Mesozoic Era were regularly swept up by flash floods. After they drowned （usually in a tangled heap）， their remains often wound up buried in the soft silt at the bottom of lakes and rivers. This is what scientists call a selection effect： billions of dinosaurs perished③ well away from water， but their bodies didnt fossilize as readily. Also， the fact that a particular dinosaur drowned is no evidence that it couldnt swim. After all， even experienced human swimmers have been known to go under！

With all that said， there is some tantalizing④ fossil evidence for swimming dinosaurs. A dozen preserved footprints discovered in a Spanish basin have been interpreted as belonging to a medium-sized theropod gradually descen-ding into the water. As its body was buoyed up， its fossilized footprints become lighter and those of its right foot begin to veer off⑤. Similar footprints and track marks from Wyoming and Utah have also occasioned speculation about swimming theropods， though their interpretation is far from certain.

At this point， you may be wondering about the swimming abilities of the biggest dinosaurs of them all， the hundred-ton sauro-pods and titanosaurs of the later Mesozoic Era. A few generations ago， paleontologists believed that the likes of Apatosaurus and Diplodocus spent most of their time in lakes and rivers， which would have gently supported their vast bulks. A more rigorous⑥ analysis showed that the crushing water pressure would have virtually immobilized these huge beasts. Pending further fossil evidence， the swimming habits of sauropods will have to remain a matter of speculation！

注：① hedgehog 刺猬

② intrinsically 本質地

③ perish 死亡

④ tantalizing 诱人的

⑤ veer off 转向

⑥ rigorous 严谨的