I realize that the large sea-critters contemporaneous with the dinosaurs – ichthyosaurs, plesiosaurs, mosasaurs, etch. – were only distant kin. Have there been any actual dinosaur species found that had adapted to life in the oceans / rivers / lakes? Or amphibious forms? I may be missing something obvious, but I can’t think of any right now. Any info would be appreciated!
Spinosaurus, one of the largest if not the largest carnivorous dinosaurs, appears to have been at least semi-aquatic.
This page
lists dinosaurs it calls “Aquatic”. The reasoning seems to be that such dinosaurs as Baryonyx and Suchomimus were known to be fish-eaters, and the others have similar cone-shaped teeth. Their jaws seem to resemble those of crocodiles.
aside from that, they don’t seem extremely aquatic – they don’t have fins or indications of webbed feet. They just spent a lot of times in (I assume) lakes and rivers.
When I was a kid, they showed sauropods (brontosaurs, diplodocus, and the like) and hadrosaurs (the now-dubious trachodont, lambeosaurs, and the like) dwelling in swamps and marshes. In the former case, it was claimed that they needed the water to help support their prodigious weight, but that idea seems to have gone into the dustbin of prehistory.
It’s a good question. Is it possible that some already known dinosaurs were “aquatic” in some sense, but we haven’t noticed the clues?
Penguins are fairly aquatic… oh, you mean extinct dinosaures.
But there don’t seem to have been any ocean-going dinosaurs.
In the latter case, I suspect it was for no better reason than that some had ducklike bills. The famous “Trachodon mummy” was supposed to show webbed forefeet, but that’s been shown to be a misinterpretation.
Unlikely that they would be strongly aquatic for well-known species. There would be evidence from both the depositional environment, plus anatomy (such as denser bones), as well as trackways.
Normally, you’d get the gold medal for cleverest response today in GQ, but the Romanian judge deducted a point for the spelling error. 
Seriously, though, that was good. Nice catch!!
Correct. Spinosaurus may have lived in coastal environments, however.
My FIL, a geologist, held the opinion that Brontosaurus could not support it’s weight on land.
That’s clearly and thoroughly refuted by the existence of many sauropod trackways that were made on land.
It was formerly thought that the lack of tail dragging marks indicated the animals were wading with the tail floating, but it is now thought that they habitually carried their tails off the ground.
Of course, it’s still possible that they were primarily terrestrial, but still liked going swimming, like elephants. Or many other points in between exclusively one or the other. All land tracks actually prove is that they weren’t obligate aquatic.
This depends on what you call Dinosaurs.
I’d say yes. There was a large 25-foot monitor lizard called a Megalania in Australia that was believed to be around when modern human emerged 50,000 years ago. The aminal was believed to be sluggish, and the humans of the time set fire to the grass, killing them off.
Of course, you have crocodiles and Alligators, two large reptilian animals that were around for about 180 million years!
Scientists believe 100+ million years ago, the oxygen level was around 30% of the atmosphere, which helped larger creatures get enough of it to move around. Today it’s about 18% of the atmosphere.
Not too far back, the Coelacanth fish was discovered, who shared waters with dinosaurs.
Well, I can’t believe that I forgot about spinosaurus. Good one!
Swam or waded?
Although I have seen video of elephants swimming.
It is generally believed today that sauropods were primarily terrestrial, based on various lines of evidence including anatomy and probable diet. Some did, however, live in wet environments such as river margins or near marshes. (Of course, these are the environments in which fossils are most likely to be created as well.)
Trackways have been found in which only the forefeet show, implying that the animal was partly floating with its hind feet off the bottom. However, it has also been claimed that in some circumstances the front feet would leave marks when the hind feet might not even on land (due to the different structure).
This would have baffled the French judge, who was prepared to award an extra deux points for correct spelling.
Scientists have a very strict definition of what is a dinosaur. They and crocodiles and alligators, and the lizards [megalania is was pretty much a komodo dragon on steroids] are all reptiles, and share a common ancestor, but the line recognised as dinosaurs branched off 200+ million years ago. While big and scary and doubtless an apex predator in its environment, megalania and dinosaurs were only distant cousins.
We don’t have any direct evidence that humans interacted with Megalania, much less any evidence of what methods would have been used to hunt them.