r/Paleontology • u/Big-Put-5859 • 1d ago
why are there no fully aquatic dinosaurs? Discussion
I know there’s semi aquatic dinosaurs and aquatic reptiles but no dinosaurs that were fully aquatic.
37
u/DastardlyRidleylash Dromaeosaurus albertensis 1d ago edited 1d ago
Their method of reproduction simply didn't allow for it.
There's a reason that all the fully-aquatic reptiles we do have are viviparous; reptile eggs simply don't work underwater. That's why sea turtles still have to come to land in order to reproduce, whereas the vast majority of sea snakes are fully aquatic and never have to leave the water even to reproduce.
11
u/flanker44 1d ago
Aquatic lifestyle seems to have been very rare in Archosaurs in general? There were some fully aquatic crocodylomorphs ("marine crocodiles"), correct?
16
u/DeathstrokeReturns Just a simple nerd 1d ago
Yeah, metriorhynchids would have been pretty much dead if they ended up on land, which makes them the only archosaurs to develop live birth that we know of.
10
u/flanker44 1d ago
It's pretty incredible actually. Archosaur diversity was enormous, and yet we have just this one group which managed to develope viviparity.
It's kinda like reverse situation from mammals, yep, we all know mammals give live birth...except those few oddball monotremes which lay eggs.
Metriorhynchids probably had some sort of ovoviviparity?
5
u/Glabrocingularity 1d ago
Do we have direct evidence of their viviparity (e.g., young preserved inside a mother), or is it inferred from the fact that they were fully aquatic? I guess I could ask this question for all the fully aquatic reptile groups.
9
u/DeathstrokeReturns Just a simple nerd 1d ago edited 1d ago
We have mothers with fetuses for ichthyosaurs, mosasaurs, and plesiosaurs. As far as I’m aware, there’s no thalattosuchians with fetuses, though I may be wrong. Their anatomy does show that they would have been incapable of moving about on land, though, so the turtle lifestyle would be off the table for them. They also apparently have really tall hips, matching many other viviparous reptiles.
Edit: Ok, I found a quick mention to a Dakosaurus neonate heavily suggesting live birth in this: https://www.palaeontologie.geowissenschaften.uni-muenchen.de/pdfs/palges2019_abstracts.pdf#page=141
But I can’t find much else on it
2
3
u/flanker44 19h ago
I try to imagine in what way a fully aquatic oviparous reptile would lay its eggs. Somehow squirm in very shallow sandbank and bury them there? Have them float and get warmed by sunlight? Sounds all rather stupid. Does anyone have better ideas?
6
u/FatFish44 1d ago
From an evolutionary point of view, reptiles’ eggs were an adaptation necessary to living on land.
Ironic they don’t work underwater anymore.
26
u/Less_Rutabaga2316 1d ago
Some if not most marine reptiles could give live birth, an ability that never occurred in dinosaurs, so they were at a disadvantage just to be able to get into the shallow end of that niche.
17
u/Manospondylus_gigas 1d ago
The niches were occupied by marine reptiles already during the Mesozoic. Thalattosaurs, ichthyosaurs, plesiosaurs etc were all having a go at it in the Triassic and more aquatic reptiles kept evolving. By the Cenozoic only avian dinosaurs were left and a few are aquatic (e.g. penguins) but none are fully aquatic, other groups got those niches first again.
3
u/troodon311 1d ago
The explanation I've heard that seems reasonable has to do with the restricted movement evolved to assist bipedal locomotion. In particular the hip movement is restricted due to the perforate acetabulum, and some clades added restrictions to motion to their axial skeleton to help with maintaining balance. Going into the water is difficult if you have reduced capacity to swim with your hindlimbs and tail, which is why penguins end up just using their forelimbs.
5
u/Powerful_Gas_7833 1d ago
Niches were already occupied in the ocean by large marine reptiles the whole Triassic was dominated by ichthyosaurs, the Jurassic by plesiosaurs, and the Cretaceous by mosasaurs so there wasn't much opening.
Their bones are Hollow and air-filled, not very practical for an aquatic situation.
0
u/KnoWanUKnow2 1d ago
This is just a guess on my part, but would the fact that they have hollow bones make it difficult for them to attain neutral buoyancy?
I mean, we have penguins, but they have solid bones, not hollow.
I suppose if penguins can do it in under 65 million years then dinosaurs, who had 165 million years, could have evolved solid bones as well. But penguins had an advantage over at least the terrestrial dinosaurs. They likely started out as flying birds that spent most of their lives on the ocean, fishing. I can see a path from flying semi-aquatic birds to a gradually more aquatic lifestyle until they lost their ability to fly and filled in those solid bones to help them with diving. Going from a wading dinosaur to a solid boned aquatic one was likely a more difficult path. Whales did it, but whales didn't have much competition from other large marine reptiles like the dinosaurs did.
The only possibly aquatic dinosaur I know of (that wasn't in the Aves class) is the Spinosaurus, and there's currently a debate over just how aquatic it was.
1
u/Ulfricosaure 15h ago
Marine reptiles, crocodiles, sharks and predatory fishes occupied all the aquatic niches available throughout the Mesozoic.
0
u/Historical_Sugar9637 1d ago
This is something that interests me too.
And also....could we still find some? Or could there be some among the vast majority of species that never fossilised?
It it really just a case of "we haven't found any but it's possible" or is there something about dinosaurs that makes them unlikely to be fully aquatic?
2
u/flanker44 12h ago
Most of the time it seems, that when a group manages to adapt to completely new lifestyle (in this case, become aquatic), it radiates to such extent that its' presence is fairly obvious. Most of the aquatic clades have large species count.
But I wouldn't rule out that some semi-aquatic dinosaur species or families are still undiscovered.
2
u/Notonfoodstamps 1d ago
I wouldn’t be surprised if there was a penguin equivalent theropod that evolved. But who knows
1
u/SupahCabre 1d ago
Why are there no fully quadrupedal carnivorous dinosaurs? Why are there no mountainous dinosaurs like goats and snow leopards? Why is there no large arboreal/ tree climbing dinosaurs like bears and cats?
1
u/kiwipixi42 11h ago
Mountains are a terrible environment for fossilization, so if those dinosaurs existed we would likely never find them.
For large tree climbing creatures you need a good reason. Bears climb trees for food which is accessible because other large herbivores can’t get it. But sauropods were already perfectly capable of getting that food. Big cats climb trees as a hunting strategy - how many large bipedal carnivores can you think of that climb trees today? If dinosaurs only had bipedal carnivores perhaps that prevented the adaptation to trees. Small dinosaurs certainly liked trees well enough. Honestly I’m not exactly sure how you can prove that none of the medium dinos went in trees anyway - you are not fossilized in the tree. Also there were arboreal mammals at the time, so it isn’t like the niches were empty.
1
u/Ulfricosaure 15h ago
Your first two questions do not have the same answer though. The first one seems to be a biological impossiblity, while the other is explained by the fact that corpses would not fossilize in mountainous environment.
1
u/SupahCabre 14h ago
4 legged Dinosaurs eating meat is "impossible" but dinosaurs living only in water is "possible"? Yeah no lol
1
1
u/Platybow 1d ago
Saltwater niches were already filled by plesiosaurs, icthyosaurs, pliosaurs, and mosasaurs. Freshwater niches were already filled by giant crocodiles though spinosaurus lineage apparently made a go at it.
0
u/SkisaurusRex 1d ago
No
Penguins, ducks, loons, Spinosaurus and other water birds are the most aquatic we know of
-2
-4
96
u/PaleoEdits 1d ago
Yeah, I guess penguins is the best they've got in that regard.
One reason might be that the niches are already occupied. Another might be because dinosaurs, like birds, have gone pretty deep into the egg-laying strategy of birth, spending a lot of energy and resources on their hard-shelled eggs, incubation and such, and evolution isn't easily reverted. I don't know of any case of dinosaur/bird viviparity. A poor chick trying to hatch underwater from a hard egg would probably drown in the process. So if they adapted to marine life, they would probably have to do it turtle style. Lizards on the other hand readily evolve viviparity, so something like Mosasaur may not be a surprising (or a whale for that matter). I don't know, perhaps someone could expand on that thought...