Were there any marine/aquatic dinosaurs?

Question

Most dinosaurs were terrestrial, but there were a couple of groups of arboreal and flying dinosaurs (microraptors, birds etc).

I have read that the theory that Brachiosaurids were aquatic has been largely discredited¹, but that Spinosaurids were probably semi-aquatic².

Excluding modern marine birds (e.g. penguins etc.), were there any other marine/aquatic dinosaurs?

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^{Note: I am aware that the marine reptiles mesosaurs, phytosaurs, mosasaurs, dolichosaurs, Icthyosaurs, thalattosaurs, Sauropterygia (placodonts, nososaurs, plesiosaurs etc), Choristodera were not part of Dinosauria.}

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^1.

^{• Three new sauropod dinosaurs from the Upper Jurassic of Colorado, Great Basin Naturalist.
 • The Physiology of Dinosaurs: _{Circulatory and Respiratory Function in the Largest Animals Ever to Walk the Earth}, Respiratory Care
 • A note on the habits of sauropods, Annals and Magazine of Natural History
 • Tipsy punters: sauropod dinosaur pneumaticity, buoyancy and aquatic habits, Proceedings of the Royal Society of London}

^2.

^{• Oxygen isotope evidence for semi-aquatic habits among spinosaurid theropods, Geology
 • Semiaquatic adaptations in a giant predatory dinosaur, Science
 • Convergent evolution of jaws between spinosaurid dinosaurs and pike conger eels, Acta Palaeontologica Polonica
 • Functional Morphology of Spinosaur ‘crocodile-Mimic’ Dinosaurs, Journal of Vertebrate Paleontology}

Answer 1

There are a couple of specimens of a dinosaur species named Liaoningosaurus paradoxus that have been found with fish in their stomachs and skeletal features suggesting it was at least semi-aquatic. It is a member of the ankylosaurian dinosaurs from the early Cretaceous period of China.

From Fish Hunting Ankylosaurs( Dinosauria,Ornithischia) from the Cretaceous of China:

With elongate and fork-like denticles of cheek tooth crowns,L. paradoxus has a dentition capable of penetrating into animals like small fishes. The carnivorous adaptation of the dinosaur is also supported by the ungual modification to a sharp claw in both the fore-and hind-limbs. The evolution of a shield-like ventral armor plate and the loose sacrum-pelvic connection suggest that L. paradoxus may have adopted an aquatic way of life,using the ventral armor plate to protect the body from underwater attacks; as such,the open suture between the neural arch and centrum of the vertebrae cannot be used to indicate the juvenile nature of the type specimen. L. paradoxus is the first carnivorous ornithischian dinosaur since dinosaur was first known in the 18th century and represents not only the first aquatic or semi aquatic example of armored dinosaurs but also the smallest species of ornithischian dinosaur so far known.

Image Source: [Paleontology • 2016] Liaoningosaurus paradoxus • Fish Hunting Ankylosaurs (Dinosauria, Ornithischia) from the Cretaceous of China

Answer 2

The only example I could find for the moment are Hesperornithes. Hesperornithes are actually part of the Aves clade (see here on tolweb.org) but they are not modern bird species as they existed during the Cretaceaus.

Note that there are examples of aquatic species in Squamata (Squamata are not Dinosauria) though such as the mosasaurs for example.

Answer 3

I found this wikipedia page which lists a few more examples:

Only a few nonavian dinosaurs are thought to have been semiaquatic. The combination of being oviparous and endothermic seems to have prevented the evolution of fully aquatic dinosaurs.

• Theropoda
  • Paraves
    • Avialae
      • †Ichthyornis - a toothed ornithuran analog of modern seabirds such as gulls and petrels
      • †Hesperornithes - a mostly flightless group of diving bird-like ornithurans
    • †Dromaeosauridae
      • †Halszkaraptor - a mallard-sized basal dromaeosaurid from Mongolia with flipper-like forelimbs
  • †Megalosauria
    • †Spinosauridae - thought to be piscivores, the group had crocodile-like skulls and includes some of the largest known carnivorous dinosaurs
• †Ornithischia
  • †Ornithopoda
    • †Hadrosauriformes
      • †Lurdusaurus - an unusually heavy-bodied and short-limbed iguanodont conjectured to have been similar in lifestyle to the modern hippopotamus

List of semi-aquatic tetrapods: Non-avian dinosaurs

Chapter 23 of New Perspectives on Horned Dinosaurs: The Royal Tyrrell Museum Ceratopsian Symposium, "A Semi-Aquatic Life Habit for Psittacosaurus, describes the evidence suggesting Psittacosaurus was semi-aquatic.

P57-59 of Discovering Dinosaurs: Evolution, Extinction, and the Lessons of Prehistory suggests some hadrosaurs may have been semi-aquatic:

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Icthyornis:

Hesperornithes:

Halszkaraptor:

Spinosaurus:

Lurdusaurus:

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Psittacosaurus: