page_title: List of semiaquatic tetrapods
section_title: Nonavian dinosaurs
hierarchical_section_title: List of semiaquatic tetrapods / Nonavian dinosaurs
caption_attribution_description: Hesperornis regalis, a bird from the Late Cretaceous of North America, pencil drawing
context_page_description: This is a list of tetrapods that are semiaquatic; that is, while being at least partly terrestrial, they spend part of their life cycle or a significant fraction of their time in water as part of their normal behavior, and/or obtain a significant fraction of their food from an aquatic habitat. The very earliest tetrapods, such as Ichthyostega, were semiaquatic, having evolved from amphibious lobe-finned fish.
  Some marine mammals, such as the marine otter, the polar bear and pinnipeds, are semiaquatic, while others, such as the sea otter, cetaceans and sirenians, are fully aquatic. The only fully aquatic nonmarine mammals are several manatees and certain small cetaceans. No bird species is fully aquatic, as all must lay and incubate their amniotic eggs, as well as begin raising their young, on land or ice. Similarly among marine reptiles, sea turtles are almost fully aquatic, but must come ashore to lay eggs. Marine iguanas and partly marine crocodiles are all semiaquatic. Most sea snakes are ovoviviparous and fully aquatic. A few freshwater snakes are also ovoviviparous and fully aquatic, but the majority are semiaquatic.
context_section_description: Only a few nonavian dinosaurs are thought to have been semiaquatic. While the Mesozoic had many types of marine reptiles, the combination of being oviparous and endothermic seems to have prevented the evolution of fully aquatic dinosaurs, as in birds.
  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 partially piscivorous; the group had crocodile-like skulls, paddle-like tails and includes some of the largest known carnivorous dinosaurs. The preference for a semiaquatic lifestyle probably varied by species.
  †Megalosauridae - coastal predators and scavengers.
  †Ornithischia
  †Ornithopoda
  †Hadrosauriformes
  †Lurdusaurus - an unusually heavy-bodied and short-limbed iguanodont conjectured to have been similar in lifestyle to the modern hippopotamus
caption: Hesperornis restoration
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