Are marine reptiles dinosaurs?
Mosasaurus and plesiosaurs belong outside the dinosaur group. The Natural History Museum identifies mosasaurs as marine squamates, the wider reptile group that includes lizards and snakes. Plesiosaurs represent a different marine-reptile branch.
Compare anatomy through the fossils
Plesiosaurs used four flipper-like limbs. Some had long necks and small heads, while others had shorter necks and large jaws. That variation is a reason to check each profile rather than give every prehistoric sea reptile the same body form.
Sources: [1]
Check the scope of a sea-creature reconstruction
Open an exhibit to see which species or specimen anchors its reconstruction. Size estimates, diets and reproduction fields carry their own evidence notes. A partial skeleton and a preserved pregnancy answer different questions; the fictional collecting-game egg does not replace the research profile.
Prehistoric fish have their own directory below. Separating fish, marine reptiles and turtles makes it easier to follow an animal's scientific identity even when their game companions share the same collection.
Explore marine reptiles by name
16 linked profiles. Choose a name for the complete exhibit and sources.
A
C
Cryptoclidus
A small-headed, long-necked plesiosaur whose flippers have been studied through detailed muscle reconstructions. [1]
Cymbospondylus
A giant ichthyosaur whose Nevada skull documents very early marine-reptile gigantism. [1]
D
Desmatochelys
An Early Cretaceous marine turtle whose Colombian fossils include a female with preserved eggs. [1] [2]
Dolichorhynchops
A long-snouted, short-necked plesiosaur known from Kansas marine skeletons. [1]
I
Ichthyosaurus
Ichthyosaurus was a streamlined marine reptile. A Somerset specimen of I. somersetensis preserves an embryo, providing direct evidence of live birth. [1]
M
Mosasaurus
Mosasaurus was a large marine lizard with four flippers, a long tail and toothed jaws. Its size is reconstructed from incomplete fossils. [1]
O
Ophthalmosaurus
A large-eyed ichthyosaur whose British fossils must be separated from debated North American relatives. [1]
P
Pelagosaurus
A slender-snouted marine crocodylomorph whose bones suggest an amphibious life along shallow Jurassic seas. [1]
Platecarpus
A streamlined mosasaur known from an exceptional fossil preserving skin traces and digested fish. [1]
Pliosaurus
A large-skulled pliosaur whose Dorset holotype provides a detailed view of a Jurassic marine predator. [1]
Polycotylus
A short-necked plesiosaur whose pregnant fossil provides direct evidence of live birth. [1]
Protostega
A giant Late Cretaceous marine turtle with large front flippers and a reduced bony shell. [1] [2]
S
Stenopterygius
A fish-shaped marine reptile with direct pregnancy fossils and teeth that changed as it grew. [1]
T
Temnodontosaurus
An Early Jurassic ichthyosaur whose jaws and teeth reveal specialized prey-processing anatomy. [1]
Tylosaurus
A marine lizard with an elongated adult snout tip, whose fossil identities are still being revised. [1]
Sources for this guide
- What is a plesiosaur?Natural History Museum, London · Accessed 2026-10-09
- What is a mosasaur? Facts about Mosasaurus and its relativesNatural History Museum, London · Accessed 2026-10-09