What Is a Marine Reptile? Land Animals That Went Back to the Sea

Three groups dominate the record and they are not closely related to each other. Ichthyosaurs had a body shape closely resembling that of a dolphin, with a streamlined form, a dorsal fin, a crescent tail and limbs converted into rigid flippers, and they had abandoned any connection to land entirely, giving birth to live young at sea. Plesiosaurs kept a broader body and swam with four large wing-like flippers, dividing into forms with very long necks and small heads and forms with short necks and enormous heads. Mosasaurs appeared much later and were lizards closely related to monitor lizards and snakes, swimming with a laterally flattened tail. Alongside these sat marine crocodile relatives, marine turtles and various smaller groups, so the sea contained a considerable variety of reptiles for well over a hundred million years.

Each group had to solve the same problems and the solutions are recognisably similar:

• •Streamlining, since drag dominates swimming and body shape converged on similar forms in unrelated lineages
• •Limbs converted to flippers, with the bones shortened, flattened and in several groups increased in number
• •Live birth, since an egg cannot be laid at sea, which is directly evidenced by fossils preserving embryos and one caught mid-birth
• •Salt management, requiring glands to excrete the excess taken in with food and water
• •Breathing air while diving, with adaptations to oxygen storage that are inferred rather than observed
• •Dense or lightened bone depending on the way of life, since bone density controls buoyancy

The point is worth stating clearly because the confusion is near universal. Dinosaurs are defined by descent from a specific ancestor and by features of the hip and limb that permitted an upright posture, and no marine reptile group falls within that. Ichthyosaurs and plesiosaurs branched from the reptile tree before the dinosaur lineage. Mosasaurs are lizards. Pterosaurs, which are also routinely called dinosaurs, are the closest of the lot, being on the same branch, and are still outside the group. The animals shared their world with dinosaurs and were the dominant large predators in the sea while dinosaurs dominated land, which is presumably why they get bundled together in popular accounts. The one genuinely marine group descended from dinosaurs is the seabirds, which is a fact that surprises people when stated that way.

Reptiles were not the first or last vertebrates to reverse the move onto land, and the repeated pattern is one of the clearer demonstrations of how predictable evolution can be under strong constraints. Whales descend from land mammals and passed through a documented series of intermediate forms, retaining reduced hip bones that record the transition. Seals, sea cows, otters and several others made partial returns and show the process at various stages. Penguins converted wings into flippers. Sea snakes and marine iguanas did it more recently. In each case the same changes recur, namely streamlining, limbs to flippers, dense or lightened bone, salt excretion and modified breathing, arrived at independently because the physics of moving through water is the same for everyone. The extinct marine reptiles simply went further and, in the case of ichthyosaurs, arrived at a shape almost indistinguishable from an animal separated from them by hundreds of millions of years.

These animals were central to the founding of palaeontology as a science, earlier than the dinosaurs. Specimens collected from the coastal cliffs of southern England in the early nineteenth century, substantially by Mary Anning, provided the first well-understood examples of large extinct animals unlike anything alive, and they were described and debated by the leading anatomists of the period at a time when extinction itself was still a contested idea. The completeness of those specimens, which came from finely laminated marine mudstone that preserves whole skeletons, made them far more informative than the fragmentary land material available then. Anning's position, as a working-class woman selling fossils to gentlemen who published on them without crediting her, has been reassessed extensively, and her contribution to the foundations of the subject is now generally acknowledged as having been substantial and systematically understated.

Ichthyosaurs, plesiosaurs and mosasaurs are three unrelated groups that independently converged on streamlined bodies, flipper limbs and live birth, with a fossil preserving one caught mid-birth. None is a dinosaur, since all branched from the reptile tree elsewhere, and the only marine dinosaurs are seabirds. These animals founded the science, from cliffs in southern England, before dinosaurs were recognised.