Protostega
| Protostega Temporal range: Late Cretaceous, | |
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| Mounted skeleton | |
Scientific classification  | |
| Kingdom: | Animalia |
| Phylum: | Chordata |
| Class: | Reptilia |
| Order: | Testudines |
| Suborder: | Cryptodira |
| Family: | †Protostegidae |
| Genus: | †Protostega Cope, 1872 |
| Type species | |
| †Protostega gigas Cope, 1872 | |
Protostega ('first roof') is an extinct genus of sea turtle containing a single species, Protostega gigas. The species lasted from the Coniacian to the Campanian stages of the Late Cretaceous. Its fossil remains have been found in the Smoky Hill Chalk formation of western Kansas (Hesperornis zone, dated to 83.5 million years ago), time-equivalent beds of the Mooreville Chalk Formation of Alabama and Campanian beds of the Rybushka Formation (Saratov Oblast, Russia). It is also known from the middle Campanian Pembina member of the Pierre shale in Canada, Fossil specimens of this species were first collected in 1871, and named by Edward Drinker Cope in 1872. With a total length of 3.9 metres (13 ft), it is the second-largest sea turtle that ever lived, second only to the giant Archelon, and one of the three largest turtles of all time alongside Archelon and Gigantatypus.