| Meiolaniidae Temporal range: Middle Eocene to Holocene
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| Restoration of the head of various meiolaniid species | |
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| Skeleton of Meiolania platyceps | |
Scientific classification 
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| Domain: | Eukaryota |
| Kingdom: | Animalia |
| Phylum: | Chordata |
| Class: | Reptilia |
| Clade: | Pantestudines |
| Clade: | Testudinata |
| Clade: | Rhaptochelydia |
| Clade: | Mesochelydia |
| Clade: | Perichelydia |
| Clade: | †Meiolaniformes |
| Family: | †Meiolaniidae Lydekker, 1887 |
| Genera | |
Meiolaniidae is an extinct family of large, probably herbivorous stem-group turtles with heavily armored heads and clubbed tails known from South America and Australasia. Though once believed to be cryptodires, they are not closely related to any living species of turtle, and lie outside crown group Testudines, having diverged from them around the Middle Jurassic. They are best known from the last surviving genus, Meiolania, which lived in Australia from the Miocene until the Pleistocene, and insular species that lived on Lord Howe Island and New Caledonia during the Pleistocene and possibly the Holocene for the latter. Meiolaniids are part of the broader grouping of Meiolaniformes, which contains more primitive turtles species lacking the distinctive morphology of meiolaniids, known from the Early Cretaceous-Paleocene of South America and Australia.
Meiolaniidae includes a total of five different genera, with Niolamia and Gaffneylania native to Eocene Patagonia and the remaining taxa, Ninjemys, Warkalania and Meiolania being endemic to Australasia. The group is believed to have evolved on the continent of Gondwana prior to its split into South America, Australia and Antarctica. For this reason it is speculated that meiolaniids were also present on the latter, although no fossils of them have yet been found there. Furthermore, meiolaniids may have been present on New Zealand based on the discovery of turtle remains as part of the St Bathans Fauna.
Meiolaniids were large animals, with the bigger species reaching total lengths of perhaps up to 2–3 m (6.6–9.8 ft). Meiolaniid remains can easily be identified by their skulls, which are covered in distinctive scale patterns and formed elaborate head crests and horns that vary greatly between genera. While some such as Niolamia had massive frills and sideways facing, flattened horns, others like Meiolania had cow-like, recurved horns. They also had long tails that were covered in spiked rings of bones that, at least in some genera, transitioned into a tail club towards the tip.
While their lifestyle was long debated, current research indicates that they were terrestrial herbivores with a keen sense of smell that may have used their heavily armored bodies in intraspecific combat, perhaps during mating season.