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.

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.

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 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.

The research history of meiolaniids is long and at times complicated, with especially the early years suffering from poor records, incorrect identifications and loss of information.

Some of the earliest supposed discoveries made by western scientists are said to date to the middle of the 19th century, with writings suggesting that various locals and visitors of Lord Howe Island, situated off the eastern coast of Australia, discovered the remains of large turtles.

It was based on this material that Owen named the genus Meiolania in 1886 to include two species, M. platyceps and M. minor, believing it to be a small relative of the mainland specimen.

Having heard of Ameghino's Niolamia argentina, the researcher concluded that Roth's turtle represented the same species, but placed both in the genus Miolania (likely a misspelling of Meiolania).

Instead, the vast quantity of fossil material collected on Lord Howe Island led to a series of major publications penned by Eugene S. Gaffney, now renowned for his work on this group.

Split across three papers published in 1983, 1985 and 1996, Gaffney described in great detail the skull,[15] vertebrae[16] and finally the shell and limbs[17] of Meiolania platyceps, providing the most extensive look at this taxon to date.

[19] Only two new taxa have been named since this boom in the 1990s, with ?Meiolania damelipi representing an uncertain member of this group from Holocene of Vanuatu and Fiji and Gaffneylania being a second genus from the Eocene of Argentina in addition to Niolamia.

The oldest unnamed meiolaniid from Australia, known based on shell remains, osteoderms and a tail ring, dates to the Late Eocene and has been discovered in the Rundle Formation of Queensland.

In meiolaniids, the individual plates that form the skull are highly ankylosed, meaning they are fused with each other to a degree that typically makes it impossible to determine where one element ends and the other begins.

Despite the absence of such sutures however, researchers can readily distinguish the different genera and species through the presence of marks left by the overlying scale areas, with are either present through faint grooves or raised ridges.

To simplify diagnosis and create a consistent naming scheme, these scale areas are labeled with capital letters, a system already used in a similar form during early research and later refined by Gaffney.

Generally speaking, the A horn is a singular element located at the back of the skull that ranges from forming a large, frill-like structure to an almost vestigial shelf.

Sterli and de la Fuente conclude that the presence of well defined scale areas present on the skull may have been plesiomorphic for all turtles, and was simply lost and re-evolved repeatedly in the crown group.

[31][32][26][7] Pleurodira Cryptodira Chubutemys copelloi Mongolochelys efremovi Peligrochelys walshae Patagoniaemys gasparinae Otwayemys cunicularius Kallokibotion bajazidi Niolamia argentina Ninjemys oweni Warkalania carinaminor Meiolania platyceps According to research by Sterli and colleagues, meiolaniids derive from the Meiolaniformes, a group of turtles that likely evolved during or prior to the Early Cretaceous with a ghost lineage stretching as far back as the Early Jurassic.

It is thought that meiolaniids evolved from meiolaniiforms in the approximate region of where the continents South America, Antarctica and Australia connected prior to the separation of these landmasses in the Late Eocene.

[25] While the early distribution of the family is easily explained by continental drift, several competing ideas exist in regards to their further dispersal across the islands of the South Pacific.

Some researchers, in particular those in support of an aquatic lifestyle, have proposed that meiolaniids actively crossed oceans to arrive on distant islands, either by swimming, wading or floating.

[7] In addition to citing many of the same reasons that render active swimming unlikely (the insufficient buoyancy of the shell and heavy build), Brown and Moll argue that adults would struggle with finding rafts large enough, while juveniles would be easy prey to any marine or island predators.

[17] Sterli however argues that this model is limited in its ability to explain distribution, as many of the island chains meiolaniid remains were found on run parallel to mainland Australia, rather than moving away from it.

[7] Multiple elements of Meiolania's skeleton, such as the domed shell, robust forelimbs and anatomy of the shoulder girdle, all compare favorable to terrestrial tortoises rather than aquatic terrapins or turtles.

Brown and Moll further criticize the methodology and sample size of Lichtig and Lucas specifically, pointing out that their publication worked with a single juvenile specimen, which was a composite and thus didn't reflect actual Meiolania proportions, much less those of an adult.

[7] Part of the reason for this is the limited range of motion provided by their neck and the heavy skulls, which are not suited for an animal that would have to consistently keep its head raised to feed.

Chemical signals can induce aggressive and combat behavior in modern tortoises, which may respond with a variety of shell-based maneuvers like pushing, ramming, knocking and in the case of meiolaniids the use of their the clubbed tail.

[33] The wide and oftentimes isolated nature of meiolaniid distribution means that their extinction was not a singular event but rather the combination of several factors that gradually caused their disappearance from different landmasses.

The gradually cooling of Earth's climate following the Eocene Optimum put pressure on the turtles native to Patagonia, which failed to cope with the changing conditions.

While South America generally remained in the same place, Australia would continuously drift northward, entering higher latitudes and subsequently compensating for the global drop in temperatures, allowing meiolaniids to survive past the Oligocene and into the Pleistocene to Holocene.