Meiolaniidae
Meiolaniidae | |
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Restoration of the head of various meiolaniid species | |
Skeleton of Meiolania platyceps
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Scientific classification | |
Domain: | Eukaryota |
Kingdom: | Animalia |
Phylum: | Chordata |
Class: | Reptilia |
Clade: | Pantestudines |
Clade: | Testudinata |
Clade: | Rhaptochelydia
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Clade: | Mesochelydia |
Clade: | Perichelydia |
Clade: | †Meiolaniformes |
Family: | †Meiolaniidae Lydekker, 1887 |
Genera | |
Meiolaniidae is an extinct family of large, probably herbivorous
Meiolaniidae includes a total of five different genera, with
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
History of discovery
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. The first well supported finds came just prior to the 1880s, when a large skull of what is now known as Ninjemys was discovered in Queensland and sent to paleontologist Richard Owen.[1] Although the fossils was correctly identified by its collector, G. F. Bennett, Owen instead believed the skull to have belonged to a type of lizard. Combining the skull with the vertebrae of the giant monitor lizard Megalania and the foot bones of a marsupial, Owen came to believe that the bones represented a type of giant thorny devil.[2][3]
By 1884 better recorded fossil discoveries had been made on Lord Howe Island, with multiple shipments being sent to Owen in London. Again, the material had been correctly identified as having belonged to turtles by local collectors and researchers, but was then misattributed to lizards by Owen. 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.[4] As Owen had given the name Megalania (great roamer)[5] to the chimeric material from the mainland, he subsequently named the Lord Howe material Meiolania (small roamer). This has however led to some confusion, as the etymology of Meiolania was never specified in the actual publication. Eugene S. Gaffney would later suggest that "-lania" actually translated to "butcher", a notion later contested in the works of Juliana Sterli.[6]
Owen's identification was soon criticized by other scientists in London, who agreed with the Australian researchers in that these remains were actually those of turtles, not lizards. Just one year after Meiolania was named,
In spite of Owen's conviction, more and more researchers published on the turtle identity of Meiolania.
Parallel to the later stages of this initial burst of revisions, the remains of a third meiolaniid were discovered in 1898 across the
No new species were named between 1938 and the 1990s. 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. This detailed look at the type species ran in tandem with several studies examining meiolaniid fossils from other localities. 1992 saw the description of three new meiolaniid taxa in the span of a single year, consisting of the new species Meiolania brevicollis from mainland Australia,[18] Ninjemys as a new name for Meiolania oweni and Warkalania, a new genus with reduced horns.[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.[6]
Species
Genus | Species | Age | Location | Notes | Image |
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Gaffneylania[6] | Gaffneylania auricularis | Eocene | Argentina (Sarmiento Formation) | An early, but relatively poorly understood meiolaniid, Gaffneylania was named in honor of Eugene S. Gaffney for his contributions to the understanding of this group. While geographically close and having similar B-horns to Niolamia, its exact phylogenetic position is unclear due to the fragmentary material. | |
Meiolania brevicollis[18] | Miocene | Australia (Camfield Beds) | Named for its short neck, M. brevicollis is the oldest species of Meiolania and among the most complete. It shows more slender and more strongly curved horns compared to M. platyceps and further is clearly geographically separate, as it was found on mainland Australia. | ||
?Meiolania damelipi[20][21] | Holocene | Vanuatu (Efate Island) | Known from archaeological sites on Vanuatu and Fiji, ?M. damelipi is known primarily from limb elements that show clear signs of butchering and burning. However, the absence of skulls, horns or tail rings has led some researchers to question if this turtle was actually a meiolaniid or some other, unrelated type of turtle. Sterli further points at several anatomical features that do not match meiolaniid anatomy.[7] | ||
Meiolania mackayi[9][17] | Pleistocene | New Caledonia (Walpole Island) | A potentially dubious species of Meiolania, Gaffney argues that the material is insufficient to distinguish it from M. platyceps of Lord Howe Island. However, the name is retained regardless due to its importance for communication, making it easier to clarify which island's turtles are referred to. Furthermore, Gaffney concurs that it may have been a "biological species", meaning it could have been genetically distinct given the large distance between the Walpole population and those of New Caledonia and Australia.[17] This notion would later be echoed by Sterli, who reasoned that the two populations would have been unable to maintain gene flow between another.[7] | ||
Meiolania platyceps[4] | Pleistocene | Lord Howe Island | Meiolania platyceps is the type and best known species of Meiolania, known from several hundred individuals found during almost uninterrupted collection on Lord Howe Island. It is the only meiolaniid of which the entire skeleton is known and thus one of the main sources for information on this group. | ||
Wyandotte species[22][23] | Pleistocene | Australia (Wyandotte Creek) | The largest form of Meiolania, the Wyandotte species remains unnamed and is at times tentatively assigned to M. platyceps. It has some of the largest horns among Meiolania species. | ||
Ninjemys[1] | Ninjemys oweni | Late Pleistocene | Australia (Darling Downs) | Named for the Teenage Mutant Ninja Turtles, Ninjemys is the basal-most meiolaniid of Australasia. This is evident through the anatomy of its horns, which bears closer resemblance to Niolamia. Ninjemys was among the largest meiolaniids, rivaling the contemporary Wyandotte species. | |
Niolamia[24][12] | Niolamia argentina | Eocene | Argentina (Sarmiento Formation?) | Niolamia has a long and complex history strongly tied to the rivalry between the researchers that named and described it. The subsequent confusion extends to the type locality, for which contradictory information exists, however, recent research suggests that the fossil likely originated in the Sarmiento Formation. Crossochelys corniger, another Argentinian meiolaniid, was found to be a juvenile of Niolamia. | |
Warkalania[19] | Warkalania carinaminor | Oligocene - Miocene | Australia (Riversleigh Station) | Warkalania shows the least elaborate head gear among meiolaniids, with the horns and shields seen in other genera being reduced to a continuous shelf of horns that spans the back of the head. Warkalania is the oldest named meiolaniid from Australia. |
Indeterminate remains
As meiolaniid fossils are often found in the form of broken horn cores and tail rings, much of the collected material is only present in the form of fragmentary remains too scrappy to be named or even assigned to any existing species. Due to this, much of meiolaniid diversity is only known to science in the form of various fossils designated Meiolaniidae indeterminate. However, even if fragmentary, this material nonetheless shows that members of this group were diverse and widespread throughout Cenozoic Australia.
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.[25] Remains found in Early Miocene Canadian Lead near Gulgong (New South Wales) seem to belong to an intermediate taxon, combining the flattened horns of taxa like Niolamia and Warkalania with the recurved horns of Meiolania. Other continental remains were found in the Late Oligocene Etadunna Formation and Namba Formations (South Australia), the Early Miocene Carl Creek Limestone of the Riversleigh (Queensland), the Middle Miocene Wipajiri Formation (South Australia) and the Pliocene Chinchilla Sands (Queensland).[26][17] Some of these may have beend alongside named genera, indicating that two or more meiolaniids could be found in the same environment. The indeterminate Riversleigh meiolaniid for instance likely coexisted with Warkalania, which is clearly differentiated through the horn core anatomy.[19]
Indeterminate remains from islands have been discovered in the Pleistocene to Holocene
Description
The most defining feature of meiolaniids is the presence of clearly defined scale areas covering the skull. 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. These scale areas, at times also simply referred to as scales or scutes, are largely homologous with one another and can easily be compared. 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. Some consistent features of these scales include the presence of paired G and D scales covering the roof of the skull, a singular X scale sitting at the center of these scales which varies in size between basal and derived genera and unpaired Y and Z scales that sit between the eyes and over the nose.[24][1]
The most prominent scale areas are those designated A to C in order from the back-most area to the front-most pair. These scale areas, commonly referred to as horns or horn cores due to their size and shape, are very pronounced and highly distinct in the individual genera and even species. 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. The B scales are paired and appear more horn-like in their morphology, while the C horns are typically reduced and knob-like. Niolamia possesses the most elaborate A horn, which forms a structure somewhat resembling the frill of a
Aside from the large horns present on the skulls, meiolaniids are also characterized by their heavily armored tails. It is believed that the entirety of the tail in meiolaniids was covered in bony rings flanked by at least two pairs of spikes. Such bone rings are known from even the most basal genus, Niolamia, and surrounds the entire circumference of the tail in it and Ninjemys.
Other parts of the skeleton are harder to compare due to the incomplete nature of most meiolaniids, with much of the information stemming from Meiolania platyceps itself. The shell of Meiolania is domed rather than flattened, one of several traits indicative of a terrestrial lifestyle. However, the
Meiolaniids were large and robust animals. Even the smaller species, namely Meiolania mackayi, have been estimated to have reached a carapace length of 0.7 m (2 ft 4 in). Meiolania platyceps could have reached a carapace length of 1 m (3 ft 3 in) and Niolamia was estimated at 1.2 m (3 ft 11 in). The largest sizes were seemingly reached by Ninjemys and the Wyandotte species. Both were estimated to have reached a similar weight and the latter was estimated to have reached a carapace length of up to 2 m (6 ft 7 in).[28] Notably, these length estimates are restricted to the bony shell and do not factor in the combined length of the head, neck and long tail. This may indicate that meiolaniids could have reached lengths of up to 3 m (9.8 ft).[29]
Phylogeny
Phylogenetic analysis consistently recovers Meiolaniidae as a
Pictured below is the phylogenetic tree recovered in Sterli, de la Fuente and Krause in 2015. Other than the wildcard Gaffneylania, the phylogenetic tree matches with prior work by Gaffney.[6]
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Creating a phylogenetic tree for the individual species of Meiolania is theoretically possible, however as discussed by Gaffney, the results of doing so are highly questionable. Only two species would be complete enough to provide valuable characters, as M. mackayi is likely a synonym of M. platyceps and the wyandotte species is only represented through horn cores. This renders the morphology of the B horns the only way to possibly determine relationships within the established Meiolania species, a trait that has been proven to be highly variable even within a single species. Doing so regardless would yield the following results, with the groupings being entirely based on the length to width ratio of the horns.[17] In 2015, Sterli recovers M. platyceps and M. mackayi as sister taxa, with M. brevicollis being the basalmost Meiolania species. The Wyandotte species was not used in this analysis due to it being too fragmentary.[7]
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While their internal relationships are relatively well understood, their relation to other turtles has long remained elusive. Throughout their history, they've been variable considered
Fossil discoveries made since them have drastically changed this however. Several genera of Mesozoic turtles have been found to share similarities to meiolaniids, giving crucial insight into the potential origin of the group. The first instance of this was recognized as early 1987, when Ckhikvadzé grouped
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Evolution and dispersal
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.[26] Although some fossil evidence may suggest a cosmopolitan distribution, the majority of their known diversity could be found on the southern supercontinent Gondwana. 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.[17] This would account for the immense distance that today separates the areas where these turtles have been found. The fossil record of that time period is however scant and little is known about the early history of meiolaniids. It is therefore not certain whether they originated in South America and dispersed towards Australia, dispersed from Australia into South America or even originated in Antarctica and spread from there.[6][7] The best fossils derive from the middle Eocene of Argentina, where Niolamia and Gaffneylania have been found, with the discovery of an isolated tail ring confirming the group's presence in Eocene Australia as well.[25]
While the early distribution of the family is easily explained by
Bauer and Vindum (1990) on the other hand suggest that rather than spreading naturally, the last meiolaniids were helped in their dispersal by humans bringing them along as a food source. Historically, tortoises have been used as living provisions by seafarers
The final hypothesis, and the one favored by Brown and Moll, proposes that meiolaniids primarily arrived on distant islands through travel over land. Among these, one possible explanation can be found in the "escalator hopscotch" model. According to this, an island chain may undergo a process that see part of the chain submerged by water on one end and new land emerge on the other. Through this, a terrestrial animal may move over time from island to island, with its final distribution being much further off shore than where it started. Additionally, this could explain the precense of a relatively ancient lineage on what is a comparably young landmass. This has been suggested for Meiolania platyceps, as Lord Howe Island is a volcanic island situated atop the Lord Howe Rise.[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] Another hypothesis ties the distribution of island meiolaniids to the continent Zealandia. In this scenario, meiolaniids were possibly more widespread across this continent and were eventually restricted to isolated island ecosystems once the continent was submerged by the sea.[29] This could find support in turtle remains discovered on New Zealand, which have been interpreted by Worthy and colleagues to represent a potential meiolaniid. Little is known about this form, but it is argued that the presence of a large terrestrial tortoise dispels the hypothesis that New Zealand was entirely flooded in this time period.[27]
Paleobiology
Lifestyle
The lifestyle of meiolaniids has historically often been questioned. Even during the earliest discoveries on Lord Howe Island, the idea that they were marine animals was proposed by scientists like
Despite the reoccurring notions of semi-aquatic or even aquatic habits in meiolaniids, most historic and contemporary research favors an exclusively terrestrial lifestyle.[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. Features such as the osteoderm-covered limbs, limited range of motion of the neck, large and heavy skulls and the almost anchor-like clubbed tail have all been cited as being detrimental to an animal living in the water, as they would be a hinderance when the animal were to try and swim between islands or try to reach its head above water. 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.[29]
An additional point in favor of terrestrial life was recovered when the nasal cavities and inner ears of several meiolaniids were analysed. The study found that meiolaniids had enlarged nasal cavities, larger than even those of modern tortoises, which could be indicative of an enhanced sense of smell. While other possible uses of an enlarged nasal cavity are also considered, including sound production and thermoregulation, the benefits to the sense of smell is considered to be the most likely cause. Compared to this, smell plays a very minor rolle in the lives of aquatic turtles, which subsequently have a much smaller nasal cavity. The vestibulum of the nose is elongated and although this is associated with trunk-like structures in modern
Diet
Meiolaniids are thought to have been
Social behavior and reproduction
Study on the
Several egg clutches are known from Meiolania platyceps, which indicate that they made their nests in high moisture environments to prevent water evaporation from drying out the clutch. Suitable environments would include beaches, which is where the nests have been found on Lord Howe Island. Individual eggs were roughly spherical and measured 53.9 mm (2.12 in) across, making them comparable in size to those of modern giant tortoises. A single clutch of Meiolania eggs seems to have consisted of ten eggs, which were organized into two layers of a single nest.[33]
Extinction
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. Meiolaniids probably disappeared from South America at some point during the middle Eocene. The gradually cooling of Earth's climate following the
Australasian meiolaniids meanwhile would fare better, in part due to the continent they inhabited not being as stationary. 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.
Evidence for hunting may be found in the case of ?Meiolania damelipi, which preserves signs of being butchered by human settlers on Vanuatu. The material of this turtle, consisting primarily of the meaty limbs, were discovered in the remains of a human settlement dating to 2,800
The youngest confirmed meiolaniid remains were recovered from Pindai Caves and have been dated to 1720 ± 70 years BP (160–300 AD) via uncalibrated radiocarbon dating and 1820–1419 years BP (130–531 AD) through calibrated 14C dating.
References
- ^ a b c d e f g h Gaffney, E. S. (1992). "Ninjemys, a new name for "Meiolania" oweni (Woodward), a Horned Turtle from the Pleistocene of Queensland" (PDF). American Museum Novitates (3049): 1–10.
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- ^ Ameghino, F. (1899). "Sinopsis geológico-paleontológica de la Argentina. Suplemento (adiciones y correcciones)". Imprenta la Libertad (Author Edition). La Plata, Argentina.
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- ^ Simpson, G.G. (1937). "New reptiles from the Eocene of South America" (PDF). American Museum Novitates (927).
- ^ Simpson, G.G.; Williams, C.S. (1938). "Crossochelys, Eocene horned turtle from Patagonia" (PDF). Bulletin of the AMNH. 74 (5).
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- ^ a b c d Gaffney, Eugene S.; Archer, Michael; White, Arthur (1992). "Warkalania, a New Meiolaniid Turtle from the Tertiary Riversleigh Deposits of Queensland, Australia" (PDF). The Beagle, Records of the Northern Territory Museum of Arts and Sciences. 9 (1): 35–48.
- ^ PMID 20713711.
- ^ PMID 27922064.
- ^ McNamara, G.C. (1990). "The Wyandotte Local Fauna: A New, Dated, Pleistocene Vertebrate Fauna from Northern Queensland". Memoirs of the Queensland Museum. 28: 285–297.
- ^ Gaffney, E.S.; McNamara, G. (1990). "A meiolaniid turtle from the Pleistocene of Northern Queensland". Memoirs of the Queensland Museum. 28 (107–113).
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- ^ Townsend, Charles Haskins (1925). "The Galapagos tortoises in their relation to the whaling industry: a study of old logbooks". Zoologica. 4: 55–135.
- ^ Lichtig, A.J.; Lucas, S.G. (2018). "The ecology of Meiolania platyceps, a Pleistocene turtle from Australia". New Mexico Museum of Natural History and Science Bulletin. 79: 363–368.
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: CS1 maint: multiple names: authors list (link