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| Elephant birds | |
|---|---|
 | |
| Aepyornis maximus skeleton and egg | |
Scientific classification | |
| Kingdom: | Animalia |
| Phylum: | Chordata |
| Class: | Aves |
| Infraclass: | Palaeognathae |
| Clade: | Novaeratitae |
| Order: | †Aepyornithiformes Newton, 1884[1] |
| Type species | |
| †Aepyornis maximus Hilaire, 1851 | |
| Genera | |
Elephant birds are extinctflightless birds belonging to theorderAepyornithiformes that were native to the island ofMadagascar. They are thought to have gone extinct around 1000 CE, likely as a result of human activity. Elephant birds comprised three species, one in the genusMullerornis, and two inAepyornis.Aepyornis maximus is possibly the largest bird to have ever lived, with their eggs being the largest known for anyamniote. Elephant birds arepalaeognaths (whose flightless representatives are often known asratites), and their closest living relatives arekiwi (found only in New Zealand), suggesting that ratites did not diversify byvicariance during the breakup ofGondwana but insteadconvergently evolved flightlessness from ancestors that dispersed more recently by flying.
Elephant birds have been extinct since at least the 17th century.Étienne de Flacourt, a French governor of Madagascar during the 1640s and 1650s, mentioned an ostrich-like bird, said to inhabit unpopulated regions, although it is unclear whether he was repeating folk tales from generations earlier. In 1659, Flacourt wrote of the "vouropatra – a large bird which haunts the Ampatres and lays eggs like the ostriches; so that the people of these places may not take it, it seeks the most lonely places."[2][3] There has been speculation, especially popular in the latter half of the 19th century, that the legendaryroc from the accounts ofMarco Polo was ultimately based on elephant birds, but this is disputed.[4]
Between 1830 and 1840, European travelers in Madagascar saw giant eggs and eggshells.[3] British observers were more willing to believe the accounts of giant birds and eggs because they knew of themoa in New Zealand.[3] In 1851 the genusAepyornis and speciesA. maximus were scientifically described in a paper presented to theParis Academy of Sciences byIsidore Geoffroy Saint-Hilaire, based on bones and eggs recently obtained from the island, which resulted in wide coverage in the popular presses of the time, particularly due to their very large eggs.[4]
Two whole eggs have been found in dune deposits in southernWestern Australia, one in the 1930s (the Scott River egg) and one in 1992 (theCervantes egg); both have been identified asAepyornis maximus rather thanGenyornis newtoni, an extinct giant bird known from the Pleistocene of Australia. It is hypothesized that the eggs floated from Madagascar to Australia on theAntarctic Circumpolar Current. Evidence supporting this is the finding of two freshpenguin eggs that washed ashore on Western Australia but may have originated in theKerguelen Islands, and anostrich egg found floating in theTimor Sea in the early 1990s.[5]
Like theostrich,rhea,cassowary,emu,kiwi and extinctmoa, elephant birds were ratites; they could not fly, and theirbreast bones had nokeel. Because Madagascar andAfrica separated before the ratitelineage arose,[6] elephant birds are thought to have dispersed and become flightless and giganticin situ.[7]
More recently, it has been deduced from DNA sequence comparisons that the closest living relatives of elephant birds are New Zealandkiwi,[8] though the split between the two groups is deep, with the two lineages being estimated to have diverged from each other around 54 million years ago.[9]
Placement of Elephant birds within Palaeognathae, after:[10][11]
| Paleognathae |
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The ancestors of elephant birds are thought to have arrived in Madagascar well afterGondwana broke apart. The existence of possible flyingpalaeognathae in the Miocene such asProapteryx further supports the view that ratites did not diversify in response tovicariance. Gondwana broke apart in the Cretaceous and their phylogenetic tree does not match the process ofcontinental drift. Madagascar has a notoriously poor Cenozoic terrestrial fossil record, with essentially no fossils between the end of the Cretaceous (Maevarano Formation) and the Late Pleistocene.[12] Complete mitochondrial genomes obtained from elephant birds eggshells suggest thatAepyornis andMullerornis are significantly genetically divergent from each other, withmolecular clock analyses estimating the split at around 27-30 million years ago, during theOligocene epoch.[9][13]
Up to 10 or 11 species in the genusAepyornis have been described,[14] but the validity of many have been disputed, with numerous authors treating them all in just one species,A. maximus. Up to three species have been described inMullerornis.[15] A major systematic review by Hansford and Turvey (2018) based on morphological analysis recognised only four valid elephant bird species,Aepyornis maximus,Aepyornis hildebrandti,Mullerornis modestus, and the new species and genusVorombe titan to accommodate the largest elephant bird remains.[16] However, the validity ofVorombe titan was later questioned by genetic sequencing data, which did not findVorombe distinct fromAepyornis, and it has been suggested that specimens assigned toVorombe merely represent large (perhaps female) specimens ofA. maximus.[13] Eggshells that have had their DNA sequenced from the far north of Madagascar may represent a third species ofAepyornis, due to their genetic distinctiveness from the other two species, but the lack of skeletal remains from this region renders this currently inconclusive.[13]
All elephant birds are usually placed in the single family Aepyornithidae, but some authors suggestAepyornis andMullerornis should be placed in separate families within the Aepyornithiformes, with the latter placed into Mullerornithidae.[13]
Elephant birds were large sized birds (the largest reaching 3 metres (9.8 ft) tall in normal standing posture) that had vestigial wings, long legs and necks, with small heads relative to body size, which bore straight, thick conical beaks that were not hooked. The tops of elephant bird skulls display punctuated marks, which may have been attachment sites for fleshy structures or head feathers.[18]Mullerornis is the smallest of the elephant birds, with a body mass of around 80 kilograms (180 lb),[13] with its skeleton much less robustly built thanAepyornis.[19]A. hildebrandti is thought to have had a body mass of around 230–285 kilograms (507–628 lb).[13] Estimates of the body mass ofAepyornis maximus span from around 275 kilograms (606 lb)[20] to 700–1,000 kilograms (1,500–2,200 lb)[13] making it one of the largest birds ever, alongsideDromornis stirtoni andPachystruthio dmanisensis.[21][22] Females ofA. maximus are suggested to have been larger than the males, as is observed in other ratites.[13]
Examination of brainendocasts has shown that bothA. maximus andA. hildebrandti had greatly reducedoptic lobes, similar to those of their closest living relatives, the kiwis, and consistent with a similarnocturnal lifestyle. The optic lobes ofMullerornis were also reduced, but to a lesser degree, suggestive of a nocturnal orcrepuscular lifestyle.A. maximus had relatively largerolfactory bulbs thanA. hildebrandti, suggesting that the former occupied forested habitats where the sense of smell is more useful while the latter occupied open habitats.[23] Based on the proportions of their leg bones, unlike most living ratites such as ostriches, emus and rheas, but similar to moas, elephant birds are thought to have walked with a relatively slow graviportal locomotion, though the smallerMullerornis may have been capable of somewhat more agile locomotion than the largerAepyornis species.[18]
A 2022 isotope analysis study suggested that some specimens ofAepyornis hildebrandti were mixed feeders that had a large (~48%)grazing component to their diets, similar to that of the livingRhea americana, while the other species (A. maximus,Mullerornis modestus) were probablybrowsers.[24] It has been suggested thatAepyornis straightened its legs and brought its torso into an erect position in order to browse higher vegetation.[25] Some rainforest fruits with thick, highly sculpturedendocarps, such as that of the currently undispersed and highly threatenedforest coconut palm (Voanioala gerardii), may have been adapted for passage through ratite guts and consumed by elephant birds, and the fruit of some palm species are indeed dark bluish-purple (e.g.,Ravenea louvelii andSatranala decussilvae), just like many cassowary-dispersed fruits, suggesting that they too may have been eaten by elephant birds.[26]
Elephant birds are thought to have had ak-selective life strategy, taking at least several years from hatching to reaching maximum body size, as opposed to taking around a year to reach maximum body size from hatching as is typical of birds.[27][19] Elephant birds are suggested to have grown in periodic spurts rather than having continuous growth.[19] An embryonic skeleton ofAepyornis is known from an intact egg, around 80–90% of the way through incubation before it died. This skeleton shows that even at this early ontogenetic stage that the skeleton was robust, much more so than comparable hatchling ostriches or rheas,[28] which may suggest that hatchlings wereprecocial.[19]
The eggs ofAepyornis are the largest known for anyamniote, and have a volume of around 5.6–13 litres (12–27 US pt), a length of approximately 26–40 centimetres (10–16 in) and a width of 19–25 centimetres (7.5–9.8 in).[19] The largestAepyornis eggs are on average 3.3 mm (1⁄8 in) thick, with an estimated weight of approximately 10.5 kilograms (23 lb).[13] Eggs ofMullerornis were much smaller, estimated to be only1.1 mm (3⁄64 in) thick, with a weight of about 0.86 kilograms (1.9 lb).[13] The large size of elephant bird eggs means that they would have required substantial amounts of calcium, which is usually taken from a reservoir in the medullary bone in the femurs of female birds. Possible remnants of this tissue have been described from the femurs ofA. maximus.[19]
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It is widely believed that the extinction of elephant birds was a result of human activity. The birds were initially widespread, occurring from the northern to the southern tip ofMadagascar.[29] The late Holocene also witnessed the extinction of other Malagasy animals, including several species ofMalagasy hippopotamus, two species of giant tortoise (Aldabrachelys abrupta andAldabrachelys grandidieri), thegiant fossa, over a dozen species ofgiant lemurs, the aardvark-like animalPlesiorycteropus, and the crocodileVoay.[25] Several elephant bird bones with incisions have been dated to approximately 10,000 BCE which some authors suggest are cut marks, which have been proposed as evidence of a long history of coexistence between elephant birds and humans;[30] however, these conclusions conflict with more commonly accepted evidence of a much shorter history of human presence on the island and remain controversial. The oldest securely dated evidence for humans on Madagascar dates to the mid-first millennium AD.[31]
A 2021 study suggested that elephant birds, along with the Malagasy hippopotamus species, became extinct in the interval 800–1050 CE (1150–900 yearsBefore Present), based on the timing of the latest radiocarbon dates. The timing of the youngest radiocarbon dates coincided with major environmental alteration across Madagascar by humans changing forest into grassland, probably for cattlepastoralism, with the environmental change likely being induced by the use of fire. This reduction of forested area may have had cascade effects, like making elephant birds more likely to be encountered by hunters,[32] though there is little evidence of human hunting of elephant birds. Humans may have utilized elephant bird eggs. Introduced diseases (hyperdisease) have been proposed as a cause of extinction, but the plausibility for this is weakened due to the evidence of centuries of overlap between humans and elephant birds on Madagascar.[25]
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