Elephant bird
Elephant birds are extinct flightless birds belonging to the order Aepyornithiformes that were native to the island of Madagascar. They are thought to have gone extinct around 1000 AD, likely as a result of human activity. There are three currently recognised species, one in the genus Mullerornis, and two in Aepyornis. ''Aepyornis maximus'' is possibly the largest bird to have ever lived, with their eggs being the largest known for any amniote. Elephant birds are palaeognaths, and their closest living relatives are kiwi, suggesting that ratites did not diversify by vicariance during the breakup of Gondwana but instead convergently evolved flightlessness from ancestors that dispersed more recently by flying.
Discovery
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." There has been speculation, especially popular in the latter half of the 19th century, that the legendary roc from the accounts of Marco Polo was ultimately based on elephant birds, but this is disputed.Between 1830 and 1840, European travelers in Madagascar saw giant eggs and eggshells. British observers were more willing to believe the accounts of giant birds and eggs because they knew of the moa in New Zealand. In 1851 the genus Aepyornis and species A. maximus were scientifically described in a paper presented to the Paris Academy of Sciences by Isidore 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.
Two whole eggs have been found in dune deposits in southern Western Australia, one in the 1930s and one in 1992 ; both have been identified as Aepyornis maximus rather than Genyornis newtoni, an extinct giant bird known from the Pleistocene of Australia. It is hypothesized that the eggs floated from Madagascar to Australia on the Antarctic Circumpolar Current. Evidence supporting this is the finding of two fresh penguin eggs that washed ashore on Western Australia but may have originated in the Kerguelen Islands, and an ostrich egg found floating in the Timor Sea in the early 1990s.
Taxonomy and biogeography
Like the ostrich, rhea, cassowary, emu, kiwi and extinct moa, elephant birds were ratites; they could not fly, and their breast bones had no keel. Because Madagascar and Africa separated before the ratite lineage arose, elephant birds are thought to have dispersed and become flightless and gigantic in situ.More recently, it has been deduced from DNA sequence comparisons that the closest living relatives of elephant birds are New Zealand kiwi, 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, during the early Eocene epoch.
Placement of Elephant birds within Palaeognathae, after:
The ancestors of elephant birds are thought to have arrived in Madagascar well after Gondwana broke apart. The existence of possible flying palaeognathae in the Miocene such as Proapteryx further supports the view that ratites did not diversify in response to vicariance. Gondwana broke apart in the Cretaceous and their phylogenetic tree does not match the process of continental drift. Madagascar has a notoriously poor Cenozoic terrestrial fossil record, with essentially no fossils between the end of the Cretaceous and the Late Pleistocene. Complete mitochondrial genomes obtained from elephant birds eggshells suggest that Aepyornis and Mullerornis are significantly genetically divergent from each other, with molecular clock analyses estimating the split at around 27-30 million years ago, during the Oligocene epoch.
Species
Up to 10 or 11 species in the genus Aepyornis have been described, 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 in Mullerornis. A major systematic review by Hansford and Turvey based on morphological analysis recognised only four valid elephant bird species, Aepyornis maximus, Aepyornis hildebrandti, Mullerornis modestus, and the new species and genus Vorombe titan to accommodate the largest elephant bird remains. However, the validity of Vorombe titan was later questioned by genetic sequencing data, which did not find Vorombe distinct from Aepyornis, and it has been suggested that specimens assigned to Vorombe merely represent large specimens of A. maximus. Eggshells that have had their DNA sequenced from the far north of Madagascar may represent a third species of Aepyornis, due to their genetic distinctiveness from the other two species, but the lack of skeletal remains from this region renders this currently inconclusive.- Order Aepyornithiformes Newton 1884
- * Genus Aepyornis Geoffroy Saint-Hilaire 1850
- ** Aepyornis hildebrandti Burckhardt, 1893
- ** Aepyornis maximus Hilaire, 1851
- * Genus Mullerornis Milne-Edwards & Grandidier 1894
- ** Mullerornis modestus Hansford & Turvey 2018
Description
Elephant birds were large sized birds 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 skulls of elephant bird species differ little from each other except in size, though the front part of the skull in Mulleronis is less robustly built than in Aepyornis. The tops of elephant bird skulls display punctuated marks, which may have been attachment sites for fleshy structures or head feathers. The spinal column is suggested to have been made up of 24 free unfused vertebrae, 16-17 cervical vertebrae in the neck, and 6-7 thoracic vertebrae. The pelvis of Aepyornis is robust and all its constituent elements heavily fused to each other. The pelvis of Mulleronis is generally 3 times wider than long. The hindlimb bones, especially the femur and tibiotarsus, are very robust in Aepyornis, with the femur of Aepyornis being distinctly short and wide. The hindlimb bones were somewhat more gracile in Mullerornis. The terminal claw-bearing phalanges in the feet are uncurved and relatively broad, though somewhat more elongate and sharper in shape in Mullerornis.''Mullerornis is the smallest of the elephant birds, with a body mass of around, with its skeleton much less robustly built than Aepyornis. A. hildebrandti is thought to have had a body mass of around. Estimates of the body mass of Aepyornis maximus span from around to making it one of the largest birds ever, alongside Dromornis stirtoni and Pachystruthio dmanisensis. Females of A. maximus'' are suggested to have been larger than the males, as is observed in other ratites.Biology
Examination of brain endocasts has shown that both A. maximus and A. hildebrandti had greatly reduced optic lobes, compared to most other ratites, similar to those of their closest living relatives, the kiwis, and consistent with a similar nocturnal lifestyle. The optic lobes of Mullerornis were also reduced, but to a lesser degree, suggestive of a nocturnal or crepuscular lifestyle. A. maximus had relatively larger olfactory bulbs than A. hildebrandti, suggesting that the former occupied forested habitats where the sense of smell is more useful while the latter occupied open habitats. Elephant birds probably heavily relied on their sense of smell. Morphological analysis of their ears suggests that they had relatively poor hearing. 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 smaller Mullerornis may have been capable of somewhat more agile locomotion than the larger Aepyornis species.Diet
A 2022 isotope analysis study suggested that some specimens of Aepyornis hildebrandti were mixed feeders that had a large grazing component to their diets, similar to that of the living Rhea americana, while the other species were probably browsers. It has been suggested that Aepyornis straightened its legs and brought its torso into an erect position in order to browse higher vegetation. Some rainforest fruits with thick, highly sculptured endocarps, such as that of the currently undispersed and highly threatened forest coconut palm, 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, just like many cassowary-dispersed fruits, suggesting that they too may have been eaten by elephant birds.Growth and reproduction
Elephant birds are thought to have had a k-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. Elephant birds are suggested to have grown in periodic spurts rather than having continuous growth. An embryonic skeleton of Aepyornis 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, which may suggest that hatchlings were precocial.The eggs of Aepyornis are the largest known for any amniote, and have a volume of around, a length of approximately and a width of. The largest Aepyornis eggs are on average thick, with an estimated weight of approximately. Eggs of Mullerornis were much smaller, estimated to be only thick, with a weight of about. 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 of ''A. maximus.''