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☀Moa skeletons were traditionally reconstructed in an upright position to create impressive height, but analysis of their vertebral articulations indicates that they probably carried their heads forward, in the manner of a kiwi. The spine was attached to the rear of the head rather than the base, indicating the horizontal alignment. This would have let them graze on low vegetation while being able to lift their heads and browse trees when necessary. This has resulted in a reconsideration of the height of larger moa.
No records survive of what sounds moa made, some idea of their calls can be gained from fossil evidence. The trachea of moa was supported by many small rings of bone known as tracheal rings. Excavation of these rings from articulated skeletons has shown that at least two moa genera (Euryapteryx and Emeus) exhibited tracheal elongation, that is, their trachea was up to 1 m (3 ft) long and formed a large loop within the body cavity. They are the only ratites known to exhibit this feature, which is also present in several other bird groups, including swans, cranes, and guinea fowl. The feature is associated with deep resonant vocalizations that can travel long distances.
Evolutionary relationships
A comparison of a kiwi, ostrich, and Dinornis, each with its egg
The moa's closest relatives are small terrestrial South American birds called the tinamous, which can fly. Previously, the kiwi, the Australian emu, and cassowary were thought to be most closely related to moa.
Although dozens of species were described in the late 19th and early 20th centuries, many were based on partial skeletons and turned out to be synonyms. Currently, 11 species are formally recognized, although recent studies using ancient DNA recovered from bones in museum collections suggest that distinct lineages exist within some of these. One factor that has caused much confusion in moa taxonomy is the intraspecific variation of bone sizes, between glacial and interglacial periods (see Bergmann’s rule and Allen’s rule) as well as sexual dimorphism being evident in several species. Dinornis seems to have had the most pronounced sexual dimorphism, with females being up to 150% as tall and 280% as heavy as males—so much bigger than they were formerly classified as separate species until 2003. A 2009 study showed that Euryapteryx Curtis and E. Gravis were synonyms. A 2010 study explained size differences among them as sexual dimorphism. A 2012 morphological study interpreted them as subspecies, instead.
Ancient DNA analyses have determined that a number of cryptic evolutionary lineages occurred in several moa genera. These may eventually be classified as species or subspecies; Megalapteryx Benham (Archey) is synonymized with M. Didymus (Owen) because the bones of both share all essential characters. Size differences can be explained by a north-south cline combined with temporal variation such that specimens were larger during the Otiran glacial period (the last ice age in New Zealand). A similar temporal size variation is known for the North Island's Pachyornis mapping.Some of the other size variations for moa species can probably be explained by similar geographic and temporal factors.
The earliest moa remains come from the Miocene Saint Bathans Fauna. Known from multiple eggshells and hind limb elements, these represent at least two already fairly large-sized species.