Moa


Moa are an extinct group of flightless birds formerly endemic to New Zealand. During the Late Pleistocene-Holocene, there were nine species, in six genera. The two largest species, Dinornis robustus and Dinornis novaezelandiae, reached about in height with neck outstretched, and weighed about ; the smallest, the bush moa, was about the size of a turkey. Estimates of the moa population when Polynesians settled New Zealand circa 1300 range from 58,000 to 2.5 million.
Moa are traditionally placed in the ratite group. Genetic studies have found that their closest relatives are the flighted South American tinamous, once considered a sister group to ratites. The nine species of moa were the only wingless birds, lacking even vestigial wings, which all other ratites have. They were the largest terrestrial animals and dominant herbivores in New Zealand's forest, shrubland, and subalpine ecosystems until the arrival of the Māori, and were hunted only by Haast's eagle. Moa extinction occurred within 100 years of human settlement of New Zealand, primarily because of overhunting.

Etymology

The word moa is a Polynesian term for domestic fowl. The name was not in common use among the Māori by the time of European contact, likely because the bird it described had been extinct for some time. Nevertheless, knowledge was passed down through traditional stories and proverbs, such as kua ngaro i te ngaro o te moa. The earliest written record of the name was by missionaries William Williams and William Colenso in January 1838; Colenso speculated that the birds may have resembled gigantic fowl. In 1912, Māori chief Urupeni Pūhara claimed that the moa's traditional name was te kura. The North Island giant moa is also called kuranui.

Description

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. However, Māori rock art depicts moa or moa-like birds with necks upright, indicating that moa were capable of assuming both neck postures.
No records survive of what sounds moa made, though some idea of their calls can be gained from fossil evidence. The trachea of moa were supported by many small rings of bone known as tracheal rings. These rings were preserved within some articulated skeletons, and shows that at least two moa genera exhibited tracheal elongation, where their trachea, up to long, 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 vocalisations that can travel long distances.

Feathers and soft tissues

Several examples of moa remains have been found with soft tissues preserved through desiccation after the bird died at a dry site ; these specimens are a form of natural mummy. Most were found in the semiarid Central Otago region, the driest part of New Zealand, but others are known from outside the Central Otago region. These include:
  • Dried muscle on bones of a female Dinornis robustus found at Tiger Hill in the Manuherikia River Valley by gold miners in 1864
  • Several bones of Emeus crassus with muscle attached, and a row of neck vertebrae with muscle, skin, and feathers collected from Earnscleugh Cave near the town of Alexandra in 1870
  • An articulated foot of a male D. giganteus with skin and foot pads preserved, found in a crevice on the Knobby Range in 1874
  • The type specimen of Megalapteryx didinus found near Queenstown in 1878
  • The lower leg of Pachyornis elephantopus, with skin and muscle, from the Hector Range in 1884;
  • The complete feathered leg of a M. didinus from Old Man Range in 1894
  • The head of a M. didinus found near Cromwell sometime before 1949.
  • A complete foot of M. didinus found in a cave on Mount Owen near Nelson in the 1980s
  • A skeleton of Anomalopteryx didiformis with muscle, skin, and feather bases collected from a cave near Te Anau in 1980.
In addition to these specimens, loose moa feathers have been collected from caves and rock shelters in the southern South Island, and based on these remains, some idea of the moa plumage has been achieved. The preserved leg of M. didinus from the Old Man Range reveals that this species was feathered right down to the foot. This is likely to have been an adaptation to living in high-altitude, snowy environments, and is also seen in the Darwin's rhea, which lives in a similar seasonally snowy habitat.
Moa feathers are up to long, and a range of colours has been reported, including reddish-brown, white, yellowish, and purplish. Dark feathers with white or creamy tips have also been found, and indicate that some moa species may have had plumage with a speckled appearance.

Classification

Evolutionary relationships

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 the 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 recognised, 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, 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 that they were classified as separate species until 2003. A 2009 study showed that Euryapteryx curtus 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.
Analyses of ancient DNA have determined that a number of cryptic evolutionary lineages occurred in several moa genera. These may eventually be classified as species or subspecies; Megalapteryx benhami is synonymised with M. didinus 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. Similar temporal size variation is known for the North Island's Pachyornis mappini. Some of the other size variation 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.

Phylogeny

Because moa are a group of flightless birds with no vestiges of wing bones, questions have been raised about how they arrived in New Zealand, and from where. Many theories exist about the moa's arrival and radiation in New Zealand, but the most recent theory suggests that they arrived in New Zealand about 60 million years ago and split from the "basal" moa species, Megalapteryx, about 5.8 Mya instead of the 18.5 Mya split suggested by Baker et al.. This does not necessarily mean there was no speciation between the arrival 60 Mya and the basal split 5.8 Mya, but the fossil record is lacking and most likely the early moa lineages existed, but became extinct before the basal split 5.8 Mya. The presence of Miocene-aged species certainly suggests that moa diversification began before the split between Megalapteryx and the other taxa.
The Oligocene Drowning Maximum event, which occurred about 22 Mya, when only 18% of present-day New Zealand was above sea level, is very important in the moa radiation. Because the basal moa split occurred so recently, it was argued that ancestors of the Quaternary moa lineages could not have been present on both the South and North Island remnants during the Oligocene drowning. This does not imply that moa were previously absent from the North Island, but that only those from the South Island survived, because only the South Island was above sea level. Bunce et al. argued that moa ancestors survived on the South Island and then recolonised the North Island about 2 Myr later, when the two islands rejoined after 30 Myr of separation. The presence of Miocene moa in the Saint Bathans fauna seems to suggest that these birds increased in size soon after the Oligocene drowning event, if they were affected by it at all.
Bunce et al. also concluded that the highly complex structure of the moa lineage was caused by the formation of the Southern Alps about 6 Mya, and the habitat fragmentation on both islands resulting from Pleistocene glacial cycles, volcanism, and landscape changes. The cladogram below is a phylogeny of Palaeognathae generated by Mitchell with some clade names after Yuri et al.. It provides the position of the moa within the larger context of the "ancient jawed" birds:
The cladogram below gives a more detailed, species-level phylogeny, of the moa branch of the "ancient jawed" birds shown above:

Taxonomy

The currently recognised genera and species are:
  • Order †Dinornithiformes Ridgway 1901
  • *Family Dinornithidae Owen 1843
  • **Genus Dinornis
  • ***North Island giant moa, Dinornis novaezealandiae
  • ***South Island giant moa, Dinornis robustus
  • *Family Emeidae
  • **Genus Anomalopteryx
  • ***Bush moa, Anomalopteryx didiformis
  • **Genus Emeus
  • ***Eastern moa, Emeus crassus
  • **Genus Euryapteryx
  • ***Broad-billed moa, Euryapteryx curtus
  • **Genus Pachyornis
  • ***Heavy-footed moa, Pachyornis elephantopus
  • ***Mantell's moa, Pachyornis geranoides
  • ***Crested moa, Pachyornis australis
  • *Family Megalapterygidae
  • **Genus Megalapteryx
  • ***Upland moa, Megalapteryx didinus
Two unnamed species are also known from the Saint Bathans Fauna.