Moa - Taxonomy and Evolution

Taxonomy and Evolution

The kiwi were formerly regarded as the closest relatives of the moa, but comparisons of their DNA in a paper published in 2005 suggested moa were more closely related to the Australian emu and cassowary. However research published in 2010 found that the moa's closest cousins were not the emu and cassowary but smaller terrestrial South American birds called the tinamous which are able to fly.

Although dozens of species were described in the late 19th century and early 20th century, many were based on partial skeletons and turned out to be synonyms. Currently, eleven 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 inter-glacial 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 that they were formerly classified as separate species until 2003. A 2009 study showed that Euryapteryx curtus and Euryapteryx 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 there were a number of cryptic evolutionary lineages in several moa genera. These may eventually be classified as species or subspecies; Megalapteryx benhami (Archey) which is synonymised with M. didinus (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). Similar temporal size variation is known for the North Island Pachyornis mappini. Some of the other 'Large' ranges in variation for moa species can probably be explained by similar geographic and temporal factors.

The currently recognised genera and species are:

• Order †Dinornithiformes - Moa
  • Family Dinornithidae
    • Genus Dinornis
      • North Island Giant Moa, Dinornis novaezealandiae (North Island, New Zealand)
      • South Island Giant Moa, Dinornis robustus (South Island, New Zealand)
      • Dinornis new lineage A (South Island, New Zealand)
      • Dinornis new lineage B (South Island, New Zealand)
  • Family Megalapterygidae
    • • Upland Moa, Megalapteryx didinus (South Island, New Zealand)
  • Family Emeidae
    • • Bush Moa, Anomalopteryx didiformis (South Island, New Zealand)
      • Coastal Moa, Euryapteryx curtus (North and South Island, New Zealand)
      • Eastern Moa, Emeus crassus (South Island, New Zealand)
    • Genus Pachyornis
      • Heavy-footed Moa, Pachyornis elephantopus (South Island, New Zealand)
      • Mantell's Moa, Pachyornis geranoides (South Island, New Zealand)
      • Crested Moa, Pachyornis australis (South Island, New Zealand)
      • Pachyornis new lineage A (North Island, New Zealand)
      • Pachyornis new lineage B (South Island, New Zealand)

The following cladogram shows the genetic relationships within Dinornithidae, based on Bunce et al. (2009).

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. There are many theories about the moa's arrival and radiation on New Zealand, but the most recent theory suggests that the moa arrived on New Zealand about 60Mya and split from the basal moa species, Megalapteryx about 5.8 Mya instead of the 18.5Mya split suggested by Baker et al. (2005). 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 is it most likely that early moa lineages existed but went extinct before the basal split 5.8 Mya. The Oligocene drowning maximum about 22Mya, was a period of time where only 18% of present day New Zealand was above sea level, and is very important in the moa radiation. Because the basal moa split occurred so recently (5.8 Mya), 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 absent from one island, but that only those from the South island survived because only the South Island was above sea level. Bunce et. al (2009) argued that moa ancestors survived on the South Island and then recolonized the North Island about 2 My later, when the two islands rejoined after 30 My of separation. 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 My, and the habitat fragmentation on both islands resulting from Pleistocene glacial cycles, volcanism, and landscape changes.