Right so, imagine a cross between an Ostrich and an Emu, make it bigger and hey presto you have a Moa! This is not scientifically what a Moa (Order: Dirnornithiformes) is, however genetic studies have found that its closest relatives are small South American birds called the Tinamous (Order: Tinamiformes)[1]^,[2], despite the fact Moas used to live on New Zealand.

The Dinornis Robustus, commonly known as the South Island Giant Moa is the largest out of the nine recognised species, with a height of 2 meters (to their back), they could reach foliage up to 3.6 meters off the ground (See Figure 2 for visual reference). This makes them the tallest bird species known[3]. On the other hand, the Bush Moa (Anomalopteryx didiformis), is another Moa species however the size of this is comparable to a turkey[4].

Despite the large range in species size, determining what fossil belongs to what species is incredibly difficult for multiple reasons. Firstly, if the fossil assemblage is not complete, or there are a range of bones from other species it is difficult to determine the specific species. The Moa fossils we have in the collections (below) are some limb bones. Due to the fossils being incomplete it is difficult to determine the size or age of the Moa in question.

Another factor exacerbating the identification of species in fossils is variation of bone sizes between glacial and interglacial periods. Glacial periods are intervals of time where temperatures are much colder and glaciers advance. An interglacial is the opposite of a glacial. This period separates two glacial periods[5][6]. We are currently in an interglacial period known as the Holocene[7].

The variation in Moa bone sizes between glacial and interglacial periods can be explained through Bergmann’s rule. Bergmann’s rule states that warm blooded vertebrate species are larger in colder climates than warmer climates[8]. Gui (2000) study on Moa eggshells highlights how the further south the fossils were found, the larger the Moa[9]. Due to the large latitudinal range of New Zealand, there is a significant change in climate as you progress south; this supports Bergmann’s rule[10].

Another factor that until recently made determining species and gender of Moa fossils is sexual dimorphism. This is where the gender size stereotypes are reversed, with females being larger than males[11]. Studies have found that the giant Moa (Dinornis) had the most significant sexual dimorphism, the difference being so large that until 2003, they were formerly classed as two separate species[12][13][14].

These factors make determining species of all fossils difficult. In terms of the Moa, studies are being carried out on the biogeography and morphology of the fossils[15]. By using information about present day birds and mammals, palaeontologists can gain a better insight into this extinct species.

By Anjali Dhunna, Gallery Steward

[1] Allentoft, M.E.; Rawlence, N.J. (2012-01-20). “Moa’s Ark or volant ghosts of Gondwana? Insights from nineteen years of ancient DNA research on the extinct moa (Aves: Dinornithiformes) of New Zealand” (PDF). Annals of Anatomy – Anatomischer Anzeiger. 194 (1): 36–51. doi:10.1016/j.aanat.2011.04.002

[2] Mitchell, K.J.; Llamas, (et al) (2014-05-23). “Ancient DNA reveals elephant birds and kiwi are sister taxa and clarifies ratite bird evolution” (PDF). Science. 344 (6186): 898–900.

[3] Davis, W.E., 2003. RATITES AND TINAMOUS: Tinamidae, Rheidae, Dromaiidae, Casuariidae, Apterygidae, Struthionidae. The Wilson Journal of Ornithology, 115(2), pp.217-218.

[4] “Little bush moa | New Zealand Birds Online”. nzbirdsonline.org.nz. Retrieved 2020-07-24.

[5] J. Severinghaus; E. Brook (1999). “Abrupt Climate Change at the End of the Last Glacial Period Inferred from Trapped Air in Polar Ice”. Science. 286 (5441): 930–4. doi:10.1126/science.286.5441.930

[6] Christopher M. Fedo; Grant M. Young; H. Wayne Nesbitt (1997). “Paleoclimatic control on the composition of the Paleoproterozoic Serpent Formation, Huronian Supergroup, Canada: a greenhouse to icehouse transition”. Precambrian Research. Elsevier. 86 (3–4): 201.

[7] Walker, Mike; Johnsen, Sigfus; Rasmussen, (et al) (2009). “Formal definition and dating of the GSSP (Global Stratotype Section and Point) for the base of the Holocene using the Greenland NGRIP ice core, and selected auxiliary records” (PDF). Journal of Quaternary Science. 24 (1): 3–17.

[8] Meiri, S. and Dayan, T., 2003. On the validity of Bergmann’s rule. Journal of biogeography, 30(3), pp.331-351.

[9] Gui, B.J., 2000. Morphometrics of moa eggshell fragments (Aves: Dinornithiformes) from Late Holocene dune‐sands of the Karikari Peninsula, New Zealand. Journal of the Royal Society of New Zealand, 30(2), pp.131-145.

[10] Worthy, T. H. 1988: An illustrated key to the main leg bones of moas (Aves: Dinornithiformes). National Museum of New Zealand Miscellaneous Series 17: 1-37.

[11] Armenta J. K.; Dunn P. O.; Whittingham L. A. (2008). “Quantifying avian sexual dichromatism: a comparison of methods”. The Journal of Experimental Biology. 211 (15): 2423–2430. doi:10.1242/jeb.013094

[12] Bunce, M., Worthy, T.H., Ford, (et al), 2003. Extreme reversed sexual size dimorphism in the extinct New Zealand moa Dinornis. Nature, 425(6954), pp.172-175.

[13] Huynen, L., Lissone, I., Sawyer, S. and Lambert, D., 2008. Genetic identification of moa remains recovered from Tiniroto, Gisborne.

[14] Baker, A.J., Huynen, L.J., Haddrath, (et al), 2005. Reconstructing the tempo and mode of evolution in an extinct clade of birds with ancient DNA: the giant moas of New Zealand. Proceedings of the National Academy of Sciences, 102(23), pp.8257-8262.

[15] Worthy, T.H.; Scofield, R.P. (2012). “Twenty-first century advances in knowledge of the biology of moa (Aves: Dinornithiformes): A new morphological analysis and moa diagnoses revised”. New Zealand Journal of Zoology. 39 (2): 87–153. doi:10.1080/03014223.2012.665060