South Island giant moa (Dinornis giganteus/robustus) Dinornithiformes

Biology of the South Island giant moa

Most prevalent in lowland regions of Canterbury region, but also found in some South Island subalpine caves, this large-bodied bird had a surprisingly slender neck and head.  This species reached a body weight of 242 kg for females and 85 kg for males, indicating pronounced reversed sexual dimorphism (Tennyson and Martinson, 2006).  The eggs reached 240 mm by 178 mm in size.  The powerful jaws and many gizzard stones suggest a diet rich in fibre and analysis of stomach contents found twigs and berries, but few herbs and grasses (Tennyson and Martinson, 2006).

Brain of the South Island giant moa

Both male and female specimens were available for this species.

AV8713 is a female from the Christchurch Museum collection (with thanks to RP Scofield).  The endocranial volume is 60.0 ml for an encephalization quotient (EQ) range of 0.388 to 0.748.  The forebrain axis is not as retroverted as in other moa species, with an angle between the forebrain and brainstem axes of + 9.7 degrees.  

 AV29786 is a male, also from the Christchurch Museum collection.  The endocranial volume is 43.0 ml, for an encephalization quotient (EQ) range of 0.503 to 0.846.  As for the female, there is minimal retroflexion, with the forebrain and brainstem aligned.

 The features of the telencephalic surface (olfactory bulb, parts of the pallium) have left impressions on the skull interior (see Figure 1a to d; 2a to d).  The optic foramen transmits the optic nerve (2n).  The optic lobe is composed of the soft tissue optic tectum, a part of the midbrain which projects ventrolaterally caudal to the telencephalon in birds.  The wulst is a dorsal projection of the telencephalon which is made up of the dorsal pallium (or hyperpallium) and the parahippocampal area.  The wulst is separated from the rest of the pallial surface by a groove (vallecula), which may be divided into medial and lateral divisions in some avian species, but these divisions are not visible on the moa endocranium.  A substantial proportion of the wulst is concerned with vision, so the large wulst but small olfactory bulbs in moa suggest a greater reliance on vision relative to olfaction.

 The foramen magnum transmitting the spinal cord (Figure 1e) is quite small for the endocranial volume and directed posteriorly.  Note also that the occipital condyle is directed posteriorly rather than posteroinferiorly as in extant ratites.  Both of these features reflect the more horizontal carriage of the head on the upper cervical vertebrae in giant moa compared to the angulation at the C1/occipital joint in extant ratites such as the ostrich (Ashwell and Scofield, 2008).

Figure Legends

Figure 1 shows images of 3D reconstructions of the skull of a female South island giant moa (AV8713).  The fossil was scanned at Christchurch Radiology Group, St George’s Hospital (Siemens Somatom Definition, 120 kV, 400 mA, 0.6 mm slice thickness).  Impressions on the skull interior from soft tissue structures have been labelled.

 Figure 2 shows images of 3D reconstructions of the skull of a male South Island giant moa (AV29786).  Details of scanning as for Figure 1. Impressions on the skull interior from soft tissue structures have been labelled.

References

Ashwell KWS, Scofield RP (2008) Big birds and their brains: Paleoneurology of the New Zealand Moa. Brain Behavior and Evolution 71, 151-166.

Tennyson A, Martinson P (2006) Extinct Birds of New Zealand. Wellington, NZ: Te Papa Press.