The origin of giant birds: Dinornis, the giant hoatzin

There have been several giant birds
in the fossil record (Fig. 1), and a few, like the ostrich, Struthio, are still alive today. In the next few days we’ll examine several giant birds and their smaller progenitors.

Figure 1. Giant Dinornis compared to chicken-sized Opisthocomus to scale and similar in torso length.

Figure 1.
Giant Dinornis compared to chicken-sized Opisthocomus to scale and similar in torso length.

Dinornis, the giant hoatzin
Dinornis maximus (Owen 1843; recently extinct; 3.6m tall), the moa, is the tallest bird that ever lived. Females were apparently distinct and larger than males. Long considered a ratite based on its resemblance to Struthio, the ostrich, here in the large reptile tree (LRT, 1119 taxa, subset Fig. 3) Dinornis nests between the hoatzin, Opisthocomus (see below) and parrots like Ara. Feather color was similar to the hoatzin, but perhaps more hair-like in substance.

Opisthocomus hoazin (Müller 1776, Hoatzin) The extant hoatzin, or stink bird, is a chiefly arboreal bird capable of short flights. A sister to Gallus and DinornisOpisthocomus is famous for having chicks with claws on two of the wing digits that metamorphosize into the standard fused bird hand as adults. This appears to be an atavism, or reappearance, of a trait that is lost in all other birds. The hoatzin is an herbivore. The feet are large. The taxonomic positiion of Opisthocomus has been debated, but it appears to be a primitive member of extant birds, close to the much larger Dinornis (below).

Here, another big flightless bird:

Figure 3. The extinct and flightless Cnemiornis (at right) compared to the extant and volant Cereopsis, the New Zealand goose.

Figure 2. The extinct and flightless Cnemiornis (at right, note the little forelimbs) compared to the extant and volant Cereopsis.

This is Cnemiornis
(Fig. 3, right; Owen 1866; recently extinct; 1 m tall), the New Zealand goose, a very large and flightless goose with reduced forelimbs, reduced sternum and not much webbing between the toes. The neck is longer and pedal digit 1 is absent.

As in the unrelated Dinornis, relative to Opisthocomus,
the skull is relatively smaller, the neck is longer and has more cervicals, the dorsal neural spines are taller, the pelvis is less ossified, the forelimbs are vestiges, the patella is larger, and pedal digit 1 is smaller (actually absent in Cnemiornis).

Figure x. Bird giants in the bird subset of the LRT.

Figure 3. Bird giants in the bird subset of the LRT.

References
Owen R 1843. On the remains of Dinornis, an extinct gigantic struthious bird. Proceedings of the Zoological Society of London: 8–10, 144–146.
Owen R 1866. XI. On Dinornis (Part X.): containing a Description of part of the Skeleton of a flightless Bird indicative of a New Genus and Species (Cnemiornis calcitrans, Ow.) Journal of Zoology 1866 The Zoological Society of London.
Statius Müller PL 1776. Des Ritters Carl von Linné Königlich Schwedischen Leibarztes &c. &c. vollständigen Natursystems Supplements- und Register-Band über alle sechs Theile oder Classen des Thierreichs. Mit einer ausführlichen Erklärung. Nebst drey Kupfertafeln.Nürnberg. (Raspe).

wiki/Hoatzin
wiki/Dinornis
wiki/Cnemiornis

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