The problematic adzebill (genus: Aptornis)

Updated October 31, 2020, with the addition of more taxa now nesting Aptornis with the similarly flightless moa, Dinornis, both close to the hoatzin, OpisthocomusLink here.

If you’ve never heard of the adzebill…
I’m with you. I never heard of it before either. The adzebill (genus: Aptornis, sometimes Apterornis; early Miocene to Holocene; Owen 1844; Figs. 1,2) is a recently extinct large (80 cm in length) flightless bird found only in New Zealand. The question is, what is it?

FIgure 2. Aptornis skeleton and parts.

Figure 1. Aptornis skeleton and parts. This taxon is more derived, more extreme, less pleisomorphic. than the extant kagu.

Figure 1. Aptornis skull. Note the sharp downturned beak, concave premaxilla, lack of prefrontal and the upper temporal fenestra confined by the postorbital and squamosal, very rare in birds.

Figure 2. Aptornis skull. Note the sharp downturned beak, concave premaxilla, lack of prefrontal and the upper temporal fenestra confined by the postorbital and squamosal, very rare in birds.

Figure 4. Dinornis skull for comparison to Aptornis.

Figure 3. Dinornis skull for comparison to Aptornis.

A little backstory.
An adze is a tool similar to an ax with an arched blade at right angles to the handle, used for cutting or shaping large pieces of wood. This bird, like the extant kagu, uses its beak as an adze, but the beak and the extant bird are not as derived as in the extinct bird. 

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

Figure 4. Giant Dinornis compared to chicken-sized Opisthocomus to scale and similar in torso length.

Wikipedia reports:
Aptornis has “been placed in the Gruiformes (cranes) but this is not entirely certain.” The report also includes possible relationships to the kagu (Rhynochetos), trumpeters (Psophia), moas (Dinornis, Figs. 3, 4), and the sunbittern (Eurypyga). None of these birds are related to each other in the LRT.

Musser 2017 reports:
“Past morphological studies placed Aptornis as a sister taxon to Rhynochetos jubatus, but recent genomic studies reveal R. jubatus and Eurypyga helias to be sister taxa, and posit that Aptornis falls within Gruoidea.” Musser’s study found strong support for a sister relationship between the kagu, Rhynochetos, and the sunbittern, Eurypgya, but Aptornis nested with the trumpeter, Psophia.

Aptornis defossor
(Owen 1844; 80 cm in length) is the extinct flightless adzebill, which nests with the extinct moa, Dinornis. The rostrum is sharp, short and turns down. The hind limbs are robust. The wings are vestiges.

Figure 3. Subset of the LRT focusing on birds. Nahmavis is highlighted in yellow. Color clades include extant taxa.

Figure x. Subset of the LRT focusing on birds. Nahmavis is highlighted in yellow. Color clades include extant taxa.

New Caledonia
is the closest island to New Zealand. These islands which were once contiguous when ocean levels were lower.


References
Musser GM 2017. Resolving the radiation and phenotypic evolution of basal neoaves: beginning construction of a new morphological dataset and a novel sister taxon for Aptornis. Journal of Vertebrate Paleontology abstracts 2017: 167.
Owen R 1844. On Dinornis, an extinct genus of tridactyle struthious birds, with descriptions of portions of the skeleton of five species which formerly existed in New Zealand. (Part I.) Transactions of the Zoological Society of London, 3(3): 235–275,

wiki/Adzebill

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