The adzebills, genus Aptornis, were two closely related bird species, the North Island adzebill, (Aptornis otidiformis), and the South Island adzebill, (Aptornis defossor), of the extinct family Aptornithidae. The family was endemic to New Zealand. A tentative fossil species, (Aptornis proasciarostratus), is known from the Miocene Saint Bathans fauna.[3]
Adzebills were first scientifically described in 1844 by biologist Richard Owen, who mistook them for a small species of moa.[4] The first species named was Dinornis otidiformis (later Aptornis), with the specific epithet referring to its similarity in size to the great bustard (Otis tarda).[5]
They have been placed in the Gruiformes but this is not entirely certain. It was also proposed to ally them with the Galloanserae.[6] Studies of morphology and DNA sequences place them variously close to and far off from the kagu of New Caledonia,[7] as well as the trumpeters.[8] However, on first discovery of fossils, they were mistaken for ratites, specifically small moa. Its morphological closeness to the kagu may be the result of convergent evolution, although New Zealand's proximity to New Caledonia and shared biological affinities (the two islands are part of the same microcontinent) has led some researchers to suggest they share a common ancestor from Gondwana. The Gondwanan sunbittern is the closest living relative of the kagu, but these are not close to the Gruiformes proper (i.e. cranes, rails and allies).[9][10]
A 2011 genetic study found A. defossor to be a gruiform. There are no available DNA sequences for A. otidiformis, but it was assumed the two species were more closely related to each other than to other birds.[11]
In 2019 two studies came forth with more in-depth phylogenetic methods. The first from Boast et al. (2019) using data from near-complete mitochondrial genome sequences found adzebills to be closely related to the family Sarothruridae, the flufftails.[12] Shortly after another study by Musser and Cracraft (2019), using both morphological and molecular data, found support for adzebills to be closely related to trumpeters of the family Psophiidae instead.[8] The authors took account of Boast et al. (2019) dataset and found it took 18 more steps to support the Aptornithidae-Sarothruridae clade than for Aptornithidae-Psophiidae.
The adzebills were about 80 centimetres (31 in) in length with a weight of 18 kilograms (40 lb), making them about the size of small moa (with which they were initially confused on their discovery) with enormous downward-curving and pointed bill, and strong legs.[13] They were flightless and had extremely reduced wings, smaller than those of the dodo compared to the birds' overall size, and with a uniquely reduced carpometacarpus.[14]