Neck length and neck posture are both relevant to give the appropriate position of the head during all kinds of behaviors . For activities such as mating or defense the neck has the ability to perform more complex movements  linked to the cervical morphology. The head centre of gravity over the feet is deeply correlated with the erect posture on land and also depends on the movements of the neck. In normal stance birds (and all amniotes) maintain vertical its cervical column .

Size, morphology and number of vertebrae are three important variables involved in the motion of the neck . Birds have a highly variable number of cervical vertebrae (9–11 in some parrots, 23–25 in swans) but, whatever the number, their necks look shorter or longer depending on the neck-folding (Fig. 1). All birds have a flexible “S” shaped neck but the flexion capacity varies between them and for sure, the arrangement is a consequence of an intricate system made by muscles, ligaments and complex articulations. As flexion increases, the distance between the head and the thorax will be shorter and, under these circumstances the external profile of the animal’s neck region shortens .

Each vertebra articulates with the adjacent through a saddle-shaped region (articulatio intercorporalisin the sense of  located at the base of the vertebral body (corpus vertebrae) (Fig. 2) and two additional sliding surfaces located in the top (articulatio zygapophysialis cranialis and caudalis) . This morphology allows large dorsoventral movements of the neck but prevents rotation, which is generally small.

Andalgalornis steulleti (Kraglievich 1931), from the upper Miocene–lower Pliocene (≈6 million years ago) of Argentina, was a medium-sized patagornithine phorusrhacid of about 40 kg body mass, 1.4 m height, and 370 mm total skull length . Together with the long legs, the atypical large skull with high and narrow beak is a characteristic feature of all phorusrhacids . The Phorusrhacidae are often regarded as apex predators of the South American Tertiary environments sharing this role with large now-extinct marsupials . Popularly known as “terror birds” (a name applied at least to the giant forms) there are no close modern analogs which facilitate interpretations about their biology.

Here we analyze the flexion patterns of the neck of Andalgalornis based on the neck vertebrae morphology and biometrics. Complete columns among phorusrhacids remains are scare and in most cases in those specimens where the column is quite complete, the vertebrae are severely damaged. So far, the column of Andalgalornis is the most complete and well preserved known of a terror bird. The study here is the first interpretation of the potential performance of the neck of Andalgalornis in its entirety and we considered this an important starting point to understand and reconstruct the entire neck of other phorusrhacids from which the neck is unknown or severely damaged.

The vertebral morphology of Andalgalornis has not been extensively described in the literature. In their original description of the only complete column known for the species, Patterson and Kraglievich recognized the presence of 17 presynsacral vertebrae (11 cervicals, 2 cervicodorsals and 4 dorsals) of which 13 form the neck. Their description is limited to the size comparison of the vertebral body and only in a few vertebrae did they also compare the height of the neural spine.

We will dwell on the comparison between several extant birds of different size and habits in order to establish a correlation between cervical morphology and mode of life. Such anatomical analysis is not available for any phorusrhacid, while it is a first requirement for any quantitative analysis in comparative, functional or ecological morphology.

Research work of : Claudia P. Tambussi^1,2*, Ricardo de Mendoza¹, Federico J. Degrange^2,3, Mariana B. Picasso¹

1 División Paleontología Vertebrados, Museo de La Plata, Paseo del Bosque s/n, La Plata, Argentina, 2 Consejo Nacional de Investigaciones Científicas y Técnicas CONICET, Ciudad Autónoma de Buenos Aires, Argentina, 3 Centro de Investigaciones en Ciencias de la Tierra CICTERRA, Córdoba, Argentina .Published in PLOs One .