Background Andalgalornis steulleti from the upper Miocene–lower Pliocene (≈6 million years ago) of Argentina is a medium-sized patagornithine phorusrhacid. It was a member of the predominantly South American radiation of ‘terror birds’ (Phorusrhacidae) that were apex predators throughout much of the Cenozoic. A previous biomechanical study suggests that the skull would be prepared to make sudden movements in the sagittal plane to subdue prey.Methodology/Principal FindingsWe analyze the flexion patterns of the neck of Andalgalornis based on the neck vertebrae morphology and biometrics. The transitional cervical vertebrae 5th and 9th clearly separate regions 1–2 and 2–3 respectively. Bifurcate neural spines are developed in the cervical vertebrae 7th to 12th suggesting the presence of a very intricate ligamentary system and of a very well developed epaxial musculature. The presence of the lig. elasticum interespinale is inferred. High neural spines of R3 suggest that this region concentrates the major stresses during downstrokes.Conclusions/SignificanceThe musculoskeletal system of Andalgalornis seems to be prepared (1) to support a particularly big head during normal stance, and (2) to help the neck (and the head) rising after the maximum ventroflexion during a strike. The study herein 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 flexion pattern of other phorusrhacids from which the neck is unknown.

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Cayaoa bruneti, from the early Miocene Gaiman Formation of Chubut, Argentina, provides the earliest evidence for diving behaviour and the earliest example of flight loss within Anseriformes. It is also the only known diving duck from South America. A new phylogenetic analysis using only morphological characters places Cayaoa bruneti as part of a single radiation of diving ducks within Anatinae, as sister group to the Erismaturinae. Partition analysis of subsets of characters shows that the humerus and femur have the strongest effect in generating this phylogeny, while the skull characters result in groupings closer to those seen in analyses with molecular data. When the analysis is constrained by enforcing the molecular tree as a backbone, Cayaoa bruneti emerges as a basal branch within the Erismaturinae. These results make Cayaoa bruneti an independent and early example of the recurring evolution of flightlessness in large marine Anatidae.

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