Abstract
The South American phorusrhacid bird radiation comprised at least 18 species of small to gigantic terrestrial predators for which there are no close modern analogs. Here we perform functional analyses of the skull of the medium-sized (∼40 kg) patagornithine phorusrhacid Andalgalornis steulleti (upper Miocene–lower Pliocene, Andalgalá Formation, Catamarca, Argentina) to assess its mechanical performance in a comparative context. Based on computed tomographic (CT) scanning and morphological analysis, the skull of Andalgalornis steulleti is interpreted as showing features reflecting loss of intracranial immobility. Discrete anatomical attributes permitting such cranial kinesis are widespread phorusrhacids outgroups, but this is the first clear evidence of loss of cranial kinesis in a gruiform bird and may be among the best documented cases among all birds. This apomorphic loss is interpreted as an adaptation for enhanced craniofacial rigidity, particularly with regard to sagittal loading. We apply a Finite Element approach to a three-dimensional (3D) model of the skull. Based on regression analysis we estimate the bite force of Andalgalornis at the bill tip to be 133 N. Relative to results obtained from Finite Element Analysis of one of its closest living relatives (seriema) and a large predatory bird (eagle), the phorusrhacid's skull shows relatively high stress under lateral loadings, but low stress where force is applied dorsoventrally (sagittally) and in “pullback” simulations. Given the relative weakness of the skull mediolaterally, it seems unlikely that Andalgalornis engaged in potentially risky behaviors that involved subduing large, struggling prey with its beak. We suggest that it either consumed smaller prey that could be killed and consumed more safely (e.g., swallowed whole) or that it used multiple well-targeted sagittal strikes with the beak in a repetitive attack-and-retreat strategy.
Highlights
Phorusrhacids were a predominantly South American radiation of gruiform birds known from middle Paleocene to lower Pleistocene deposits [1,2] and most closely related to seriemas (Cariamidae) among extant birds [3,4,5,6]
Divergent skull morphology and loss of cranial kinesis The nature of the connections between the skull bones is important to the analysis of mechanical behavior, because these connections influence the relative movement of areas of the skull [10] and the distribution of stress and strain [11,12]
We assessed the detailed morphology of these features in Andalgalornis and some other phorusrhacids using computed tomographic (CT) scanning, supplementing these findings with observations of other taxa
Summary
Phorusrhacids were a predominantly South American radiation of gruiform birds known from middle Paleocene to lower Pleistocene deposits [1,2] and most closely related to seriemas (Cariamidae) among extant birds [3,4,5,6]. Their often gigantic body sizes, large skulls, and carnivorous lifestyles have resulted in phorusrhacids being popularly termed ‘‘terror birds.’’ Andalgalornis steulleti [7], from the upper Miocene–lower Pliocene (
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