Article

Mechanical Analysis of Feeding Behavior in the Extinct “Terror Bird” Andalgalornis steulleti (Gruiformes: Phorusrhacidae)

CONICET-División Paleontología Vertebrados, Facultad de Ciencias Naturales y Museo, Museo de La Plata, Universidad Nacional de La Plata, La Plata, Argentina.
PLoS ONE (Impact Factor: 3.53). 08/2010; 5(8):e11856. DOI: 10.1371/journal.pone.0011856
Source: PubMed

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 (approximately 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.

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    • "Psilopterus lemoinei (and all the phorusrhacids), as we stated previously , lack both hinges. This suggests that cranial kinesis was lost secondarily in the history of the Phorushracidae (Degrange et al., 2010). "
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    • "In a broad sense, we may assume that the skulls of other large phorusrhacids, characterized by high and compressed beaks, would respond similarly to that of Andalgalornis (body mass, 40 kg). Phorusrhacids chased and killed their prey using their huge beaks as a hatchet (Degrange et al., 2010). "
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    • "Psilopterus lemoinei (and all the phorusrhacids), as we stated previously , lack both hinges. This suggests that cranial kinesis was lost secondarily in the history of the Phorushracidae (Degrange et al., 2010). "
    [Show abstract] [Hide abstract]
    ABSTRACT: Psilopterus lemoinei, the largest species in this genus, was a small terror bird weighing 8–9 kg, and was a ground bird with functionally tridactyl feet. New remains of this phorusrhacid, including an exceptionally preserved anterior part of a skull recovered from the Patagonian Killik Aike Norte locality (Santa Cruz Formation, late early Miocene), is now available for study. The main purpose of this paper is to provide a detailed morphological description of Psilopterus lemoinei. The new fossils show for the first time that the internal structure of the beak is hollow and reinforced with thin-walled trabeculae. The absence of zona flexoria palatina and zona flexoria arcus jugalis are key features related to the evolution of cranial akinesis. Homologies of the narial and fenestra antorbitalis boundaries have been clarified. Our re-examination allows the establishment of primary osteological homologies useful in comparative anatomy, functional morphology, and phylogenetic studies.
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