Flexibility along the Neck of the Neogene Terror Bird Andalgalornis steulleti (Aves Phorusrhacidae)

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 FINDINGS: We 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/SIGNIFICANCE: The 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

Textural ageing in Pygoscelis antarctica (Aves, Sphenisci­formes): a new comparative scale for penguin bones

Textural ageing is a technique that recognizes the bone texture as a clear indicator of the post-natal ontogenetic stages in birds. Although it was only tested in a few groups of birds, the results obtained in the pioneer works were widely applied. Though convinced of the effectiveness of this technique, we also believe that given the variability that exists between birds, its application should be done using a reliable comparative scale. In this contribution, a post-natal ontogenetic series for Pygoscelis antarctica was built to analyze the textural ageing in penguins. We used the same descriptive terms to characterize each texture as in previous contributions, to make the scales comparable to each other. The variations were standardized, and a new comparative scale for penguins was propose

Angles between vertebrae taken with goniometer.

<p>– Not measured.</p

Schematic hypothetical lateral reconstruction of neck muscles and ligaments in <i>Andalgalornis</i> based on sites of muscle origin and insertions identified in the holotype and according [<b>10]</b>, [22], [30<b> </b>].

<p>A–C represents successively superficial layers. Arrow indicates cranial direction.</p

Schematic vertebrae showing the measurements and angles taken.

<p>A, cranial view; B, dorsal view; C–D, lateral view. ML maximum length, MH maximum height, LF in dorsal view, distance between the <i>facies articularis craniali</i>s and <i>caudalis</i>, L1 in dorsal view, distance between the cranial and posterior borders of the <i>foramen vertebrale</i>, L4 in ventral view, maximum length of the <i>corpus</i>, AW cranial width, Fw1 distance between both <i>zygapophysis cranialis</i>, Cw caudal width. L1, L4 and Fw1 are following Guinard <i>et al.</i> (2009: Fig. 2), I1 development and cranial projection of <i>facies articularis cranialis</i> (I1 =  L4-L1/L4), I2 elongation index (I2 = L4/Fw1), the later in the same way as Guinard <i>et al.</i> (2009), da dorsal angle, va ventral angle, c. circa because deformation.</p

Transitional vertebrae of <i>Andalgalornis steulleti</i> FMNH - P14357.

<p>A, E dorsal view; B, F ventral view; C, G right lateral view; D, H cranial view. Scale 1 cm.</p

Complete neck of <i>Andalgalornis steulleti</i> FMNH - P14357 and selected vertebrae.

<p>A, neck with R2 in maximum dorsal flexion. R1 region 1, R2 region 2, R3 region 3, VT cervico dorsal vertebrae; B, vertebra 3 (R1); C, vertebra 6 (R2); D, vertebra 11 (R3). Top line, dorsal view; Midline, right lateral view; Bottom line, cranial view. Scale 1 cm.</p

Measurements and indices of 3^rd to 13^th<i>Andalgalornis steulleti</i> cervical vertebrae. Abbreviations in Figure 3.

<p>Measurements and indices of 3^rd to 13^th<i>Andalgalornis steulleti</i> cervical vertebrae. Abbreviations in <a href="http://www.plosone.org/article/info:doi/10.1371/journal.pone.0037701#pone-0037701-g003" target="_blank">Figure 3</a>.</p

Comparison of the number of vertebrae per region of <i>Andalgalornis</i> with selected taxons with and without bifurcated neural spines.

<p>BNS vertebrae with bifurcated neural spines, T total vertebrae of the neck, v vertebrae.</p