Comparative axial morphology in pinnipeds and its correlation with aquatic locomotory behaviour

Abstract

Regional variation in the axial skeleton of pinnipeds (seals and walruses) and its correlation with aquatic locomotory behaviour is examined using vertebral functional profiles. The results demonstrate clear morpho-functional differences in the thoracolumbar region of modern pinnipeds (Phocidae, Otariidae, Odobenus) that can be strongly linked to swimming style. Phocid seals have a rigid thoracic region attached to a highly flexible lumbar region with long muscular lever arms providing the necessary mobility and leverage to perform pelvic oscillations. Conversely, otariid seals have extremely flexible inter-vertebral joints along the length of the column which should enhance manoeuvrability and turning performance. They also have greater muscular leverage in the anterior thoracic region to support pectoral oscillations. Odobenus (walrus) shows vertebral characteristics most similar to phocids, but with some otariid qualities, consistent with an intermediate or mixed form of aquatic locomotion, with pelvic oscillation dominating over pectoral oscillation. Comparison of the vertebral functional profiles in the fossil taxon Allodesmus kernensis with those of modern pinniped clades reveals that this extinct pinniped may also have used a combination of pectoral and pelvic oscillatory movements during swimming, but in a manner opposite to that of Odobenus, with pectoral oscillatory movements dominating. This study raises questions about the evolution and diversification of pinniped locomotory behaviours, but also provides the necessary framework to begin to examine axial mechanics and locomotory stages in other fossil pinnipedimorphs and their relatives in more detail.

Vertebral bending mechanics and xenarthrous morphology in the nine-banded armadillo (Dasypus novemcinctus)

“…Dorsoventral transverse process/diapophysis angle and inter-zygapophyseal length have both been positively correlated with dorsoventral range of motion, and pre-zygapophyseal angle positively correlated with range of motion in lateroflexion. Centrum length, neural spine length, craniocaudal transverse process/diapophysis angle and neural spine angle have also been shown to be negatively correlated with range of motion in the sagittal plane (Buchholtz and Schur, 2004;Granatosky et al, 2014;Long et al, 1997;Molnar et al, 2014Molnar et al, , 2015Pierce et al, 2011). Metapophyses and anapophyses and their corresponding linear and angular measures are, with some exception, specific to the xenarthrous region, and their influence on intervertebral mechanics is the focus of the present study.…”

“…Nonetheless, our data do implicate xenarthrous articulations and post-diaphragmatic zygapophyses with an increase in range of motion, as PC1, which was heavily weighted in xenarthrous and zygapophyseal measures, was found to be predictive of range of motion in all three bending directions (Tables 4, 5). It appears that, rather than acting as bony stops, xenarthrous articulations may help to amplify the observed effects of the curved, protruding postdiaphragmatic zygapophyses in mammals, which permit and secure flexion by increasing the surface area upon which vertebrae can slide without disarticulating, thereby enhancing range of motion (Filler, 2007;Jenkins, 1974;Pierce et al, 2011;Russo, 2010;Shapiro, 1995). In functioning in this way, xenarthrous articulations may extend beyond a purely stabilizing role and increase range of motion by providing greater surface area for movement.…”

“…Boszczyk et al, 2001;Buchholtz, 2001;Cullinane and Bertram, 2000;Filler, 2007;Gál, 1993a,b;Gaudin and Biewener, 1992;Hebrank et al, 1990;Hildebrand, 1959;Jones and German, 2014;Jones and Pierce, 2016;Molnar et al, 2014;Pierce et al, 2011;Rockwell et al, 1938;Schilling, 2011;Shapiro, 1995;Slijper, 1946;Ward and Mehta, 2014). Vertebral evolution is facilitated by regionalization, the Hox-mediated division of the column into morphological and functional units (Head and Polly, 2015;Schilling, 2011;Wellik, 2007).…”

“…In mammals, the thoracic and lumbar regions are foremost implicated in locomotion, prompting investigations into the functional link between thoracolumbar morphology and mechanical properties of intervertebral joints (e.g. Boszczyk et al, 2001;Gál, 1993a,b;Gaudin and Biewener, 1992;Granatosky et al, 2014;Jones, 2016;Long et al, 1997;Nyakatura and Fischer, 2010;Pierce et al, 2011;Russo, 2010;Shapiro, 1995Shapiro, , 2007Slijper, 1946). These studies illustrate the morphological and mechanical diversity of thoracolumbar vertebrae within and between mammals, and demonstrate that morphological predictors of mechanics can vary according to size, taxon and mode of locomotion.…”

Gross Morphology and Morphometry of Thoracic and Lumbar Vertebrae in a Mugger Crocodile (Crocodylus palustris)

“…The regional differentiation in the crocodilian vertebral column is not as pronounced as seen in mammals (Shapiro, 2007;Pierce et al, 2011;Molnar et al, 2015). However, the primary function of thoracic and lumbar vertebrae in crocodiles is to provide body support and mobilization during terrestrial locomotion.…”