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Comparative Study
. 2011 Oct;219(4):502-14.
doi: 10.1111/j.1469-7580.2011.01406.x. Epub 2011 Jun 14.

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

S E Pierce  1 , J A Clack, J R Hutchinson
Affiliations
Comparative Study

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

S E Pierce et al. J Anat. 2011 Oct.

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.

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Figures

Fig. 1
Fig. 1
Biomechanically informative measurements. CH: centrum height (taken on posterior endplate). CL: centrum length (taken along the ventral margin). CW: centrum width (taken on posterior endplate). LW: lamina width (taken posterior to the transverse processes). NPLA: neural spine lever arm (taken from centre of endplate). NSA: neural spine angle (taken from the horizontal plane with anterior set to 0°). PZA: prezygapophyseal angle (taken between prezygapophyses). TPAP: transverse process anteroposterior projection (taken from the horizontal plane with anterior set to 0°). TPDV: transverse process dorsoventral projection (taken from the sagittal plane with dorsal set to 0°). TPLA: transverse process lever arm (taken from centre of endplate). ZL: zygapophyseal length (taken as maximum distance). Inter-zygapophyseal length (I-ZL) (or degree of zygapophyseal overhang) was calculated by subtracting CL from ZL. The images represent the second lumbar vertebra from Halichoerus grypus (Grey seal), UMZC K.7943.
Fig. 2
Fig. 2
(A) Results from principal components analysis of log (size normalised) linear measurements. (B–H) Linear functional profiles plotted as a percentage of thoracolumbar length. Suggested functional implications of each measurement are depicted on the right-hand side of each graph. The dashed line indicates the transition between the thoracic and lumbar regions; this transition takes place one step/vertebra earlier in Odobenus rosmarus and Hydrurga leptonyx. Note that the first lumbar transverse process lever arm datum point for H. leptonyx was not used when calculating the phocid average for this measurement.
Fig. 3
Fig. 3
(A–D) Angular functional profiles plotted as a percentage of thoracolumbar length. Suggested functional implications of each measurement are depicted on the right-hand side of each graph. The dashed line indicates the transition between the thoracic and lumbar regions; this transition takes place one step/vertebra earlier in Odobenus rosmarus and Hydrurga leptonyx.
Fig. 4
Fig. 4
Centrum shape, as defined by relative centrum length [2CL/(CH+CW)], plotted as a percentage of thoracolumbar length. Direction of shape change is depicted on the right-hand side of the graph. The dashed line indicates the transition between the thoracic and lumbar regions; this transition takes place one step/vertebra earlier in Odobenus rosmarus and Hydrurga leptonyx.
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