A comparative morphological study of the ligamentum lamina nuchae and ligamenta interspinalia reveals enthesis patterns at the sites of attachment from the second to seventh cervical vertebrae in four extant species of Equus

Abstract

<p>Morphological variation in the attachment sites of the <i>ligamentum lamina nuchae</i> and <i>ligamenta interspinalia</i> to the 2nd to 7th cervical vertebrae is described in four species of <i>Equus</i>, and enthesis patterns in relation to these attachment sites are explored. This comparative study provides new insights with respect to this variation in the zebrine and asinine clades within <i>Equus</i>. In zebrines (<i>Equus caballus</i>, <i>E. quagga boehmi</i> and <i>E. przewalskii</i>), the <i>ligamentum lamina nuchae</i> attaches to the eminence of the cervical dorsal spines from either the 2nd to 5th, and or 2nd to 7th. These attachments resemble digits and between each digit an aperture is apparent. The <i>ligamenta interspinalia</i> attach to the base of each cervical dorsal spine from the 2nd to 7th before progressing caudally onto the thoracic dorsal spines. Three enthesis patterns per cervical dorsal spine correspond to the size and shape of these ligaments. In contrast, in asinines (as represented by <i>Equus asinus</i>), the <i>ligamentum lamina nuchae</i> attaches without digitation to the complete sagittal ridge of the cervical vertebra including the dorsal spine and exhibits no <i>ligamenta interspinalia</i>. Consequently, the enthesis pattern is represented by a raised sagittal ridge only, excluding the 7th cervical dorsal spine. These differences between the <i>ligamentum lamina nuchae</i> and <i>ligamenta interspinalia</i> attachment sites create distinct enthesis patterns that differentiate zebrines from asinines. Findings include: differing ligamentous morphology; associated enthesis patterns corresponding to attachment sites of the ligaments to the cervical vertebrae; influences in dorsal spine morphology subject to the presence or absence of the ligaments; and the postulation of evolutionary functional adaptions in response to environmental influences between the two <i>Equus</i> clades. These results may be useful for palaeontologists in the identification of isolated cervical vertebrae.</p>