A comparative morphological study of the sixth and seventh spinous processes of the cervical vertebrae in extinct and extant species of Equus

Abstract

<p>The lateral profile of the 6th and 7th cervical spinous processes (CSPs) were examined in four extant species of <i>Equus</i> (n=33); <i>E. caballus</i> (n=26), <i>E. przewalskii</i> (n=3), <i>E. quagga boehmi</i> (n=1), <i>E. asinus</i> (n=3) and compared to pre-domesticated <i>Equus</i> specimens (n=66) representing three known species; <i>E. occidentalis</i> (n=56), <i>E. mosbachensis</i> (n=2), <i>E. curvedins/insulatis</i> (n=1) and unknown <i>Equus</i> species (n=7) from five museums. Six common morphological profiles were revealed: cuneate, curvate, falcate, rudimentary, scalenate, and truncate. For the 6th CSP, the distribution of these morphologies amongst extant <i>Equus</i> is: cuneate, only <i>E. asinus</i>; curvate, <i>E. caballus</i> and <i>E. przewalskii</i> always in combination with <i>ligamentum lamina nuchae</i> (<i>lig. lamina nuchae</i>) attachments from the 2nd through to 7th CSP inclusive; falcate, E. caballus and E. przewalskii always in combination with lig. lamina nuchae attachments from the 2nd through to 7th CSP inclusive; rudimentary, <i>E. caballus</i> always associated with <i>lig. lamina nuchae</i> attachments from the 2nd through to 5th CSP inclusive; scalenate, <i>E. caballus</i> in association with <i>lig. lamina nuchae</i> attachments from the 2nd through to 5th (n=11) or 7th (n=4) CSP inclusive; truncate, not present. The 6th CSP in museum specimens of <i>Equus</i> exhibits one of four profiles: cuneate (n=10), curvate (n=14), scalenate (n=11) and truncate (n=4). For the 7th CSP, the distribution of these morphologies amongst extant <i>Equus</i> is: curvate, <i>E. caballus</i> mostly associated with <i>lig. lamina nuchae</i> attachments from the 2nd through to 5th CSP inclusive, falcate, <i>E. caballus</i> mostly associated with <i>lig. lamina nuchae</i> attachments from the 2nd through to 7th CSP inclusive, scalenate, <i>E. caballus</i> associated with <i>lig. lamina nuchae</i> attachments from the 2nd through to 5th CSP inclusive. Only Rancho La Brea and Tar Pits Museum provided samples suitable for examination of the 7th CSP. These exhibited four profiles: curvate (n=7), falcate (n=11), scalenate (n=2), and truncate (n=7). These findings suggest that the lateral profile of the 6th CSP is of potential use in identification of species; attachments of the lig. lamina nuchae alter the morphology of the 6th and 7th CSP; and that attachments of the <i>lig. lamina nuchae</i> on the 2nd to 7th CSP were likely present in species of <i>Equus</i> prior to domestication.</p>