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

Sharon May-Davis,Robert Hunter,Wendy Brown

doi:10.18435/vamp29373

Abstract

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

Highlights

The number of cervical vertebrae in mammals became fixed more than 200 million years ago; the canonical number of seven cervicals is well established by palaeontological evidence (Buchholtz et al 2012; Arnold 2020)
Functional Anatomy: The lig. lamina nuchae consists of a bilateral sheet of yellow elastin ligamentous fibres that arise from the spinous processes of the 2nd and 3rd thoracic vertebrae and the ligamentum funiculus nuchae, after which they traverse cranioventrally and insert on the Cervical spinous processes (CSPs) from the 2nd to 7th cervicals in E. przewalskii, E. quagga boehmi and E. asinus
It should be noted that E. przewalskii, E. quagga boehmi and E. asinus present with lig. lamina nuchae attachments from the 2nd to 7th CSPs

Summary

Introduction

The number of cervical vertebrae in mammals became fixed more than 200 million years ago (mya); the canonical number of seven cervicals is well established by palaeontological evidence (Buchholtz et al 2012; Arnold 2020). The mammalian cervical spine, considered as a morphologically static feature (Arnold et al 2017), typically includes dorsally positioned spinous processes on some or all of the 2nd to 7th cervical vertebrae (Zsoldos and Licka 2015; Arnold 2020). Hyracotherium (Buchholtz et al 2012; MacFadden 1992) bears blade-like spinous processes on the 4th to 7th cervical vertebrae (Wood et al 2011) (Fig. 1A). This morphological trait is still retained in two of three extant perissodactyl families, namely Tapiridae and Rhinoceridae (Fig. 1C, D). Cervical spinous processes (CSPs) provide important attachment sites for neck and back musculature, as well as ligamentous structures such as the lig. lamina nuchae

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Conclusion