The Disappearing Lamellae: A Modern and Historical Study into the Morphological Variations of the Equine Ligamentum Lamina Nuchae and its Attachments to the Cervical Vertebrae

Abstract

<p>The fossil remains of North American <i>Hyracotherium</i>, the progenitor to modern day <i>Equus</i> from 55 million years ago (mya), are the earliest known representatives of perissodactyls. This diminutive browser presented full <i>ligamentum lamina nuchae</i> (nuchal ligament lamellae - NLL) attachments from the 2nd to 7^th cervical vertebrae (C2–C7), and early anatomists from the 18^th and 19^th century identified a similar number of attachments in <i>Equus caballus</i>. Yet a recent study of modern and ancient breeds of <i>E. caballus</i> reported a lack of functional NLL attachments at the<i>6th</i> and 7^th cervical vertebrae (C6 and C7) in 35/35, indicating a morphological change that has not been scientifically investigated. This thesis presents 6 peer reviewed studies that examine the morphological range of NLL attachments in the current population of<i>E. caballus</i>; it then compares the relevant findings from C6 and C7 to extinct pre-domestic <i>Equus</i> specimens looking for evidence of NLL attachments.</p> <p>The first two studies examined through gross anatomic observations the NLL in four species of <i>Equus</i> (<i>n</i>=153):<i>E. caballus</i> (<i>n</i>=148), and three species of sister taxa; <i>Equus asinus</i> (<i>n</i>=2), <i>Equus przewalskii</i> (<i>n</i>=2), and <i>Equus quagga boehmi</i> (<i>n</i>=1).<i>E. caballus</i> and one subspecies were further categorised into 23 breeds designated into 3 categories: modern breeds developed post 1500s (breeds <i>n</i>=18; specimens <i>n</i>=132), ancient breeds developed pre 1500s (breeds <i>n</i>=3; specimens <i>n</i>=10), a primitive breed (autochthonic) (breed <i>n</i>=1; specimens <i>n</i>=5) and a close descendant of the subspecies - <i>Equus ferus ferus</i> (breed <i>n</i>=1; specimen <i>n</i>=1) (refer to Appendix B for details of breed, dissections and<i> in situ</i> observations). Of the 153 specimens examined, the NLL attached from either C2–C5, C2–C6 or C2–C7. The results identified attachments in the following specimens: from C2–C5 in 140/153 (modern <i>n</i>=128; ancient <i>n</i>=10; primitive <i>n</i>=2); from C2–C6 in 3/153 (modern <i>n</i>=3), and from C2–C7 in 10/153 (modern 1; primitive <i>n</i>=3; close descendant <i>E. f. ferus</i> <i>n</i>=1; and sister taxa <i>n</i>=5). Although these two studies experienced a limitation in numbers associated with the primitive breed (specimens <i>n</i>=5) and species of sister taxa (species <i>n</i>=3, specimens <i>n</i>=5), these 10 specimens still offered more NLL attachments from C2–C7 (8/10) than modern and ancient breeds combined. This implies the absent C6 and C7 NLL attachments are mostly associated with modern and ancient breeds of<i>E. caballus</i>.</p> <p>The 3^rd and 4^th studies (specimens separate to the previous two studies) sought to enable the morphological investigations of NLL attachments in the live horse with ultrasound. The 3^rd study described a methodology that established the protocols of identifying the NLL attachments to the cervical vertebrae in the live horse (<i>n</i>=2), followed by gross examination. This allowed the investigation of NLL attachments in those breeds that are endangered, isolated or experience limited numbers, to be investigated without gross anatomic examination. The4^th study examined an isolated and endangered breed of <i>E. caballus</i> (Yonaguni Pony) by examining<i> in situ</i> remains (<i>n</i>=3) and the now validated protocol using ultrasound in a well-handled individual. The<i> in situ</i> specimens provided evidence of NLL attachments from C2–C7, and these findings corresponded with the ultrasound images in the live horse.</p> <p>However, identifying NLL attachments in extinct pre-domestic <i>Equus</i> specimens required innovative research, and the 5^th and 6^th studies investigated these possibilities. The 5^th study identified the NLL attachments to the cervical spinous processes (CSPs) by a comparative gross study in four species of <i>Equus</i>. This methodology examined the NLL attachments at the CSPs in equids presenting from C2–C5 and C2–C7, and identified their corresponding enthesis patterns. During the study, differing anatomic shapes associated with the lateral CSPs profiles of C6 and C7 were noted, and appeared dependant on the presence/absence of NLL attachments. This became the focus for the 6^th study. The apparent association between NLL attachments and lateral CSPs profiles of C6 and C7 were examined in extant <i>Equus</i> and compared with extinct <i>Equus</i> specimens. Four extant species of <i>Equus</i> (<i>n</i>=33) were investigated -<i>E. caballus</i> (<i>n</i>=26), <i>E. asinus</i> (<i>n</i>=3), <i>E. przewalskii</i> (<i>n</i>=3) and <i>E. q. boehmi</i> (<i>n</i>=1), (C6 <i>n</i>=33; C7 <i>n</i>=33). Six primary lateral CSPs profiles were identified by shape and named accordingly - cuneate, curvate, falcate, rudimentary, scalenate, and truncate. These profiles were compared to extinct <i>Equus</i> specimens (<i>n</i>=66), (C6 <i>n</i>=39; C7 <i>n</i>=27), and revealed 5/6 primary profiles were present, all except rudimentary. The lateral CSPs profiles of C6 and C7 in extinct <i>Equus</i> specimens indicated NLL attachments were likely present, with the profile cuneate only identified in <i>E. asinus</i> and extinct <i>Equus</i> on C6. </p> <p>In conclusion, this thesis proposes the NLL attachments were from C2–C7 in extinct <i>Equus</i>, thus concurring with early anatomic literature pre 1910. Thereafter, the reduction of NLL attachments in extant species became increasingly apparent with modern and ancient breeds of<i>E. caballus</i> primarily presenting from C2–C5. Causal factors indicating why modern and ancient breeds were the most represented was not evident, so a timeline introducing key historical events potentially coinciding with the reduction was considered. From this, it was proposed that World Wars I and II, a post war genetic bottleneck and a change in modern horse usage were potential factors. Additionally, the thesis discussed potential ramifications associated with the NLL reduction and reasoned caudal cervical vertebral destabilisation and/or associative osteoarthritis might be of concern. Further studies are needed to establish whether the observed reduction in NLL attachments may be a contributing factor in these conditions.</p>