Morphology of the Ventral Process of the Sixth Cervical Vertebra in Extinct and Extant Equus: Functional Implications

1. Introduction

In the mammalian neck evolutionary processes have produced a wide variety of morphological specialisations that include three complex cervical modules [1,2,3]. With few exceptions, the three modules comprise of seven cervical vertebrae (CV) - upper (C1, C2), middle (C3–C5) and lower (C6, C7). Each vertebrae in the upper and lower modules present highly specialised conserved traits specifically designed for a diverse range of daily functions [1,2,3]. When conserved traits are shared by a group of phylogenetically related animals, they are referred to as a ‘body plan’ [4]; as demonstrated in mammals by the specialised ventral process of C6 [1,2,3,5,6,7,8,9,10,11,12].

In Equus ferus caballus, veterinary text defines C6 as an ‘atypical’ vertebra inclusive of C1, C2 and C7; whereas C3–C5 are ‘typical’ [7,8,12,13,14,15]. The differentiation between CV that are typical and atypical are - a typical CV is relatively unspecialised; while an atypical CV present highly specialised structural features for soft tissue attachment/s critical for specific head and neck biomechanics [1,2,3,12,13,14,15,16,17,18,19]. In C6, its atypical feature is a bilateral ventral projection traversing the entire length of the vertebral body which is referred to in the literature by several names; ventral process, ventral lamina (lamina ventralis), ventral transverse process and or ventral tubercle [6,7,8,12,13,14,15,16,18,19,20,21,22,23,24,25,26,27,28,29]. For the purpose of this study the ventral projection will be referred to as the ventral process of C6.

The ventral process of C6 demonstrates two specifc osteological features; a cranial ventral tubercle (CrVT) and caudal ventral tubercle (CVT) and each provides attachments for the deep perivertebral muscle, longus colli [7,12,13,14,15,16]. Rhombach, Stubbs and Clayton [16] demonstrate the CrVT in E. ferus caballus is the attachment site for the cervical portion of the multi bundled longus colli muscle; while the CVT is the insertion point for the single bundled thoracal tendon of the longus colli muscle that extends caudally to either T5 or T6. This muscle aids in the fixation, stabilisation, rotation and flexion of the CV; and also acts as a site of force redirection during muscle contraction cranial to, and or caudal to C6 [12,13,14,15,18].

In domestic mammals, the literature primarily describes the ventral process of C6 relative to educational instruction or veterinary management [6,7,8,12,20,21,22,23,24,25,26,27,28,29]. Although, extensive literature on pathologies and peer reviewed journals describing anomalous presentations are well documented in some domestic mammals [15,19,24,27,28,30,31,32,33,34,35,36,37,38,39,40]; historically, congenital malformations in the fossil record are rare [41]. Therefore, when two domestic species presenting similar congenital malformations in the cervicothoracic junction involving the ventral process of C6 are reported, namely Bos taurus domesticus [30,31,32] and E. ferus caballus [34,35,36,37,38,39,40], further investigations into the fossil record are warranted.

Clinically, the condition in B. taurus domesticus, known as complex vertebral malformation (CVM) is fatal, with the animal either stillborn or spontanesouly aborted during gestation [31,32]. Hence, preservation in the fossil record is unlikely due to fragmentaion of the associative growth plates relative to the ventral process of C6 [7], plus poor preservation of the specimen, and or predation [41]. However, in modern E. ferus caballus, the condition is not known to be fatal, and the absent CVT on the ventral process of C6 is evident in mature adults, and therefore preservation is more likely [34,35,36,37,38,39,40,41]. Although clinically the absent CVT on C6 is yet to be defined, several authors have postulated, or documented via case studies symptoms conducive to the congenital malformation [34,38,40]. Therefore, the question is - is this congenital malformation of the absent CVT in C6 a recent event in modern E. ferus caballus or a normal variation in the population?

To examine this question further, this study will examine the fossil record in extinct and extant Equidae. It would be expected that such evidence, if evident, would be determinable as other anomalous variations of the CVT in C6 have been reported in the fossil record. For example; cervical rib facets on the CVT of C6 in Pleistocene Coelodonta antiquitatis [42] and Mammuthus promigenius [43], and a complete absence of the right CVT in C6 of Dendrohyrax arboreus [44]. Therefore, the purpose of this study is to determine if anomalous variations of the ventral process in C6 associated with extant modern E.ferus caballus is strictly a post-domestication abnormality with possible functional implications, or is a normal variation in the population.

2. Materials and Methods

2.1. Ethical Statement

No equids were euthanized for the purpose of this study and observational research was conducted from specimens in museum collections and educational facilities.

2.2. Terminology

Nomenclature is derived from Getty [7], Popesko [20] and Gasse [45]. When a specific term for a structural profile or shape required further descriptive terminology, this was derived from either biological or botanical nomenclature, or previous literature.

2.3. Materials

Twenty-five applications to examine extinct and extant Equus collections were made to various museums and research/educational facilities. Collections only housing modern E. ferus caballus were excluded and for reasons of logistics, availability, and suitability of the collection, only 9 museums and three educational/research facilities were utilised for this study. Museums - American Museum of Natural History, New York, USA (AMNH), Monrepos Museum for Human Behavioural Evolution, Neuwied, Germany (MMHBE), Natural History Museum, Berlin, Germany (NHMB), Natural History Museum of Los Angeles County, USA (LACM), Oxford University Museum of Natural History, UK (OUMNH), Naturalis Biodiversity Center, Leiden, The Netherlands, (NBC), Smithsonian Institute, Washington, USA (USNM), The Page Museum Rancho La Brea, Los Angeles, USA (RLB), Yokohama Horse Museum, Yokohama, Japan (YHM). Educational facilities included the Australian College of Equine Podiotherapy, Yarck, Australia (ACEPT), Equine Studies, Asch, The Netherlands (ES), and the University of Florida, USA (UF).

Peer reviewed publications that nominated catalogued specimens presenting evidence of complete ventral processes of C6 were also examined.

To be eligible for the study, the specimens required minimal damage to at least one ventral process where a clear structural definition of the CrVT and CVT was determined. The purpose was to ascertain a normal and or anomalous presentation (Figure 1).

In those specimens where the ventral process is clearly divided into a CrVT and CVT, such as Pliohippus pernix (Figure 2), and where portions of either the CrVT or CVT on one side are lost to various pressures such as taphonomy or predation, then a revised observation was documented outlining the intact tubercles.

Collectively, museums, educational/research facilities and peer reviewed literature yielded 68 extinct Equus specimens, 12 extant Equus specimens (non E. ferus caballus) and three peer reviewed pubications describing specimens - totalling 83 specimens. The age of each specimen at expiration was defined by the complete ossification of the caudal growth plate (epiphysis) on C6 and subsequently included. Adults were defined by a closed epiphysis with no delineation, while sub adults were determined by incomplete epiphyseal ossification and obvious delineation. When classification of a species was undifferentiated, the specimen was labelled Equus sp.

In museum specimens, 12 species of extinct Equus were examined totalling 68 specimens with geologic dates. Eighteen specimens were undifferentiated and categorised as Equus sp., according to the museum’s records (Table 1).

In extant Equus, five non E. ferus caballus species were examined with one undifferentiated specimen, totalling 12 specimens. In museums, specimens were recorded according to their catalogue number in the collection, except in one (NBC – Equus sp., mounted skeleton not E. ferus caballus). In educational facilities specimens were classified by the animal’s name pre-mortem (Table 2).

Gidley (1903), Wood et al (2011), and Franzen and Haberstzer (2017) presented three peer reviewed publications describing three extinct Equus species - Neohipparion whitneyi. Hyracotherium grangeri, and Eurohippus messelensis respectively (Table 3) [46,47,48].

2.3. Methods

The ventral process of C6 was observed, described, and the left or right lateral profile/s were digitally recorded in extinct and extant species of Equus (not E. ferus caballus).

The assigned shape/s describing the lateral profile of the ventral process of C6 were derived from either botanical and or biological nomenclature, or from previous publications describing CV.

3. Results

3.1. General Anatomy

In all 83 specimens, the ventral process of C6 appeared on the lateral ventral border in an antero-posterior orientation separate and distal to the transverse process. In 82/83, the CrVT was anterior to the transverse process while the CVT remained posterior. Eurohippus messelensis was the exception and did not present a CrVT nor CVT according to the depictions and radiographs [48]. Excluding Eurohippus messelensis, in 82 specimens, the lateral profile of the ventral process depicts a separation between the CrVT and CVT that is noted by a convexity in the morphology directly distal to the transverse process. The extent of the convexity was not uniform between specimens and ranged from a lesser convexity between the CrVT and CVT to a distinct hourglass convexity (Figure 1a and Figure 2).

From the ventral view, the 82 specimens presented a tube-like morphology of the ventral process with a central constriction distal to the transverse process. Enthesis patterns were evident on both tubercles in relation to the attachment of the longus colli muscle – the cranial longus colli attachment to the CrVT and the thoracal tendon of the longus colli to the CVT. The CVT appeared more expanded than the CrVT, and when both cranial and caudal ventral tubercles were present, the overall outside width between left to right tubercles were greater across the CrVTs (Figure 4).

No congenital malformations or anomalous presentations as previously described were evident in the ventral process of C6 in the 83 specimens, nor in those specimens unsuitable for the study.

3.2. Lateral profile of the ventral process

In all 82/83 specimens, the CVT appeared longer than the CrVT. The size of the convexity in the ventral process presented three distinct morphological variations that were determined by size, and simply described as small, medium, and large (Figure 5).

In extinct Equus, the 71 specimens from museums and the literature presented a total of 38 small, 19 medium, and 13 large convexities with Eurohippus messelensis presenting a morphologically different bilateral ventral process with no convexities (Table 4).

Eurohippus messelensis presented bilateral ventral processes with no convexities that appeared as well-developed ventrolateral appendages from the vertebral body [48], similar to a trapezoid in profile. Gidley’s (1903) description of the ventral process describes the ventral surface of the 6th cervical as flat, turning downward laterally into the wing-like transverse processes, which are more strongly developed than in Equus [46]. Old literature describes these as cranial and caudal ventral transverse processes [49].

All 12 specimens in extant Equus presented small convexities in the ventral process, yet the demarcation between the CrVT and CVT was still clear (Table 5).

3.3. Variations of the Epiphyseal

During the examination of extinct and extant Equus variations of the CVT epiphyseal could be noted in sub adults as evident or absent. The absence was not anomalous variation, but a lost portion of the epiphyseal through destructive process such as degradation or human intervention through preparation of the vertebra (Figure 6).

In the extant specimen of Przewalskii’s horse from Equine Studies (Rideg), the caudal epiphyseal was intact upon dissection and maceration, its loss can be attributed to final preparations, such as degreasing.

4. Discussion

This study examined the morphology of the ventral process of C6 in extinct and extant Equus. The findings revealed the mammalian body plan for this specialised atypical structure remained a conserved trait in Equidae from its earliest ancestor – Hyracotherium grangeri [47]. The only noted variation in morphology was the size of the convexity between the CrVT and CVT receding from large to small over 55 million years, except in Eurohippus messelensis where a descent to modern E. ferus caballus could not be established [48].

The relationship between the convexity of the ventral process and its recedence might be functionally related to the increase in size of the head/neck, change in feeding regimes from predominantly browsing to predominantly grazing, and or environmental pressures such as predation [50]. The altered morphology of the ventral process led to an elongated CrVT and CVT with a reduced convexity, thus providing a longer attachment site for the Longus colli muscle to adequately support head/neck function. However, to verify this further morphometric studies would be necessary.

As for the variations seen in individual cervical vertebrae (C1–C7), they can be described in terms of intravertebral versus intervertebral modules, and defined as a component of a developing organism (e.g., an embryo) that are semi-autonomous relative to pattern formation and differentiation [51,52]. Arnold (2021) mapped the different possible modular schemes for cervical vertebrae based on developmental, morpho-functional and paleontological perspectives [1]. Similarly, Randau and Goswami (2017) demonstrated patterns of shape integration reflected modular organisation in felids [52], while in other carnivores, larger modules supported the integration between adjacent vertebrae to meet locomotory and functional demands [53].

In Equidae, as per most mammals, the cervical spine is held as vertically as possible to reduce the distance between the weight of the head and the sustaining cervicothoracic junction (C5–T2) [1,2]. This leads to the stereotypical vertical, s-shaped and self-stabilising resting posture of the cervical spine [54,55]. As head/neck movements start from this posture, orientation and gaze changes in the sagittal plane are restricted to the occiput to C2 module and C6–T1 module [1,2]. Arnold (2021) states that this functional modularity is supported by two prominent bony processes in mammals that provide major muscle attachment sites for head and neck motion: the enlarged spinous process on C2, and the ventral process of C6 [1]. These findings support Bainbridge’s (2018) comments that the ventral process of C6 acts as a site of force redirection during muscle contraction cranial to, and or caudal to C6 [15].

In modern E. ferus caballus, reports of the ventral process of C6 being unilaterally or bilaterally absent have been reported in multiple breeds from numerous countries [33,34,35,36,37,38,39,40]. Clinical symptoms are yet to be defined through longitudinal studies, yet the presentation across breeds and geographical regions suggests a familial connection [34]. With respect to the concepts of cervical modularity, this anomalous variation of the ventral process of C6, not seen in ancestral Equus, infers head and neck function could be compromised through the limitation, or lack thereof, for the attachment site of the thoracal longus colli muscle. May-Davis and Walker (2015) concurred through case studies based on gross examination of the thoracal Longus colli muscle, that it either and or, relocated with altered tendon morphology, hypertrophied on the affected side, and the authors also described potential symptomatic observations of afflicted horses bearing this anomalous variation [40]. Similar studies concurred with these findings and further described ataxic behaviours [34,38,56].

The findings from this study indicate the anomalous variations reported in the ventral process of C6 in modern E. ferus caballus is a recent event and not indicative of a normal variation within the population of Equus. Nor was it present in pre-domestic Equus or ancestral Equus. However, the limitations in this study were identified as the small number of extant Equus (non E. ferus caballus), the limitation of complete specimens in the fossil record and access to further specimens. Even so, the evidence to date suggests this anomalous variation of the ventral process in C6, was not present in the fossil record (n=71). Therefore, in modern E. ferus caballus, when the CVT of C6 is absent, then functional ramifications could be of concern when the structural integrity of the caudal cervical module in head/neck function is compromised. These findings might benefit equine practitioners when reporting on diagnostic images of C6, and especially for potential buyers in pre-purchase cases.

5. Conclusions

The ventral process of C6 is a conserved trait in most mammals and its highly specialised atypical structure is present in extinct and extant Equus. Functionally, the muscles attaching to this process aid in flexion, stabilization, fixation, and force redirection of the cervical vertebrae, which are essential for head/neck posture and locomotion. When the CVT of the ventral process is absent, as reported in modern E. ferus caballus, there could be consequences relevant to normal function. This study provided evidence that absent CVTs are not present in extinct Equus, nor in a limited number extant species of Equus, only E. ferus caballus.

Therefore, as an integral part of the caudal cervical module, any anomalous variations in the ventral process of C6 might lead to dysfunctional ramifications of the cervicothoracic junction. This would require further studies, morphologically and biomechanically, to understand full implications.