The total average number of bones seen in the axial skeleton of this species was 75 (Table 1). This comprised bones of the Skull, Mandible, sternum, ribs, cervical, thoracic, lumbar, sacral and caudal vertebrae.
The Skull
This presented an elongated structure made up of caudally located cranial bones and rostrally located facial bones. The bones were flat and joined together by sutures. Caudally, the cranial cavity was bounded by an unpaired occipital bone that presented a dorsal quadrilateral squamous part, lateral parts and a ventral basilar part (Fig. 4). The quadrilateral squamous part presented a convex external surface roughened at its centre, cranial border that joined the parietal bone cranially and lateral borders that joined the lateral parts of occipital. The lateral parts were formed by the lateral paramastoid process and medial condyle. They were separated by a deep condyloid fossa that lodged two foramina medially. The ventral basilar part met cranially forming a synchondrosis with the basi-sphenoid (Fig. 2). These three parts of the occipital bone formed the boundaries of the large foramen magnum (Fig. 4). Dorsocaudally, a single parietal bone formed the middle part of the roof of the cranial cavity. It extended laterally and rostrally to articulate with the squamous part of temporal bone and paired frontal bones respectively (Fig. 1). The interparietal bone was absent. The paired frontal bone (separated by the interfrontal suture) formed the dorso-rostral part of the cranial cavity and major part of the orbital wall. Each frontal bone extended a zygomatic process caudo-laterally that articulated with that of the malar (Fig. 1). They also bore the supraorbital foramen dorsally and articulated rostrally with the paired nasal bones (via the fronto-nasal suture) and lacrimal bones (via the fronto-lacrimal suture) (Fig. 1).
The paired temporal bone formed the ventro-lateral boundaries of the cranium composing of a petrous and squamous part (Fig. 2). The squamous part presented a temporal fossa dorsally and an articular surface ventrally which extends a rostral zygomatic process to the malar. It also presented a caudo–lateral temporal canal caudal to the articular surface for mandibular condyle articulation (Fig. 3). The petrous part was ventrally located and composed of a medial massive tympanic bulla, styloid process and mastoid process (Fig. 2). Between the basilar part of occipital and the tympanic bulla was the position of the rostral lacerated foramen and the caudal jugular foramen (Fig. 2).
The sphenoid bone (Fig. 2) formed part of the ventral surface of the cranial cavity. It continued rostral to the occipital (basilar part) to present a rostral part (pre-sphenoid), a caudal part (basiphenoid), medial vertical wings and lateral horizontal wings (which bore the cranial and caudal large orbitorotundum and oval foramina respectively). The optic foramen opened cranial to the orbitorotundum foramen. The vomer was placed cranial to the presphenoid projecting rostro-medially into the floor of the nasal cavity (Fig. 2).
The ethmoid bone was placed cranial to the cranium projecting the crista galli medio-caudally, perpendicular plate medio-cranially and the cribiform plate laterally. The palatine bone, also located ventrally was composed of a vertical and horizontal part. The vertical part formed the boundary of the posterior choanae while the horizontal part bore the palatine foramen. Placed between the vertical wing of sphenoid and vertical palatine was the pterygoid bone, a small flat thin quadrilateral bone ending ventrally in a Hamulus.
The nasal bone (Fig. 3) was rostral to the frontal bone articulated to it, via the fronto-nasal suture. Rostrally, its ventrolateral border joins with the dorsal border of the maxilla and premaxilla. The paired lacrimal bone formed the caudolateral aspect of the nasal cavity and rostral margin of the orbit (Fig. 3). It presented a triangular outline and articulated rostro-ventrally with the maxilla and malar bone. Its caudal border presented a lacrimal foramen and a bulla further into the orbital cavity. The frontal, nasal and lacrimal bones formed the caudal (serrated), dorsal and ventral borders respectively of a triangular shaped sinus that exposed the dorsal nasal concha.
The malar bone was located on the ventral part of the orbit and lateral to the lacrimal. It projected a caudal zygomatic process and a dorsal orbital process that met with the zygomatic process of the temporal and orbital process of the frontal respectively. Its ventral border articulated with the dorsocaudal border of the maxilla (Fig. 3).
The maxilla (Fig. 3) was located on the latero-ventrally and presented at its ventral margin, alveolar processes and alveoli for dental insertions. Its ventral palatine process formed the rostral part of the hard palate while its dorsal border articulated with the malar, lacrimal and nasal bones via the malo-maxillary, lacrimo-maxillary and naso-maxillary sutures respectively. Rostroventrally to the first premolar, the maxilla presented two (large and small) infraorbital foramina (Fig. 3).
The premaxilla was an irregularly shaped bone articulating with the rostral border of the maxilla. It composed of a body laterally, nasal process dorsally and palatine process ventrally on which the palatine fissure was located (Fig. 1). The three scroll-like (dorsal, middle and ventral) turbinate bones were located on the medial aspect of the maxilla composing of numerous fenestrations (Fig. 5).
The mandible (Fig. 6) presented a v-shaped appearance having two halves joined rostrally to form a symphysis. The body located rostrally, presented dorsal alveoli for insertion of the incisors while bearing a lateral mental foramen. The horizontal rami located caudal to the body presented alveoli on their dorsal margin for insertion of the molars and premolars. The diastema, a space between the incisors and the first premolar was bounded ventrally by the dorsal mandibular crest. The vertical rami projected a curved coronoid process, rostrally and a slightly bent mandibular condyle, caudally. These were separated by a sigmoid notch. The medial aspect of this rami, bore the mandibular foramen.
The Vertebral Column: This comprised of 7 cervical, 13 thoracic, 6 lumbar, 4 sacral and 10–14 caudal vertebrae, giving a formula of C7, T13, L6, S4, C10-14. They all consisted of a body with an arch constructed dorsally to enclose the median vertebral foramen. Emanating from these were the spinous processes dorso-medially, articular processes dorso-laterally and transverse process laterally. Some specific vertebrae bore foramina, and other processes (mammillary and ventral).
The cervical vertebrae formed the skeletal framework of the neck. The first cervical vertebra, the atlas (Fig. 7) presented an almost non-existent body forming a strong ring (enclosed the vertebral foramen) from which a pair of modified and undivided transverse process, the wings projected laterally. Convex and concave articular surfaces of the body featured cranially and caudally, respectively. It also presented dorsal and ventral median tubercles. The dorsal median tubercle was devoid of ridges. Each wing presented a convex dorsal surface and a concave ventral depression, the atlanta fossa. Its cranial aspect presented an inner lateral vertebral foramen and an outer alar foramen.
The second cervical vertebra, the axis (Fig. 8), projected craniomedially, the odontoid process having a dorsal concave and ventral convex surfaces. The lateral vertebral foramen perforated the arches to open into the large medial vertebral foramen. The caudal articular process of the axis projected dorso-laterally from the arch while the transverse process projected only caudo-laterally. The spinous process ran dorsally on the length of the arch forming a crest that hung over the odontoid process.
The 3rd to 6th cervical vertebrae (Figure 9) presented the typical configuration of a vertebra in addition to a transverse foramen placed laterally in between the transverse process and the arch parallel to the body. Their spinous processes increased caudally. The 5th and 6th cervical vertebrae presented a ventral process each, that projected cranially only in the 5th. The 7th cervical vertebra appeared like the thoracic vertebrae as it possessed a longest spinous process than the 3rd to 6th, caudal coastal facet and a rudimentary transverse foramen that was only visible cranially (Figure 9).
The thoracic vertebrae (Fig. 10) formed the dorsum of the thoracic cavity presenting well developed bodies with convex cranial and concave caudal surfaces both bearing coastal facets laterally for head of rib articulation. However, the last thoracic vertebra lacked a caudal coastal facet. The spinous processes increased from T1 to T4, reaching maximum height in the T4. From here, the spinous processes decreased caudally. They were oriented dorso-caudally from T1 to T11. Spinous process of T12 was vertical, while that of T13 was oriented dorso-cranially. These spinous processes increased in length caudally, peaking at T4 before diminishing towards the last. Mammillary processes (between the cranial articular process and transverse process) were present on T1 to T7 (Fig. 11). The cranial and caudal articular processes were not as prominent as those of the cervical vertebrae.
The lumbar vertebrae (Fig. 11) formed the dorsum of the abdomen presenting, in addition to the features of a typical vertebra, long dorso-ventrally flattened transverse processes that curved cranially and lacked the transverse foramen. The length of the transverse process of L1 and L6 were the same, and smaller than the length of those of L2-L5. The cranial articular processes had concave facets that articulated with convex articular facet of the caudal articular process of the preceding vertebrae. Their spinous processes were short, broad, and ended sharply as a crest (Fig. 11).
The sacrum (Fig. 12) composed of 4 fused bones with the first being the largest and featuring similar processes like the lumbar vertebra. Their spinous processes fused to form a median sacral crest. Three dorsal and ventral sacral foramina were identified laterally on each side of the vertebrae.
The caudal vertebrae (Fig. 13) presented a cranial portion having the typical vertebral features that diminished gradually through the series. Only the bodies of the last few caudal vertebrae were evident.
The ribs were 26 in number (13 pairs). The dorsal parts were bony while the ventral parts were cartilaginous presenting surfaces for attachment to the sternum. The proximal extremity of each rib was made up of a cranially located head and neck, and a caudally located tubercle. The cranial border of the shaft was concave while its caudal border was convex in appearance. The lateral and medial surfaces were lined by the coastal groove.
Table 1
Number of bones of the axial skeleton of the sitatunga (Tragelaphus spekii)
Bones
|
Number
|
Skull
|
1
|
Mandible
|
1
|
Sternum
|
5
|
Ribs
|
26 (13 pairs)
|
Cervical vertebra
|
7
|
Thoracic vertebra
|
13
|
Lumbar vertebra
|
6
|
Sacral vertebra
|
4
|
Caudal vertebra
|
10–14
|
Total average:
|
73–77 (Av. 75)
|