THE CRANIO-CERVICAL SKELETON OF EMYDURA SUBGLOBOSA

The skull of Emydura subglobosa is twice as long as high. The maximum skull width is between the squamosal wings, which participate in the formation of the otic capsule. The snout region is round in frontal view (Figure 17.2), and the posterior part of the skull is dorsoventrally flattened (Figure 17.4). The adductor (mandibulae) chamber is bordered by the orbital region anteriorly, the lower jaw and the pterygoid ventrally, by the otic capsule posteroventrally, and by a bony bridge – mainly formed by the squamosal – laterally in the temporal region. Separated by the otic capsule, the adductor chamber is formed by two regions, an anteroventral postorbital cavity, and a posterodorsal temporal cavity (Figure 18.3). In the fossa orbitalis, 13 scleral bones are present (Figure 17.1, Werneburg et al. 2009). As in all Testudines (Mickoleit 2004), eight cervical vertebrae exist (Figure 18.3). Posteroventral of the first cervical vertebra (atlas), a separated "processus odontoideus" (sensu Ogushi 1911, see also Kasper 1903) is visible. It represents the untercentrum of the atlas (C1' of Herrel et al. 2008; Hoffstetter and Gasc 1969).

Neurocranial Elements

Basisphenoid. The unpaired and flat basisphenoid is situated posteroventrally to the eye region and has an oval shape that is broader than long. Anteriorly, it contacts the pterygoids; anterolaterally, it contacts the prootics, and posteriorly, it comes in contact to the basioccipital (Figure 18.1-2).

Basioccipital. The unpaired basioccipital has an oval shape that is broader than long. Anteriorly, it contacts the basisphenoid. Laterally, it has a narrow connection to the opisthotic and posteromedially, it contacts the exoccipitals each side. Posteromedially, it forms a broad processus that forms the lower part of the condylus occipitalis (Figure 18.1-2, Figure 27.2). Dorsally, this processus meets the condylus processi of both exoccipitals.

Prootic. Together with the quadrate, the prootic forms the posteroventral margin of the whole adductor chamber. Anteriorly and anterodorsally, it sutures with the parietal. It sutures with the quadrate ventrolaterally. In its posterior region, it contacts the opisthotic, and posteromedially it contacts the supraoccipital. Posterolaterally, the prootic meets the squamosal and ventromedially, it meets the basisphenoid. Anteroventrally, it participates to the formation of the foramen nervi trigemini.

Opisthotic. The opisthotic forms the posteroventral margin of the temporal cavity. Anteriorly, it contacts the prootic, laterally the squamosal, dorsomedially the supraoccipital, posteromedially the exoccipital, anteroventrally the basisphenoid, and posteroventrally the basioccipital.

Exoccipital. Except for the dorsalmost part – which is formed by the supraoccipitals – the exoccipitals form the surrounding margin of foramen magnum. Dorsally, the exoccipital contacts the supraoccipital, and laterally it contacts the opisthotic. Ventrally, it forms a posterior processus that forms the dorsolateral part of the condylus occipitalis. This processus meets the basioccipital ventrally (Figure 18.1-2, Figure 27.2).

Supraoccipital. The unpaired supraoccipital posteroventrally forms the dorsal most margin of the foramen magnum. Anterodorsally, it contacts to the parietals in their posteromedial region, and in dorsal view, the supraoccipital is wedge-shaped. On both sides, the bone anteriorly contacts the vertical plates of the parietals and anteroventrally it contacts the prootics. Posteroventrally, it contacts the dorsal parts of the opisthotic and the exoccipitals. The crista supraoccipitalis is not elongated posteriorly and in lateral view, the supraoccipital and squamosal form a consistent curvature (Figure 17.1).

Splanchnocranial Elements

First Pharyngeal Arch: Palatoquadrate Element

Quadrate. The quadrate forms the dorsal, ventral, anterior, and medial wall of the otic capsule. Laterally, the quadrate forms an anterior and a ventral vaulting that results in a concave, chalice like shape of the otic capsule. In its posteroventral region, the quadrate is separated by a groove that continues mediad along the curvature of the quadrate and forms the incisura columellae auris; herein, the columella auris is situated. Posterodorsally, the quadrate has a broad circular contact area with the squamosal. Anteromedially and dorsomedially, it meets the prootic and posteromedially, it meets the opisthotic. Anteroventral of its medial face, it meets the posterolateral most edge of the pterygoid. Together with the prootic, the quadrate forms the posteroventral margin of the whole adductor chamber.

First Pharyngeal Arch: Mandibular Elements

Cartilago meckeli. The cartilago meckeli is situated in the primordial channel of the lower jaw, which is formed mainly by the medial face of the dentary, but also partly by prearticular and angular.

Articular. The articular forms the posterior most part of the lower jaw, and it articulates with the anteroventral curvature of the quadrate. Therefore, it forms a depression posterodorsally. Anteriorly, it contacts the dentary, anterodorsally, and laterally the surangular, ventrally the angular, and anteromedially and medially the prearticular.

Second Pharyngeal Arch: Hyomandibular Elements

Columella auris. The columella auris extends into the cavum tympani of the otic capsule as a bar-shaped bone (Figure 30.4). It passes through the incisura columellae auris that is formed by the quadrate. Laterally, it connects with the extracolumella. Within the pericapsular recess of the inner ear (Wever 1978), the bar-shaped columella broadens as a disk that attaches the cochlear duct.

Extracolumella. The extracolumella (Wever 1978) is formed as an oval disk-shaped cartilage that connects laterally to the tympanic membrane (Figure 22.5, Figure 23.3, Figure 29.1, Figure 29.3, Figure 30.2-3). Medially, in the centre of the disk, it forms a process that connects to the columella auris.

Second Pharyngeal Arch: Hyal Element

Cornu hyale. The short-bared cartilaginous cornu hyale is sitated below the suture of palatine and pterygoid in the closed mouth. Proximally, it articulates to the lateral process of anterior region of corpus hyoidei (Figure 22.4, Figure 23.2).

Second to Fourth Pharyngeal Arch: Basibranchial Elements

Corpus hyoidei. In the studied specimens, the flat unpaired corpus hyoidei is ossified only in its posterior region, and the ossification has a convex shape (Figure 18.1-2, Figure 19). Together with the corni hyale, the corni branchial, and the hypoglossum, it forms the hard tissue base of the mouth cavity. It is situated ventrally to the basisphenoid and pterygoid. Anteriorly, the cartilaginous region of corpus hyoidei forms the processi lateralis anterior that articulate to the corni hyale. Between those processi, the oval fenestra ossis hyoidis is formed, having an anterior-posterior extension. In the anterior cartilaginous region, posterior to the articulation with the corni hyale, the corpus hyoidei articulates directly with the cornu branchial-I; only a very small processus lateralis intermedius is suggestively recognisable. In the posterior region, lateral to the ossification centre, the corpus hyoidei laterally articulates with the corni branchial-II at the cartilaginous processus lateralis posterior. Anterodorsally, the corpus hyoidis forms an unpaired medial processus lingualis that is situated above the hypoglossum.

Third and Fourth Pharyngeal Arch: Branchial Elements

Cornu branchial-I. The well ossified cornu branchial-I is situated below the pterygoid and the otic capsule. It articulates medially with the medial part of the corpus hyoidei. When the mouth is closed, its distal – corpus hyoidei opposing – part extends caudad beyond the skull up to the level of the first cervical vertebra. Here it is connected to the posteriormost edge of the squamosal via a short and strong tendon.

Cornu branchial-II. In subadult specimens, the cornu branchial-II is still partly cartilaginous ((Figure 19, Figure 20, Figure 21). It is situated below the otic capsule and it reaches the anterior neck region ventromedially. Medially, it articulates with the posterior part of corpus hyoidei. As for cornu branchial-I, the distal part of the element extends caudad shortly beyond the posterior border of the skull.

Skeletal Derivates of the Posterior Pharyngeal Arches

Cartilago thyreoidea. Dorsal to the fenestra ossis hyoidis of the corpus hyoidei, the tubular larynx is situated. It is formed by three cartilages that are very similar in shape when compared to those referred for E. krefftii (Siebenrock 1899). The short tube-like cartilago thyreoidea is the largest element of the laryngeal apparatus, and it is built of about five partly fused tracheal rings (Figure 19). Its dorsal region remains open, forming a longitudinal groove, the rima glottidis (Figure 25.1). It is closed in its posterior most part only. Posteriorly, it contacts the trachea. From its middle to anterior region, it is truncated from dorsal by the adjacent cartilagi arytaenoidea, resulting in an acuminated anteroventral tip. In the medial region of its dorsal groove, cartilago thyreoidea encloses the cartilago cricoidea and contacts it laterally.

Cartilago arytaenoidea. Dorsally to the cartilago thyreoidea, the paired triradiate cartilagi arytaenoidea are situated. Each of them has a short lateral processus muscularis in its anterior half. Ventrally, the cartilage proceed in a processus vocalis that directly contacts the anterior truncation of cartilago thyreoidea. The caudad-leading processi articularis of the contralateral cartilagi arytaenoidea do not contact each other, but their posterior tips contact the cartilago cricoidea.

Cartilago cricoidea. This very small oval and unpaired cartilage is situated within the medial part of rima glottitis. It contacts the cartilago thyreoidea laterally each side and the cartilagi arytaenoidea anterolaterally.

Trachea.  In the subadult, at least four tracheal rings show ossifications that may be descendens of posterior pharyngeal ("gill") arches (Figure 19).

Dermatocranium

Skull Roof

Nasal. The nasal is a rectangular element and about half the length of the prefrontal. Compared to all other Emydura species described or depicted (Schumacher 1954-55b; Gaffney 1975, 1979), the dorsoventral altitude is more flattened. The contralateral nasals meet each other anteriorly in a medial suture. Posteromedially, the nasal contacts a narrow anterior process of the frontal, and posteriorly it sutures the prefrontal.

Prefrontal. The dorsal plate of the prefrontal is small and rectangular. Its posterior most part forms a part of the dorsal margin of the orbit. The larger ventral plate of the bone extends ventrally and forms the anterior wall of the fossa orbitalis. Anteriorly and posteriorly, the prefrontal meets along transverse sutures with the nasal and frontal, respectively. Ventrally, it contacts the medial plate of the premaxilla. Dorsally, the prefrontals do not meet each other, and they are medially separated by anterior processi of the frontals.

Frontal. The frontal is an elongated flat triangular bone that forms the major part of the dorsal margin of the fossa orbitalis, and it is three times as long as the prefrontal. The contralateral frontals are sutured medially. Posterolaterally, they are in contact with the postorbitals and posteriorly with the parietals. In its anterior region, the frontal forms a median processus that is laterally bordered by the prefrontal and anteriorly by the nasal. Laterally, the frontal forms the slightly vaulted dorsal rim of the orbit.

Parietal. The parietal consists of two plates. Dorsally, the flat horizontal plate is twice as long as the frontal, it has a rectangular shape and forms the major part of the skull roof. This dorsal plate contacts the frontal anteriorly, the postorbital anterolaterally, the supraoccipital posteromedially, and the squamosal posterolaterally. Ventrolateral of the horizontal plate, the vertical plate of the parietal forms the lateral wall of the braincase. Anteriorly, the vertical part of the parietal contacts the postorbital, posteriorly the supraoccipital, and posteroventrally the prootic. Ventrally, it continues as a concave, mediad leading bone lamella that contacts the pterygoid dorsally. Posterior to the suture with the pterygoid and ventral to the suture with the prootic, the vertical plate of the parietal contributes to the formation of the foramen nervi trigemini.

Temporal Region

Postorbital. The postorbital contacts the parietal posterodorsally, the frontal anterodorsally, and the jugal ventrolaterally. It forms the posterodorsal wall of the fossa orbitalis and the anterodorsal border of the adductor chamber. A ventral-bar shaped part of the bone ventrally contacts the processus trochlearis of the pterygoid at its anterodorsal edge.

Jugal. In lateral view, the jugal is a bar-shaped bone with a lateral, convex curvature. It forms the major part of the posterior rim of the fossa orbitalis and the anterior border of the adductor chamber. Dorsally, it contacts the postorbital, ventrally the posterodorsal part of the maxilla. Ventromedially, the jugal forms a triangular processus, which dorsally contacts the postorbital and ventrally the maxilla. Ventrally of the postorbital, a tip of the jugal contacts the processus trochlearis pterygoidei. In E. subglobosa, as in other Emydura species, a quadratojugal is missing, and no zygomatic arch is formed.

Squamosal. As a vaulted posterodorsal part of the otic capsule, the anterior part of the squamosal contacts the quadrate at its posterodorsal region. Anteriorly, it contacts the prootic. Posteriorly and medially, it sutures the opisthotic. Dorsolaterally, a bone lamella is developed that extends dorsomediad in a convex course. The lamella narrows and contacts a short bone lamella formed by the posterolateral part of the parietal (horizontal plate).

Palatal Elements

The unpaired upper ramphotheca forms the anterior most tip of the head and covers most parts of the premaxilla and the maxilla.

Premaxilla. The triangular premaxilla forms a part of the floor of the apertura narium externa (internal bony opening of the nares). Anteriorly, ventrally, and laterally, it is covered by the upper ramphotheca. Anteriorly, it forms a ventral ridge forming the anterior border of the mouth cavity. Laterally, it contacts the maxilla in a broad suture; medially, it sutures the contralateral premaxilla.

Maxilla. The maxilla has a lunate shape in lateral view, and it is laterally mostly covered by the upper ramphotheca. Its vertical arm contacts the nasal dorsally and the prefrontal posteriorly. The horizontal arm meets the jugal and the palatine posteriorly and forms a medial bone plate that anterolaterally contacts the premaxilla, the contralateral maxilla, and the vomer. Posterolaterally, this bone plate contacts the palatine, and in its medial region, it forms a part of the border of foramen orbito-nasale.

Vomer. The unpaired, bar-shaped vomer borders the foramen interorbitale ventrally. Anteriorly, it contacts the maxillae. Lateral to its anterior half, the foramen orbito-nasale is formed. In its posterior half, the vomer is thickened and laterally it contacts the palatines. Its posterior most tip reaches a point, where also the contralateral palatines and pterygoids meet.

Palatine. The triangular palatine forms the lateral border of the foramen orbito-nasale and the major part of the floor of fossa orbitalis. The palatine contacts the maxilla laterally, the vomer posteromedially and the pterygoid posteriorly. Posterolaterally, the palatine forms a short ascending process that contacts the jugal at its ventral tip.

Pterygoid. The flat and rectangular pterygoid builds the major part of the skull floor and is anteriorly connected to the palatine. At its anterolateral edge, it is connected to the prootic, jugal, and maxilla. Medially, it sutures with the contralateral pterygoid for a long distance. Posteriorly, it meets the oval basisphenoid. Posterolaterally, the bone is elongated and reaches the medial face of the quadrate at its anteroventral side. Together with the prootic, pterygoid, and parietal, it encloses the foramen nervi trigemini. The lateral part of the pterygoid vaults dorsolaterad and forms the processus trochlearis pterygoidei.

Lower Jaw Elements

Dentary. The unpaired lower ramphotheca covers most of the dentary, and only the posterior and posterodorsal parts of the dentary are visible in lateral view. The dentary is the largest bone of the lower jaw and contacts the coronoid posterodorsally, the surangular posterolaterally, the angular posteroventrally, the prearticular posteromedially, and the articular posteriorly. The contralateral dentaries are fused in an anteromedial symphysis.

Surangular. The surangular forms a major part of the lower margin of the adductor chamber, and it is situated at the posterodorsal part of the lower jaw. Anterodorsally, it sutures with the coronoid, anteroventrally with the dentary, posteromedially with the articular, and ventromedially with the prearticular.

Prearticular. It is situated medially in the lower jaw and contacts the dentary anteriorly, the angular ventrally, the articular laterally and posterolaterally, the surangular dorsolaterally, and the coronoid posteroventrally.

Coronoid. The coronoid forms the dorsal most curvature of the lower jaw and contacts the dentary anteriorly and ventrally, the surangular posteroventrally, and in its posterior part the prearticular ventromedially.

Angular. Together with the dentary, the angular forms the ventral margin of the lower jaw. Anteriorly and laterally, it contacts the dentary, dorsally the prearticular, and posteriorly and posterodorsally the articular.

Additional Hard Tissue Elements in the Head

Hypoglossum. The flat, unpaired, and cartilaginous hypoglossum ("entoglossum" in several studies, e.g., Gräper 1932) is mainly situated below the anterior part of the corpus hyoidei, and it covers most of the fenestra ossis hyoidis ventrally. Only the anterior and anterolateral part extend below the corpus hyoidei. In ventral view, it has an anterior-posterior compressed, rectangular shape.

Cartilago transiliens. Short before its insertion to the lower jaw, the coronar aponeurosis forms the internal cartilago transiliens that articulates with the processus trochlearis pterygoidei ventrally.

Cervical Vertebrae

E. subglobosa has eight cervical vertebrae (Figure 18.3). Dorsally of the centra, they proceed in a neural arch, that anterolaterally and posterolaterally forms zygophyses on each side (Herrel et al. 2008). Laterally, the vertebrae proceed in a transverse process each side. Among each other, the cervical vertebrae articulate over the anterior and posterior articular facet of the centrum. Posterior and anterior zygapophyses of two following vertebrae articulate over their articular facet. Anteriorly, the articular facet of the first vertebra centrum articulates with the cranium at the condylus occipitalis. Posteroventral of the first cervical vertebra, the intercentrum of the atlas is situated that is not fused with the remainder of the atlas. The eighth cervical vertebra articulates posteriorly with the first dorsal vertebra, which is connected to the carapace and the first rib.