Osteology III: Craniofacial bones in lateral view

The bones seen when a tyrannosaurid skull is viewed from the side, © Dino Pulerà.The mesethmoid is not included (see text for explanation). Carbon dust plate by Mr. Dino Pulerà.

Introduction

When the skull is considered as a single functional unit, it is clear that aside from their flexible interfaces with each other, bones as parsed segments are an illusion. However, dinosaurs and all other osteichthyans are descendants of a common ancestor whose head and pharynx were enclosed externally and supported internally by separate bones. Although separate bones are the result of historical constraint, their separateness provides us the opportunity to access the library of evolutionary information they hold.

The goal of this section is to provide a summary of the bones of the craniofacial skeleton that can be seen from the side. The bones are organized by alphabetical order, and their contacts, general structure, and functional significance are described.

Description

Ectopterygoid: The ectopterygoid is a paired palatal bone that extends caudoventrally below the facial skeleton as a stout hook. Laterally it is braced against the medial surface of the jugal; ventrally, medially, and dorsally it joins the pterygoid. In terms of the skull frame, it separates the subtemporal fenestra from the suborbital fenestra. This bone connected the facial skeleton with the pterygoid close to the junction of that bone with the braincase and quadrate. The surface of the ectopterygoid that can be seen below the facial frame is coarsened by an extensive muscle attachment surface, presumably for a slip of the pterygoideus that inserted onto the mandibular ramus.

Frontal: The frontal is a paired bone of the dorsal skull roof, that caudally joins the parietal, caudoventrally the laterosphenoid, laterally the postorbital, rostrolaterally the lacrimal, rostromedially the prefrontal, ventrally the orbitosphenoid, rostroventrally the mesethmoid, and rostrally the nasal. In lateral view it is seen to roof the orbital space that contained the eyeball and its associated neurovasculature and musculature, and that it formed the rostral extent of a large basin (dorsotemporal fossa) from which originated parts of the adductor musculature. Also from the side, it is seen that the frontal anchored the temporal and antorbital regions at a common point above the orbital fenestra; the frontal forms a narrow slip of the fenestra between the postorbital and lacrimal in species where those bones are separate from each other.

Jugal: In lateral view, the jugal is a paired bone that overlaps the maxilla rostroventrally and the lacrimal rostrodorsally; it is overlapped in turn by the lacrimal ahead of the orbital fenestra; caudodorsally it is overlapped by the postorbital, which extends a short distance medial to it behind the orbital fenestra; caudally the jugal is overlapped by a long process from the quadratojugal; medially the jugal abuts the ectopterygoid below the orbital fenestra.

From the side, the jugal is seen to participate in many openings, including much of the ventral margin of the orbitotemporal region, and it extends forward into the caudoventral corner of the antorbital region. Rostrally it forms the caudoventral corner of the antorbital fenestra, dorsally it forms the ventral margin of the orbital fenestra, and caudally most of the rostral and ventral margins of the laterotemporal fenestra. Its position relative to the ectopterygoid shows that it formed the rostrolateral wall of the adductor chamber and the lateral wall of the suborbital space. The jugal extends below the ventral margin of the facial skeleton as a low ornamental bump.

Lacrimal: The lacrimal is a paired bone that separates the orbital fenestra behind from the antorbital fenestra ahead. Dorsally it inserts into the frontal caudomedially, joins the prefrontal caudoventrally, the nasal rostrodorsally, and the maxilla rostroventrally. The ventral end of the bone overlaps the lateral surface of the jugal caudally and is overlapped by it rostrally; its rostromedial surface joins the palatine. The lacrimal forms the rostrodorsal and rostral margins of the orbital fenestra, and the caudodorsal, caudal, and part of the caudoventral margin of the antorbital fenestra.

The lacrimal extends above the dorsal surface of the skull as a prominent bump; its coarse surface is continuous with that of the rugose dorsal surface of the nasal ahead, and with the rough lateral surface of the postorbital behind. Therefore, in additional to its structural necessity in the facial frame, its upper end passively transmitted information to conspecifics regarding relative maturity and species identity.

Maxilla: The maxilla is a paired bone; it is the largest bone of the facial skeleton, when it is seen from the side. It contacts the premaxilla rostrally, the nasal dorsally, the jugal caudoventrally, the lacrimal caudodorsally, the palatine caudomedially, the vomer rostroventromedially, and its complement rostromedially. The maxilla forms the deeply concave rostral margin of the antorbital fenestra, and the rostrolateral margin of the bony choana.

It contains all except four of the teeth contained in the upper jaw, and forms part of the ventral margin of the orbital region. The maxilla is an important part of the facial frame above and below the antorbital fenestra. Above, it contributes to the dorsal skull roof with the lacrimal and nasal, and ventrally the lower bar of the snout with the lacrimal, jugal, and (out of the plane of view) the palatine. The lateral surface of the bone is excavated by the extensive antorbital fossa, which is the large depression that also extends onto the lacrimal and jugal.

Nasal: In lateral view, the nasal forms over half of the antorbital region. In tyrannosaurids it is a single unit bone, but open sutures rostrally and caudally indicate that it develops from separate ossifications that later fuse along the midline. Caudolaterally the nasal contacts the lacrimal, ventrally the maxilla, and rostrally the premaxilla. The nasal contacts the premaxilla at two points, above and below the bony naris, of which it forms its caudomediodorsal and caudolateroventral margins. The complementary nasals are joined along the dorsal midline to receive the caudodorsally extending process from the premaxilla, whereas each nasal separately extends a long process along the rostrodorsal surface of the maxilla to join the corresponding process from the premaxilla to enclose the bony naris. The nasals enclose the airway from above and support a coarse surface externally that was a part of the extensive signaling structure that is largely concentrated along the top of the orbital and antorbital regions of the skull.

Orbitosphenoid: The orbitosphenoid is a small oosification located ahead of the laterosphenoid, below the frontal, and behind the mesethmoid (not pictured because it was not preserved in any of the specimens used to produce the image). Although often described as paired bones, the orbitosphenoid in tyrannosaurids is seen as a single unit structure. The dorsal surface of the orbitosphenoid contributes to the enclosure of a canal that in life extended branches of the olfactory nerve (CN I). The orbitosphenoid was also located above the opening for the branches of the optic nerve (CN II). The orbitosphenoid, like the mesethmoid, can be thought of as ossified proximal portions of the continuous interorbital and internasal septa.

Otoccipital: In lateral view, this large paired bone is largely concealed by the surrounding palate and frame of the facial skeleton. Although its rostrodorsal part can be seen through the laterotemporal fenestra, its distalmost end forms the caudalmost point of the skull, which gives an indication of its true extent and structural importance.

The otoccipital extends medial to the prootic rostrally, the basisphenoid rostroventrally, the squamosal caudodorsolaterally, the quadrate caudodorsally, and the basioccipital ventromedially. Much of the lateral surface of the otoccipital forms part of the medial wall of the middle ear and it is penetrated by the associated pneumatic recesses, receives the stapes, and its caudoventral corner serves as the origin for the mandibular depressor, the muscle that assists gravity in opening the jaw.

Palatine: In lateral view, the palatine is the dominant bone of the palate when seen through the antorbital fenestra. This paired bone articulates caudomedially with the pterygoid, medially with its complement, rostromedially with the vomer, ventrolaterally with the maxilla, and caudolaterally with the lacrimal. Between the maxilla and lacrimal, there may be a contact with the jugal.

The palatine participates in the boundaries of several openings, including the caudal, caudodorsal, and caudoventral margins of the bony choana (internal bony naris), the rostral margin of the suborbital fenestra, and the rostrodorsal margin of the palatine fenestra. The palatine fenestra can be seen in lateral view, between the pterygoid caudodorsally and the palatine rostrally.

The palatine is an important structural component of the craniofacial skeleton, where it forms part of a massive strut that braces the antorbital region transversely (from side-to-side), and also participates in a strut that extends for nearly the entire length of the skull, starting caudally with contralateral flanges that extend rostromedially from each quadrate that continue through the pterygoids before converging at the palatine, extending rostrally through the vomer and finally below the palatal shelf of the maxilla and possibly the premaxilla. The palatine connects the facial skeleton to the palate into a mediolaterally stable frame.

Parietal: The parietal is a single unit bone, and it is the caudalmost bone of the dorsal skull roof. In lateral view, the parietal is saddle shaped, but it is mostly blocked from view by the dorsal temporal bar, except for the dorsally extending rostral extreme of the sagittal crest and the tall nuchal crest that extends above the entire dorsal skull roof. The parietal contacts the frontal rostrally, the postorbital rostrolaterally, the laterosphenoid rostroventrally, the prootic ventrally, the otoccipital caudoventrally, the squamosal ventrolaterally, and the supraoccipital caudally. It also forms the caudal half of the ceiling of the endocranial cavity (the space contains the brain and its surrounding venous and dural structures).

In lateral view, two primary landmarks of the parietal can be seen, namely the sagittal crest and the nuchal crest. The sagittal crest extends from the rostral end of the parietal along the midline to the rostral surface of the nuchal crest, where it fades. The sagittal crest forms the sharp midline boundary between the paired dorotemporal fossae that served as the surface of origin for the adductor (jaw closing) muscles.

The nuchal crest is the stable bony interface that separates the adductor musculature rostrally from the cervicocranial musculature caudally. The latter muscles extend from the vertebrae caudally onto the occipital surface of the skull rostrally. In lateral view, the surface of the nuchal crest that can be seen is a part of the dorsotemporal fossa; neck muscles would have inserted on the concave caudal surface that cannot be seen from the side, except for a marginal insertion surface along its caudolateral edge.

Parabasisphenoid: The parabasisphenoid is a single unit bone that is one of the largest bones of the braincase; however, very little of it can be seen in lateral view aside from a slip of its caudoventral corner between the quadrate and jugal within the laterotemporal fenestra, and nearly the entirety of the sphenoid rostrum within the orbital fenestra. The parabasisphenoid contacts the laterosphenoid rostrodorsally, the prootic dorsally, and the otoccipital caudally. This bone forms the ventral and rostral margins of the basicranium (the region below the endocranial cavity).

The slip that can be seen through the laterotemporal fenestra pertains to the caudoventral margin of the basicranium between the basal tuber caudodorsally and the basipterygoid process rostroventrally. The structures are blocked from view by the quadrate and jugal, respectively.

The sphenoid rostrum is a midline structure that is excavated by a concavity caudodorsally, which is called the Turkish saddle (sella turcica) that lodged part of the pituitary gland from behind and below. The rostrum extends rostroventrally into the orbital space. Ahead of the saddle, a long furrow excavates the entire dorsal surface of the rostrum to its tip, whereas its ventral surface is convex and smooth. The cartilaginous interorbital septum almost certainly fit ventrally into the rostrum and was stabilized from above by the orbitosphenoid, mesethmoid, frontal, and nasal. If so, then the septum was much wider dorsally than it was ventrally.

Postorbital: The postorbital is a paired bone that is triradiate in shape, which sends a process rostrally that articulates with the frontal rostromedially, the parietal caudally, and the laterophenoid caudoventrally; it sends a process caudally that fits into a deep and narrow slot in the lateral surface of the squamosal to complete the dorsal temporal bar; and it sends a process ventrally that fits onto the rostrolateral surface of the jugal to complete the postorbital bar. In some tyrannosaurines the rostral process of postorbital reaches far enough forward to contact the lacrimal.

In lateral view, the postorbital separates the laterotemporal and orbital fenestrae, where it forms the rostrodorsal corner, and the caudodorsal and caudal margins of the openings, respectively. The lateral surface that extends between the ventral and rostral processes carries an ornamental structure, the cornual process of the postorbital. This is seen in all derived tyrannosauroids, except in the least mature specimens. In Albertosaurus libratus, as pictured here, the process it situated along the orbital fenestra and along the ventrolateral edge of the rostral process. In other tyrannosauroids, this ornament is much larger and it is situated caudal to the margin of the fenestra. The lateral surface of the ventral process has a coarse texture, which is continuous with texture that also covers the entire antorbital region.

The postorbital forms the dorsal margin of the skull between the squamosal caudally and the frontal rostrally (lacrimal in some tyrannosaurines). The rostral half of the bone flanks the orbital space, whereas it caudal half flanks the adductor chamber. Also, the postorbital is limited ventrally by the jugal to approximately the midheight of the skull frame, where it stops above the level of the tooth-bearing regions of the maxilla and premaxilla.

The ventral edge of the rostral process and the concave margin between the rostral and ventral processes form the caudodorsal margin of the orbit proper (the part that actually surrounded the eyeball in life). The irregular and vertically oriented rostroventral margin of the ventral process flanked the space situated below the eyeball. Pterygoideus musculature that originated from the palatine and extended caudally and ventrally over the ectopterygoid to insert onto the lower jaw, occupied this space; this group of muscles would have almost certainly provided speed to the bite, as is seen in modern crocodilians.

Prefrontal: In lateral view, the prefrontal is a small triangular wedge of bone situated at the rostrodorsal corner of the orbital fenestra; it is a paried bone. Caudally it inserts into the frontal and rostroventrally it fits into a groove in the caudomediodorsal surface of the lacrimal. The caudoventral edge of the bone forms part of a strut that extends rostrolaterally from the cranial crest (crista cranii) of the frontal to the orbitonasal ridge of the lacrimal. Regionally, this part of the prefrontal separates the orbital space caudolaterally from the nasal airway rostromedially. The prefrontal is situated below the rostrolateral surface of the frontal, and so it does not reach the midline of the skull. The prefrontal is a deep structure that does not contribute to the lateral surface of the face or to the margin of the orbital fenestra.

Premaxilla: The premaxilla is a paired midline bone that surrounds the bony naris rostrodorsally, rostrally, and rostroventrally; it contacts the nasal caudodorsally, caudally, and the maxilla caudoventrally. The premaxilla also contains the mesialmost part of the tooth row. The contact between the premaxilla and the maxilla is separated into dorsal and ventral parts by the subnarial foramen.

In lateral view, the caudodorsal surface of the premaxilla is flattened, smoothed, and excavated by the narial fossa. The coarse subcutaneous surface covers the rostral surface of the bone, which extends caudoventrally below the subnarial foramen onto the maxilla, and dorsally along the bony naris onto the nasal. Large foramina penetrate the subcutaneous surface and narial fossa, a rostral continuation of the row of foramina that extends along the ventral margin of the maxilla.

Prootic: The prootic is an extensive paired bone that is situated on the lateral surface of the braincase, and much of it can be seen through the laterotemporal fenestra. The prootic contacts the laterosphenoid rostrally, the parabasisphenoid rostroventrally, the otoccipital caudoventrally and caudally, the squamosal caudodorsally, and the parietal rostrodorsally. The prootic overlaps the lateral surface of the otoccipital and a gap separates them at the rostral end of their contact.

The prootic is a structurally important bone, where in lateral view it surrounds the rostrodorsal part of the middle ear, forms part of the bony boundary between the dorsotemporal fossa and the middle ear, and between the middle ear and the orbital space. The rostral end of the auditory (hearing) ossicle, the stapes, is lodged between the otoccipital medially and the prootic laterally.

Pterygoid: In lateral view it can be seen that the paired pterygoid is one of the longest bones of the skull, and the longest of the palate. The pterygoid is represented by dashed lines in the carbon dust plate of A. libratus, because I did not have a specimen of a pterygoid available to me at the time I drafted the line drawing, some time between 1996 and 1998.

The pterygoid contacts the quadrate caudodorsally, the parabasisphenoid caudomedially, the ectoptergyoid laterally, its complement rostromedially, the palatine rostrolaterally, and the palatine and vomer rostrodorsally. In lateral view, the pterygoid is seen to form the caudodorsal boundary of the palatine fenestra.

Quadrate: In lateral view, the ventral and rostral parts of the paired quadrate can be seen. The quadrate contacts the squamosal dorsolaterally, the otoccipital dorsomedially, the quadratojugal caudodorsally and caudoventrally, the pterygoid rostrolaterally, and ventrally the mandibular ramus (surangular laterally and articular medially).

When seen from the side, the long and deep orbital process can be seen extending through the adductor chamber. The orbital process would have been positioned deep (medial) to the adductor musculature. At the same time, the process bounded the middle ear space laterally, in life partly shielding its contents from the bulging contractions of the jaw closing muscles. Also, the orbital ramus has an extensive articulation with pterygoid (not shown here, for the reason given above). As such, this part of the quadrate is a functional and structural part of the palate.

The ventrally extending part of the quadrate is the mandibular process; this structure is separated by a groove into a lateral and medial condyle. A ridge formed by the articular fits into the groove between the condyles. The groove extends rostromedially, which constrained the excursion of the mandibular ramus upon opening and closure. As a result, the high mediolateral forces that occurred at this joint during the capture and rendering of prey were prevented from dislocating the jaw.

Quadratojugal: The quadratojugal is the paired L-shaped bone that forms the caudoventral corner of the facial skeleton. This bone forms the caudoventral margin of the skull, a shallow concavity that in life was situated ahead of the ear canal (external auditory meatus). The quadratojugal also forms the ventral margin of the skull for a short distance ahead of the jaw joint; the bone also completes the caudoventral corner of the laterotemporal fenestra. The quadratojugal overlaps all of its surrounding bones: the squamosal dorsally, the quadrate caudodorsally and caudoventrally, and the jugal rostroventrally. The squamosal and jugal are overlapped in the vertical and rostrocaudal planes, whereas the quadrate is overlapped in the mediolateral and vertical planes. Also, a blade-like process from the quadrate extends laterally through a slot at the caudodorsal corner of the bone.

The quadratojugal is composed of two primary processes: a dorsal process that expands rostrally toward the squamosal and a long rostral process that covers the lateral surface of the jugal. The dorsal process and squamosal together form a wide flange that nearly cuts the laterotemporal fenestra in two. The quadratojugal and jugal form the lower temporal bar. Based on its position and contacts, the quadratojugal helps to stabilize the jaw joint between facial skeleton laterally and dorsally, and the braincase dorsomedially.

Squamosal: In lateral view, the squamosal is a complex paired bone that can be divided into three processes, including a rostral process that forms most of the upper temporal bar and receives the postorbital on its lateral surface, a caudal process that overlaps the otoccipital, and a rostroventral process that extends along the top of the quadratojugal into the laterotemporal fenestra.  The squamosal contacts the postorbital rostrally, the parietal, otoccipital, and prootic medially, the otocciptial caudally, the quadrate ventrally, and the quadratojugal rostroventrally.

The squamosal is extensive, where it forms the entire caudodorsal corner of the craniofacial skeleton, including its lateral, caudal, and dorsal surfaces. Although it is adjacent to the adductor chamber, the internal surface of the bone almost certainly was not occupied by musculature; instead, it appears that it was a part of the middle ear cavity. The squamosal forms the tallest part of the temporal region and extends caudoventrally along the upper temporal bar to the otoccipital. Together these bones form a process that extends caudally above the region of the external ear opening.

The squamosal is a structurally important bone in that it stabilized the top of the quadrate against itself and the lateral surface of the braincase. It also completed the connection between the lower and upper temporal bars, and enclosed the adductor and middle ear spaces caudodorsally.

Vomer: In lateral view, the single unit and midline vomer is seen through the antorbital fenestra, where it extends rostrally from the medial surface of the palatine to the ventral surface of the palatal shelf of the maxilla; caudally the voner contacts the pteryoid, a contact that is concealed by the palatine. The vomer forms most of the dorsomedial margin of the bony choana and it forms the ventral bony part of the internasal septum with the palatine.

The vomer completes the axial strut that begins caudally with the widely separate quadrates and rostrally-converging pterygoids. The vomer would have helped to resist dorsoflexion (extension) of the antorbital region.