Osteology VI: Craniomandibular skeleton in ventral view

The skull and jaws of a juvenile Tyrannosaurus rex (CMNH 7541) seen in ventral view, © Dino Pulerà. The mandibular bones are labeled on the left, whereas the craniofacial bones are labeled on the right. The septum between the basiphenoid pneumatic foramina has been altered from the original specimen to show its correct midline orientation, which in the specimen is displaced to the side. Uniform gray shows regions restored with plaster and paired hatch marks on grey indicate matrix. This is a colorized version of the original carbon dust plate that first appeared in Carr (1999).

Introduction

Although incompletely prepared, this specimen provides an excellent view of the relationships between the bones of the skull and jaws when they are in occlusion. It is worth noting that the caudal region of the skull – including the braincase and suspensorium – has been displaced rostrally. For example, this has shifted the basisphenoid unnaturally ahead relative to the pterygoids, and the basioccipital ahead of the otoccipitals.

In this specimen, the relationship between the skull and closed jaws can clearly be seen. For example, the region of the greatest depth of the jaws (where the prearticular and splenial meet) is positioned below the medial edge of the midlength of the palatine. It is seen that the palatal process of the maxilla extends medial to the mandible, which is consistent with the position of depressions that received the teeth along the medial alveolar process of the maxilla. Also, the dorsal edge of the mandible extended within a wide slot formed by the jugal laterally and the ectopterygoid medially along its course to the jaw joint. Finally, the slip of bone lateral to the surangular and angular is the ventral edge of the jugal.

Description

SKULL BONES

Basioccipital: In ventral view, the basioccipital is seen situated below the otoccipital and caudal to the basisphenoid. It forms the caudoventral region of the occipital condyle, the ventral part of the neck of the occipital condyle, the caudoventral region of the basicranium, the caudoventral edge of the basicranium, and the midregion of the caudal wall of the basisphenoid recess. The basioccipital is positioned between the otoccipital and basisphenoid, except for the caudodorsal region that culminates in the occipital condyle. The basioccipital does not contribute to the caudoventrolateral corner of the basicranium; instead, the otoccipital and basisphenoid form the corner where they contact each other distal to the basioccipital. The external surface of the basioccipital occupies four planes: caudoventral at the occipital condyle, caudoventral below the occipital condyle, ventral between the basal tubera, and rostral at the basisphenoid recess.

In ventral view, the basal tuber can be seen, and the prominent ascending scar blocks the region of the subcondylar fossa from view. The extensive muscle attachment surface below the occipital condyle dominates the basioccipital. It can also be seen that the funnel-like ceiling of the basisphenoid recess produces a concave surface in the rostral surface of the bone.

Basisphenoid: In ventral view, the basisphenoid is broadly exposed to view, especially the basisphenoid recess and its contributions to the basal tuber complex. The bone contacts the basioccipital caudally, the otoccipital caudodorsolaterally, and the pterygoid rostroventrolaterally. With the otoccipital the bone forms the caudoventrolateral corner of the basicranium. Although somewhat difficult to determine from this image, the basisphenoid extends steeply rostroventrally from the basal tuber complex to the basipterygoid process. As noted, the caudal end of this skull is displaced rostrally; in its natural position, the basipterygoid process would be situated above – not rostral to – the stout, caudally- extending process from the pterygoid.

Ectopterygoid: Qlthough the ectopterygoid is often pictured lying flat, in ventral view its true orientation is seen, where it extends steeply rostrodorsally. This position accounts for its appearance in lateral view below the jugal as a prominent hook-like process. Although concealed by matrix, the distal end of the thin and rostrodorsally extending jugal process is apposed to the medial surface of the jugal at the caudal end of the ventral cornual process of that bone. The caudomedial and caudal edges of the ectopterygoid have an extensive overlapping contact with the pterygoid. The bone is in apposition medially with a stout lateral process of the pterygoid.

Maxilla: In this view, only the rostral part of the maxilla is in view, where it extends lateral to the dentary; tts contacts with other facial bones cannot be seen. With the dorsal processes of the palatines and the vomer concealed by matrix, it can be seen that the maxilla can be considered as a part of the  palate. The palatal process (=shelf) of the maxilla is completed caudomedially by the palatine, which continues this flat and ventromedially facing surface caudally to the pterygoid.

Otoccipital: In ventral view, the otoccipital is tilted rostroventrally at a steep angle. The bone articulates ventrally with the basioccipital for most of its width medially, and with a slip of the basisphenoid laterally. The otoccipital forms the dorsolateral part of the neck of the occipital condyle, and that of the condyle itself. In this specimen, the basioccipital is displaced the rostrally relative to the otoccipital, exposing the joint surface for the basioccipital to view, especially on the right side. The otoccipital extends caudodorsolaterally, until the articular bone of the lower jaw conceals its lateral end from view. As the otoccipital extends laterally, it becomes progressively vertically (caudally) oriented such that it appears to taper along its course. In contrast, the ventromedial corner of the bone does taper to a point as it extends toward the basal tuber.

Palatine: In ventral view, most of the pterygoid process and the rostral end of its maxillary process of the palatine can be seen although the bone is largely concealed by the mandibular ramus and matrix. The palatine contacts the pterygoid caudomedially and it extends along the maxilla rostromedially. The pterygoid process is wide, rodlike, and it extends caudomedially, where it is underlapped by the pterygoid. The palatine sends a small slip along the rostrolateral edge of the pterygoid; otherwise it is completely covered by that bone. The pterygoid is positioned distally on the process; i.e., the pterygoid does not reach the intersection between the three primary processes (vomeropterygoid, maxillary, pterygoid) of the palatine.

In contrast, the maxillay process in this view is a narrow and rostromedially tapering slip that extends along the medial edge of the palatal process of the maxilla. Therefore, the palatine contributes to the caudomedial edge of the palatal shelf.

On the midline, the complementary pterygoid and maxillary processes surround a relatively wide space that separates the two bones. This stands out here because the dorsally extending vomeropterygoid processes of the bone are not prepared to view. If they were, it would be seen that they would have extended toward each other and made contact dorsally, with their rostral margins defining the caudal edge of the bony choana.

Pterygoid: In ventral view, the pterygoid is a complex bone that divides from a common point into four processes, including the quadrate process that extends caudodorsolaterally, the basisphenoid process that extends caudolaterally, the ectopterygoid process that extends laterally, and the main body of the bone that extends rostrally.

The complementary bodies of the bone are separated by a long interpterygoid vacuity, which extends rostrally between the palatines and maxillae. The body of the bone is a long structure that extends ahead of the lateral process to underlap the caudoventral surface of the pterygoid process of the palatine.

The lateral process underlaps the caudoventral surface of the ectopterygoid, and twists along its long axis into a steep caudoventral to rostrodorsal orientation. The ventral surface between the lateral process and the body is deeply concave; it is possible that this fossa is pneumatic and associated with the large ectopterygoid pneumatic recess.

The basisphenoid process is the shortest of the processes, which extends slightly caudolaterally to underlap the basipterygoid process of the basisphenoid at a very loose basal joint. The lateral process extends ventrolaterally relative to this process. In contrast, the quadrate process extends steeply rostrodorsolaterally relative to the short process. In this view, only the ventral edge of the dorsal process can be seen, extending along the ventral surface of the orbital process of the quadrate.

Quadrate: In ventral view, the quadrate is a narrow rostromedially extending strut that overlaps the tip of the dorsal process of the quadrate. In part, this is an artifact of the sediment-filled spaces in the skull.

MANDIBULAR BONES

Angular: In ventral view, the angular is a dominant bone of the caudal half of the ventrolateral and ventral surfaces of the mandibular ramus. It contacts the surangular caudomedially, the dentary rostrolaterally, the prearticular dorsally and medially, and the splenial rostromedially.

Caudally it overlaps the lateral surface of the surangular, which separates the angular from the ventral margin of the bone in this region; i.e., the caudal tip of the angular extends caudodorsally onto the lateral surface of the surangular. Rostrally the angular extends along the prearticular before it continues medially onto the ventral surface of that bone, where it forms the ventral margin of the mandibular ramus. The angular then extends rostrodorsally out of view between the dentary and splenial. The angular curves laterally along its course.

Articular: The articular forms the caudomedial end of the mandibular ramus, and it is the widest part of the structure. In ventral view, the articular contacts the prearticular rostromedially and the surangular rostrolaterally. The suture with the prearticular is not easily seen in this specimen, so the placement and course shown here is approximate. The articular is relatively widely exposed in ventral view and it extends the mandibular ramus medially, where it constricts the caudal region of the nasopharynx down to the width of the snout. Together, the articular and prearticular form a prominent medial bar that extends below the Meckelian fossa.

Dentary: In ventral view, the dentary is the longest bone of the mandibular ramus, where it contacts the angular caudomedially and the splenial medially. The dentary follows the curvature of the snout, where its caudal two thirds extends caudolaterally and its rostral third extends nearly directly rostrally to the symphysis. The change in direction occurs far rostrally, approximately at the midlength of the maxillary tooth row. The row of neurovascular foramina that penetrates the ventrolateral surface of the bone is in view along the rostral half of the bone; the nerve branches that extended from these openings to innervate the skin would have sent tactile information to the brain.

The dentary is widest along the rostral third of the bone, which corresponds to the dentigerous region of the bone. This approximately matches the region of the largest maxillary teeth; the rostral end of the bone attenuates at a blunt tip that is flattened medially by the symphysis. The caudal half of the bone is much narrower than the part below the teeth; this region does not continuously reduce in width, but it is abruptly attenuated where it extends onto the lateral surface of the angular.

The rostral end of the dentary abuts the lingual surface of the premaxillary teeth, indicating an overbite when the mouth is closed. It is also seen that the maxillary teeth are positioned lateral to the dentary, indicating that contact between the dentary and maxillary teeth does not occur in tyrannosaurids; likewise, there is no occlusion between the dentary and premaxillary teeth in tyrannosaurids. The medial edge of the dentary is continuous except caudally, where it deviates caudolaterally at the angular.

Intercoronoid: Nearly the entire caudal extent of the intercoronoid is seen in ventral view at the rostrodorsal limit of the Meckelian fossa. Put another way, the Meckelian fossa separates the caudoventral edge of the intercoronoid from the caudodorsal edge of the prearticular. This small part of the bone is overlapped rostromedially by the prearticular and it is apposed to the medial surface of the surangular. This part of the intercoronoid is triangular in shape, flat, and faces ventromedially, in plane with the rest of the mandibular bones in the midregion of the skull. When the jaws are closed, the caudal end of the intercoronoid is positioned lateral to the ectopterygoid, below its jugal process and above its caudolateral process.

The rostral part of the intercoronoid can be seen above the splenial; the suture between them was difficult to trace rostrally, so in the illustration a dashed line marks its approximate course. The rostral part of the bone completes the upper part of the medial wall of the dental alveoli (tooth sockets) in the form of a long strap that is dorsoventrally shallow and mediolaterally thin. The ventral edge of the intercoronoid extends along the rostrodorsal edge of the splenial; its rostral end is blocked from view by the dentary as it twists along its long axis into a vertical orientation.

Prearticular: In ventral view, the prearticular tilts medioventrally, showing it is the longest bone of the caudal region of the mandibular ramus, which extends for half of its length. The prearticular contacts the articular caudally, the surangular caudolaterally, the angular laterally and ventrally, the splenial rostroventrally, and the intercoronoid caudodorsally. The prearticular forms the ventrla margin of the ramus along its course between the surangular and the region where it is undrlapped by the angular.

The prearticulosplenial contact is separated for a short distance rostrodorsally by the internal mandibular fenestra. The prearticular is mediolaterally wide caudally, whereas it is thin rostrally. With the jaws closed, the rostrodorsal end of the bone is positioned below the base of the pterygoid process of the palatine.

Caudally the prearticular overlaps the rostroventromedial surface of the articular, whereas it is overlapped caudoventrolaterally by the surangular, and ventrally by the angular. The rostroventral edge of the bone extends along the edge of the splenial. Rostrodorsally the bone tapers toward the dorsal tip of the splenial.

Splenial: In ventral view, the medial surface of the splenial tilts mediodorsally into view, showing it is the longest mandibular bone in the midregion of the ramus. The splenial contacts the prearticular caudodorsally, the angular caudoventrally, the dentary ventrally, and the intercoronoid rostrodorsally. The caudodorsal margin of the bone is deeply notched by the internal mandibular fenestra, which separates it from the prearticular. At no point does the splenial does not reach the ventral margin of the mandibular ramus. The medial surface of the bone is essentially flat, except caudally where a ridge can be seen.

The splenial starts below the rostral end of the Meckelian fossa between the prearticular and the angular, and it extends rostrally where it stops below the midlength of the dentigerous region of the snout. The bone tapers rostrally below the intercoronoid and above the dentary. It is perforated rostrally by the large rostral mylohyoid foramen. With the mouth closed, the foramen is positioned below the palatal process of the maxilla, ahead of the palatine bone. The dorsal apex of the splenial is positioned between the pterygoid and maxillary processes of the palatine, ahead of the prearticular.

Surangular: In ventral view, only the caudoventral corner of the surangular can be seen. The bone contacts the articular caudoventromedially, the prearticular ventromedially, and the angular ventrolaterally. The lateral surface of the surangular is deeply creased at the caudal end of the surangular shelf. Despite its small size, the surangular forms the caudoventral corner of the mandibular ramus, and virtually prevents the articular from lateral exposure. A small tab from the surangular extends medially to underlap the region where the articular and prearticular join each other; in this region the surangular manages to form the ventral surface of the ramus.