Osteology VII: Mandibular ramus in lateral view.

Mandibular ramus of a subadult Albertosaurus libratus (ROM 1247) in right lateral view,

© Dino Pulerà. This illustration was our first professional collaboration.

Introduction

The image in this post is of the mandibular ramus of a subadult Albertosaurus libratus in right lateral view, based on the bones preserved in ROM (Royal Ontario Museum) 1247. This carbon dust plate was the first collaborative work - back in 1993 - between myself and Dino Pulerà. We chose the ramus as the starting point for our collaboration for several reasons: in contrast to the skull, the ramus required less time for me to draft a line drawing, and less time for Dino to transfer and render. Not only does the ramus have fewer bones than the skull, it is also essentially a flat, two-dimensional surface, aside from the teeth, foramina, the upper region of the surangular, and the rostroventral region of the dentary.

This was also a test of the effectiveness of the carbon dust medium for this type of subject matter. I learned very quickly that carbon dust can achieve what other media can’t - it can capture a bone’s entire surface and texture, in addition to its contours, within the limits of the tooth (texture) of the illustration board. The end result is as high resolution as can be hoped to achieve.

One of the most important learning experiences for me was that this was a collaborative endeavor - along the way Dino noticed many details of the bones that I had overlooked, and I’ve learned from him to view bone with intense scrutiny. I also picked up the habit of ‘viewing’ bones with my fingertips, an approach that Dino used to verify if a surface was truly featureless or to follow the course of subtle features. In the end, this image was the most accurate depiction that we could produce within our abilities at the time. The articular is not pictured here because that bone is missing from the specimen.

DESCRIPTION

General form of the mandibular ramus: The mandibular ramus of tyrannosaurids is longer than deep, where the region of maximum depth is situated caudal to the midlength of the ramus. The dentigerous (tooth-bearing) region occupies nearly the entire rostral half of the ramus. With the ventral margin of the dentary held level, the jaw joint is situated far above the level of the tooth row. The ramus is penetrated by two relatively large openings, namely the caudal surangular foramen (csf) and the external mandibular fenestra (emf).

Caudal surangular foramen—The csf penetrates the ramus rostroventral to the glenoid, ventral to the surangular shelf, and above the angular. Dorsoventally, the csf is positioned above the level of the tooth row. In lateral view, and more obviously in medial view, it is located ahead of the point of divergence between a pair of prominent ridges; this perforation was not situated in place of structural weakness.

External mandibular fenestra—The emf is a gap that opens into the lower part of the Meckelian (=prearticular) fossa between the rostroventral margin of the surangular, rostrodorsal edge of the angular, and caudoventral margin of the dentary. This aperture is situated close to the ventral margin of the ramus at – approximately - its caudal third above the end of the dentary. Dorsoventrally, the emf is positioned below the level of the tooth row. Although the prearticular can be seen through the gap, it does not contribute to its margin; instead, the bone is situated far medially, where it encloses the Meckelian fossa between itself and the laterally positioned bones.

Margins of the ramus—The caudodorsal, caudal, and caudoventral margins of the ramus are formed by the surangular. Ahead of the surangular, the prearticular forms the caudoventral margin of the ramus for a short distance, which is followed rostrally for a shorter distance by the angular, and finally, the rest of the ventral margin is formed by the dentary. The dentary also forms the rostroventral margin, and greater than the rostral half of the dorsal margin of the ramus. Although the splenial is a large bone in tyrannosaurids, it is not exposed laterally.

Distinct landmarks of the ramus—In broad analogy with mammals, the ramus can be thought of as having a ‘chin’ and an ‘angle’, where the chin is at the intersection of the rostroventral and ventral margins of the dentary, and the ventral mandibular angle is at the change in direction of the ventral margin between the horizontal dentary and the caudodorsally extending postdentary moiety. As illustrated, the chin in ROM 1247 is below the 4^th alveolus, and the angle is below the rostral end of the emf.

The Bones and Landmarks of the Mandibular Ramus

Angular: In lateral view, the angular is third largest bone, which forms most of the caudoventral quadrant of the ramus to the exclusion of the surangular and prearticular. It has an extensive contact with the surangular dorsally, a narrow contact with the prearticular ventrally and medially, and is rostrally overlapped by the dentary. Regardless, most of the bone is exposed laterally.

The angular closely approaches the csf and forms the concave caudoventral margin of the emf. Despite its size and position, the angular only forms a small part of the ventral margin of the ramus between the dentary and prearticular. The angular extends caudodorsally from the ventral angle of the mandibular ramus.

Caudal mandibular angle: In lateral view, this is a convexity in the dorsal margin of the surangular at the rostral end of the coronoid region, an extensive muscle insertion surface that covers the dorsolateral surface of the surangular ahead of the glenoid fossa.

Chin: The abrupt change in direction that separates the caudoventrally extending rostral margin of the dentary from the horizontally oriented ventral margin of the bone.

Coronoid process: A low, dorsally extending ridge along the medial edge of the surangular that bounds the coronoid region medially.

Dentary: In lateral view, the dentary is the largest bone of the mandibular ramus, where it makes up nearly three quarters of its length. The caudoventral part of the bone forms the angle of the mandibular ramus. The dentary forms the rostral margin of the emf, where the opening usually notches the bone; this condition is not seen in ROM 1247. When viewed from the side, the dentary contacts the surangular caudodorsally and the angular ventromedially.

In a general sense, the dentary is shaped like an hourglass set on its side, where the dorsal and ventral margins are concave, and the long axis deepens rostrally and caudally. The caudal end of the bone is deeper than the rostral end, reaching its maximum height at its contact with the surangular. Rostrally, the bone is deepest in the neighborhood of the fourth-sixth tooth positions. The rostral end of the lateral alveolar margin is convex, which becomes concave from approximately the eighth tooth position to the caudal end of the tooth row. The alveolar margin has a scalloped margin of low convexities that are positioned beside each alveolus.

The rostral margin of the bone extends abruptly caudoventrally before abruptly extending caudally. The point of change in direction is here termed the chin, which is usually below alveolus four and, less frequently, it occurs below the third tooth position. The transition point also marks the approximate location of the caudal limit of the symphysis. The rostral margin of the bone is largely convex.

The ventral margin of the dentary is sinuous, where its rostral extent is gently convex before becoming concave for most of the length of the bone. Caudally, the ventral margin extends caudodorsally at the ventral angle of the mandibular ramus. As such, the caudoventral corner of the bone is situated on the lateral surface of the angular.

The caudal edge of the dentary is overlapped by the surangular, although this overlap might not be as extreme as seen in the illustration presented here. Caudodorsally, the dentary in turn overlaps the surangular with a stout process.

Over half the length of the dentary is occupied by the tooth row. It is evident that the extensive and complex lateral contact with the surangular, and the broad medial contact with the angular served to stabilize the bone on the postdentary moiety during forceful bites.

Among the mandibular bones, the dentary is perforated by the greatest number of neurovascular foramina. These are concentrated rostrally, parallel to the tooth row, and along the ventrolateral margin of the bone, stopping ahead of the level of the last teeth. If the branches of sensory nerves entered these foramina from the skin, then the oral margin, tip of the jaw, and ventrolateral surface of the dentigerous region were highly sensitive to touch in life.

In contrast, only a single large neurovascular foramen penetrates the rostrodorsolateral surface of the surangular, which is presumably represents the caudalmost foramen of the dorsal row. Otherwise, the caudal region of the dentary and the entire postdentary moiety is imperforate and presumably insensitive to tactile information.

Many minute foramina penetrate the upper edge of the dentary along the tooth row; presumably these were primarily vascular structures located in a region that would have required nearly constant remodeling due to the compressive forces and lesions sustained during killing and feeding.

The lateral surface of the dentigerous region is convex and bulges slightly laterally below the dorsal row of foramina. In contrast, the caudal region of the bone is flat, continuous with the condition seen in the surangular and angular behind it.

Glenoid fossa: In lateral view, the margin of the jaw joint as formed by the dorsal margin of the surangular bone.

Postdentary moiety: The complex of bones caudal to the dentary, including the surangular, articular, angular, and prearticular. This region and the dentary are connected by the surangular, prearticular, angular, splenial, and intercoronoid. The only bone that is not involved in this complex connection is the articular. The postdentary moiety bears the mandibular portion of the jaw joint and houses the massive Meckelian fossa, in addition to its connection with the dentary.

Prearticular: Although in lateral view only a narrow slip of the prearticular is seen, it forms most of the caudoventral margin of the mandibular ramus between the surangular caudally and the angular rostrally. The prearticular does not reach the ventral angle of the ramus. Most of the prearticular extends along the angular, except for its caudodorsal extent that extends below the surangular; the surangular forms a short and narrow wedge that separates the prearticular from the angular.

The prearticular can be seen in the ventral quadrant of the emf, where its rostral ramus begins to extend rostrodorsally along its course toward the splenial and interocornoid. The dorsal margin of the bone represents the ventromedial margin of the Meckelian fossa. This low rim formed by the prearticular along the ventromedial edge of the fossa attests to the massive and extensive musculature that in life filled the medial surface of the postdentary moiety.

Retroarticular process: The short, fan-like process that extends caudoventrally from the caudal end of the surangular.

Rostral mandibular angle: The abrupt change in direction in the dorsal margin of the ramus, between the caudal end of the tooth row and the postdentary moiety, at a convexity in the rostral end of the surangular.

Rostral surangular foramen: A neurovascular canal that opens at a low angle along the rostrodorsolateral surface of the surangular caudal to the joint surface for the tab-like process of the dentary. This foramen is almost certainly the caudalmost foramen of the dorsal row of foramina that penetrates the dentary.

Surangular: In lateral view, the surangular forms most of the caudodorsal section of the postdentary moiety, and accounts for approximately two-thirds of the height of the ramus through its maximum height. The surangular forms the dorsal, caudal, and caudoventral margins of the mandibular ramus; it also completes the glenoid fossa in forming its lateral half, although this is out of the plane of the illustration. The caudoventral region of the bone is overlapped by the angular, and rostrodorsally it is overlapped by a short process of the dentary.

Caudoventrally, the surangular is reduced to narrow slip between the prearticular medially and the angular laterally. These bones contact rostrally, pinching out the surangular between them, where the prearticular extends laterally to cup the angular from below.

Ventral mandibular angle: The abrupt change in direction of the ventral margin of the bone, between the horizontally oriented dentary and the caudodorsally oriented postdentary moiety. The angle proper occurs at the dentary.