Osteology V: Craniomandibular skeleton in caudal view

The skull and jaws of a juvenile Tyrannosaurus rex (CMNH 7541) in caudal 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. Gray indicates missing bone, hatch marks indicate broken surfaces, small double hatch marks on gray indicate matrix. Openings in the skull are filled with white, whether or not they lead into a closed chamber. This specimen is less complete than is shown in Bakker et al. (1988); regardless, all of the important anatomical details can be seen, except for the dorsolateral margins of the nuchal crest. The pterygoid is not shown. This is a colorized version of the carbon dust plate by Mr. Dino Pulerà that first appeared in Carr (1999).

Introduction

This image shows the braincase and the mandibular ramus in caudal view. The bones are labeled generally on the right side, whereas subordinate structures that involve multiple bones are labeled on the left side. In my view, one of the best works on the structure of the tyrannosaurid basicranium is Bakker et al. (1988) and so I make extensive use of their terminology here. I see no reason to follow subsequent revisions of Bakker et al.’s (1988) terms, which are novel, descriptive, and useful.

CRANIAL SKELETON

Basal tuber: In caudal view, the basal tuber is a muscle insertion scar positioned ventrolateral to the occipital condyle; the tuber is plesiomorphic for tetrapods, where it serves as the attachment for subvertebral musculature.

In tyrannosaurids, the tuber covers the ventral tip of the otoccipital, caudoventral tip of the basisphenoid, and the ventrolateral tip of the basioccipital. The tuber delimits the ventrolateral corner of the subcondylar fossa. Contractions of muscle upon one the tuber would pivot the head - at the occipital condyle - downward and to the side, whereas a simultaneous contraction upon both would pull the head caudoventrally.

Basioccipital: In caudal view, the basioccipital forms the ventral midline of the foramen magnum and it encloses the basisphenoid recess caudally. The basioccipital contacts the otoccipital dorsolaterally at the occipital condyle and within the subcondylar recess, and it contacts the basisphenoid rostrolaterally beside the basisphenoid recess and caudoventrally at the basal tuber.

The basioccipital can be divided into two major regions: the occipital condyle and a ventral plate. The contribution to the occipital condyle is extensive, where the bone forms nearly the entire caudoventral part of the joint surface for the atlas. In most tyrannosaurids, the basioccipital spans the entire height from the foramen magnum to the ventral edge of the condyle; the otoccipital only forms the dorsolateral corner of the condyle. Despite its extensive contribution to the condyle, the basioccipital makes only a minor contribution to the foramen magnum along its ventral midline. However, the basioccipital’s contribution to the condyle is wider than the foramen magnum.

The ventral plate of the basioccipital is extensive, and forms the caudoventral portion of the basicranium. This region is broadly triangular in shape, with the apex pointing dorsally. The plate widens as it extends ventrolaterally to form the caudomedial part of the basal tuber. The subcondylar recess excavates the dorsolateral surface of the plate, the fossa is a pneumatic excavation that leads to a large pneumatic foramen. The foramen enters the bone’s interior, which is located ventrolateral to the occipital condyle. The foramen cannot be seen in the specimen figured here.

Prominent ascending scars (Bakker et al., 1988) separate the subcondylar recess from the apneumatic region of the ventral plate. This surface – along with the ascending scars - almost certainly was the insertion surface for subvertebral musculature, given their generally coarse texture. The ascending scars converged dorsally, fading at the stout neck that extends caudally from the plate to the occipital condyle.

Basisphenoid: In caudal view, the basisphenoid extends rostroventrally below the basioccipital; this extensive bone forms the rostroventral region of the basicranium. Also, a small slip of the bone separates the tips of the otoccipital and basioccipital to form the midregion of the basal tuber. The basisphenoid is apposed to the rostral surface of the basioccipital and to the rostroventral edge of the otoccipital, relationships that can best be seen from the side.

The contribution of the basisphenoid to the basal tuber is small, where it completes the convex surface of this subvertebral muscle insertion scar. The basal tuber extends rostroventrally along the basisphenoid as the oval scar (Bakker et al., 1988). In contrast, the rostroventral region of the bone is massive and has the form of a caudoventrally open box. The empty part of the box is represented by the basisphenoid recess, which is bounded laterally by a prominent ridge, the basicranial boxwork wall (Bakker et al., 1988) and rostrally by the basipterygoid web (Bakker et al., 1988).

The rostroventral corner of the bone culminates in the basipterygoid process, which articulates with the dorsal surface of a small process from the pterygoid, forming the so-called basal joint. This connection between the basicranium and the palate almost certainly provided axial stability to the skull frame during biting in addition to providing one of the five primary anchoring points (in addition to the vomeromaxillary, palatomaxillary, pterygoquadrate and epipterygolaterosphenoid contacts) for the palate onto the braincase and facial skeleton. In contrast to the other joints, the basiphenoidopterygoid joint is loose, where the bones are not tightly apposed to each other. This suggests that there was mobility between the palate and cranium at the basal joint.

Basisphenoid recess: In caudal view, the rostral and medial surfaces of the basisphenoid recess can be seen. The rostral surface is penetrated by a pair of large pneumatic foramina, but they will be dealt with in a future post. The basisphenoid recess is the large cavity that hollows out the ventral surface of the bone. However, the recess is not entirely enclosed by the basisphenoid, where the basioccipital forms its caudal wall. The recess is hypothesized to be produced by the median pharyngeal air sac system (Witmer et Ridgely, 2009). More on this will be given later in a post that deals specifically with pneumatic features.

Dorsotemporal fossa: The dorsotemporal fossa is seen caudally at the dorsomedial corner of the squamosal. The fossa serves as the origin for the adductor musculature that closed the jaws upon contraction.

Foramen magnum: The foramen magnum is the large opening located at the center of the occiput above the occipital condyle. The opening is an actuality a rostrocaudally short canal whose caudal edge represents the boundary between the rostral space that contained the brain and cranial nerves, and their associated dural and vascular structures (endocranial space) and the caudal space outside of the skull that contained spinal cord, spinal nerves, and their associated structures (vertebral canal). The margin of the foramen is formed by the supraoccipital dorsally; the otoccipital dorsolaterally, laterally, and ventrolaterally; and the basioccipital ventrally.

Foramen of the vagal canal: In caudal view, this large foramen opens from the occiput ventrolateral to the occipital condyle and below the stalk that extends to it from the caudal surface of the otoccipital. In life the tenth cranial nerve, the vagus nerve (CN X), and the eleventh cranial nerve, the accessory nerve (CN XI), left the braincase through the opening (Witmer et Ridgley, 2009). Also, the posterior cerebral (=jugular) vein left the braincase through this opening as well (Witmer et Ridgley, 2009).

Occipital condyle: The occipital condyle is located at the center of the occiput below the foramen magnum and above the basicranium. The dorsal surface of the condyle is flat, forming the rostral end of the floor of the vertebral canal. The condylar surface is widely exposed in caudal view, which is convex horizontally and vertically. The condyle extends rostroventrally toward the occiput.

The occipital condyle faces caudally and ventrally, and its curvature suggests that side-to-side rotation was more limited than its ability to make dorsoventral excursions. The truncated (flat) dorsal margin of the condyle prevented hyperextension. The form of the condyle corresponds to the bean-shaped condylar surface of the atlas, which cradled the condyle caudoventrally.

Otoccipital: In caudal view, the otoccipital is one of the largest bones of the occiput, which forms almost its entire midregion. The otoccipital contacts the supraoccipital mediodorsally, the parietal caudodorsally, the squamosal rostrodorsally, the basioccipital medioventrally, and the basisphenoid laterally and ventrolaterally. The medial edge of the foramen magnum is notched by the foramen magnum above its caudally extending contribution to the occipital condyle.

The otoccipital extends laterally as the deep and laterally extensive paroccipital process, and ventrally as the spike-like metotic strut. The bone lies mostly in the vertical plane, and it is nearly flat, with the exception of the caudally extending process that forms the dorsolateral corner of the occipital condyle.

The paroccipital process was almost certainly the insertion point for several supravertebral muscles, whose contractions swung the head to the side. Its ventrolateral corner was the origin point for the depressor mandibulae, whose contractions opened the mouth.

Paraquadrate foramen: The paraquadrate foramen is a large opening that separates the quadratojugal laterally from the quadrate medially. These bones are in tight apposition with each other above and below the foramen. The foramen occurs at the midheight of the conjoined bones and it is taller than wide.

Paraquadrate fossa: In caudal view, the paraquadrate fossa surrounds the paraquadrate foramen, where it excavates the caudomedial surface of the quadratojugal and the caudolateral surface of the quadrate. The fossa tends to be more deeply excavated into the quadratojugal.

Parietal: In caudal view, the parietal is the largest single unit midline bone that forms the dorsal third of the occiput as a fan like structure, the nuchal crest. The parietal contacts the supraoccipital caudally, the squamosal rostrolaterally, and the otoccipital caudoventrolaterally. The nuchal crest received a variety of muscles from the neck, whose contractions would have pivoted the head upwards upon the occipital condyle. A small ventrolaterally extending process contributed to the dorsal surface of the paroccipital process of the otoccipital.

The caudal surface of the nuchal crest was concave on either side of the midline, and a vertical ridge extends ventrally between the fossae. The midline ridge abutted the dorsal surface of the supraoccipital, which formed a prominent ridge along the ventral half of the nuchal crest.

Quadrate: In caudal view, the quadrate is a vertical shaft that contacts the squamosal dorsally, the otoccipital dorsomedially, the quadratojugal dorsolaterally and ventrolaterally, the articular ventromedially, and the surangular ventrolaterally. The quadrate is notched laterally at its midheight (its narrowest region in caudal view) by the paraquadrate foramen, and its caudolateral surface is gently excavated by the paraquadrate fossa. The margin of the paraquadrate foramen is medially concave.

The quadrate is the primary bone of the cranium that forms the jaw joint. In caudal view, its stout and columnar form can be seen. Although it cannot be seen in the image, the quadrate forms the entire contribution made to the jaw joint by the skull, whereas two bones form its mandibular complement.

Quadratojugal: In caudal view, the quadratojugal forms the lateral part of the suspensorium (the complex of bones that attaches the jaw joint of the skull to the braincase, palate, and facial skeleton). The quadratojugal contacts the squamosal dorsomedially, and the quadrate caudodorsomedially and caudoventromedially. The dorsal and ventral parts of the bone are relatively flat and extend caudomedially, whereas the midregion, beside the paraquadrate foramen, is columnar. The margin of the paraquadrate foramen is laterally concave. The caudomedial surface of the bone is deeply excavated by the paraquadrate fossa.

Squamosal: In caudal view, the squamosal caps the caudodorsolateral corner of the skull, where it bulges dorsally above the parietal, otoccipital, and quadratojugal. The squamosal contacts the parietal caudomedially, the otoccipital caudolaterally, the quadratojugal lateroventrally, and the quadrate ventrally.

Subcondylar recess: In caudal view, the subcondylar recess is an oval, lateroventrally extending pneumatic fossa that excavates the otoccipital and basioccipital. It is possible that a diverticulum of the cervical air sac system produced this depression. Dorsomedially, the recess leads into a pair of relatively large pneumatic foramina, one pierces the otoccipital, whereas the other penetrates the basioccipital; these foramina cannot be seen in the image. The foramina lead into pneumatic chambers within each bone. The recess is located, as its name implies, ventrolateral to the occipital condyle, and below a fossa that the foramina of several cranial nerves penetrate.

The lateral margin of the fossa tends to be less distinct than its medial margin, which is bounded by the prominent ascending scar. The basal tuber delimits the ventrolateral corner of the fossa. The otoccipital and basioccipital contact each other as a ridge along the long axis of the fossa.

Supraoccipital: In caudal view, the supraoccipital is a small single-unit midline bone that extends from the foramen magnum to the nuchal crest. The supraoccipital contacts the parietal rostrodorsally and the otoccipital ventrolaterally. It forms the dorsal margin of the foramen magnum, the midregion of the bar-like occipital surface between the paroccipital processes, and a prominent process at the base of the nuchal crest. The supraoccipital can be divided into two parts: a block-like dorsal process and ventral body, which extends laterally on each side as a stout process. The bone contributes to the nuchal crest and to the medial part of the paroccipital process.

MANDIBULAR SKELETON

Articular: In caudal view, the articular is the dominant bone of the lower jaw, which forms all but its lateral edge. The articular is apposed to the surangular laterally, and dorsally it articulates with the medial region of the mandibular process of the quadrate. Its caudal surface is broadly concave, a surface called the caudal fossa, which received the insertion of the depressor mandibulae, the muscle whose contractions opened the mouth. A flange of the surangular overlaps the caudolateral surface of the articular.

Surangular: In caudal view, only a narrow flange of the surangular can be seen that overlaps the caudolateral surface of the articular. Outside of the plane of view, the surangular also articulates with the quadrate dorsally.

References cited

Bakker, R. T., P. J. Currie, and M. Williams. 1988. Nanotyrannus, a new genus of pygmy tyrannosaur, from the latest Cretaceous of Montana. Hunteria 1:1, -30.

Carr, T. D. 1999. Craniofacial ontogeny in Tyrannosauridae (Dinosauria, Theropoda). Journal of Vertebrate Paleontology 19:497, -520.

Witmer, L. M., R. C. Ridgely. 2009. New insights into the brain, braincase, and ear region of tyrannosaurs (Dinosauria, Theropoda), with Implications for sensory organization and behavior. The Anatomical Record 292:1266-1296.