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.

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.

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Mr. Joseph Frederickson (UW-Milwaukee, 2011) presenting the results of his Independent Study in Dinosaur Ontogeny and Evolution at the 2010 Annual Meeting of the Society of Vertebrate Paleontology. Although Joseph is a UW-Milwaukee graduate, he completed the entire paleontology track at Carthage College. Joseph completed his MSc degree at Temple University in Spring 2013; he is now a doctoral candidate at the University of Oklahoma.

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1) Comparative Anatomy of the Vertebrates (Fall, Sophomore year), an upper-level biology course; this is the prerequisite for all subsequent courses in the track.

2) Dinosaur Evolution and Extinction (Sophomore or Junior year); this is an upper level biology course. This includes the opportunity to prepare dinosaur fossils in the lab.

3) Field course (summer, Freshman or Sophomore years); a three week gen ed course, where students join me to dig up dinosaurs in the Hell Creek Formation of Montana from mid July to early August: http://www.carthage.edu/live/news/151-productive-dinosaur-dig-has-students-buzzing. Exceptional paleo track students are invited to participate in succeeding field seasons as paid field assistants.

4) Independent Study (Spring, Junior year) in Dinosaur Ontogeny and Phylogeny; this is an upper-level biology course. This is only for students who are headed for graduate school (either Master's or Doctoral programs) who have a high record of academic achievement. In this course, students get one-on-one training with me on an actual research project that they can develop into their senior thesis. They also receive an intensive primer in phylogenetic systematics (cladistics).

Students are required to present the results of their project at the Natural Sciences Division Colloquium and, in their senior year, present their results as a poster or platform presentation at the Annual Meeting of the Society of Vertebrate Paleontology and/or the Midstates Math and Science Consortium.

Beyond that, the Independent Study is intended to give students an early start on their research that they will develop further in graduate school.

5) Senior seminar (Fall, Senior year); this is an expansion of the Independent Study project, ideally based on a museum visit over the winter break.

Exceptional students in this track are candidates for the Paleobiology Achievement Award.

I must emphasize that this track is for students who are truly committed to pursuing vertebrate paleontology as their research career; this track is demanding, yet rewarding. Its entire purpose is to train students for graduate school, so that by their senior year they will have the background, experience, and maturity to meet potential graduate advisers.

Interested?! For more information, please contact me (tcarr@carthage.edu)!

Q &A II: The dueling tyrannosaurid

Introduction

Last month my reader from South Korea asked my opinion regarding the tyrannosaurid skeleton that was found associated with a ceratopsid skeleton; the so-called Dueling Dinos, which are presently up for sale.

Q: Do you think the dueling tyrannosaurid is just a large juvenile Tyrannosaurus rex?

A: On the face of it, that is a straightforward question, but it is complicated for a couple of reasons:

(1) The word “just” implies that if the specimen is a juvenile then somehow its scientific importance is diminished. That couldn’t be further from the truth – if the specimen is a juvenile or subadult, then its scientific importance increases because we are presently have a deficient sample size for juvenile and subadult tyrannosaurids from the Hell Creek Formation (and its lateral equivalents in the American and Canadian West). In my view, every juvenile and subadult tyrannosaurid is important, no matter what unit they were collected from. The early interval of growth in these dinosaurs is virtually a black box, which is why there is so much disagreement at the present time over tyrannosaurid diversity in the Hell Creek Formation. We need more juveniles and subadults – lots of them!

(2) This is a privately held specimen that is up for sale. I will not comment directly on it because (a) I have not seen it first hand, and (b) aside from that – and more importantly - it is not in a legitimate museum collection, where access to it is guaranteed to all researchers, including myself.

Certainly a lot of attention has swirled around the Dueling Dinos, but it is not ethical for me to offer my views on it – no matter how well informed – until it is in a legitimate repository. My position is consistent with Section 4 of the Society of Vertebrate Paleontology’s Bylaw on Ethics:

"Scientifically significant fossil vertebrate specimens, along with ancillary data, should be curated and accessioned in the collections of repositories charged in perpetuity with conserving fossil vertebrates for scientific study and education (e.g., accredited museums, universities, colleges and other educational institutions)."

 and also Section 6:

“The barter, sale or purchase of scientifically significant vertebrate fossils is not condoned, unless it brings them into, or keeps them within, a public trust. Any other trade or commerce in scientifically significant vertebrate fossils is inconsistent with the foregoing, in that it deprives both the public and professionals of important specimens, which are part of our natural heritage.”

Until the tyrannosaurid is in a legitimate repository, I consider the specimen to be in limbo, perilously caught between the Scylla of commerce ($7-8 million price tag) and Charybdis of commodification (scientifically uninformed hype) - which is frustrating for me as a tyrannosaurid researcher given its apparent completeness, quality of preservation, and ontogenetic stage.

In addition to that, I do not want to say anything that could be construed as either enhancing its commercial value or endorsing the sale of a scientifically important specimen; ergo, the absence of an image of it above. I take the view that if a specimen is scientifically important, then it should not be up for sale.

Therefore, I encourage all of those involved in trying to sell the Dueling Dinos to do the right thing: donate the specimens to a legitimate museum in exchange for the expense taken to collect and prepare the skeletons. Fossils like the Dueling Dinos must not be on the open market where they are perched above the abyss of disappearing into the hands of a private individual or an illegitimate institution – fossils are nonrenewable resources, and it would harm science if they are sold beyond its reach.

Sources cited

http://vertpaleo.org/The-Society/Statements-and-Guidelines/Bylaw-on-Ethics-Statement.aspx

Field notes I: 2013 - summer of Tyrannosaurus rex

Right frontal in dorsal view of a juvenile Tyrannosaurus rex (DDM-344.1) from the Hell Creek Formation of southeastern Montana that I found last August. Rostral is to the right, medial is toward the top of the image. Abbreviation: Dinosaur Discovery Museum, Kenosha WI.

Introduction

This series of posts will describe the upcoming Carthage College Expedition to the Hell Creek Formation of southeastern Montana. We have been collecting there since 2006 on exposures that are on sections managed by the Bureau of Land Management, and we leave for there in the morning!

Little Clint Quarry

This year we (myself, my technician, 10 students, 2 field assistants, 2 volunteers) resume excavation of a multitaxic and monodominant bonebed, the Little Clint Quarry (named for Thescelosaurus expert Dr. Clint Boyd).

Little Clint is the nickname of a partial skeleton of a juvenile T. rex that led to the bonebed. So far, we have collected a frontal, rib, tibia, and pedal phalanx of the specimen. We have also found a partial maxilla, a pair of tibiae, and a metatarsal III (see image below) belonging to a large T. rex, and a tooth and pedal phalanx of a medium-sized T. rex that we have nicknamed ‘Big Clint’.

In addition to T. rex, we have recovered a complete hadrosaurid fibula, a partial skeleton of Triceratops that includes cranial and postcranial bones, and two Thescelosaurus femora. The bonebed also includes nondinosaurian amniotes, including isolated bones of crocodilians and turtles. In some parts of the quarry the dinosaur bones are so densely concentrated that they are stacked upon each other. This year we will resume excavation of this lag deposit, and I am optimistic that we will recover at least a few additional bones of the large T. rex.

Partial left metatarsal III (DDM-35.131) in anterior view of a large Tyrannosaurus rex from the Little Clint Bonebed.

Little Hyslop Locality

Last summer I went prospecting with one of my long-standing volunteers, Mr. Andy Prell (Kenosha, WI) a few hours after the students left for home. About 15 minutes from camp I walked up a ravine, and to my astonishment, I saw a small tyrannosaurid frontal bone lying upside down on the slope (see image above). Andy joined me a few minutes later and searched upslope, where he found a partial pedal ungual (D I-1) of a small T. rex. Moments later I found a small T. rex tooth a short distance north of the frontal (see image below).

The bones and tooth correspond to the same size of animal, between 3 and 6 meters long, and so there is a good chance that at least a partial skeleton lies below the surface. Andy and I searched intently on the hillside for more evidence of the skeleton, without success. Several days later I returned to the locality with our team of volunteers, but again without finding anything else. I hope that this year’s spring rains have brought more of the specimen to the surface, unless Andy and I found the last of it. The Little Hyslop locality is named for Mr. Dan Hyslop (UW-Madison graduate).

The tooth (DDM-344.3) of a juvenile Tyrannosaurus rex that I found at the Little Hyslop Locality. It is consistent in size with the frontal bone, which suggests that there is more of the specimen to find.

I will make the best effort to keep you posted on our progress in the field, as far as T. rex discoveries are concerned!

Q&A I: Teratophoneus curriei ontogeny

 

The skull of the type specimen of Teratophoneus curriei in left lateral view, modified from Carr et al. (2011).

Introduction

If I have sufficient time, I’ll answer questions put to me directly that illuminate aspects of tyrannosauroid paleobiology. Recently I was asked about Teratophoneus curriei, a new genus and species of tyrannosaurine that I named recently (2011) with Thomas Williamson, Brooks Britt, and Ken Stadtman. These questions come from a reader in South Korea.

Q: If the base of the interfenestral strut (see diagram) in T. curriei is concave, and that is considered to be a Stage 3 (adult) adult feature of Albertosaurus libratus (Carr, 1999), then why in T. curriei is it considered to be a subadult feature?

A: In the article our goal was to establish the relative maturity of the holotype specimen based on my earlier work (Carr, 1999) on other tyrannosaurids, particularly A. libratus. Since we were working on the only specimen available to us at the time, we could not establish its maturity with precision. In addition to a high number of immature features, it turned out that the specimen had several that clearly indicated that it was neither a young juvenile nor a full adult. So what about that nagging strut?

The table in Carr (1999) shows that the strut is flat in small stage 1 (i.e., juveniles), whereas it is concave in large stage 1 specimens. Ergo, the presence of a concave strut in T. curriei is consistent with what is seen in relatively immature A. libratus. However, in A. libratus, this is not a perfectly clean pattern, because individuals of greater maturity (Stage 2; subadults) also have the flat condition, whereas the concave condition is seen in the most mature specimens (Stage 3; adults). So where does that leave us?

Keep in mind that in Carr (1999) I established growth stages – large intervals of ontogeny - that would assist in distinguishing ontogenetic variation from phylogenetically informative variation. However, many specimens can be grouped into the categories of small juvenile, large juvenile, subadult and adult, indicating that growth stages have limited resolution because the relative maturity of the specimens within those categories is not specified. This indicates that the growth stage categories themselves are arbitrarily defined. Is there a way to solve these issues of resolution and arbitrariness?

Clearly this is an area where a more rigorous approach to ontogeny is required, which, in my view, is solved by cladistic analysis of ontogenetic characters (Carr and Williamson, 2004; Carr 2010). It is only through this approach, which solves both problems in one stroke, can we distinguish ontogenetically informative variation from individual variation. I have work in progress for all of Tyrannosauridae, which I have presented over the past several years at the annual meeting of the Society of Vertebrate Paleontology.

In the meantime, I suspect that the concave condition does indicate a relatively mature condition (it is not seen in the smallest, presumably least mature, juveniles), but that its timing is individually variable, somewhat like tooth eruption in people (for example, none of my ‘wisdom teeth’ have erupted and I am middle aged!). We’ll have to wait and see what the results of the analyses show. I’ll be able to answer your question with more clarity and depth once that work is published.

Q: Teratophoneus has a low tooth count, a condition that reflects its short snout. What do you expect the tooth count to be at the opposite extremes of its ontogeny?

A: One trend in tyrannosaurids is to increase tooth count, then reduce it (Carr, 1999); in others the tooth count is somewhat stable (Tsuihiji et al., 2009). I expect that T. curriei will show the latter pattern, where it will show little if any variation in tooth count. The short snout imposes a limit on the number of alveoli (tooth sockets) early in ontogeny, and I do not expect that would change as the animals increased in size. However, if T. curriei increased the size of its teeth in the manner of T. rex, then I would expect a reduction in tooth count with increasing maturity. Presently the answer awaits new specimens.

References cited

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

Carr, T. D. 2010. A taxonomic assessment of the type series of Albertosaurus sarcophagus and the identity of Tyrannosauridae (Dinosauria, Coelurosauria) in the Albertosaurus bonebed from the Horseshoe Canyon Formation (Campanian–Maastrichtian, Late Cretaceous). Canadian Journal of Earth Sciences 47:1213-1226.

Carr, T. D. and T. E. Williamson. 2004. Diversity of Late Maastrichtian Tyrannosauridae from western North America. Zoological Journal of the Linnean Society 142:479-523.

Carr, T. D., T. E. Williamson, B. B. Britt, and K. Stadtman. 2011. Evidence for high taxonomic and morphologic tyrannosauroid diversity in the Late Cretaceous (Late Campanian) of the American Southwest and a new short-skulled tyrannosaurid from the Kaiparowits formation of Utah. Naturwissenschaften 98:241-246.

Tsuihiji T, M. Watabe, K. Togtbaatar, T. Tsubamoto, R. Barsbold, S. Suzuki, A. H. Lee, R. C. Ridgely, Y. Kawahara, and L. M. Witmer. 2011. Cranial osteology of a juvenile specimen of Tarbosaurus bataar (Theropoda, Tyrannosauridae) from the Nemegt Formation (Upper Cretaceous) of Bugin Tsav, Mongolia. Journal of Vertebrate Paleontology 31: 497–517.

Osteology IV: Craniofacial skeleton in dorsal view

 

The bones of the craniofacial skeleton of Albertosaurus libratus seen in dorsal view, © Dino Pulerà. The bones of the palate are not shown (see text for details). Carbon dust plate by Mr. Dino Pulerà.

Introduction

This image shows the bones of the braincase and facial skeleton in dorsal view; it does not show the quadrate or the palate. The missing bones reflect the limited amount of material that I had to work with at the time I drafted the line drawing on which this rendered imaged is based. However, in future posts of this series each bone will be featured in multiple views, which hopefully will compensate for the deficits in this otherwise informative image.

Description

Basioccipital: In dorsal view, the basioccipital is represented by a sliver of bone that extends along the dorsal midline of the occipital condyle (the ball joint that articulates with the first vertebra of the neck) between the overlying otoccipitals. The basioccipital is a single unit and midline bone.

Frontal: The paired frontal bone contacts the nasal rostrally, the prefrontal rostrolaterally, the lacrimal laterally, the postorbital caudolaterally, the parietal caudally, and its complement medially. It contributes a slip to the margin of the orbital fenestra and forms a part of the rostromedial margin of the dorsotemporal fenestra. The frontal forms most of the interorbital region of the dorsal skull roof and the rostral part of the temporal region.

The dorsotemporal fossa covers approximately the caudal half of the bone, whereas the rostral half is covered by smooth subcutaneous surface. The dorsotemporal fossa is a depression with an irregular surface that functioned as the origin of attachment for the adductor (jaw closing) muscles in life. The triangular rostral end of the bone that lies outside of the fossa is informally termed here the forehead.

Jugal: In dorsal view, the jugal is a mediolaterally narrow strap that bounds the orbital fenestra ventrolaterally. It extends from caudolaterally to rostromedially along its course from the wide temporal chamber caudally to the base of the snout rostrally. The temporal part of the jugal extends nearly directly caudally, whereas the orbital region extends rostromedially. The jugal gives the orbital fenestra the rostral vector of its orientation. Caudally the jugal is overlapped laterally by the quadratojugal, whereas rostrally it overlaps the caudal end of the maxilla as a narrow wedge. The postorbital and lacrimal conceal the jugal’s contact with those bones.

Lacrimal: In dorsal view, the lacrimal is a large block-like structure with a rapidly tapering rostral process. Caudally the lacrimal inserts into the frontal, medially it extends along the prefrontal, and rostrally it inserts into the dorsolateral surface of the nasal; however, its ventral contact with the jugal is blocked from view by the lacrimal and its contact with the maxilla is out of the plane of view.

The lacrimal is a dominant structure above the transition between the orbital region and the base of the snout. It is widest in the region of two confluent ornamental structures, the caudolateral shelf and the cornual process. The shelf extends laterally over the orbital fenestra, whereas the cornual process extends rostrolaterally between the orbital and antorbital fenestrae.

Laterosphenoid: The laterosphenoid is paired bone that forms the rostrodorsolateral corner of the braincase. In dorsal view, it forms the rostromedial margin of the dorsotemporal fenestra. It contacts the frontal rostrodorsally, the parietal caudodorsally, and the prootic caudally. The part that can be seen forms a laterally extending ridge that is situated above the joint surface for the epipterygoid, a bone of the palate; as such, the laterosphenoid shielded that delicate bone from the adductor musculature.

Maxilla: In dorsal view, the maxilla forms nearly the entire length of the lateral surface of the snout; it is excluded from the dorsum of the snout by the premaxilla and nasal. Caudally the maxilla is overlapped by the jugal, dorsally by the nasal, and rostrodorsally and rostromedially by the premaxilla. Although the maxilla does contact the lacrimal caudodorsally, it disappears below the nasal above the antorbital fenestra. Like the jugal, the maxilla slopes slightly ventrolaterally, bringing it more broadly into view.

Below the jugal, the caudalmost part of the maxilla extends caudolaterally toward the orbital region. In this region the maxilla extends further laterally than it does medially. When seen from above, the maxilla forms the ventrolateral and rostroventral margins of the antorbital fenestra.

Nasal: The nasal is the longest bone of the dorsal skull roof, where it extends from the rostral part of the orbital region to nearly the tip of the snout. The nasal contacts the frontal caudally, the prefrontal caudolaterally, the lacrimal laterally and dorsally, the maxilla ventrally and rostrolaterally, and the premaxilla rostrally. Between the lacrimal caudally and the maxilla rostrally, the nasal spans the width of the snout above the antorbital fenestra and part of the antorbital fossa.

In dorsal view, the nasal is seen to be a long and narrow structure that forms the dorsomedial margin of the antorbital fenestra and the caudolateral and caudomedial margins of the bony naris. The nasal bears the coarsest region of the rugose ornamental surface of the facial skeleton, a condition that is also seen from the side.

Otoccipital: The otoccipital is an extensive bone mediolaterally and dorsoventrally, such that it requires two labels here. The otoccipital contacts the basioccipital ventrally, the supraoccipital dorsally, the parietal dorsolaterally, and the squamosal rostrally.

The otoccipital forms nearly all of the occipital condyle in dorsal view, each side separated from each other by a narrow midline strip of the basioccipital. The otoccipital extends at a low caudolateral angle behind the squamosal, distally forming the caudolateral corner of the skull.

Parietal: In dorsal view, the parietal is an extensive bone that contacts the frontal rostrally, the laterosphenoid rostroventrally, the prootic caudoventrally, the squamosal caudolaterally, the otoccipital caudoventrally, and the supraoccipital caudally. The parietal is widely exposed between the dorsotemporal fenestrae; in contrast, it is narrowly exposed along and above the occiput, where it extends mainly dorsally in the vertical plane as the tall nuchal crest. The parietal is the widest of the bones of the dorsal skull roof, which reaches its greatest breadth across the caudolateral processes that extend between the squamosals and otoccipitals.

The rostral part of the parietal lies entirely within the dorsotemporal fossa. The sagittal crest separated the complementary fossae along the dorsal midline. In this region, the parietal is dorsoventrally deep, curving lateroventrally to the laterosphenoid, prootic, otoccipital, and squamosal. Depite its depth, it only forms a short rostromedial extent of the margin of the dorsotemporal fenestra.

The caudal surface of the nuchal crest received vertebrocranial musculature, whereas its rostral surface anchored adductor musculature. In life, muscular contractions on the nuchal crest from behind lifted the head, which pivoted on the occipital condyle.

Postorbital: In dorsal view, the postorbital is a slim bone that extends caudolaterally from the frontal to the squamosal. The postorbital contacts the frontal rostromedially and the squamosal caudomedially.

The postorbital delimits the dorsotemporal fossa rostrolaterally by a ridge, which does not extend onto the frontal. Although it is tilted out of the plane of view, the postorbital overlaps the lateral surface of the squamosal. In the image it appears that the postorbital overlaps the squamosal medially; but this is not the case – a V-shaped notch splits the rostral end of the squamosal in this region, exposing the postorbital to view ahead of the laterally overlapping contact.

As seen from above, the cornual process of the bone can only marginally be seen. In contrast, the dorsotemporal fossa is the most widely exposed part of the bone in this view.  Behind this, the postorbital forms the mediolaterally narrow rostral end of the upper temporal bar.

Prefrontal: The prefrontal is divided into two processes, and the dorsal of these is seen from above, where it is situated between the lacrimal laterally, the frontal caudally, and the nasal medially. The prefrontal is the smallest of the dorsal skull roof bones and it is located at the level of the rostral end of the interorbital region. This bone is shaped somewhat like a teardrop, where the wide end is positioned caudally and the tapering end points rostrally, pinched between the lacrimal and nasal. The prefrontal is smooth and is not coarsened by the rugose ornamental surface that is seen in the lacrimal and nasal.

Premaxilla: In dorsal view, the premaxilla forms the front of the snout, where it articulates with the maxilla laterally, and the nasal caudolaterally and caudodorsally. It forms the rostroventral and rostral boundaries of the bony naris. Most of the dorsal surface of the bone is flattened and smoothed by the narial fossa, except its rostral surface is covered by the coarse subcutaneous texture. In Albertosaurus libratus, as pictured here, the tips of the separate premaxillary diverge from each other such that a wedge from the nasal separates them; in other taxa these processes are apposed to their tips.

Prootic: Only a narrow slip of the prootic can be seen in dorsal view, where it forms the caudomedial margin of the dorsotemporal fenestra. This part of the bone forms a ledge that would have deflected adductor musculature away from the space and contents of the middle ear.

Quadratojugal: In dorsal view, the quadratojugal caps the caudolateral corner of the craniofacial skeleton. The vertical stalk of the bone is seen, as well as its long rostral process that extends onto the lateral surface of the jugal. As such, the quadratojugal forms the lateral half of the lower temporal bar.

Squamosal: In dorsal view, the squamosal completes the mediolaterally narrow upper temporal bar; it contacts the postorbital rostrolaterally, the otoccipital caudomedially, and the parietal rostromediodorsally. The medial process of the bone is wedged in a groove between the parietal dorsally and the prootic ventrally. The bone widens caudally, where it forms the flat surface of the dorsotemporal fossa, which is bounded laterally by a ridge.

Supraoccipital: The supraoccipital is a single unit midline bone; it contacts the otoccipital ventrally and the parietal rostrally. In dorsal view, it forms the midregion of a transverse (mediolateral) bar that extends caudally above the foramen magnum and lower half of the occiput (caudal surface of the braincase), and it forms a rostrodorsally-inclined rectangular block that extends a short distance up the midline of the nuchal crest.