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.