Osteology II: Craniofacial frame and openings in dorsal view

The craniofacial skeleton of a subadult Albertosaurus libratus seen in dorsal view, 

© Dino Pulerà. Labels emphasize the overall structure of the skull and are explained below. Carbon dust plate by Mr. Dino Pulerà.

Introduction

When viewed from above, the function of the tyrannosaurid skull is starkly clear: it is a supreme biological weapon honed by natural selection that is adapted to effortlessly aim, strike, and grip living animals with immobilizing power.

Directional terms

Lateral: A point of reference toward or at the side of a given structure (in this case, the craniofacial skeleton).

Medial: A point of reference toward or at the rostrocaudal midline of a given structure (in this case, the craniofacial skeleton).

Rostrolateral: A direction or point of reference that is both ahead of and external to another point of reference. Variation: caudolateral.

Rostromedial: A direction or point of reference that is both ahead of and internal to another point of reference. Variation: caudomedial.

DESCRIPTION

AOFEN - Antorbital fenestra: In dorsal view, the antorbital fenestra is a long and narrow slot that extends along the lateral surface of the snout. It is surrounded ventrally and rostrally by the maxilla; medially and dorsally by the nasal rostrally, and by the lacrimal caudally; and the jugal contributes to its caudoventral corner. A wide axial strut separates the contralateral fenestrae dorsally, which is dominated by the nasal rostrally, but with caudolateral contributions by the lacrimals.

AOR – Antorbital region: When seen from above, the antorbital region narrows as it extends rostrally from the orbital region to form the parallel-sided snout.

BN - Bony naris: In dorsal view, the bony naris is a long and narrow opening situated somewhat toward the dorsal midline. It is enclosed rostrolaterally and ventrally by the premaxilla, caudolaterally and ventrally by the nasal, and medially and dorsally by the nasal.

DTFEN - Dorsotemporal fenestra: The dorsotemporal fenestra is a hole through the dorsal skull roof that permits the adductor (jaw-closing) muscles access to a high surface area of attachment. This fenestra is the largest opening that is seen in dorsal view, and it easily exceeds all others in terms of length, width, and cross sectional area. The fenestra narrow medially and it widens rostrally and caudally as it extends laterally.

The fenestra is surrounded by several bones, including: the frontal rostromedially; the postorbital rostrolaterally; the squamosal caudolaterally and caudally; the prootic caudomedially; the laterosphenoid rostromedially; and also the parietal rostromedially, between the laterosphenoid and frontal.

The dorsotemporal fenestra differs from the other openings described previously in that its circumference does not lie in a single plane; for example, the squamosal and postorbital are situated dorsal to the level of the prootic and laterosphenoid. The fenestra is a side effect of looking down into an irregular tunnel composed of many bones. Regardless, the dorsotemporal fenestra has a large cross sectional area in the horizontal plane, which indicates in the living animal a high bite force exerted from each side of the head.

LTFEN - Laterotemporal fenestra: In dorsal view, only the ventral part of this opening can be seen, where it is reduced to a narrow slot.

NB – Narial bar: When viewed from above, the narial bar is much a much wider structure than it appears to be in lateral view. The proximal end of the bar is wide and vaulted, and it gradually narrows and flattens as it extends rostroventrally to the dorsal surface of the premaxilla. The narial bar is dominated by the nasal, whereas the premaxilla is limited to the rostral end of the bar. The tip of each premaxilla is gripped laterally and medially by the nasal; the medial slip of the nasal is not seen in all tyrannosaurids.

OFEN - Orbital fenestra: In dorsal view, the orbital fenestra is a small triangular opening situated toward the lateral surface of the skull that is surrounded laterally and rostromedially by the jugal; medially and dorsally by the lacrimal rostrally, and the frontal caudally; and caudolaterally by the postorbital.

OR – Orbital region: When seen from above, the orbital region is much smaller than it appears in lateral view. It is roofed by the frontal, prefrontal, lacrimal, postorbital, and nasal; it is bounded caudally by the postorbital and jugal, ventrally by the jugal, and rostrally by the lacrimal, although some of these are not in the plane of view.

ORTR – Orbitotemporal region: In dorsal view, the orbitotemporal region surrounds the dorsotemporal and orbital fenestrae. This region has a complex three dimensional shape and it includes many bones, including the jugal, maxilla lacrimal, prefrontal, frontal, parietal, postorbital, squamosal, prootic, laterosphenoid, supraoccipital, basioccipital, quadratojugal, pterygoid, epipterygoid, and ectopterygoid (the last three bones are not illustrated here).

TR - Temporal region: When seen from above, the temporal region is the largest part - over half - of the orbitotemporal region. Its size directly reflects the great amount of adductor musculature that was present in life. In contrast to the dominant temporal region, the small orbital region is virtually a bare necessity required for aiming the snout and exerting the massive bite forces stored in the caudal quarter to third of the skull.

UTB - Upper temporal bar: In dorsal view, the upper temporal bar is a very narrow strut that extends between the relatively thin paroccipital complex (parietal + squamosal + otoccipital) caudally and the stable dorsal skull roof rostromedially. The squamosal forms the caudal part of the bar, whereas the postorbital completes it rostrally.

The squamosal is apposed to the rostrolateral surface of the paroccipital process of the otoccipital, and it extends rostrally along that structure as a tapering fingerlike process that fits into a long and deeply incised groove between the parietal above and the prootic below. The surface area of the connection between the squamosal and otoccipital is quite high, ensuring a stable contact between their otherwise lightly textured and gently undulating surfaces. Rostrolaterally the squamosal receives the squamosal process of the postorbital into a long and deep cleft in its lateral surface.

The postorbital extends rostromedially to articulate with the frontal along a complex and stable contact with the frontal dorsally and the laterosphenoid caudoventrally. Rostrally the postorbital and frontal abut each other, whereas caudal to this they are secured to each other by prominent and complementary ridges and slots. Caudoventrally the laterosphenoid inserts a stout process into a deep, cup-like process in the medial surface postorbital below the frontal. In some large adult tyrannosaurines, the postorbital and lacrimal are joined to each other above the orbital fenestra (Currie, 2003). In these cases, the upper temporal is incorporated into the orbital region.

References cited

Currie, P. J. 2003. Cranial anatomy of tyrannosaurid dinosaurs from the late Cretaceous Alberta, Canada. Acta Palaeontologica Polonica 48:191-226.