Osteology I: Craniofacial frame and openings in lateral view

Composite skull of a subadult Albertosaurus libratus in right lateral view with the major openings and struts labeled,

© Dino Pulerà. Abbreviations are defined below. Carbon dust plate by Mr. Dino Pulerà.

Introduction

The osteology series of this blog is a primer for enthusiasts who are not necessarily paleontologists, but who want to know the definitions of the anatomical terms that they read about in technical articles and books. This series will begin with the craniofacial skeleton, followed by its individual bones, and then it will move on to the mandibular ramus, and so on.

Each entry of this section will start with a labeled figure, where spaces will be indicated by a leader line ending with a dot, and structures with lines that end in an arrowhead. The descriptions will be organized alphabetically, where each term will be defined and described. The spaces and structures will be based on the literature; novel terms will not appear here. Hopefully this approach will result in a clear and useful guide to derived tyrannosauroid (Bistahieversor + Tyrannosauridae) osteology.

Institutional abbreviations: AMNH FARB, American Museum of Natural History Fossil Amphibians and Reptiles (New York); ROM, Royal Ontario Museum (Toronto); TMP, Royal Tyrrell Museum of Palaeontology (Drumheller); USNM, United States National Museum (Washington).

A note on the figure

The figure of Albertosaurus libratus seen in this entry is a colorized version of the grayscale carbon dust plate that appeared in Carr (1999). This illustration is a composite of several specimens: AMNH FARB 5664, postorbital; ROM 1247, maxilla, nasals, lacrimal, jugal, braincase; ROM 1422, premaxilla, squamosal; TMP 1990.036.0500, premaxilla; USNM 12814, palatine, quadratojugal. The rationale for the decision to draft a composite was to present an approximation of what a “large stage 1” A. libratus would look like, where each of the specimens included are of the same relative maturity (Carr, 1999). The length of the skull is approximately 750 mm.

My illustrator, Mr. Dino Pulerà, and I worked closely together on this illustration. I first drafted a line drawing in pencil that was traced from photographs of the specimens, which Dino then transferred onto illustration board. He then covered the board in frisket (a sheet of low tack removable adhesive plastic), and cut out the space of the skull, leaving covered the spaces that extend through the skull (e.g., bony naris, antorbital fenestra). At that point illustration commenced, where Dino applied layers of graphite dust by brush onto the exposed board surface, gradually blocking out the form and shadows. By the convention of scientific illustration, the light source is from the upper left and skulls are usually depicted in right lateral view.

Once the form of the skull was achieved by brush, the intensive process in adding details began. To ensure accuracy Dino worked with the specimens (AMNH FARB, ROM) in hand illuminated with a single light source, and with me acting as art director over his shoulder. This approach ensured every detail seen in the specimens would be included in the image. Carbon dust has an advantage over line-and-stipple and coquille board in that the entire surface of a bone can be rendered, where no detail is lost to the low resolution and extensive white highlights that compromise the other approaches. With carbon dust, only the tooth (coarseness) of the board limits the level of detail in the illustration. This part of the process took the greatest amount of time, between 40 and 80 hours. In the end, the frisket was peeled and the image scanned for digital cleanup and layout.

Description

Directional terms are used throughout the osteological entries, and they are defined here.

Caudal: A point of reference at or toward the back of the skull.

Dorsal: A point of reference at or toward the top (upper surface) of the skull.

Rostral: A point of reference at or toward the front of the skull.

Ventral: A point of reference at or toward the bottom (lower edge) of the skull.

AOFEN – Antorbital fenestra: In terms of area, the antorbital fenestra (“window ahead of the eye”) is the largest opening of the skull. It is surrounded rostrodorsally, rostrally, and rostroventrally by the maxilla, caudoventrally by the jugal, and caudally and caudodorsally by the lacrimal. This large space is an osteological correlate for the paranasal sinus that extended laterally from the nasal airway to breach the bony enclosure of the snout. Several bones can be seen within the antorbital fenestra, including the conjoined pterygoid, palatine, and vomer. Also, the bony choana and palatine fenestra can be seen as gaps below and between the palatal bones.

The antorbital fenestra splits the facial skeleton into a set of dorsoventrally shallow struts; the dorsal strut is formed by the conjoined lacrimal, maxilla, and nasal, which form the dorsal skull roof in this region. The ventral strut is formed by the jugal and maxilla on each side of the snout.

AOR – Antorbital region: This region is equivalent to the vernacular term ‘snout’, the region of the face ahead of the orbital fenestra. The antorbital region is flanked laterally by (from caudal to rostral) the lacrimal, maxilla, and premaxilla; dorsally it is covered by the nasal; and ventrally and medially it is composed of (rostrally to caudally) the vomer, palatines, and pterygoids. The antorbital region is an open structure that is penetrated by several large openings, including the bony naris, antorbital fenestra, bony choana, and palatine fenestra. Importantly, this region also includes the teeth that extend from its lower margin and continue across the front. The outer surface of this region is coarse, especially its dorsal surface where primary ornamental structures are located. The coarse surface surrounds the smooth antorbital fossa.

Functionally the antorbital region is complex; it includes the nasal airway, pneumatic sinuses, the palate, the upper jaw, ornamental structures, and a concentration of openings for tactile sensory nerves. Taken together, it is reasonable to regard the antorbital region as simultaneously a massive fingertip, shield, signal platform, nasal passage, and penetrating pincer.

BCH – Bony choana: This opening corresponds to the vernacular ‘internal nostril’, the bony caudal end of the nasal passage. In life, soft tissue would have extended this passage further caudally. The bony choana is surrounded rostrolaterally by the maxilla; rostromedially by the vomer; and caudolaterally, caudally, and caudomedially by the palatine.

BN – Bony naris: The bony naris corresponds to the vernacular ‘nostril’, the rostral end of the nasal passage. In life, soft tissue would have positioned the external opening toward the rostroventral region of the bony naris. The bony naris is surrounded rostrodorsally, rostrally, and rostrolaterally by the premaxilla, and caudolaterally, caudally, and caudodorsally by the nasal.

LTB – Lower temporal bar: The lower temporal bar forms the ventral boundary of the laterotemporal fenestra, and it is composed rostrally by the jugal and caudally by the quadratojugal. The presence of this bar is typical of the diapsid condition seen in amniotes. The jugal component is formed by a pair of caudally extending processes that extend deep to a single rostrally extending paddle-like process of the quadratojugal. The lower temporal bar forms much the ventral margin of the skull in the temporal region. It is also an important strut that extended lateral to the jaw-closing muscles and connects the suborbital region with the suspensorium (quadrate + quadratojugal + squamosal).

LTFEN – Laterotemporal fenestra: This fenestra opens onto the adductor chamber, a space through which the jaw closing muscles extended, as well as structures associated with the outer and middle ears. This opening is typical of diapsid amniotes. The laterotemporal fenestra is surrounded rostrodorsally by the postorbital, rostroventrally by the jugal, caudoventrally by the quadratojugal, and caudodorsally by the squamosal. Parts of several structures can be seen through the fenestra, including the parietal, laterosphenoid, prootic, otoccipital, quadrate, and pterygoid.

The fenestra in derived tyrannosauroids is distinct in that a large rostrally extending flange composed of the squamosal above and quadratojugal below nearly cuts the fenestra into a pair of subordinate openings. However, in no tyrannosauroids is the opening so completely separated.

NB – Narial bar: The narial bar is the strut that encloses the bony naris dorsally and rostrodorsally. It is comprised by the nasal rostrally, and by the nasal and premaxilla caudally, where the premaxilla extends along the lateral surface of the nasal, eventually tapering and stopping at the dorsal margin of the bony naris.

OFEN – Orbital fenestra: The orbital fenestra surrounds the former location of the eyeball and its associated structures. It is surrounded dorsally by the frontal (sometimes excluded by the conjoined postorbital and lacrimal), rostrodorsally, rostrally, and rostroventrally by the lacrimal, ventrally by the jugal, and caudally and caudodorsally by the postorbital. Although the prefrontal is seen in lateral view in the rostrodorsal corner of the orbit, it does not contribute to the margin of the fenestra.

In large tyrannosauroids such as Bistahieversor and Tyrannosaurus, the fenestra is keyhole shaped, with a circular dorsal region and a triangular ventral region. The upper part surrounded the eyeball, whereas the ventral region did not. Parts of several structures can be seen through the fenestra, including the mesethmoid, orbitosphenoid, sphenoid rostrum, prefrontal, and pterygoid.

ORTR – Orbitotemporal region: This region surrounds the orbital and laterotemporal fenestrae, and comprises approximately the caudal half of the craniofacial skeleton.

PALFEN – Palatine fenestra: This opening occurs between the pterygoid and palatine and it is seen within the antorbital fenestra.

POB – Postorbital bar: This wide bar separates the laterotemporal and orbital fenestrae, and so forms their rostral and caudal margins, respectively. The postorbital forms the rostrodorsal half of the structure, whereas the jugal forms its caudoventral part. The postorbital bar flanks the adductor chamber laterally, and only its rostrodorsal part encloses the caudodorsal corner of the orbital region.

TR – Temporal region: The temporal region represents the caudal half of the orbitotemporal region and it surrounds the adductor chamber, through which extended the adductor musculature in life. The temporal region is comprised by the braincase medially, the suspensorium caudolaterally, and the postorbital and jugal rostrolaterally. The space is crossed medially by the vertical strut formed by the quadrate and pterygoid, which separates the medial tympanic (middle ear) space from the lateral channel for the adductor musculature.

UTB – Upper temporal bar: The upper temporal bar bounds the laterotemporal fenestra dorsally and connects the suspensorium with the postorbital bar. The squamosal forms the caudal part of the bar and extends medial to the postorbital, which forms the rostral part of the bar.  The squamosal receives the postorbital in a deep, v-shaped groove. When articulated, the squamosal has the appearance of splitting into a pair of processes above and below the postorbital. In most cases the dorsal process extends to the rostral end of the bar, whereas the ventral bar stops short of it.

References cited

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