Osteology VIII: The antorbital cavity

Digital carbon dust plate of the skull and mandibular ramus of an adult Tyrannosaurus bataar (PIN 551-3) in left lateral view © Dino Pulerà. The antorbital cavity is highlighted in color; plaster is a uniform dark gray. Among derived tyrannosauroids, the antorbital cavity of T. bataar is maximally enclosed by the surrounding subcutaneous bone and so has a reduced surface area when viewed from the side. The skull is 1130 mm long from the premaxilla to the quadrate (Maleev, 1974). Abbreviation: PIN, Palaeontological Institute, Russian Academy of Sciences, Moscow.

Introduction

In the last lecture of my Dinosaur Evolution and Extinction course, I walked the students through the Extant Phylogenetic Bracket (Witmer, 1990), and I concluded by placing a cast of an ankylosaurid skull in the middle of the table and saying (paraphrased here), “Skeletons are an illusion; bones are closer to the hole than to the donut - they are mostly shaped by the soft tissues that attach to them and permeate through their interiors. When you gaze upon bones you do not see the animal laid bare, you see a naked thing from which nearly all of the information has been stripped away.”

In this vein, my goal in this post is to draw attention to the skull as a living structure that is shaped by specific soft tissues and soft tissue systems. This will be a departure from the bone-by-bone approach that I have taken in the preceding posts, but both views are illuminating and I think this is a worthwhile change in perspective.

By the way, much of what is presented here follows the work of Dr. Lawrence M. Witmer (Ohio University, Athens), who innovated the Exant Phylogenetic Bracket (EPB) method for inferring the soft tissues that shaped the skeletons of extinct amniotes. I first saw him present his intellectually exciting research, which established the pneumatic null hypothesis for the antorbital cavity, at the 4^th Symposium of Mesozoic Terrestrial Ecosystems back in 1987 at the Tyrrell Museum of Palaeontology (Drumheller, Alberta). Ever since then I have respected and admired the innovative and illuminating work done by Larry and his students. As such, a strong EPB theme runs through the course of these posts.

A note on the illustration—The image is a digital carbon dust plate rendered by Mr. Dino Pulerà. This is our most recent collaboration; I drafted the line drawing from photographs that I took of the original specimen, when it was at the Royal Ontario Museum (ROM) as part of a temporary vertebrate fossils exhibit from Russia. I was fortunate enough to have after hours access to the exhibit  – for several nights – to take notes and measurements, and to sketch and photograph the skull. I produced the line drawing by standing directly over the skull with the photographs in hand; I was able to exactly draw all of the plaster reconstruction, foramina, sutures, and damage onto each photo. The resulting line drawing was the most exact that I could produce, under what were the best possible conditions.

By the time Dino and I agreed that we’d like to collaborate on a digital carbon dust, departing from the traditional graphite dust on illustration board, the original skull had long since been shipped to another venue. Fortunately, the ROM has a cast of the skull in its vertebrate paleontology collections (see photograph below). In 2008, Dino and I worked on the plate in the darkened collections room for several evenings over a week. Neither of us considers this work finished, but the results are promising enough to debut our new approach to illustration here.

Dino Pulerà at work on the digital carbon dust of PIN 551-3 during January, 2008. He is working from a cast in the collections room of the Department of Vertebrate Paleontology at the Royal Ontario Museum. Dino is crafting the illustration in Photoshop using a WACOM tablet.

On a less formal note, up until the ROM exhibit I had only seen the pen-and-ink illustration and the photographic plate of the skull in Maleev (1974), and the low-resolution casts of the skull in the holdings of the ROM, American Museum of Natural History, and the Canadian Museum of Nature. I have to admit that when I saw the specimen in person for the first time, it was a breath-catching moment - it is a very beautiful fossil, where the preservation has an unexpected level of anatomical detail. It is our hope to capture a sliver of that quality in the digital image.

TERMINOLOGY

The osteological terms used here follow those established by Witmer and Baumel (1993), and Witmer (1997) for archosaurian anatomy; anglicized versions of Latin terms in those sources are used when an English equivalent is not provided.

Antorbital cavity: The space ahead of the orbital fenestra, lateral to the nasal airway and medial to the external surface of the bone (Witmer, 1997). Laterally this space is enclosed by the external antorbital fenestra, and the dorsolateral surfaces of the palatine bone and the palatal process of the maxilla bound it ventromedially (Witmer, 1997). The cavity includes the external and internal antorbital fenestra, maxillary and promaxillary fenestrae, and the antorbital fossa and its subordinate openings (Witmer, 1997).

Antorbital fossa: The smooth surface that excavates the bony surfaces situated between the edges of the external and internal antorbital fenestrae (Witmer, 1997). This excavated surface (relative to the external surface of the bone) is also termed the medial lamina (Witmer, 1997).

Antorbital sinus: Term for the soft tissue structure that produced the bony antorbital cavity (Witmer, 1997).

Basal derived tyrannosauroids: This phrase refers to the Bistahiversor + Albertosaurus grade, excluding Tyrannosaurinae unless defined otherwise.

Derived tyrannosaurines: This phrase denotes the Daspletosaurus + Tyrannosaurus clade, and all descendants of their common ancestor.

Derived tyrannosauroids: This phrase refers to the Bistahieversor + Tyrannosauridae clade, which is the phylogenetic limit of content in this post unless stated otherwise.

External antorbital fenestra: The outer edge of the antorbital fossa (Witmer, 1997).

Internal antorbital fenestra: The margin of the antorbital fenestra that is separated from the external antorbital fenestra by a bony surface that is excavated by the antorbital fossa (Witmer, 1997). The fenestra is positioned lateral to the bony choana, the internal nostril (Witmer, 1997).

Lateral lamina (=lamina lateralis; Witmer, 1997): The external, subcutaneous surface of the maxilla ahead of the antorbital fossa, which may undercut the surface of the bone and extend ahead of the external antorbital fossa. In tyrannosaurids this takes the form of a strut that conceals the promaxillary fenestra from lateral view.

Maxillary antrum: The chamber within the medial half of the maxilla into which the maxillary fenestra opens; positioned caudal to the promaxillary recess (Witmer, 1997). The chamber extends ventrally between tooth root bulges as pneumatic interalveolar recesses (Witmer, 1997).

Medial lamina: See Antorbital fossa.

Promaxillary recess: The chamber ahead of the maxillary antrum into which the promaxillary fenestra opens; the recess is expressed externally as the vestibular bulla (Witmer, 1997).

Subcutaneous surface: The textured external surface of the facial and mandibular bones that was attached in life to the overlying skin.

Ventral ramus of the lacrimal (=jugal ramus; Witmer, 1997): This is the vertically oriented bar of the lacrimal that separates the orbital fenestra and the antorbital cavity from each other.

DESCRIPTION

General features of the antorbital cavity: In lateral view, the antorbital fossa is a smooth depression that is inset relative to the subcutaneous surface of the facial skeleton and surrounds the internal antorbital fenestra. The fossa is an osteological correlate the soft tissue antorbital sinus that is composed of epithelium, which removes bone (Witmer, 1987). As such, the bones that bear the fossa are described as excavated, in view of the destructive process that produced it in life (Witmer, 1987).

In terms of its relationships to bony structures, this cavity in derived tyrannosauroids extends laterally through the internal antorbital fenestra, rostrally into the maxillary antrum through the caudal fenestra of the maxillary antrum, medially through the maxillary fenestra into the maxillary antrum, and rostrally through the promaxillary fenestra into the promaxillary recess; it also extends ventromedially into the interior of the palatine bone through one or two large pneumatic recesses.

In tyrannosaurids, the fossa excavates the ventral half of the rostral ramus (the part that extends rostrally above the antorbital fenestra) of the lacrimal, the rostral surface of the ventral ramus of the lacrimal (although this cannot be seen from the side), the rostroventral quadrant of the ventral ramus of the lacrimal, the rostrodorsal quadrant of the maxillary ramus of the jugal, and the lateral surface of the maxilla that surrounds the antorbital fenestra, and extends rostrally to produce a region that is penetrated by two subordinate openings. Most of these features can be seen in Tyrannosaurus bataar, except that the cavity is enclosed rostrally and ventrally in the maxilla.

Even in a taxon such as T. bataar where it is partly enclosed, the cavity is extensive: rostrocaudally it occupies a third of the total skull length and over half of the length of the snout, and dorsoventrally it exceeds two-thirds of the preorbital skull height.

Antorbital fossa: In lateral view, the antorbital fossa is the smooth surface that separates the external and internal antorbital fenestrae; it is the surface that has been excavated by the epithelium of the antorbital air sac. The fossa is usually inset relative to the subcutaneous surface of the facial bones; however, in some places the surfaces grade into each other. The antorbital fossa excavates the lacrimal, jugal and maxilla as a continuous, unbroken surface that twists along its course around the internal antorbital fossa. Although most of the fossa faces laterally, in all derived tyrannosauroids it flattens the rostral surface of the ventral ramus of the lacrimal and so it is not in view.

Ontogeny of the antorbital fossa—During ontogeny, the rostral margin of the antorbital fossa becomes enclosed by the caudally extending rostrodorsal edge of the external antorbital fenestra, which conceals the promaxillary fenestra from view (Carr, 1999).

Taxonomic variation of the antorbital fossa—The fossa does not contact the nasal in Bistahieversor (Carr et Williamson, 2010) or in Daspletosaurus adults (Russell, 1970); this may also be the case in Raptorex (Sereno et al., 2009) and in juvenile T. bataar (Tsuihiji et al., 2009). The fossa has a very short contact with the nasal in Albertosaurus libratus (Carr, 1999) and juveniles of D. torosus (Currie, 2003). In contrast, the fossa has an extensive contact with the nasal in juvenile and adult T. rex (Carr, 1999; Carr et Williamson, 2004), and in adult T. bataar (Maleev, 1974).

In derived tyrannosaurines, the fossa faces ventrally along the midlength of the rostral ramus of the lacrimal such that the subcutaneous surface reaches the ventral margin of the ramus. In T. bataar, the fossa is enclosed by the subcutaneous surface of the maxilla ventrally, such that the fossa is blocked from lateral view; this condition is seen juveniles (Currie et Dong, 2001; Tsuihiji et al., 2009).

See Closure of the antorbital cavity in T. bataar, External antorbital fenestra.

Antorbital fenestra: See Internal antorbital fenestra.

Closure of the antorbital cavity in T. bataar: Arguably, T. bataar has a condition that is convergent upon what is seen in derived ornithischians, namely the external closure of the antorbital cavity. The external antorbital fenestra of T. bataar differs from that of other tyrannosauroids in that it is partly closed by the subcutaneous flange, a tall ridge that extends dorsally along the ventral margin of the fenestra (Carr, 2004). The flange blocks the ventral extent of the antorbital fossa of the maxilla from view, and entirely so below the internal antorbital fenestra.

In dorsolateral view, the flange produces a deep slot between itself and the antorbital fossa proper. This feature gives the snout of T. bataar is distinctive appearance in having what appears to be an unusually deep proximal dentigerous region, in contrast to what is seen in its closest relatives, including T. rex. Since tooth root bulges do not fill the slot, the flange is a dorsalward extension of the subcutaneous surface and is not produced by unusually deep tooth roots.

Distal accessory recess of the lacrimal—This opening occurs on the ventrolateral surface of the rostral ramus of the lacrimal at least halfway along the rostral ramus.

Taxonomic variation of the distal accessory recess of the lacrimal—This opening is seen in Alioramus, Teratophoneus, Daspletosaurus and in Tyrannosaurus Brusatte et al., 2012; Carr et Williamson, 2004, Carr et al., 2011). In Daspletosaurus, both this opening and the proximal accessory recess occur together (Carr et Williamson, 2004).

Dorsal fossa of the lateral interfenestral strut (=pneumatic excavation of the ascending ramus; Witmer, 1997): The rationale for the name change from Witmer (1997) is to express the exact topological position of this structure. This fossa is absent or shallow in relatively immature derived tyrannosauroids, whereas it is deep in adults (Carr, 1999; Carr et Williamson, 2004).

External antorbital fenestra: This feature corresponds to the edge of the antorbital fossa along the subcutaneous surface of the facial skeleton. It may seem to be a subtle distinction from the antorbital fossa, but the difference becomes clear in mature tyrannosaurids, where the subcutaneous surface becomes strutlike – especially rostrally – and partly encloses the perimeter of the antorbital fossa.

This rim extends along the lateral surface of the maxilla, jugal, and lacrimal, In some taxa, including T. rex and T. bataar, it extends along the ventrolateral edge of the nasal bone. Over much of its course the fenestra is a distinct rim, but in some taxa (e.g., Tyrannosaurus) it is only a subtle demarcation between the coarse subcutaneous surface and the smooth antorbital fossa in several regions, such as on the maxilla along the ascending ramus, the lacrimal at the lacrimal pneumatic recess, and the jugal on either side of the jugal pneumatic recess.

On the lacrimal, the rim occurs on the lateral surface of the bone at the angle between the ventral and rostral rami. The caudodorsal edge of the rim forms the external margin of the lacrimal pneumatic recess. The entire rim may be undercut by the antorbital fossa, or the subcutaneous and fossa surfaces might not be separated from each other (Carr, 1999).

The rim usually extends along the lateral surface of the rostral ramus; it may also extend along the ventrolateral edge of the ramus. As such, in some tyrannosauroids (Bistahieversor, Albertosaurus spp.), the antorbital fossa is widely exposed on the ramus in lateral view, whereas it faces largely or entirely ventrally in others (T. bataar, Daspletosaurus).

The rim extends along the leading edge of the ventral ramus of the lacrimal, before extending caudoventrally toward the jugal. In this region, the antorbital fossa returns to view on the lateral surface of a small flange, the rostroventral ala. As it extends from the lacrimal to the jugal, the rim may be distinct, where the antorbital fossa undercuts the subcutaneous surface of the lacrimal and jugal (Albertosaurus spp.) or it might be indistinct (Tyrannosaurinae), where the undercut is absent.

The rim of the fenestra extends around the rostrodorsal corner of the jugal, extending between the lacrimal dorsally and the maxilla rostrally. The rim forms the distinct caudoventral edge of the jugal pneumatic recess, which is undercut by an invasive sinus that inflates the body of the jugal bone.

Ontogeny of the external antorbital fenestra—In small juveniles of A. libratus, the rostroventral margin of the fenestra is distinct, whereas in more mature specimens it is indistinct and the subcutaneous surface and the antorbital fossa grade into each other. Also, in A. libratus juveniles, the rostral margin of the ventral ramus of the lacrimal is convex along its course; in contrast, the rostrodorsal margin is distinctly concave in subadults and adults (Carr, 1999).

See also Antorbital fossa.

Internal antorbital fenestra: This is the conventional ‘antorbital fenestra’ of most workers, which is usually not distinguished from the external antorbital fenestra. The internal fenestra is a large aperture that is enclosed by the maxilla rostrally, the lacrimal caudodorsally and caudally, and by the jugal caudoventrally. In some taxa the contribution of the jugal may be very short. Of these bones, the maxilla has the greatest contribution to the fenestra, followed by the lacrimal, and then the jugal.

Ontogeny of the internal antorbital fenestra—In tyrannosaurids the general trend is for the fenestra to deepen, increasing the ratio of its height to length (Carr, 1999; Currie, 2003). This trend pertains to all tyrannosaurids, including tyrannosaurines (Tsuihiji et al., 2009).

Taxonomic variation of the internal antorbital fenestra—The fenestra of basal derived tyrannosauroids is longer than tall, whereas among tyrannosaurines the fenestra is as long as tall, or taller than long (Carr, 1999). In contrast to other derived tyrannosauroids, the contribution of the jugal to the margin of the fenestra is greatly reduced – nearly excluded by the lacrimal and maxilla - in T. rex (Carr, 1999).

Jugal pneumatic recess: This recess penetrates the caudoventral corner of the antorbital fossa, ahead of or slightly below the ventral ramus of the lacrimal and caudal to the division between the lateral and medial maxillary processes of the jugal (Carr, 1999). The jugal pneumatic recess leads into an extensive chamber that hollows the body of the bone below the orbital fenestra and in some cases extends a short distance up into the postorbital process of the jugal.

Lacrimal antorbital fossa: The antorbital fossa covers the ventral region of the rostral ramus of the lacrimal, which is continuous rostrally with that of the maxilla. The antorbital fossa of the lacrimal is penetrated by up to three pneumatic foramina, in addition to the lacrimal pneumatic recess (Carr et Williamson, 2004).

Ontogeny of the lacrimal antorbital fossa—In juvenile A. libratus, the antorbital fossa on the rostral ramus is widely exposed to lateral view, whereas in subadults and adults, it is nearly excluded by the ventrally extending subcutaneous surface (Carr, 1999). In A. libratus, the antorbital fossa of the lacrimal is not undercut where it extends along the subcutaneous surface of the ventral ramus in juveniles; in contrast, the fossa is deeply excavated in this region (Carr, 1999).

Lacrimal pneumatic recess (=dorsolateral lacrimal fossa; Witmer, 1997): This opening occurs at the caudodorsal corner of the antorbital fossa, where it pierces the crux between the ventral and rostral rami of the lacrimal. The chamber it opens into hollows the supraorbital region of the bone and extends a short distance into the ventral ramus.

Ontogeny of the lacrimal pneumatic recess—In derived tyrannosaurines, the opening is large in juveniles; in contrast, in adults it is reduced in size in part by the inflation of surrounding pneumatic sinuses within the bone.

Taxonomic variation of the lacrimal pneumatic recess—In basal derived tyrannosauroids (Apalachiosaurus, Bistahieversor) and in derived tyrannosaurines (Daspletosaurus, Tyrannosaurus), the opening is small. In contrast, the opening is large in Albertosaurus, Alioramus, and Teratophoneus.

Lateral casement of the antorbital sinus: In modern birds, the facial skin covers the antorbital sinus (Witmer, 1997); based on recent common ancestry, there is no reason to think that this condition was not also in extinct archosaurs. This drum-skin arrangement was vulnerable to injury - I have seen many tyrannosaurids with extensive lesions on the antorbital fossa; some of these penetrate through the maxillary bone (Peterson et al., 2009). To my mind, it is notable that tyrannosaurids survived direct injuries (that presumably were open to the outside) to this part of their respiratory airway.

Maxillary fenestra: This opening is seen in all tyrannosaurids, and is the second largest opening in the antorbital fossa. The maxillary fenestra is the largest of the subordinate openings that can be seen in lateral view. It is a window that opens into the maxillary antrium, a box enclosed medially by thin bone that is penetrated by three additional medial openings.

The fenestra in tyrannosaurids has a distinct shape, where the caudodorsal and caudoventral margins are nearly straight and converge caudally, producing a caudally pointing V (Brochu, 1993). It is this V-shaped margin that defines the dorsal and ventral limits of the lateral interfenestral strut; ergo, the dorsoventral height of the fenestra is equivalent to that of the strut. The caudal extreme of the ‘v’ represents the narrowest point of the interfenestral strut. The rostroventral margin usually curves rostrodorsally along a steep angle to the convex rostrodorsal margin.

Ontogeny of the maxillary fenestra—In most derived tyrannosauroids the fenestra is small in juveniles, where it is located centrally in the antorbital fossa and does not approach the rostrodorsal, rostral, or ventral margins of the depression (Carr, 1999). In A. libratus the fenestra enlarges, where it extends rostrally, but without closely approaching the rostral margin of the antorbital fossa (Carr, 1999). In contrast, the opening is large in juveniles of Raptorex and T. bataar (Sereno et al., 2009; Tsuihiji et al., 2009).

In adults of basal derived tyrannosauroids (Bistahieversor, Albertosaurus) the fenestra is larger than the condition seen in juveniles, but it is still centrally located in the antorbital fossa and does not approach the rostral margin of the fossa. In tyrannosaurine adults, the fenestra closely approaches (Daspletosaurus) or extends ahead (Tyrannosaurus) of the rostral and rostrodorsal margins of the external antorbital fenestra.

Taxonomic variation of the maxillary fenestra—In basal derived tyrannosauroids (Appalachiosaurus, Bistahieversor, Albertosaurus, Teratophoneus), the fenestra is small. In contrast, the fenestra is large in derived tyrannosaurines,where the fenestra closely approaches. (Daspletosaurus) or extends medially past the rostral margin of the antorbital fossa (Tyrannosaurus; Carr, 1999).

The condition seen in Zhuchengtyrannus is unusual, where the enlarged fenestra reaches the rostral margin of the fossa by a notch (Hone et al., 2011). The fenestra is long in Alioramus, but it is centrally located within the antorbital fossa without closely approaching its rostral margin (Brusatte et al., 2012). In fact, it is situated closer to the rostrla margin of the internal antorbital fenestra than to the external antorbital fenestra Brusatte et al., 2012).

Promaxillary fenestra: In lateral view, this opening cannot be seen in T. bataar or T. rex adults owing to the rostrally extended maxillary fenestra. However, in less mature individuals and other tyrannosaurids, the dorsal and ventral margins of the fenestra can usually be seen, as well as a shallow fossa that extends between them into the opening.

Ontogeny of the promaxillary fenestra—In juveniles of A. libratus and T. rex, the promaxillary fenestra is a narrow slit that is not recessed dorsally (Carr, 1999). In contrast, the dorsal margin of the fenestra is recessed in subadults of A. libratus, whereas in adult T. rex the opening is modified into a round foramen (Carr, 1999). The fenestra is recessed in the holotype of Alioramus altai, where its dorsal margin has the form of a distinct ridge, which is consistent with its subadult relative maturity (Brusatte et al., 2012).

Taxonomic variation of the promaxillary fenestra—In Bistahieversor, Albertosaurus, Alioramus, and Teratophoneus, this opening is a narrow teardrop shaped slit. This condition is also seen in juveniles of Tyrannosaurus, whereas it is a round foramen in adults (Carr, 1999).

The dorsoventral position of the fenestra relative to the ventral margin of the maxillary fenestra is taxonomically informative, where the opening in A. libratus and Bistahieversor is positioned dorsal to the ventral margin of the maxillary fenestra (Holtz, 2001; Carr et Williamson, 2010).

Promaxillary pillar (=pila promaxillaris; Witmer, 1997): The region of the antorbital fossa that separates the promaxillary and maxillary fenestrae (Witmer, 1997).

Ontogenetic & taxonomic variation of the promaxillary pillar—In basal derived tyrannosauroids, excluding tyrannosaurines more derived that Teratophoneus, the pillar is rostrocaudally long and widely separates the promaxillary and maxillary fenestra. This is also seen in juvenile derived tyrannosaurines, whereas in adults the pillar is nearly eliminated (Daspletosaurus) or eliminated (Zhuchengtyrannus, Tyrannosaurus) by the rostrally extended maxillary fenestra.

Proximal accessory recess of the lacrimal: This recess occurs ahead of the lacrimal pneumatic recess and they are separated by a septum that may be short or long.

Taxonomic variation of the proximal accessory recess of the lacrimal: The proximal recess is seen in A. libratus (Carr, 1999), A. sarcophagus (Carr et Williamson, 2004), Daspletosaurus (Carr et Williamson, 2004), and in T. bataar (Carr et Williamson, 2004).

Rostrodorsal recess of the maxilla: This recess is seen in Alioramus altai (Brusatte et al., 2010), Raptorex (Sereno et al., 2009), and juvenile T. bataar (Tsuihiji et al., 2009).

Ontogeny of the rostrodorsal recess of the maxilla—In juvenile T. bataar this recess is a shallow fossa (Tsuihiji et al., 2009); it culminates in adults (PIN 551-1) as a penetrating recess (Maleev, 1974).

Taxonomic variation of the rostrodorsal recess of the maxilla—This recess is seen in Alioramus, Raptorex, and T. bataar, whereas it is not seen in other derived tyrannosauroids.

Rostroventral ala of the lacrimal (=ventrolateral lacrimal fossa; Witmer, 1997): This structure is a short triangular surface that is deeply excavated by the antorbital fossa such that it is separated from the subcutaneous surface of the ventral ramus by a deep slot. The lateral surface of the ala is concave, indicating an indistinct fossa.

Secondary fossa of the jugal: This structure is a synapomorphy of Tyrannosauridae, which is not seen in Bistahieversor or less derived tyrannosauroids (Carr et Williamson, 2010). This is a discrete fossa positioned medial to the jugal pneumatic recess that has a distinctly inset dorsal margin; in contrast, the corresponding surface in Bistahieversor is flat. The secondary fossa is widely exposed to lateral view in adult tyrannosaurines, such as T. bataar, whereas in juveniles and in nontyrannosaurines, the fossa is only marginally exposed to view (Carr, 1999).

Ontogeny of the secondary fossa—In tyrannosaurine juveniles, the secondary fossa is marginally exposed to lateral view; in contrast, it is widely exposed to view, where it extends far above the lateral edge of the jugal pneumatic recess (Carr, 1999; Currie, 2003). This exposure is almost certainly the result of absorption of bone along the lateral edge of the jugal pneumatic recess.

Taxonomic variation of the secondary fossa —In nontyrannosaurines, the secondary fossa is marginally exposed above the lateral edge of the jugal pneumatic recess. In contrast, the fossa is widely exposed in tyrannosaurines, such that it is nearly circular in outline (Carr, 1999; Currie, 2003).

Subordinate fossae: In lateral view, several nonpenetrating fossae are seen within the boundaries of the antorbital fossa. These occur with regularity in tyrannosaurids, although they may vary ontogenetically and phylogenetically. These include the proximal and distal pneumatic recesses of the lacrimal, the secondary fossa of the jugal, the rostrodorsal recess on the maxilla, and the dorsal and ventral fossae of the lateral interfenestral strut.

See Dorsal fossa of the lateral interfenestral strut, Rostrodorsal recess of the maxilla, Secondary fossa, Ventral fossa of the lateral interfenestral strut.

Subordinate penetrating recesses: The antorbital fossa has several subordinate recesses; these are openings that are larger than neurovascular foramina, which lead into sinuses that hollow out the interior of bones. Tyrannosaurids have a relatively fixed set of penetrating recesses, namely the lacrimal pneumatic recess, the jugal pneumatic recess, the maxillary fenestra, and the promaxillary fenestra. In addition to these, most tyrannosaurids have one or two accessory recesses ahead of the lacrimal pneumatic recess.

The maxilla has a set of subordinate recesses; these include a recess that occurs rostrodorsal to the maxillary fenestra (Alioramus altai, Raptorex, T. bataar), another in the dorsal half of the lateral interfenestral strut, and one in the base of the interfenestral strut. The recesses of the interfenestral strut become more deeply excavated during ontogeny (Carr, 1999), until – in Tyrannosaurus – they penetrate through the basal part of the strut (Maleev, 1974; Carr et Williamson, 2004).

See Rostrodorsal recess of the maxilla, Ventral fossa of the lateral interfenestral strut.

Ventral fossa of the lateral interfenestral strut—This fossa is an ontogentically controlled feature, where it is absent from juveniles, but present in subadults and adults (Carr, 1999). In some taxa (Zhuchengtyrannus, T. bataar, T. rex) it becomes a penetrating recess late in adulthood, where it opens into the maxillary antrum (Carr et Williamson, 2004). This fossa is not connected to the dorsal fossa of the interfenestral strut.

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Maleev, E.A. (1974). [Gigantic carnosaurs of the family Tyrannosauridae.]  Joint Soviet-Mongolian Palaeontological Expedition, Transactions 1, 132-191. (In Russian with English summary)

Peterson, J.E., Henderson, M.D., Scherer, R.P., and C.P. Vittore. 2009. Face-biting on a juvenile tyrannosaurid and behavioral implications. Palaios 24, 780-784.

Russell, D.A.  1970.  Tyrannosaurs from the Late Cretaceous of western Canada. National Museum of Natural Science Publications in Palaeontology 1: 1-34.

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.

Witmer, L.M. 1990. The extant phylogenetic bracket and the importance of reconstructing soft tissues. In Functional Morphology in Vertebrate Paleontology. Edited by J.J. Thomason. Cambridge University Process. pp. 19-33.

Witmer, L.M., and Baumel, J.J. 1993. Osteologia. In Handbook of Avian Anatomy: Nomina Anatomica Avium, Second Edition. Edited by J.J. Baumel, A.S. King, J.E. Breazile, H.E. Evans, and J.C. Vanden Berge. Publications of the Nuttall Ornithological Club. Cambridge, Massachusetts. pp. 44-132

Witmer, L.M.  1997.  The evolution of the antorbital cavity of archosaurs: a study in soft-tissue reconstruction in the fossil record with an analysis of the function of pneumaticity. Journal of Vertebrate Paleontology 17 (Suppl. to No. 1): 1-73.

Osteology VII: Mandibular ramus in lateral view.

Mandibular ramus of a subadult Albertosaurus libratus (ROM 1247) in right lateral view,

© Dino Pulerà. This illustration was our first professional collaboration.

Introduction

The image in this post is of the mandibular ramus of a subadult Albertosaurus libratus in right lateral view, based on the bones preserved in ROM (Royal Ontario Museum) 1247. This carbon dust plate was the first collaborative work - back in 1993 - between myself and Dino Pulerà. We chose the ramus as the starting point for our collaboration for several reasons: in contrast to the skull, the ramus required less time for me to draft a line drawing, and less time for Dino to transfer and render. Not only does the ramus have fewer bones than the skull, it is also essentially a flat, two-dimensional surface, aside from the teeth, foramina, the upper region of the surangular, and the rostroventral region of the dentary.

This was also a test of the effectiveness of the carbon dust medium for this type of subject matter. I learned very quickly that carbon dust can achieve what other media can’t - it can capture a bone’s entire surface and texture, in addition to its contours, within the limits of the tooth (texture) of the illustration board. The end result is as high resolution as can be hoped to achieve.

One of the most important learning experiences for me was that this was a collaborative endeavor - along the way Dino noticed many details of the bones that I had overlooked, and I’ve learned from him to view bone with intense scrutiny. I also picked up the habit of ‘viewing’ bones with my fingertips, an approach that Dino used to verify if a surface was truly featureless or to follow the course of subtle features. In the end, this image was the most accurate depiction that we could produce within our abilities at the time. The articular is not pictured here because that bone is missing from the specimen.

DESCRIPTION

General form of the mandibular ramus: The mandibular ramus of tyrannosaurids is longer than deep, where the region of maximum depth is situated caudal to the midlength of the ramus. The dentigerous (tooth-bearing) region occupies nearly the entire rostral half of the ramus. With the ventral margin of the dentary held level, the jaw joint is situated far above the level of the tooth row. The ramus is penetrated by two relatively large openings, namely the caudal surangular foramen (csf) and the external mandibular fenestra (emf).

Caudal surangular foramen—The csf penetrates the ramus rostroventral to the glenoid, ventral to the surangular shelf, and above the angular. Dorsoventally, the csf is positioned above the level of the tooth row. In lateral view, and more obviously in medial view, it is located ahead of the point of divergence between a pair of prominent ridges; this perforation was not situated in place of structural weakness.

External mandibular fenestra—The emf is a gap that opens into the lower part of the Meckelian (=prearticular) fossa between the rostroventral margin of the surangular, rostrodorsal edge of the angular, and caudoventral margin of the dentary. This aperture is situated close to the ventral margin of the ramus at – approximately - its caudal third above the end of the dentary. Dorsoventrally, the emf is positioned below the level of the tooth row. Although the prearticular can be seen through the gap, it does not contribute to its margin; instead, the bone is situated far medially, where it encloses the Meckelian fossa between itself and the laterally positioned bones.

Margins of the ramus—The caudodorsal, caudal, and caudoventral margins of the ramus are formed by the surangular. Ahead of the surangular, the prearticular forms the caudoventral margin of the ramus for a short distance, which is followed rostrally for a shorter distance by the angular, and finally, the rest of the ventral margin is formed by the dentary. The dentary also forms the rostroventral margin, and greater than the rostral half of the dorsal margin of the ramus. Although the splenial is a large bone in tyrannosaurids, it is not exposed laterally.

Distinct landmarks of the ramus—In broad analogy with mammals, the ramus can be thought of as having a ‘chin’ and an ‘angle’, where the chin is at the intersection of the rostroventral and ventral margins of the dentary, and the ventral mandibular angle is at the change in direction of the ventral margin between the horizontal dentary and the caudodorsally extending postdentary moiety. As illustrated, the chin in ROM 1247 is below the 4^th alveolus, and the angle is below the rostral end of the emf.

The Bones and Landmarks of the Mandibular Ramus

Angular: In lateral view, the angular is third largest bone, which forms most of the caudoventral quadrant of the ramus to the exclusion of the surangular and prearticular. It has an extensive contact with the surangular dorsally, a narrow contact with the prearticular ventrally and medially, and is rostrally overlapped by the dentary. Regardless, most of the bone is exposed laterally.

The angular closely approaches the csf and forms the concave caudoventral margin of the emf. Despite its size and position, the angular only forms a small part of the ventral margin of the ramus between the dentary and prearticular. The angular extends caudodorsally from the ventral angle of the mandibular ramus.

Caudal mandibular angle: In lateral view, this is a convexity in the dorsal margin of the surangular at the rostral end of the coronoid region, an extensive muscle insertion surface that covers the dorsolateral surface of the surangular ahead of the glenoid fossa.

Chin: The abrupt change in direction that separates the caudoventrally extending rostral margin of the dentary from the horizontally oriented ventral margin of the bone.

Coronoid process: A low, dorsally extending ridge along the medial edge of the surangular that bounds the coronoid region medially.

Dentary: In lateral view, the dentary is the largest bone of the mandibular ramus, where it makes up nearly three quarters of its length. The caudoventral part of the bone forms the angle of the mandibular ramus. The dentary forms the rostral margin of the emf, where the opening usually notches the bone; this condition is not seen in ROM 1247. When viewed from the side, the dentary contacts the surangular caudodorsally and the angular ventromedially.

In a general sense, the dentary is shaped like an hourglass set on its side, where the dorsal and ventral margins are concave, and the long axis deepens rostrally and caudally. The caudal end of the bone is deeper than the rostral end, reaching its maximum height at its contact with the surangular. Rostrally, the bone is deepest in the neighborhood of the fourth-sixth tooth positions. The rostral end of the lateral alveolar margin is convex, which becomes concave from approximately the eighth tooth position to the caudal end of the tooth row. The alveolar margin has a scalloped margin of low convexities that are positioned beside each alveolus.

The rostral margin of the bone extends abruptly caudoventrally before abruptly extending caudally. The point of change in direction is here termed the chin, which is usually below alveolus four and, less frequently, it occurs below the third tooth position. The transition point also marks the approximate location of the caudal limit of the symphysis. The rostral margin of the bone is largely convex.

The ventral margin of the dentary is sinuous, where its rostral extent is gently convex before becoming concave for most of the length of the bone. Caudally, the ventral margin extends caudodorsally at the ventral angle of the mandibular ramus. As such, the caudoventral corner of the bone is situated on the lateral surface of the angular.

The caudal edge of the dentary is overlapped by the surangular, although this overlap might not be as extreme as seen in the illustration presented here. Caudodorsally, the dentary in turn overlaps the surangular with a stout process.

Over half the length of the dentary is occupied by the tooth row. It is evident that the extensive and complex lateral contact with the surangular, and the broad medial contact with the angular served to stabilize the bone on the postdentary moiety during forceful bites.

Among the mandibular bones, the dentary is perforated by the greatest number of neurovascular foramina. These are concentrated rostrally, parallel to the tooth row, and along the ventrolateral margin of the bone, stopping ahead of the level of the last teeth. If the branches of sensory nerves entered these foramina from the skin, then the oral margin, tip of the jaw, and ventrolateral surface of the dentigerous region were highly sensitive to touch in life.

In contrast, only a single large neurovascular foramen penetrates the rostrodorsolateral surface of the surangular, which is presumably represents the caudalmost foramen of the dorsal row. Otherwise, the caudal region of the dentary and the entire postdentary moiety is imperforate and presumably insensitive to tactile information.

Many minute foramina penetrate the upper edge of the dentary along the tooth row; presumably these were primarily vascular structures located in a region that would have required nearly constant remodeling due to the compressive forces and lesions sustained during killing and feeding.

The lateral surface of the dentigerous region is convex and bulges slightly laterally below the dorsal row of foramina. In contrast, the caudal region of the bone is flat, continuous with the condition seen in the surangular and angular behind it.

Glenoid fossa: In lateral view, the margin of the jaw joint as formed by the dorsal margin of the surangular bone.

Postdentary moiety: The complex of bones caudal to the dentary, including the surangular, articular, angular, and prearticular. This region and the dentary are connected by the surangular, prearticular, angular, splenial, and intercoronoid. The only bone that is not involved in this complex connection is the articular. The postdentary moiety bears the mandibular portion of the jaw joint and houses the massive Meckelian fossa, in addition to its connection with the dentary.

Prearticular: Although in lateral view only a narrow slip of the prearticular is seen, it forms most of the caudoventral margin of the mandibular ramus between the surangular caudally and the angular rostrally. The prearticular does not reach the ventral angle of the ramus. Most of the prearticular extends along the angular, except for its caudodorsal extent that extends below the surangular; the surangular forms a short and narrow wedge that separates the prearticular from the angular.

The prearticular can be seen in the ventral quadrant of the emf, where its rostral ramus begins to extend rostrodorsally along its course toward the splenial and interocornoid. The dorsal margin of the bone represents the ventromedial margin of the Meckelian fossa. This low rim formed by the prearticular along the ventromedial edge of the fossa attests to the massive and extensive musculature that in life filled the medial surface of the postdentary moiety.

Retroarticular process: The short, fan-like process that extends caudoventrally from the caudal end of the surangular.

Rostral mandibular angle: The abrupt change in direction in the dorsal margin of the ramus, between the caudal end of the tooth row and the postdentary moiety, at a convexity in the rostral end of the surangular.

Rostral surangular foramen: A neurovascular canal that opens at a low angle along the rostrodorsolateral surface of the surangular caudal to the joint surface for the tab-like process of the dentary. This foramen is almost certainly the caudalmost foramen of the dorsal row of foramina that penetrates the dentary.

Surangular: In lateral view, the surangular forms most of the caudodorsal section of the postdentary moiety, and accounts for approximately two-thirds of the height of the ramus through its maximum height. The surangular forms the dorsal, caudal, and caudoventral margins of the mandibular ramus; it also completes the glenoid fossa in forming its lateral half, although this is out of the plane of the illustration. The caudoventral region of the bone is overlapped by the angular, and rostrodorsally it is overlapped by a short process of the dentary.

Caudoventrally, the surangular is reduced to narrow slip between the prearticular medially and the angular laterally. These bones contact rostrally, pinching out the surangular between them, where the prearticular extends laterally to cup the angular from below.

Ventral mandibular angle: The abrupt change in direction of the ventral margin of the bone, between the horizontally oriented dentary and the caudodorsally oriented postdentary moiety. The angle proper occurs at the dentary.

Q&A III: Nanotyrannus – my turn in the hot seat.

A cast of the Cleveland Skull next to a cast of an adult Tyrannosaurus rex skull in the striking dinosaur gallery of the Royal Ontario Museum.

 Introduction

What follows are my answers to Mickey Mortimer’s extensive questions regarding my post “Nanotyrannus isn’t real, really”. I think they are excellent and require response, especially since it gives me the opportunity to clarify and expand some points regarding my work on the Cleveland skull, the increasingly irrational defenses of the taxon, recently uncovered specimens, and the inaugural article. However, I presently have three projects in the works that have direct bearing on this taxonomic issue. Therefore, I cannot state the conclusions or share data before they are published. You'll have to read the previous post for the appropriate context.

#1: Notably it's only really engaging with the first and worst arguments for Nanotyrannus' validity.

A diagnosis sets out the evidence for the validity of a taxon; for a systematist, it is transparency at its best. In my mind, there is no reason not to accept the diagnosis of Nanotyrannus (or any other taxon) as, prima facie, the best argument for establishing a scientific name. Also, a diagnosis is a hypothesis that can be tested. This is a crucial point, and my hypothesis - the Cleveland skull is a juvenile T. rex - can be falsified if it can be shown that the diagnosis of Nanotyrannus is defensible. What could be easier? Has anyone done this?

The defenders of Nanotyrannus have not even mentioned the diagnosis in their articles.

Perhaps none of them have given the diagnosis any thought, or they assume that it is perfectly fine. Instead, they have just heaped on new characters as if that’ll do the job of rescuing the taxon. In the end, it does not matter if that blind spot is out of agreement, complaisance, avoidance, ignorance, or indifference; we must all agree that the diagnosis is central to the question of the validity of Nanotyrannus, and it has been on borrowed time.

In the case of Nanotyrannus, the diagnosis is important because if the purported type specimen (the Cleveland skull) is not diagnostic, then there is no taxon to which new specimens can be referred. That is why skeletons such as Jane and the ‘dueling tyrannosaur’ are completely irrelevant to this taxonomic issue. Arguably, the diagnosis isn’t low-hanging fruit; in this case, it’s the whole orchard.

#2: Sure Bakker's 25 year old ideas are flawed, but any modern defense of the genus includes a swath of newly recognized characters, and of course Jane.

There are two flaws that I see in Bakker et al. (1988): the article splits tyrannosaurids into many taxa without first assessing ontogeny, including the splitting of Nanotyrannus from Tyrannosaurus rex; I think the last paragraph in their article reveals the philosophical view that motivated the decision. The second flaw was in not conducting a cladistic analysis of the data listed in the caption of their branching diagram. As such, their phylogenetic scheme is a subjective arrangement and not a true test of character congruence.

Aside from those issues, the rest of the article is an excellent and refreshing review of tyrannosaurid cranial osteology. For example, I use their terminology for the basicranium, because no one before or since has done a better job of clarifying the salient landmarks of that region, or in providing a more useful nomenclature for those features.

If by new characters you mean the ones in Larson (2013), then I cannot say anything more than this: I deal with them in my in-progress work on T. rex ontogeny. That article includes a review of all of the literature and ‘evidence’ advanced against the hypothesis in Carr (1999). As you might guess, when the smoke clears, that a juvenile T. rex is still standing.

Again, specimens such as Jane and the ‘dueling tyrannosaurid’ are irrelevant to the issue unless it can be shown that they are different from juvenile T. rex as represented by the Cleveland skull. In that case, we’d have a new taxon or two, but we wouldn’t have Nanotyrannus.

#3. So yes, Bakker was wrong to think the Nanotyrannus type had extensive cranial fusion. But Larson (2013) reports Jane has presacral neurocentral, scapulocoracoid and pelvic fusion. By not addressing these characters (also including glenoid position, subnarial foramen position, quadratojugal pneumaticity, maxillary tooth count, etc.), your analysis loses most of its force.

My analysis is on the way! In the meantime, I’m going to sound like a broken record here, but I do think it is worthwhile repeating: Jane is irrelevant to the Nanotyrannus issue because (a) it is not the type specimen, (b) the diagnosis of Nanotyrannus is indefensible; therefore, (c) no specimen can be referred to a taxon that does not exist in Nature.

If it can be shown that Jane is a different taxon from the Cleveland skull and adult T. rex, then it becomes the holotype of a new taxon. There is no way that Jane can be referred to Nanotyrannus, because the taxon hasn’t a single diagnostic character. Also, I did propose a hypothesis to account for the difference in tooth count (Carr, 1999).

By the way, I am at the helm of the description of Jane, and we do take care of the purported diagnostic characters listed in recently published articles (e.g., Larson, 2013). As such, my hands are tied on discussing those characters specifically until the article is published.

#4. There's also the problem of using similarity to juvenile stages of other species as an argument. In the 4 stage ontogeny of your 1999 paper, Albertosaurus simply never gets to stage 4. It stops at what is stage 3 in Daspletosaurus. Indeed, in 1996, you used this to argue Daspletosaurus are just adult Albertosaurus. You later changed your mind on this, but couldn't Nanotyrannus be a tyrannosaurine that never grows past stage 1? This makes your huge table moot, just as a huge table of stage 3 characters listed as "Yes!" in Albertosaurus would not mean it was a subadult Daspletosaurus.

The goal of Carr (1999) was to establish a baseline of what the primary growth stages of a  tyrannosaurid looks like. In this case, Albertosaurus libratus was the point of reference given the high number of specimens and completeness of the growth series. It turns out that the growth series can be blocked out into four stages (small stage 1, large stage 1, stage 2, stage 3). In this series, the stepwise transformation of the juvenile features into adult can be clearly seen. I then assumed, based on common ancestry, that this pattern is plesiomorphic for Tyrannosauridae.

Working under that parsimonious assumption, I then reviewed the evidence for pygmy tyrannosaurids, such as Nanotyrannus and Maleevosaurus (it is curious that people are a lot less upset about my hypothesis that Maleevosaurus is a juvenile T. bataar). It turns out that both have the same features as juvenile A. libratus; ergo, they must be juveniles. Since that time, every juvenile of other derived tyrannosauroids (e.g., Bistahieversor, Daspletosaurus, T. bataar) that have come to light have the same constellation of juvenile features as are seen in A. libratus, the Cleveland skull, and "Maleevosaurus". If the Cleveland skull had characters seen in adults of A. libratus, Daspletosaurus, T. bataar, or T. rex, then that would be an indication that it might be a pygmy taxon and my hypothesis would have been different, perhaps validating Nanotyrannus.

As the Cleveland skull and juvenile Daspletosaurus (Currie, 2003) show, tyrannosaurines have the same starting point as A. libratus; this is consistent with the hypothesis that the growth pattern is conserved (i.e., plesiomorphic) in Tyrannosauridae (Carr, 1999). The calibration of growth series is a major part of a manuscript that I have in progress, some of this I have presented at SVP meetings over the past few years.

As such, it turns out that tyrannosaurids went through the same ontogenetic changes in the skull. In the table I present (implicitly) the hypothesis that Daspletosaurus is peramorphic relative to A. libratus, in that the growth trends are carried further in the tyrannosaurine. That is why Daspletosaurus is under the heading “Stage 4”. In the same vein, T. rex is peramorphic relative to Daspletosaurus.

The salient point you make is the suggestion that the Cleveland skull is an adult, but it does not grow past stage 1. I think it is reasonable to expect that an adult of a true ‘pygmy’ tyrannosaurid would have all of the adult characters seen in its closest relatives. The Cleveland skull certainly does not have any adult features - of any derived tyrannosauroid - that I can see.

On a related note, the original manuscript for Carr (1999) did not have the summary table of characters. One of the insightful reviewers required that I put the reams of features mentioned in the text in one, easy-to-follow, table. I like the outcome: the table of growth characters is a useful tool that people can use for a quick assessment of the relative maturity of any new tyrannosaurid specimens.

In the table, each column of characters can be thought of as analogous to a series of lines of arrested growth. I have since tested the data for hierarchical structure using parsimony analysis for T. rex (Carr and Williamson, 2004) and A. sarcophagus (Carr, 2010), which was found to be present. A hierarchy from this sort of data is best explained by ontogeny. As such, the table – and the cladistic analyses drawn from it – summarizes the hard-wired sequence of growth changes the members of the clade inherited, and altered, from their recent common ancestor.

#5. A related point is that Larson agrees Nanotyrannus could be the sister taxon of Tyrannosaurus, so your long list of characters they share works well for both hypotheses. We don't need to assume Bakker's odd phylogeny with a basal Nanotyrannus convergent with Tyrannosaurus for the genus to be valid.

Could Nanotyrannus could be a juvenile specimen of the sister taxon of T. rex? For that to be defensible, it has to be shown that the purported new taxon is diagnostic. The weight of evidence shows that it is a juvenile T. rex, so that avenue of thought doesn’t rescue the identity of the Cleveland skull from T. rex.

With regard to Bakker et al.’s (1988) tyrannosaurid phylogeny, I do think that context matters. Their arrangement complements what they say in the text, together showing the deductions that led to their taxonomic decision. It is an excellent article in that the data are absolutely clear.

#6. Further, it implies a taxon can be invalidated merely by finding the original diagnosis wanting. Let's see how that works for Tyrannosaurus, based on Osborn's (1905) original diagnosis.
1. "Carnivorous Dinosaurs attaining very large size." Tarbosaurus got as large as the average Tyrannosaurus, so this is invalid even within Tyrannosauridae.
2. "Humerus believed to be of large size and elongate (Brown)." This was based on an incorrectly identified bone.
3. "No evidence of bony dermal plates (Brown)." This was to distinguish it from Dynamosaurus, whose holotype had incorrectly referred plates.
Well, guess Tyrannosaurus is an invalid taxon we can't refer the Cleveland skull to... 

Is there any serious reason to doubt the validity of T. rex as there is for Nanotyrannus? The diagnosis issue has to be taken on a case-by-case basis; there is no reason to think that T. rex is invalid, given the quality of specimens (including the type) and sample size that has been amassed. Certainly, the time has come to bring the diagnoses for T. rex and many other taxa up to date, as I did for Albertosaurus sarcophagus (Carr, 2010). I have this in the works for other tyrannosaurids, including T. rex.

#7. Finally, couldn't the Cleveland skull be a juvenile and a valid genus, just without known adult specimens? That's what you argue for in Alioramus, which is (1) based on two specimens of juvenile tyrannosaurid, (2) has an original diagnosis which relies entirely on characters claimed to be ontogenetic in the case of Nanotyrannus (long snout, high tooth count, compressed teeth, etc.), (3) has derived characters only known in the sympatric Tarbosaurus, so is by your reasoning (4) therefore juvenile Tarbosaurus. Yes, Alioramus has additional suggested apomorphies, but so does Nanotyrannus and they seem to be of equal import (e.g. additional pneumatic features in each). So I'd like to know why you came to divergent conclusions in these similar cases.

I was not at the helm of that project, which accounts for the difference in results (this also pertains to your recent follow-up comment on Alioramus).

#8. Is that really fair to paleontologists in 1988? Almost no theropod specimens were even recognized as juveniles, besides some Gallimimus and Allosaurus individuals. The few known juvenile tyrannosaurids (e.g. the then unnamed Jordan theropod and Shanshanosaurus) were thought to be small basal tyrannosauroids or even dromaeosaurids. Similarly, there was no consensus the smaller Nemegt tyrannosaurids were young bataar. AFAIK, no one had thought to look for immature bone grain in any Mesozoic dinosaur yet, and there was no consensus of (Alberto(Daspleto(Tarbo,Tyranno))) like we have now to use to find nested traits that suggest synonymy. The sample size of Tyrannosaurus was low enough to make the number of specimens a notable media figure, when now we have more than most species of theropod known from more than just their holotype. The argument Nanotyrannus is a juvenile Tyrannosaurus thus depends on a lot of data which didn't exist in 1988 or wasn't organized yet. Rozhdestvensky (1965) was remarkably prescient decades earlier regarding some of these issues, though his same methodology incorrectly found Lufengosaurus to be young Yunnanosaurus, so we might question whether his seemingly correct tyrannosaurid ideas were validly justified or at least partly luck.

Yes, it is fair because sufficient data was at hand (e.g., Rozhdestvensky, 1965), and there were two growth series (A. libratus, T. bataar) available with which to make comparisons. Bakker et al. (1988) had the opportunity to define tyrannosaurid ontogeny, despite the stumble over suture closure in the Cleveland skull.

#9. It's that old standing on the shoulders of giants cliche, and while you are a major giant here, suggesting Currie, Russell, Molnar, Paul, Carpenter, Bakker, etc., basically every theropod worker of the 80s, was unreasonable seems unfair.

 I did not cast shade over the entire lot; I just just think Bakker et al. (1988) were unreasonable in their approach to tyrannosaurid taxonomy; they split taxa before assessing ontogeny. Aside from that, sincerely, it is an excellent osteological work.

#10. A valid criticism is that Bakker et al. (1988) never cite or mention Molnar's (1980) description of what would become the type of Dinotyrannus, that was referred to lancensis at the time. It was published 8 years earlier in a well read journal, so should have gotten some comment. 

That’s a good point! I have to admit that I was late on the scene as well (Carr and Williamson, 2004).
 

#11. The supposed unreasonableness of Bakker et al. (1988) also affects your main argument, as some of it depends upon the truth of our current consensus. Accepting Nanotyrannus as a juvenile Tyrannosaurus seems more plausible in a world where Maleevosaurus is just a young Tarbosaurus, Stygivenator is a juvenile Tyrannosaurus, etc.. But if the well-studied Cleveland skull is valid based on newly recognized characters, what does this mean for Maleevosaurus, which has never been studied firsthand since its description to my knowledge? Larson considers Stygivenator a younger Nanotyrannus, for instance, so the former can no longer be used as an example of what juvenile Tyrannosaurus are like.

The bottom line is that the relative maturity of specimens and their taxonomic identity must be considered separately, so a change in the decision regarding the Cleveland skull does not reverse the decision on Maleevosaurus; it doesn’t change the fact that both have the same constellation of juvenile features that are seen in all other derived tyrannosauroids, nor does it change the fact that the Cleveland skull has features that are only seen in T. rex. Williamson and I (2004) showed that ‘Stygivenator’ is referable to T. rex, but Larson (2013) did not engage with the evidence that we presented.

In the same fashion, the Larson (2013) article did not deal with the diagnosis issue in Bakker et al. (1988), or the evidence presented in Carr (1999). Therefore, the taxonomic decisions in that article carry no weight because the validity of Nanotyrannus is uncritically assumed to be valid despite all of the evidence to the contrary.

I trust all of that satisfies your query.

References cited
 

Bakker, R.T., Williams, M., and Currie, P.J.  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.

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.

Currie, P.J. 2003. Cranial anatomy of tyrannosaurid dinosaurs from the Late Cretaceous of Alberta, Canada. Acta Palaeontologica Polonica 48(2): 191-226.

Larson, P. L. 2013.  The case for Nanotyrannus in J. M. Parrish, R. A. Molnar, P. J. Currie., and E. B. Koppelhus (eds.) Tyrannosaurid Paleobiology, University of Indiana Press, Bloomington and Indianapolis, pp. 15-53.
Rozhdestvensky, A. K. 1965. [Growth changes in Asian dinosaurs and some problems of their taxonomy]. Palaeontological Zhurnal 1965:95-100. (Translated from the Russian.)