In its general shape and proportions the skull of
Paarthurnax broadly resembles that of other goniopholidids and many modern semi-aquatic crocodilians, being roughly triangular in shape with a single
naris and
orbits that face upwards. The snout is flattened (platyrostral) and makes up about 65% of the total skull length. The tip of the snout, formed by the paired
premaxilla, is distinctly axe-shaped like in other goniopholidids thanks to the presence of prominent notches at the contact with the maxilla, functioning to receive an enlarged fourth dentary tooth as seen in today's crocodyloids. The snout tip is further described as deflecting downwards, essentially causing the premaxillary toothrow to be lower than the subsequent maxillary teeth. The external narial opening is entirely surrounded by the premaxillae with no contribution from the
nasal bones and surrounded by a perinarial crest, an elevated rim that is at its widest and tallest around the sides of the opening and more subtle towards the back. Where the two premaxilla meet in front of the naris the holotype preserves a small dorsal projection and behind the naris the surface of the premaxillae along the midline suture is notably depressed and features a small fossa. Looking at the maxilla from above shows a distinct outwards bulge around the position of the fourth and fifth maxillary teeth that gives the bone a slightly sinusoidal outline. Among the most notable features of the maxilla compared to other goniopholidids is the apparent absence of a distinct ovoid maxillary depression as seen in most members that traditionally make up the family. Instead a series of
neurovascular foramina eventually leads to a narrow groove in the same position that would be occupied by the depression in other taxa. In addition to some pholidosaurids, in which a similar groove is seen extending onto the
jugal bone, this feature is most similar to what has been observed in the latest Cretaceous goniopholidid
Denazinosuchus as well as the paluxysuchine
Deltasuchus. Given that both the classical goniopholidid maxillary depression and the groove of Cretaceous North American forms are restricted to the maxilla and located in the same general region it is hypothesized that the two structures are
homologues. The nasal bones stretch the dorsal-most surface of the snout from roughly the position of the maxillary bulge until their contact with the anterior process of the frontal, being both excluded from participating in the naris as well as ending well before the orbital margin. The paired bones begin as a narrow wedge inserted in-between the posterior processes of the premaxilla and grow gradually wider until being themselves bifurcated by the frontal. Towards the back of the skull the nasals come into contact with the
lacrimal and the
prefrontal bones, which form much of the anterior and lateral edges of the
orbits. Through this, the prefrontal largely excludes the frontal from the orbital margin sans for a miniscule contribution on the
skull table. The orbital margin is further characterized by an elevated rim that stretches over most of the prefrontal, continues onto the lacrimal and ends in a subtriangular lacrimal fossa. Though not preserved,
Paarthurnax likely also possessed a
palpebral, as evidenced by the fact that the prefrontal shows the articular surface for the bony eyebrow. The dagger-shaped frontal bridges the rostrum and the skull table, forming a pointed anterior process that forms much of the space between the eye sockets, extends beyond the tips of the prefrontals and separates the posterior processes of the nasals. The frontal contributes little to the orbital margin itself, barely participating enough to just prevent the prefrontals from coming into contact with the
postorbitals. The frontal houses a prominent interorbital crest that spans not only this central element but stretches across both prefrontals and much of the two lacrimals. The frontal's participating on the skull table consists of lateral processes that contact the postorbitals, its contribution to the two sub-rectangular or sub-ovoid and very large
supratemporal fenestrae and the posterior extension that contacts the
parietal. The postorbital bears two notable projections. The smaller of the two is located is located anteromedially on the intertemporal bar, a suble convex peg that extends into the posterior corner of the orbit. The anterior outer corner meanwhile features a much more pronounced and elongated process that extends forward and down, forming the lateral edge of the orbit, shielding its side and causing the eyesocket to be directed upward. Similar projections are seen in some other neosuchians, among them
Sarcosuchus and the more closely related
Paluxysuchus and
Anteophthalmosuchus. The postorbital further contributes to the
postorbital bar and overlaps the
squamosal further back on the skull table. The squamosal ultimately forms the outer corners of the skull table, forming an elongated prong-like process that extends beyond the posterior edge of the
parietal, which the squamosal contacts medially. The lateral face of the squamosal bears a shallow groove that extends forward onto the postorbital that serves as an attachment point for the upper ear-lid or earflap, a soft tissue structure also found in modern crocodiles. The parietal is the final bone to participate in the dorsal surface of the skull table, being wedged in-between the squamosals, the supratemporal fenestrae and the frontal. Both it and the squamosals overhang the occipital surface at the back of the skull and prevent the
supraoccipital from being exposed on the skull table. The jugal consists of three regions, the anterior, posterior and ascending processes. Both the anterior and posterior processes are of similar depth and contact the maxilla and quadratojugal respectively. The ascending process on the other hand is forming the lower portion of the postorbital bar, connecting with the descending process of the postorbital. The entire postorbital bar is inset relative to the remainder of the contributing bones and described as anteroposteriorly flattened. The posterior process forms the lower edge of the
infratemporal fenestra until the posterolateral corner, where it connects with the
quadratojugal. The quadratojugal is broadly divided into a lateral region which is ornamented like the adjacent jugal and an unornamented anterodorsal process that forms the back of the infratemporal fenestra and contacts both the
quadrate and the bones of the skull table. It is unclear whether or not the quadratojugal formed a spine that protruded into the fenestra. The quadrate is robust and broad and forms the point of articulation for the lower jaw without help of the quadratojugal. It can be divided into a lateral and a medial hemicondyle, which are separated from each other by a
sulgus.
Dentition Paarthurnax would have possessed five teeth in either premaxilla and at least a further 14 maxillary teeth based on the more complete right half. Only three teeth are preserved themselves, with the count being otherwise based on the present
alveoli. The teeth are conical with a circular cross section and a slight curvature inward. The teeth show distinct cutting edges medially and distally (the "front" and "back") an closely-spaced striations that run from near apex of each tooth to the base of the
tooth crown. While the lower jaw is unknown, some clues on the interaction between the dentition of the upper and lower jaw can be found in the anatomy of the upper jaw. Lingual to the premaxillary alveoli the bone clearly shows two pits that would have served as occlusal pits for the first two dentary teeth, one behind and between the first two premaxillary alveoli and another right behind the third alveolus, with the latter being shallower than the former. The enlarged pseudocanine of the lower jaw would have then slid into the prominent notch behind the premaxillary toothrow when the jaws were closed.
Osteoderms A single partial osteoderm was associated with the holotype skull of
Paarthurnax. The fragment is thought to be the anterolateral corner of a right paravertebral osteoderm, i.e. part of the central double row that runs along the back of
Paarthurnax. The overall morphology is similar to that of other goniopholidids,
pholidosaurids and armored
thalattosuchians and features a heavily pitted surface and an incomplete spike-like process that would have helped articulate the bony plate with its predecessor.
Size The skull of
Paarthurnax measures approximately in length from the tip of the snout to the end of the
occipital condyle. From this Platt, Adams and Brochu calculate a total body length of . ==Phylogeny==