Paralligator Konjukova 1954

Braincase osteology of Paralligator Meatal chamber and major neurovascular passages The meatal chamber (= outer ear cavity; Montefeltro et al. 2016) is relatively expanded in Paralligator. Its anatomy is best seen in PIN 554/1-1 (holotype of P. gradilifrons), PIN 3140/502, and PIN 3141/501. The meatal chamber (MC) is dorsoventrally deeper posteriorly, between the squamosal and quadrate and tapers anteriorly (Fig. 8). Its dorsal boundary is formed by the overhanging skull roof bones—the postorbital and squamosal. The MC is limited by the postorbital bar anteriorly and by the quadratojugal anteroventrally. The quadrate forms the ventral margin of the MC, whereas the posterolateral process of the squamosal limits the MC posteriorly. A longitudinal sulcus is present along the lateral margin of the postorbital and squamosal (Figs 5A, 7B, 8A), which is an osteological correlate for the upper earlid in crocodyliforms (Montefeltro et al. 2016). This sulcus is variably developed in Paralligator specimens. It is generally weaker anteriorly, along the postorbital, and is more pronounced posteriorly on the squamosal. In some, notably smaller-sized individuals, the sulcus is prominent along its length and expands posteriorly, at the flared lobule of the squamosal: e.g. in PIN 3140/502, PIN 3141/501, PIN 3141/502, and PIN 3905/501-1 (Figs 7B, 8A). The flared lobule of the squamosal is present in PIN 551/29-1 but the sulcus for the upper earlid does not expand posteriorly, perhaps due to the development of an external ornamentation and co-ossification of a small osteoderm to the lateral surface of the squamosal. The sulcus for the earlid is the weakest in PIN 554/1-1 (holotype of P. gradilifrons) and PIN 3726/501-1 (holotype of P. major), both of which represent potentially adult individuals and have relatively flat squamosals. For example, the sulcus fades out at the mid-length of the squamosal in PIN 554/1-1 (Fig. 5). These differences may represent ontogenetic or individual variation in Paralligator, as has been suggested by Turner (2015) and Skutschas et al. (2015). The dorsal surface of the quadrate bears a shallow depression—the periotic fossa, which marks the anterior and ventral attachment of the tympanic membrane (Montefeltro et al. 2016). The fossa is relatively expanded anteroposteriorly and distinctly bilobate in Paralligator (e.g. in PIN 3140/502, PIN 3141/501) (Figs 6C, 8A). It differs from the more restricted and rounded fossa in extant crocodylians but is similar to that of some fossil mesoeucrocodylian taxa (e.g. Araripesuchus, Sebecus, Shamosuchus: see Montefeltro et al. 2016). A single moderatelysized subtympanic foramen is consistently present at the anteroventral corner of the periotic fossa in Paralligator (e.g. PIN 554/1-1, PIN 3140/502, PIN 3141/501) (Figs 5C, 6C, 7B, 8A). The principal opening of the bony otic aperture (BOA) sensu Montefeltro et al. (2016) is the external auditory meatus (EAM). It is bordered anteriorly and ventrally by the otic incisure of the quadrate, posteroventrally by the otic buttress, and dorsally by the squamosal. The EAM of Paralligator is ovate in lateral view (Figs 5B, 7B, 8). Its posterior border is less distinct than in extant crocodylians, as the quadrate-squamosal contact is absent and the otic buttress is not prominent. The neurovascular cranioquadrate passage extends posterolaterally from the posterior corner of BOA and EAM (Figs 5B, 7C, 8). It is a dorsally open groove bounded only by the otoccipital in Paralligator. The quadrate-squamosal contact posterior to BOA and EAM is absent in all but one examined specimen. In PIN 3140/502, a brief contact partially closes the cranioquadrate passage on the left side of the skull but is lacking on the right side (Fig. 8). This represents a potential individual variation of the feature, with both conditions represented on the two sides of a single cranium. The dorsal otic incisure in the posterodorsal corner of BOA is not marked on the ventral surface of the squamosal and is not delimited from the EAM. The vascular postquadrate foramen is located deep within BOA, at its posterodorsal corner, as is seen in CT scans of PIN 554/1-1 (holotype of P. gradilifrons) (Fig. 9B). The postquadrate foramen provides passage for the stapedial/ temporoorbital blood vessels from the cranioquadrate passage into the temporal canal (seePorter et al. 2016, Kuzmin et al. 2021). The narrow temporal canal is clearly observed in CT scans of PIN 554/1-1 (Fig. 9B). It is likely limited by the prootic and otoccipital ventrally and the parietal and squamosal dorsally, but sutural relations are hard to follow due to artefacts in scanning. The temporal canal opens via the anterior temporal foramen anteriorly, on the posterior wall of the supratemporal fossa (Fig. 9A). This foramen is bounded by the parietal and squamosal dorsally and by the quadrate and prootic ventrally. On the occiput, the posttemporal fenestrae are slit-like and limited by the supraoccipital, otoccipital, squamosal, and parietal (Figs 4A, B, 6B). Mesethmoid (anterior ossification in olfactory region) The mesethmoid is a small separate ossification attached to the ventral surface of the frontal, which is preserved in PIN 3141/501 and PIN 3458/501-1 (Fig. 10). The mesethmoids in both specimens are incomplete but that of PIN 3141/501 is better preserved. Corresponding elevated ridge-like facets are present in PIN 551/29-1 and PIN 554/1-1 (holotype of P. gradilifrons), indicating that the mesethmoid is a consistent feature in Paralligator, which is either not preserved in situ and was loosely connected to the frontal or was incompletely ossified in these individuals. Similar variability has been noted in the ontogeny of the ‘ethmoid complex’ of tyrannosaurids (Ali et al. 2008). The mesethmoid is located on the ventral surface of the frontal, close to its anterior edge, and just anterior to the bases of the prefrontal pillars (Fig. 10A). It is diamond-shaped in ventral view and longer than deep in lateral view. The posterior margin of the mesethmoid forms an acute wedge that divides the impressions of the olfactory bulbs on the ventral surface of the frontal. The mesethmoid may be subdivided into the dorsal plate and the ventral median septum (see Ali et al. 2008). The dorsal plate is slightly lateromedially expanded and sutures to the frontal. The median septum of the mesethmoid is subdivided anteriorly by a deep furrow. Each lateral margin bears two or more longitudinal ridges that delineate shallow sulci. The dorsal sulcus is twice as deep dorsoventrally than the ventral sulci and is here interpreted as an osteological correlate of the medial or common nasal vein (see Porter et al. 2016). The ventral smaller sulci are considered correlates for the olfactory nerve (CN I), as in Efimov (1988) and Ali et al. (2008). Laterosphenoid The laterosphenoid is partially preserved in PIN 551/29-1 and is relatively complete in PIN 554/1-1 (holotype of P. gradilifrons), PIN 3140/502, PIN 3905/501-1, PIN 3141/501, and PIN 3141/502 (Figs 4, 5, 6, 7, 11). The isolated left laterosphenoid PIN 3141/502-9 from the Nogon Tsav locality (Nemegt Formation) is exquisitely preserved showing fine anatomical details (Fig. 11D–K). The laterosphenoid of Paralligator is generally similar to that of extant crocodylians and fossil crocodyliforms (Holliday and Witmer 2009, Kuzmin et al. 2021). It forms the anterolateral wall of the braincase and the anterior margin of the trigeminal (= maxillomandibular) fossa and foramen (Figs 5C, 7A, 11). Grooves for the corresponding branches of the trigeminal nerve (CN V) are present on the lateral surface of the laterosphenoid. The lateral bridge subdivides the passages of the ophthalmic (CN V 1) and maxillary (CN V 2) branches of CN V and associated vessels (Fig. 11F, H). The lateral bridge is completely developed and sutures to the pterygoid and quadrate ventrally (Figs 5, 7). The small caudal bridge is present ventrally to the groove for the supraorbital branch of CN V 2 (CN V so) (Fig. 11D, E). It is broken off or imperfectly preserved in most available specimens but apparently contacts the quadrate posteriorly in PIN 3140/502 and PIN 3905/501-1 (Supporting information, Figs S5, S 6). The laterosphenoid body contacts the parabasisphenoid ventrally, the prootic posteriorly, and the quadrate posterolaterally. It is internally pneumatized by the laterosphenoid pneumatic recess (present on both sides of PIN 554/1-1, PIN 3141/501, and in PIN 3141/502-9) (Fig. 11G, K). Medially, a blunt tentorial crest formed by the body delimits the cerebral fossa (Fig. 11B). Several crests extend on the lateral surface of the laterosphenoid and mark the attachment of various muscles (see Holliday and Witmer 2009, Kuzmin et al. 2021) (Fig. 11D, E). The anterior edge of the lateral bridge corresponds to the tensor crest. It is dorsally continuous with the antotic crest, which extends to the capitate process and subdivides the laterosphenoid into the temporal and orbital aspects. Dorsally, on the temporal surface, there is a faint cotylar crest—osteological correlate for M. pseudotemporalissuperficialis (Holliday and Witmer 2009). Small crests extend dorsally and ventrally to the groove of CN V so. They are anteriorly continuous with the antotic crest and posteriorly – with the crest A’ on the quadrate (Fig.5A, C). These crests delimit a shallow fossa on the lateral bridge, which is likely an osteological correlate for M. pseudotemporalis profundus (Holliday and Witmer, 2009) (Fig. 11E). The postorbital process extends anterodorsally from the laterosphenoid body and forms the temporal aspect of the bone (Fig. 11A). The dorsal surface of the laterosphenoid is sutured to the frontal anteriorly and the parietal posteriorly. The postorbital process is continuous with the anterior process anteriorly and the capitate process laterally. The capitate process terminates distally with a rounded head (capitulum)