Cranial anatomy of the mekosuchine crocodylian Trilophosuchus rackhami Willis, 1993

Abstract One of the best‐preserved crocodylian fossil specimens from the Cenozoic of Australia is the holotype of the mekosuchine Trilophosuchus rackhami, from the middle Miocene (13.56 ± 0.67 Ma) Ringtail Site at Riversleigh, northwestern Queensland. Although lacking most of the snout, the holotype skull of T. rackhami (QMF16856) has an exceptionally well‐preserved cranium. Micro‐CT scanning of the holotype has allowed for all the preserved cranial bones to be digitally disarticulated, facilitating an unprecedented insight into the cranial anatomy of not just T. rackhami, but any mekosuchine. Trilophosuchus rackhami was a small‐bodied crocodylian and one of the most morphologically distinct mekosuchines, characterized by a unique combination of cranial characteristics several of which are exclusive to the species. Fossil material that is definitively referrable to the species T. rackhami is currently known solely from the middle Miocene Ringtail Site. However, an isolated parietal from Hiatus Site at Riversleigh demonstrates that Trilophosuchus also occurred during the late Oligocene (~25 Ma), extending the range of the genus by more than 10 million years. The new description of T. rackhami also allowed for a reevaluation of its phylogenetic relationships. Our results reaffirm the placement of T. rackhami as a member of Mekosuchinae within the subclade Mekosuchini. In all analyses, Mekosuchinae was consistently found to be monophyletic and part of the larger crocodylian clade Longirostres. However, the assignment of Mekosuchinae as a subset of Crocodylidae is brought into question, suggesting that the status of Mekosuchinae as a subfamily should be reconsidered.

.5 Trilophosuchus rackhami Willis, 1993 (C) Mekosuchus sanderi Willis, 2001, QMF31166, right jugal, quadratojugal and quadrate, with the focus on the jugal pictured in lateroventral view. Alligator mississippiensis (Daudin, 1802), QMJ4850, (D) skull and mandibles in left lateroventral view with the red box highlighting the close up of the left jugal in (E). For a high-resolution version of this figure, see the PDF file of Figure S1.8 provided as a supplementary file. Abbreviations: jug ap, anterior process of the jugal; jug asp, ascending process of the jugal; jug om, orbital margin of the jugal; jug pp, posterior process of the jugal; jug vl, ventral lamina of the jugal; q bod, body of the quadrate; q lhc, lateral hemicondyle of the quadrate; q mhc, medial hemicondyle of the quadrate; q pdp, posterodorsal process of the quadrate; qj, quadratojugal.

MORPHOLOGICAL COMPARISONS BETWEEN TRILOPHOSUCHUS RACKHAMI AND OTHER AUSTRALIAN CROCODYLIANS
The main focus of this section is to compare some notable morphological features of  Willis, 1997, andQuinkana timara Megirian, 1994); and, the two extant crocodylids that inhabit Australia: Crocodylus johnstoni (Krefft, 1873) and Crocodylus porosus Schneider, 1801. Thanks to a unique combination of morphological features, Trilophosuchus rackhami is readily discernable from other crocodylians. While much of the skeleton of T. rackhami remains unknown -at present, no dental, mandibular, or postcranial remains can be referred to it with confidence -the distinctive skull morphology of the taxon is highly informative (Figs. 1 and 2).
Although several anatomical characteristics are unique to T. rackhami (such as the lateroventrally sloping cranial table, the three continuous longitudinal crests on the cranial table, the relations between the maxilla, lacrimal and jugal around the orbit, the concavity on the ventral surface of the ectopterygoid plate, and the morphology of the occipital lamina of the supraoccipital), some S1 CRANIAL ANATOMY OF TRILOPHOSUCHUS by J. Ristevski et al. 39 occur in other mekosuchines. In the following section, we discuss some of the more notable features of T. rackhami in context of other crocodylians.
Inferring the morphology of the snout in T. rackhami is somewhat difficult due to the damage on that region in the holotype specimen. Fortunately, the preserved margins of the snout allow for a relatively confident interpretation of its gross morphology. As originally remarked by Willis (1993), the snout of T. rackhami would have been relatively short. Furthermore, the crosssectional outline anterior to the orbits is strongly suggestive to have been trapezoidal (Fig. S1.2B).
Based on the better-preserved left side (comprised by the ascending process of the maxilla and the facial lamina of the lacrimal), the snout has a sub-vertical lateral margin which is consistent with an altirostral morphology. Altirostry is present in several other mekosuchines such as species of Quinkana, but also Baru and Mekosuchus. A relatively short and broad altirostral snout, similar to that of T. rackhami, is inferred for Mekosuchus inexpectatus, M. sanderi and M. whitehunterensis (Holt et al., 2007;QMF31188 and QMF31051). In contrast, species of Baru and Quinkana tend to have proportionally longer altirostral snouts than Trilophosuchus Willis, 1993 or Mekosuchus (Willis et al., 1990;Megirian, 1994;Yates, 2017).
Due to the shortness of the snout, the suborbital fenestrae of T. rackhami extend anteriorly to the level of either the sixth or seventh maxillary alveoli. In most crocodylians, the suborbital S1 CRANIAL ANATOMY OF TRILOPHOSUCHUS by J. Ristevski et al.

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fenestrae tend to reach no further than the ninth maxillary alveoli and often terminate at a more posterior level, as in most longirostrine taxa. Besides T. rackhami, other mekosuchines with suborbital fenestrae that extend far anteriorly include B. darrowi and B. wickeni (to the level of the seventh alveolus in both species; Willis et al., 1990, Willis, 1997, and Yates, 2017, M. inexpectatus (to the level of the sixth alveolus; Willis, 1997 andHolt et al., 2007), M. kalpokasi Mead et al., 2002 (to the level of the seventh alveolus; Mead et al., 2002), M. sanderi (to the level of the sixth alveolus; QMF31188), M. whitehunterensis (to the level of the seventh alveolus; QMF31051), Q. babarra (to the level of the sixth alveolus; QMF23220), and Q. fortirostrum (to the level of the seventh alveolus; Molnar, 1981).
One of the unique features of T. rackhami is the relation between the maxilla and the orbit.
No other currently described crocodylian has a maxilla that closely approaches the orbital margin only to be excluded from the latter by a narrow contact formed by the lacrimal and jugal (Fig. 6).
In other crocodylians, the maxilla is separated from the orbital margin by a comparatively wider contact between the lacrimal and jugal. The relation between the maxilla and the orbit in T.
rackhami is closest to that of Mekosuchus, where the maxilla actually forms part of the lower orbital margin -this is an autapomorphy of Mekosuchus (Balouet & Buffetaut, 1987;Willis, 1997).
On the palatal surface of the maxilla, set close to the lingual alveolar margins is the maxillary foramen for the palatine ramus of the trigeminal nerve (Figs. 4A and 4B). In T. rackhami, this foramen is enlarged and near in size to some maxillary alveoli. A similarly enlarged maxillary foramen for the palatine ramus of the trigeminal nerve is also present in other mekosuchines. This includes Baru darrowi, B. wickeni (Yates, 2017; NTM P8695-8 and NTM P91171-1), and Quinkana babarra (QMF23220). A conspicuously large foramen is also evident in M. sanderi (QMF31188) and M. whitehunterensis (QMF31051). Other Australian crocodylians, where known, possess proportionally smaller foramina on the palatal surfaces of their maxillae.
Among currently known mekosuchines, an anterior process of the frontal that is no more than 35% of the total anteroposterior length of the element is shared between Trilophosuchus and S1 CRANIAL ANATOMY OF TRILOPHOSUCHUS by J. Ristevski et al. 41 Mekosuchus. In T. rackhami (QMF16856 and QMF16857) the anterior process of the frontal is ~28-29% of the total frontal length (Figs. 7A, S2.1B and S2.1C). In M. sanderi (Fig. S1.14) and M. whitehunterensis (QMF31052; figure 3 in Willis, 1997; figure S1.11D in Ristevski et al., 2020b), the anterior process of the frontal is also quite short, ranging from 30-35% of the total length of the bone. Additionally, the anterior process of the frontal of Trilophosuchus and Mekosuchus is relatively wide and with a blunt tip. Most crocodylians, where known, possess an intermediate condition where the anterior process of the frontal is between 35% and 60% the total length of the frontal. This is true of other mekosuchines that have the anterior process of the frontal sufficiently preserved (e.g., B. wickeni, A. clarkae, Kambara spp.; see Willis & Molnar, 1991, Salisbury & Willis, 1996, Buchanan, 2009, Yates, 2017, figure S1.6 in Ristevski et al., 2020b). An exception is the condition of K. aurivellensis, where the anterior process of the frontal is extremely elongated by comprising ~67% of the total length of the frontal (Yates & Pledge, 2016 ; Fig. S3.3). No other known mekosuchine has an anterior process of the frontal as long as that of K. aurivellensis.
Outside of Mekosuchinae, a relatively short anterior process of the frontal is also present in species of Purussaurus Barbosa-Rodrigues, 1892 (see Langston, 1965 andAguilera et al., 2006). Although not as common as the moderate condition, extremely long anterior frontal processes do occur in non-mekosuchine crocodyliforms, and are here recognized in the neosuchian Theriosuchus pusillus Owen, 1878, the 'thoracosaur' eusuchian Eothoracosaurus mississippiensis Brochu, 2004, the gavialoid Piscogavialis jugaliperforatus Kraus, 1998, and the crocodylid Euthecodon arambourgi Ginsburg & Buffetaut, 1978. The combination of three continuous longitudinal crests adorning the cranial table is another autapomorphy of Trilophosuchus. On the other hand, a single midsagittal crest that extends over the frontal and/or parietal is not an uncommon occurrence among crocodylians, including some mekosuchines. The midsagittal crest of T. rackhami is prominent and continuous from the frontal and over to the parietal. In contrast, a relatively low midsagittal crest lying solely over the frontal is present in K. aurivellensis (Yates & Pledge, 2016). A midsagittal crest on the S1 CRANIAL ANATOMY OF TRILOPHOSUCHUS by J. Ristevski et al. 42 dorsal surface of the frontal also occurs in M. sanderi and M. whitehunterensis, though in the known specimens for these two species the frontal crests are reduced in both acuteness and length when compared to T. rackhami (the midsagittal frontal crest is subtle in M. sanderi specimen QMF31187 and M. whitehunterensis specimen QMF31052). Other than Trilophosuchus, a midsagittal crest over the parietal is also recognized in M. sanderi (QMF31166), however the crest in this M. sanderi specimen is discontinuous with the midsagittal crest of the frontal (Fig. S1.14).
Another peculiar feature of the parietal of M. sanderi QMF31166 is the presence of two lateral crests (Fig. S1.14; see also figure 2 in Willis, 2001). In T. rackhami, the ventral lamina of the jugal is bent ventrolaterally and bears ornamentation like on the lateral surface of the element (Fig. S1.7). This morphology is evident in other crocodylians, including some mekosuchines like A. clarkae, M. sanderi and Quinkana (Figs. S1.8-S1.10). Unlike P. vincenti, the jugal of T. rackhami does not possess a notable concavity on its ventromedial surface (see Ristevski et al., 2020a andthe 3D PDF in Ristevski et al., 2020b).
Another feature of the jugal in T. rackhami that merits commenting on is the relation between the ascending process (or, the ventral portion of the postorbital bar) and the lateral surface of the jugal.
In T. rackhami (most readily observable from the STL files of the jugals and/or the 3D PDF, provided as supplementary material), M. inexpectatus (see figures 1 and 2 in Buffetaut, 1983) and M. sanderi (see figure S1.11C in Ristevski et al., 2020b), the base of the postorbital bar is flush with S1 CRANIAL ANATOMY OF TRILOPHOSUCHUS by J. Ristevski et al. 43 the lateral surface of the jugal. On the other extreme, the base of the postorbital bar is medially inset in Baru, Kambara and P. vincenti where the postorbital bar is separated from the lateral surface of the jugal by a deep trough (Buchanan, 2009;Yates, 2017;Ristevski et al., 2020a). A somewhat intermediate condition occurs in A. clarkae (Fig. S1.8B) and Quinkana (Q. timara NTM P8697-2, Fig. S1.9B; Quinkana sp. indet. QMF1152, Fig. S1.10), where the base of the postorbital bar is mildly inset medially.
In T. rackhami, the occipital lamina of the supraoccipital is exemplified by a unique combination of anatomical features. No other known mekosuchine has a flat occipital lamina of the supraoccipital that is devoid of a nuchal crest and covered by a superficial wrinkle-like texture.
Most crocodylians (including some mekosuchines; e.g., Kambara spp., Q. timara) have a nuchal crest along the midline that is usually accompanied by a pair of concavities that flank the crest laterally. Such morphology is also present and consistent throughout ontogeny among the examined Crocodylus johnstoni and C. porosus specimens available for our assessment. Some crocodylians, like the gavialoid H. camfieldensis and the mekosuchine P. vincenti, possess a nuchal crest but lack the sub-circular concavities lateral to it. While not identical to T. rackhami, the mekosuchines A. clarkae and M. sanderi have the most comparable morphology of the occipital lamina of the supraoccipital. The holotype specimen of A. clarkae (QMF16788) also has a flat occipital lamina of the supraoccipital that lacks a nuchal crest (Fig. S3.4C). However, unlike T. rackhami the occipital lamina of the supraoccipital in A. clarkae has a smooth texture. Like T. rackhami and A. clarkae, the occipital lamina of the supraoccipital in M. sanderi is also flat and without a nuchal crest or concavities. Uniquely, the occipital lamina in M. sanderi is heavily ornamented with conspicuous sub-circular pits akin to the cranial table (Fig. S1.6).
The quadrate of T. rackhami displays a morphology that is akin to several other mekosuchines, such as Mekosuchus. Two species of Mekosuchus -M. inexpectatus and M. sanderi -have sufficiently preserved quadrates suitable for comparisons. Like T. rackhami, both M.
inexpectatus and M. sanderi have a condylar surface of the quadrate that has a dorsal peak situated S1 CRANIAL ANATOMY OF TRILOPHOSUCHUS by J. Ristevski et al. 44 closer to the medial hemicondyle than to the lateral, and the dorsal and ventral margins of the condylar surface are sub-parallel (Figs. 22B, 23B, S1.11-S1.13; see also figure 45C in Appendix 2 of Rio & Mannion, 2021). In contrast to Trilophosuchus and Mekosuchus, the condylar surface of the quadrate in P. vincenti lacks a prominent dorsal peak and the dorsal and ventral margins are concave (figure 28A in Ristevski et al., 2020a). As in T. rackhami, the body of the quadrate in species of Mekosuchus is also relatively short, with the anteroposterior length of the body from the paroccipital process to the condylar surface being less than the width at the quadrate condyles.
Indeed, this feature of the quadrate occurs in most, but not all, crocodylians including all other formally described mekosuchines that have known quadrates (e.g., B. wickeni, Kambara spp., P. vincenti). An exception among currently described mekosuchines is A. clarkae, which possesses a quadrate body that is longer than the width at the quadrate condyles (see figure 48B in Appendix 2 of Rio & Mannion, 2021).
In many mekosuchines, where known, the pterygoid process of the quadrate has exposure in occipital view, ventrolateral to the otoccipital. Such is the condition in Baru, Kambara, P.