Ultrastenos huberi ( Willis, 1997 ) Yates & Stein, 2024

Yates, Adam M. & Stein, Michael, 2024, A reinterpretation and taxonomic revision of Ultrastenos willisi Stein, Hand and Archer, 2016, a short-snouted mekosuchine crocodylian from the Oligocene of northern Australia, Palaeontologia Electronica (a 22) 27 (1), pp. 1-36 : 18-30

Ultrastenos huberi ( Willis, 1997) comb. nov.

Figures 1-8 , 11-20 , 22 , 23

1997 Baru huberi Willis , p. 429, figures 10-11.

1997 ‘Cranial Form 1’ Willis, p.435, figures 20- 22.

2016 Ultrastenos willisi Stein, Hand, and Archer , p. e1179041-2, figures 1-4.

2017 Baru wickeni : Yates, p. e3458-30, figure 16e [partim].

Holotype. QM F31060 , fragmentary rostrum.

Paratypes. QM F31061 , right premaxilla and incomplete right maxilla ; QM F31062 , right premaxilla (herein removed from the hyodigm of U. huberi ) ; QM F31063 , right maxillary fragment (herein removed from the hypodigm of U. huberi ) ; QM F31064 , right maxillary fragment ; QM F31065 , maxillary fragment ; QM F31066 , maxillary fragment ; QM F31067 , dentary ; QM F31068 , left dentary ; QM F31069 , conjoined dentary pair with attached splenials .

Type locality and horizon. White Hunter Site, Riversleigh World Heritage Area, Queensland. Unnamed fluvio-lacustrine unit, Riversleigh Faunal Zone A, Late Oligocene.

Referred material. QM F31075 , posterior skull fragment ; QM F31076 , incomplete postorbital region of skull ; QM F31077 , skull fragment ; QM F31078 , isolated parietal ; QM F42665 (holotype of Ultrastenos willisi ), fragmentary posterior skull and mandible ; QM F42669 , left atlantal neural arch (this specimen and all following specimens are paratypes of Ultrastenos willisi ) ; QM F42668 , atlantal intercentrum ; QM F42670 , caudal vertebrae series ; QM F42672 , osteoderms ; QM F42673 , metatarsal ; QM F42667 , right coracoid ; QM F42666 , right tibia . QM F61096 , mandible, missing both postdentary areas ; QM F61097 , incomplete orbital region of the skull and left pterygoid .

Localities and horizons. Type locality ( QM F31060 , QM F31061 , QM F31064 , QM F31065 , QM F31066 , QM F31067 , QM F31068 , QM F31069 , QM F31075 , QM F31076 , QM F31077 , QM F31078 , QM F61096 , QM F61097 ); Low Lion Site ( QM F42665 , QM F42669 , QM F42668 , QM F42670 , QM F42672 , QM F42673 , QM F42667 , QM F42666 ). Both sites from un-named fluvio-lacustrine units of Riversleigh Faunal Zone A, Late Oligocene .

Diagnosis. As for the genus until new species are described.

Description

General features of the skull. Several of the more informative specimens of U. huberi have been described in either Willis (1997) or Stein et al. (2016b), so the description here is focused largely upon listing character states that have been used in cladistic analyses of crocodylian relationships.

The holotype of ‘ B.’ huberi has an estimated skull length of 200 mm and a width of 110 mm ( Figure 7 ), although QM F42665 is distinctly larger, with a reconstructed posterior width of approximately 180 mm, which would indicate a skull length of about 350 mm. If the reconstruction presented here ( Figure 14 A-D) is accurate then it is a brevirostrine form with the rostrum occupying 48% of the total skull length. It is platyrostral with the depth of the maxilla approximately one third of its width at the level of the fifth maxillary alveoli. The rostrum is quite simple and lacks a median dorsal boss, anterolateral rostral ridges, and raised preorbital crests.

Dermatocranium of the dorsal skull roof. The premaxilla has a steep anterior profile due to the placement of the anterior margin of the naris close to the anterior margin of the rostrum ( Figure 1C ). The steepness of the profile matches that of Baru , however, it differs from members of that genus in being proportionally shallower ( Figure 15E ). The naris is slightly longer than wide, and it tilts to face anterodorsally ( Figure 15E ). The posterior margin of the naris lies anterior to the anterior-most extent of the maxilla. The anterior end of the bulb-shaped premaxillary fenestra is located posterior to the first pair of premaxillary alveoli while its simple, rounded posterior margin is broadly separated from the premaxilla-maxilla suture by the premaxillary symphysis ( Figure 15C ). The minimum interorbital distance is narrow and just under 20% of the width of the posterior cranial table ( Figure 5I ). The posterior cranial table bears a dorsally exposed supratemporal fossa surrounding the supratemporal fenestra with no overgrowth from the surrounding dermal bones ( Figure 3A, B ). The dorsal margin of this fossa is distinctive with the lateral, posterior, and medial margins forming a rounded, subcircular shape ( Figures 3A, B ; 5A, B ). The anterior margin, however, forms an acute point that incised into the postorbital ( Figure 3A, B ). On the palate, the position of the anterior margin of the suborbital fenestra varies from level with the eighth maxillary alveolus (e.g., QM F31060, Figure 1B ) to level with the seventh alveolus (e.g., QM F31064, Figure 15A ). In both cases the anterior margin of the suborbital fenestra lies anterior to the anterior orbital margins.

Known specimens have only four premaxillary alveoli but the number present at hatching is unknown. The penultimate premaxillary alveolus is the largest premaxillary alveolus, but it is smaller than the fifth maxillary alveolus ( Figure 15C ). The premaxillary arcade curves anteromedially so that the last alveolus is lateral to the penultimate, and the penultimate alveolus is lateral to the antepenultimate. The triangular posterodorsal processes of the premaxilla extend posteriorly to the level of the third maxillary alveoli ( Figure 1A ). The palatal premaxillomaxillary suture is largely transverse and lies anterior to the first pair of maxillary alveoli ( Figure 15C ). A ventrolaterally open arch at the lateral end of the premaxillomaxillary suture forms a notch for the reception of the fourth dentary tooth. The notch is bordered laterally and medially by a pair of ridges ( Figure 15C, E ).

The total number of maxillary alveoli would appear to be 15, but this interpretation hinges on the identity of the maxillary alveoli in QM F61097 ( Figure 16B ). We interpret the preserved alveoli of this specimen as representing the sixth through to the fifteenth, and final, alveolus. However, an alternative interpretation where the preserved alveoli represent the fifth to the fourteenth alveoli cannot be ruled out ( Figure 16B ). Thus, it is possible that the total number of maxillary alveoli for this species is actually 14. The first six maxillary alveoli are closely spaced and lack interdental reception pits ( Figure 15A ). The alveoli are bordered medially by an alveolar process (sensu Molnar, 1982). The fifth maxillary alveolus is the largest alveolus in the upper jaw and corresponds to a moderately-sized, anterior festoon ( Figure 15B ). A second, smaller peak of enlarged teeth occurs at the tenth and eleventh alveoli. The alveoli are unevenly spaced with noticeably larger diastemata between the sixth and ninth alveoli in comparison to the rest of the tooth row but are evenly and closely spaced posterior to the tenth alveolus ( Figures 15A , 16B ). All, but the last, of the maxillary alveoli are subcircular, indicating a lack of labio-lingual compression of the maxillary teeth. The alveolar row continues to the level of the posterior margin of the suborbital fenestra, without a substantial non-dentigerous process of the maxilla posterior to the last alveolus. A pair of deep interdental reception pits occupy the spaces anterior and posterior to the seventh maxillary alveolus, whereas a series of three shallower medial pits occurs posterior to these between the level of the eighth and eleventh alveoli. The part of the maxillary palate that bears these reception pits is narrow, with the distance from the suborbital fenestra to the medial side of the maxillary alveoli subequal to the width of the tenth alveolus, when measured at the level of the tenth alveolus ( Figure 15A ). There are no reception pits posterior to the eleventh aveolus ( Figure 16B ). The foramen for the palatine ramus of the trigeminal nerve (located ventromedial to the fifth alveolus) is less than 20% of the diameter of the sixth maxillary alveolus ( Figure 1B ). The maxillary margin of the suborbital fenestra is straight and does not bow medially into the fenestra. Dorsally, the maxilla lacks a posterior spur either inserting into the lacrimal or between the lacrimal and the nasal and there is a long nasolacrimal suture ( Figure 16A ).

The nasals extend all the way to the posterior margin of the naris but do not protrude into it ( Figure 1A ). The lacrimal is wider than the prefrontal with a maximum transverse width that is 1.28 times that of the latter ( Figure 16A ). An unornamented, shallow sulcus on the dorsal surface of the lacrimal extends from the anterior apex of the orbit to the anterolateral margin of the lacrimal ( Figure 16A ). This sulcus divides the lacrimal ornamentation into a posterolateral region adjacent to the lateral margin of the orbit and an anteromedial region adjacent to the prefrontal and nasal. The medial margin of the sulcus forms a weak ridge that divides the higher anteromedial region from the descending posterolateral region, thus forming a subtle ridge similar to the more pronounced version present in B. wickeni (Yates, 2017: figure 7, labelled ‘preorbital ridge’). Rio and Mannion (2021: appendix 2) distinguished two distinct forms of preorbital ridge which they treated as two independent characters (characters 27 and 30) in their matrix. The first of these (character 27) is the type seen in U. huberi and B. wickeni , where there is an angulation caused by the junction of two planar surfaces, without a projecting ridge. They somewhat confusingly applied the name ‘canthus rostralis’ to this type of ridge. ‘Canthus rostralis’ had been used previously ( Brochu, 1999) for non-homologous oblique ridges that occur on the dorsal rostral surface of some caimanines. They then restricted the term ‘preorbital ridge’ to those structures for which there is a dorsal projection of bone on the lacrimal, built above the dorsal surface of the preorbital rostrum (character 30). This type of preorbital ridge most notably occurs in some species of Crocodylus (e.g., C. porosus ). To avoid confusion we apply a new, more precise, nomenclature to these different ornamental features. We continue to use ‘canthus rostralis’ in the sense that Brochu used it, i.e., for the oblique rostral ridges seen in some caimanines ( Brochu, 1999, fig. 61). For the two distinct types of ‘preorbital ridge’ we employ two new terms: the crista preorbitalis (preorbital crest) for the raised ridges, of the type seen in C. porosus , and canthus lacrimalis (lacrimal ridge) for the ridge formed from the meeting of two angled, planar surfaces such as is seen in U. huberi ( Figures 1A , 16A ).

The anterior termination of the lacrimal forms an acute, narrow process that lies anterior to the prefrontal. The anterior termination of the prefrontal lies anterior to the anterior process of the frontal, which in turn lies anterior to the level of the anterior margins of the orbits ( Figure 16A ). The anterior process of the frontal of the holotype forms an acute point that inserts between the posterior ends of the nasals for a distance of just under 5 mm ( Figure 1A ) whereas the termination is broader in QM F F61097 and fails to insert between the nasals at all ( Figure 16A ). The frontal-nasal contact prevents the prefrontals from contacting each other. The prefrontal orbital margin is simple, lacking raised bosses, a linear sulcus or a laterally projecting bevelled flange. The dorsal surface of the frontal is flat and flush with the orbital margins. There is no sagittal keel on the frontal or parietal ( Figure 5A ). The frontoparietal suture lies entirely on the skull roof and does not reach the supratemporal fossa ( Figures 3B , 5A ). The suture is bowed posteriorly and varies from shallowly bowed (QM F31076; Figure 5A ) to deeply bowed (QM F31075; Figure 3A, B ).

The anterior termination of the jugal lies slightly anterior to the anterior orbital margin and the anterior process of the frontal ( Figure 16C ). Ventrally the anterior ramus of the jugal, adjacent to the maxilla and immediately posterior to it, bears a ventrally-facing surface that bears several ornamental pits, similar to those found on other parts of the dorsal skull roof ( Figure 16B, C ). Medially, there are a pair of enlarged foramina anterior to the base of the postorbital bar. In lateral view, the ventral margin of the lower temporal bar is mildly concave ( Stein et al., 2016b, fig. 1g). The postorbital forms the dorsal part of the slender postorbital bar. It has a rounded cross-section and lacks an anterior protuberance.

The lateral side of the squamosal forms a narrow vertical surface with a single central sulcus that maintains subparallel margins for its length ( Figures 3A, B ; 11A ). The floor of this sulcus is smooth and unornamented. Posteriorly the short posterolateral process of the squamosal descends steeply towards the dorsolateral corner of the paroccipital process, so that its dorsal profile forms a near right-angle with the dorsal surface of the skull table. There is no lamina of the squamosal extending ventrolaterally from the lateroventral corner of the paroccipital process over the dorsal surface of the quadrate. Given the anterior attenuation of the quadratojugal sutural scar on the quadrate, the anterior end of the quadratojugal probably terminates on the posterior dorsal margin of the infratemporal fenestra and fails to reach the postorbital ( Figure 3 A-D). Posteriorly, the quadratojugal completely covers the lateral surface of the quadrate condyle.

Dermatocranium of the palate. The palatine is proportionally wide at the anterior end of the palatal bar, comprising over 30% of the total with of the rostrum at the level of the eighth alveolus ( Willis, 1997, fig. 11), thus constricting the width of the suborbital fenestrae at this level. The anterior palatine process is relatively short, extending just slightly anterior to the suborbital fenestra, based on the extent of the maxillary symphysis QM F61097. The anterior tip of the maxillary ramus of the ectopterygoid forms a single point that intrudes slightly into the maxilla so that the maxilla separates the first 1.5 mm of the ectopterygoid from the margin of the suborbital fenestra ( Figure 15A ). Although the maxillary margin of the suborbital fenestra is straight, the medial margin of the maxillary ramus of the ectopterygoid is slightly bowed medially at the level of the thirteenth to fourteenth alveoli ( Figure 16A, B ). This ramus extends for just under two thirds of the length of the suborbital fenestra and has an approximately equilaterally triangular cross-section. The lateral margin of the ramus extends parallel and close to the maxillary tooth row but does not contribute to the medial walls of any maxillary alveoli. Posteroventrally, the tip of the pterygoid ramus of the ectopterygoid fails to reach the posterior tip of the pterygoid flange ( Figure 17A, B ). The pterygopalatine suture is placed well anterior of the posterior margin of the suborbital fenestra, indicating that the pterygoids contributed just over 10% of the length of the palatal bar ( Figures 14C , 17B ). The ventral surface of the pterygoid bears an anteriorly directed ridge extending from the lateral margin of the choana ( Figure 17A, B ).

Splanchnocranium. The quadrate forms the floor of the anterior temporal foramen inside the supratemporal fossa, which keeps the squamosal and the parietal widely separated ( Figures 3B , 5A ). The anterior end of the quadrate contacts the medial side of the postorbital bar at its dorsal end. The quadrate sends a small bony process up the weakly bowed posterior margin of the bony otic aperture (sensu Montefeltro et al., 2016; Figure 4B ). A broad, occipitally-facing surface of the medial process of the quadrate is exposed ventrolateral to the otoccipital ( Figure 3D ). There is a roughly triangular ventral excursion of the quadrate-pterygoid suture on the lateral braincase wall ( Figure 4D ). A small, but distinctly visible, quadrate foramen aëreum opens on the flattened medial side of the dorsal surface of the posterior ramus of the quadrate ( Figure 3C, D ). This ramus is short, with the distance between the paroccipital process and the quadrate condyle less than the transverse width of the condyle ( Figure 12 A, E ). The dorsomedial margin of the quadrate condyle, when viewed normal to its articular surface, is flattened, with a straight section extending for approximately a quarter of the mediolateral width of the condyle ( Figure 18A ). No concave notch like the one seen in M. inexpectatus ( Rio and Mannion, 2021: Appendix 2, figure 45c) is present, but the shape seen here would appear to be intermediate between the notched condition and the convex dorsomedial corner seen in B. wickeni ( Figure 18B ). The dorsal margin of the condyle is concave, producing a medial constriction between the lateral and medial hemicondyles ( Figure 18A ).

Chondrocranium. The supraoccipital forms a large, trapezoidal dorsal exposure that greatly restricts, but does not eliminate, the parietal contribution to the occipital margin of the skull table ( Figure 3A, B ). The straight, to gently convex, posterior margin of the supraoccipital hides the postoccipital processes in dorsal view. The otoccipital forms a vertical occipital face that is hidden in dorsal view. The posterior medial process of the otoccipital is reduced. In QM F31075 it is so reduced that it terminates anterior to the occipital condyle ( Figures 3C, D ; 12H ) whereas in QM F42665 the otoccipitalbasioccipital suture grazes the dorsolateral corner of the occipital condyle ( Figure 12D ). The posterior carotid foramen opens on the occipital surface of the otoccipital, far ventral to the opening of the metotic foramen and the posterior hypoglossal foramen ( Figure 3D ). The acute ventral termination of the otoccipital reaches the dorsal margin of the basal tuber ( Figure 3D ). The occipital plate of the basioccipital is dorsoventrally deeper than the occipital condyle and has lateral margins that converge ventrally. This plate bears an exceptionally tall sagittal keel at its ventral end ( Figure 12C, G ). The ventral margin of the plate is straight in occipital view. The pharygotympanic foramen opens on the lateral margin of the basioccipital plate, dorsal to the level of the median pharyngeal tube foramen ( Figure 3D ). The parabasisphenoid forms a dorsoventrally deep exposure anterior to median pharyngeal tube foramen and ventral to the basioccipital ( Figure 3D ). This exposure of the parabasisphenoid is flanked by dorsoventrally tall posterior processes of the pterygoid. The parabasisphenoid is also exposed on the lateral surface of the braincase where its anterior suture with the pterygoid is recessed within a sulcus that parallels the posterolateral margin of the braincase ( Figures 4C, D ; 12C, G ). Anteriorly the parabasisphenoid is exposed ventral to the laterosphenoids where it bears the anteriorly projecting parabasisphenoid rostrum. The base of the rostrum is simple and is not flanked on each side with a sulcus ( Figure 4D ). This anterior exposure of the parabasisphenoid does not extend onto the lateral braincase wall, anteroventral to the trigeminal foramen ( Figure 4D ). The anterior margin of the capitate process of the laterosphenoid is oblique and oriented anteromedially ( Figures 4D , 5B ). The laterosphenoid forms two complete bridges over the cavum epiptericum, the robust lateral bridge sutures to the pterygoid while the caudal bridge sutures to the quadrate. There is moderate exposure of the prootic around the trigeminal foramen within the cavum epiptericum ( Figure 4D ).

Mandible. The first and second pairs of dentary teeth are steeply angled anterodorsally ( Figure 19B ). The third dentary tooth is smaller than the fourth, which forms a large canine peak ( Figures 19 A-C; 20A, D). The third and fourth alveoli are closely spaced and almost in contact but are not confluent ( Figure 20A ), while the fifth to seventh alveoli, posterior to the fourth alveolus, are closely packed and lack diastemata ( Figure 20A, D ). The posterior festoon reaches a level higher level to the anterior ( Figure 20B ), although this is barely the case in QM F31068 ( Figure 20E ). The peak of the posterior festoon coincides with the tenth to eleventh alveoli, with the eleventh being the larger of the two ( Figures 19A, B ; 20A, B ). Further posterior, the dentary narrows into a straight alveolar row. The total number of dentary alveoli is unknown but reconstruction from non-joining fragments indicates that there were at least 16 alveoli ( Figure 19 A-C). The dentary symphysis extends posteriorly so that it is level with the sixth alveolus (e.g., QM F31069) or the margin between the fifth and sixth alveoli (QM F31068; Figures 19A ; 20A, D ). The lateral surface of the dentary curves smoothly onto the ventral surface with no prominent ventrolateral ridge. The posterior end of the symphyseal surface is bilobed in medial view with the dorsal lobe extending posterior to the ventral lobe ( Figures 19B , 20E ). The anterior end of the splenial fails to make contact with the mandibular symphysis and terminates at the level of the seventh and eighth alveoli ( Figure 20C, E, F ). The anterior termination is bifurcated with processes lying alternately dorsal and ventral to the Meckelian sulcus ( Figures 19B , 20C ). In QM F3169 the dorsal process is abbreviated so that the tip of the ventral process lies anterior to the dorsal process ( Figure 20F ), whereas the sutural scars for each process are level with one another in QM F31068 and QM F61096 ( Figure 20C, E ). Although the posterior end of the splenial is not preserved the shape of the dentary and the surangular indicate that the posterior dorsal profile of the splenial would have been straight.

Posteriorly, the mandible bears a small, oval, external mandibular fenestra with a ventral margin that weakly indents the dorsal margin of the angular ( Figures 2H , 19C ). Its length is approximately 20% of the distance between the mandibular glenoid and the posterior end of the dentary tooth row. A posterodosal process of the dentary extends along the dorsal margin of the external mandibular fenestra and terminates posterior to its posterodorsal corner ( Figures 2E , 19C ). The dorsal margin of the right angular of QM F42665 indicates that the surangular-angular suture intersects with the margin of the external mandibular fenestra on its posterior margin ( Figure 2H ). The margins of the ornamented area on the posterior mandible do not form laterally everted ridges. The smooth fossa for the origin of the M. pterygoideus ventralis on the angular is visible laterally, posteroventral to the ornamented part of this bone. The surangular bears a tall ascending process on the lateral side of the posterior wall of the glenoid fossa ( Figure 2E ). There are no fossae on the dorsal surface of the surangular, adjacent to the glenoid fossa. No anterior spur of the surangular can be seen bordering the lingual side of the posterior most dentary alveoli, but it may be masked by poor preservation. The angulosurangular suture on the internal wall of the adductor chamber contacts the articular at its anterior tip. The lingual foramen for the articular and alveolar nerve on the internal wall of the adductor chamber opens on the articulosurangular suture. Within the glenoid fossa, the surangular makes a straight suture with the articular ( Figure 2I ). The posterior process of the surangular of QM F42665 is broken anterior to its termination but remains broad along its length and shows no sign of having pinched off anterior to the posterior end of the retroarticular process. The moderately elongate retroarticular process is posterodorsally directed but the posterior dorsal tip does not extend dorsal to the level of the posterior margin of the glenoid fossa ( Figure 2E ).

Remarks

Willis (1997) listed several paratypes of ‘ B.’ huberi . These do not help unite ‘cranial form 1’ with U. huberi or bolster the case for the synonymy of U. willisi and U. huberi , but they do improve our knowledge of the osteology of the species and have been used in the description above. However, not all of the paratypes can be justifiably referred to U. huberi . The significantly informative paratypes are discussed here with justifications for their inclusion or exclusion form the hypodigm of U. huberi .

QM F31061 is a rostral fragment that includes the right premaxilla and part of the right maxilla ( Figure 15 C-F). It can be referred to U. huberi on the basis of the ridges developed along the medial and lateral margins of the lateral premaxilla-maxilla notch, the enlarged anterior maxillary neurovascular foramen and an anteriorly placed, transverse premaxilla-maxilla suture that can be projected to have been anterior to the first maxillary alveolus. It is important to note that the lateral ridge, which we propose to be an autapomorphy for the species, has been erased from the left side of the holotype due to erosion of the bone surface. It nonetheless remains present and visible on the right side.

QM F31062 ( Figure 21 A-C) is an incomplete, juvenile premaxilla that is significant for retaining a small second alveolus close to the medial margin of the third for a total of five alveoli ( Figure 21A ). However, the margins of the lateral premaxilla-maxilla notch are rounded, the anterior profile of the premaxilla is relatively deeper and the external naris lacks a strong anteroventral tilt in lateral view ( Figure 21C ). For these reasons the specimen is identified as a juvenile Baru wickeni and removed from the hypodigm of U. huberi .

The paratypes include two incomplete maxillary specimens, QM F31063 ( Figure 21D, E ) and F31064 ( Figure 15A, B ) that differ in some taxonomically significant ways. Firstly, both preserve the sutural articulation facet that receive the anterior end of the ectopterygoid. In QM F31063 this articulation intrudes deeply into the maxilla so that the anterior tip of the ectopterygoid (when in articulation) is separated from the margin of the suborbital fenestra by a lamina of the maxilla, which has a length of just over one adjacent alveolus ( Figure 21D ). A deeply intruding anterior ectopterygoid tip is an autapomorphic feature of Baru wickeni (Yates, 2017) . The ectopterygoid articulation of QM F31064 is also inset but in contrast to QM F31063, the inset is minimal with the anterior tip of the ectopterygoid separated from the margin of the suborbital fenestra by a tiny triangular process of the maxilla, with a length of just over 10% of the length of the adjacent alveolus ( Figure 15A ). The alveoli of QM F31063 exhibit marked labio-lingual compression, especially from the ninth alveolus backwards ( Figure 21D ), a feature seen in juvenile specimens of B. iylwenpeny ( Yates et al., 2023) . In contrast the alveoli of QM F31064 are subcircular as far back as the eleventh alveolus ( Figure 15A ). QM F31063 also shows a moderately large foramen ventromedial to the fifth alveolus for the palatine ramus of the trigeminal nerve, a characteristic seen in Baru ( Yates et al., 2023) but not the holotype of U. huberi . The size of the foramen in QM F31064 cannot be determined due to breakage around the margins of the foramen. Finally, the ventral margin of the sutural surface for articulation with the jugal of QM F31063 is produced into a sharp lateral keel, suggesting that it supported a lateral flange on the jugal, which is another autapomorphy of B. wickeni (Yates, 2017) . Unfortunately, the homologous region of QM F31064 is missing. From these observations, QM F31063 is removed from the hypodigm of U. huberi and is identified as a juvenile specimen of B. wickeni . QM F31064 is retained in U. huberi on the basis of its enlarged anterior maxillary neurovascular foramen.

Willis (1997) also assigned three anterior mandibular specimens (QM F31067, F31068 and F31069) to ‘ B.’ huberi . The basis for these referrals was that the mandibles of the other three crocodylian species present in the White Hunter Local Fauna were known and were demonstrably distinct from those that were referred to U. huberi ( Willis, 1997) , which is a line of reasoning that is accepted here. Willis (1997) also noted that QM F31068 ( Figure 20D, E ) is an almost exact fit for the holotype of U. huberi , further supporting this referral.

Finally, we refer two previously unmentioned specimens to U. huberi . The first of these, QM F61096 ( Figure 20 A-C), is a conjoined pair of dentaries that are referred to U. huberi for the same reason that QM F31067, F31068 and F31069 are referred to this species. Secondly, there is QM F61097 ( Figure 16 A-C), which includes the orbital region of the skull and provides anatomical details not seen in any other specimen referred to U. huberi . It can be referred to U. huberi on the basis of the distinctive dorsal ornamentation that is shared with QM F31060, F31075, and F31076 and that it can be clearly excluded from Baru wickeni , Mekosuchus whitehunterensis and Quinkana meboldi . The taxonomic referrals made in this paper are summarised in Table 2.

Figures

FIGURE 1. Ultrastenos huberi, QM F31060, holotype, maxillary rostrum. A, dorsal view. B, ventral view. C, left lateral view. Abbreviations: appal, antepenultimate premaxillary alveolus; cl, canthus lacrimalis; fpal, first premaxillary alveolus; fr, frontal; idp, interdental reception pit; ju, jugal; lpal, last premaxillary alveolus; la, lacrimal; lac, lacrimal canal; lr, lateral ridge; mal, maxillary alveolus; mt, maxillary tooth; mr, medial ridge; mx, maxilla; na, nasal; nar, naris; om, orbital margin; ppt, penultimate premaxillary tooth; pm, premaxilla; pmf, premaxillary fenestra; pf, prefrontal; rn, reception notch for dentary tooth 4; rp, reception pit; rt, replacement tooth; sof, suborbital fenestra. Note that the premaxillary teeth are not numbered because the loss of a tooth position renders numbering ambiguous. Scale bar equals 50 mm.

FIGURE 2. Ultrastenos huberi, QM F42665, significant fragments of the holotype of U. willisi Stein et al. A, B, basicranial fragment with displaced adherent left pterygoid in right lateral (A) and occipital views (B). C, D, right temporal fragment in lateral (C) and dorsal views (D). E, G, I, posterior end of left mandibular ramus in lateral (E), medial (G) and dorsal (I) views. F, H, disarticulated right suranguar and angular in medial (F) and lateral (H) views. J, right surangular in dorsal view (anterior to the left). Scale bar equals 50 mm.

FIGURE 3. Ultrastenos huberi, QM F31075, White Hunter Cranial Form 1, posterior cranial fragment. A, B, dorsal view as a photograph (A), and as an annotated photograph with interpretive overlay (B). C, D, occipital view as a photograph (C), and an annotated photograph with interpretive overlay (D). Abbreviations: bo, basioccipital; fa, foramen aereum; fm, foramen magnum; fr, frontal; mf, metotic foramen; mptf, median pharyngeal tube foramen; ncr, nuchal crest; oc, occipital condyle; oto, otoccipital; pa, parietal; parp, paroccipital process; pbs, parabasisphenoid; pcf, posterior carotid foramen; po, postorbital; pop, postoccipital process of the supraoccipital; ptf, posttemporal fenestra; ptyf, pharyngotympanic foramen; q, quadrate; qc, quadrate condyle; qjss, sutural surface for articulation with the quadratougal; so, supraoccipital; sq, squamosal; stf, supratemporal fenestra; XIIp, posterior foramen for cranial nerve XII (posterior hypoglossal foramen). Scale bar equals 50 mm.

FIGURE 4. Ultrastenos huberi, QM F31075, White Hunter Cranial Form 1, posterior cranial fragment. A, B, right lateral view as a photograph (A), and as an annotated photograph with interpretive overlay (B). C, D, right ventrolateral view as a photograph (C), and an annotated photograph with interpretive overlay (D). Abbreviations: bo, basioccipital; boa, bony otic aperture; cb, caudal bridge of the laterosphenoid; cp, capitate process of the laterosphenoid; lb, lateral bridge of the laterosphenoid; ls, laterosphenoid; mf, metotic foramen; npd, posterior wall of the nasopharyngeal duct; oc, occipital condyle; oto, otoccipital; pa, parietal; pbs, parabasisphenoid; pcf, posterior carotid foramen; po, postorbital; pro, prootic; psr, base of parasphenoid rostrum; pt, pterygoid; ptyf, pharyngotympanic foramen; q, quadrate; qc, quadrate condyle; qjss, sutural surface for articulation with the quadratougal; sq, squamosal; ss, squamosal sulcus; stf, supratemporal fenestra; styf, subtympanic foramen; suof, supraorbital foramen; V, foramen for cranial nerve V (trigeminal foramen); XIIp, posterior foramen for cranial nerve XII (posterior hypoglossal foramen). Scale bar equals 50 mm.

FIGURE 5. Ultrastenos huberi QM F31076, White Hunter Cranial Form 1, posterior cranial fragment. A, dorsal view. B, ventral view. C, occipital view. D, right lateral view. Abbreviations: bo, basioccipital; boa, bony otic aperture; cb, caudal bridge of the laterosphenoid; cp, capitate process of the laterosphenoid; cqp, cranioquadrate passage; fa, foramen aereum; fm, foramen magnum; fr, frontal; I, foramen for cranial nerve I (olfactory foramen); II, foramen for cranial nerve II (optic foramen); III+orb, common foramen for cranial nerve III (oculomotor nerve) and the orbital artery and vein; ioc, incisure of the cranioquadrate passage in the otic aperture; IV, foramen for cranial nerve IV (trochlear foramen); lb, lateral bridge of the laterosphenoid; ls, laterosphenoid; mf, metotic foramen; ncr, nuchal crest; oto, otoccipital; ott, olfactory tract trough; pa, parietal; parp, paroccipital process; pbs, parabasisphenoid; pcf, posterior carotid foramen; pfss, sutural surface for articulation with the prefrontal; plp, posterolateral process of the sqamosal; po, postorbital; pob, postorbital bar; pop, postoccipital process of the supraoccipital; ptf, posttemporal fenestra; q, quadrate; qjss, sutural surface for articulation with the quadratougal; so, supraoccipital; sq, squamosal; ss, squamosal sulcus; stf, supratemporal fenestra; V, foramen for cranial nerve V (trigeminal foramen); XIIa, anterior foramen for cranial nerve XII (anterior hypoglossal foramen); XIIp, posterior foramen for cranial nerve XII (posterior hypoglossal foramen). Scale bar equals 50 mm.

FIGURE 6. Comparison of the occipital proportions of various crocodylians. A: Osteolaemus tetraspis NTM unregistered cast, a brevirostrine form. B: Ultrastenos huberi, QM F31075. C. Tomistoma schlegelii, NTM P3142 (cast), a slender longirostrine form. Images are not to scale.

FIGURE 7. Ultrastenos huberi, QM F31060 (holotype, maxillary rostrum) and QM F31076 (posterior cranial table) placed in approximate life position, relative to each other, in dorsal view. Scale bar equals 50 mm.

FIGURE 8. Dermal bone ornamentation of Baru wickeni and Ultrastenos huberi compared. A: Explanatory drawing of the skull of Baru wickeni in dorsal view with frontal and right squamosal marked in red and the areas shown in (B) and (C) marked by boxes. B: Baru wickeni (NTM P91171-1), frontal in dorsal view showing the ornamentation. C: Baru wickeni (NTM P902-4), right squamosal in dorsal view showing the ornamentation. D: Explanatory drawing of the skull of Ultrastenos huberi in dorsal view with frontal and right squamosal marked in red and the areas shown in (E) and (F) marked by boxes. E: Ultrastenos huberi (QM F F31076), frontal in dorsal view showing the ornamentation. F: Ultrastenos huberi (QM F F31075), right squamosal in dorsal view showing the ornamentation. Scale bars in (B, C, E, F) equal 10 mm. (A, D) are not to scale.

FIGURE 11. Comparison of the squamosal in right lateral view of Ultrastenos huberi with various species of Baru, showing the angle of descent of the posterolateral process and sculptural elements of the lateral surface. A: Ultrastenos huberi, QM F31075. Note that the lateral squamosal sulcus is both broad and extensive and the posterolateral process descends at a steep angle. B: Baru wickeni, NTM P9464-10 (image reversed for comparison). Note that the lateral surface is largely flat with a short, narrow sulcus and the posterolateral process descends at a shallow angle. C: Juvenile Baru iylwenpeny (NTM P6478). Note the flat lateral surface with a short, narrow sulcus and the shallow angle of descent. D: Adult Baru iylwenpeny (NTM P6515) Note the flat lateral surface with a short, narrow sulcus and the shallow angle of descent shared with other Baru specimens. Note also the presence of a posterolateral boss shared with (B) but not (C). Dashed lines demarcate each posterolateral boss. Hatched areas demarcate lateral sulci. Scale bars equal 20 mm.

FIGURE 12. Ultrastenos huberi, shared derived character states. A-D: QM F42665, holotype of Ultrastenos willisi. A, B, Right temporal fragment in dorsal view (A) and lateral view (B). C, D, ventral portion of braincase (with attached, displaced fragment of right pterygoid) in right lateral view (C) and occipital view (D). E-H: QM F31075, ‘WH Cranial Form 1’. E, F, right temporal region in dorsal view (E) and lateral view (F). G, H, ventral portion of braincase in right lateral view (G) and occipital view (H). Numbered character states: 1, distance from the ventrolateral tip of the paroccipital process to the dorsal margin of the quadrate condyle is less than the transverse width of the quadrate condyle (shown by rectangular bracket); 2, short posterolateral process of the squamosal descends at an angle steeper than 65⁰; 3, parabasisphenoid-pterygoid suture recessed within a sulcus; 4, enlarged sagittal, ventral keel of the basioccipital prominent in lateral view; 5, otoccipital-basioccipital suture (marked with red line) fails to cross the synovial surface of the occipital condyle. Scale bars equal 20 mm.

FIGURE 13. Single most parsimonious tree found by Ristevski et al. (2023) in a traditional search of their dataset using implied weights (k = 25), showing U. huberi sharing a more recent common ancestry with mekosuchine genera such as Mekosuchus than with Baru. Large, polyspecific, named clades that do not include Mekosuchinae have been collapsed into single branches (names in bold type) to simplify the diagram. Abbreviations: CI, consistency index; MPT, most parsimonious tree; #OTUs, number of operational taxonomic units; RI, retention index. Redrawn from figure 24 in Ristevsi et al. (2023).

FIGURE 14. Reconstruction of Ultrastenos huberi based largely upon QM F31060 and QM F31076 and scaled to the size of these two specimens. A, occipital view. B, dorsal view. C, ventral view. D, right lateral view. Abbreviations: bo, basioccipital; boa, bony otic aperture; cl, canthus lacrimalis; ect, ectopterygoid; fa, foramen aëreum; fr, frontal; itf, infratemporal fenestra; ju, jugal; la, lacrimal; mx, maxilla; mal, maxillary alveolus; na, nasal; nar, naris; oc c, occipital condyle; oto, otoccipital; pa, parietal; pal, palatine; pbs, parabasisphenoid; pm, premaxilla; pmf, premaxillary fenestra; po, postorbital; pf, prefrontal; pt, pterygoid; ptf, posttemporal fenestra; q, quadrate; q co, quadrate condyle; qj, quadratojugal; rp, reception pit; so, supraoccipital; sof, suborbital fenestra; sq, squamosal; stf, supratemporal fenestra. Scale bar equals 100 mm.

FIGURE 15. Ultrastenos huberi, rostral fragments. A-B, QM F31064, fragment of right maxilla. A, ventral view. B, lateral view. C-F, QM F31061, rostral fragment, including right premaxilla and fragment of right maxilla. C, ventral view. D, dorsal view. E, lateral view. F, medial view. Abbreviations: appa, antepenultimate premaxillary alveolus; ect sut, sutural surface for articulation with the ectopterygoid; idp, interdental reception pit; lpal, last premaxillary alveolus; lr, lateral ridge; mal, maxillary alveolus; mn, margin of the naris; mr, medial ridge; msy, articular surface for maxillary symphysis; nar, naris; nc, nasal cavity; nsy, articular surface for nasal symphysis; pm, premaxilla; pmf, premaxillary fenestra; ppal, penultimate premaxillary alveolus; psy, articular surface for premaxillary symphysis; rp, reception pit; sofm, margin of the suborbital fenestra; Vpal, maxillary foramen for palatine ramus of cranial nerve V2 (maxillary division of the trigeminal nerve), note that the margins of this foramen are broken, thus enlarging the apparent size of the foramen. Scale bar equals 20 mm.

FIGURE 16. Ultrastenos huberi, QM F61097, cranial fragment. A, dorsal view. B, ventral view. C, left lateral view. Abbreviations: cl, canthus lacrimalis; ect, ectopterygoid; fr, frontal; ju, jugal; la, lacrimal; mal, maxillary alveolus; mx, maxilla; na, nasal; or, orbit; pf, prefrontal; rp, reception pit; sof, suborbital fenestra; Vpal, foramen for palatine ramus of cranial nerve V2 (maxillary division of the trigeminal nerve). Scale bar equals 50 mm.

FIGURE 17. Ultrastenos huberi, pterygoid fragments. A: QM F42665, right pterygoid in articulation with right ectopterygoid in ventral view. B: QM F31076, pterygoid pair in ventral view. Abbreviations: ect, ectopterygoid; ecss, sutural surface for articulation with the ectopterygoid; palss, sutural surface for articulation with the palatine; sofm, margin of the suborbital fenestra; vptr, ventral pterygoid ridge. Scale bar equals 10 mm.

FIGURE 18. Quadrate condyles of Ultrastenos huberi and Baru wickeni compared. A: Ultrastenos huberi, QM F31075, right quadrate condyle, photograph taken normal to the articular plane of the quadrate condyle (dorsal is towards the top of the page). The flattened dorsomedial margin is marked by a rectangular bracket and the dorsal indentation between the medial and lateral hemicondyles is marked by triangular arrow. B: Baru wickeni, NTM P911, right quadrate condyle, photograph taken normal to the articular plane of the quadrate condyle (dorsal is towards the top of the page). Note that the flattened dorsomedial margin, and the dorsal indentation are absent. The small notch visible on the medial margin is due to damage. Scale bar equals 10 mm.

FIGURE 19. Ultrastenos huberi, reconstructed left mandibular ramus based largely upon QM F61096 and QM F42665, scaled to the size of the former. A, dorsal view. B, medial view (reversed for comparison). C, lateral view. Abbreviations: an, angular; ar, articular; c, coronoid; d, dentary; dal, dentary alveolus; dsy, symphyseal surface of the dentary; emf, external mandibular fenestra; fa, foramen aëreum; fic, foramen intermandibularis caudalis; gl, mandibular glenoid; ms, Meckelian sulcus; rap, retroarticular process; sa, suraqngular; sp, splenial. Scale bar equals 50 mm.

FIGURE 20. Ultrastenos huberi, anterior mandibular fragments. A-C: QM F61096, articulated left and right dentaries. A, dorsal view. B, right lateral view. C, posterior view. Dashed line in (C) represents the anterior margin of the splenial sutural scar. D, E: QM F31068, left dentary fragment. D, dorsal view. E, medial view. White arrows in (E) indicate the anterior tip of the dorsal and ventral anterior splenial processes. F: QM F31069, symphyseal fragment of mandible in posteriomedial view. Abbreviations: avp, anterior ventral process of the splenial; d, dentary; da, dentary alveolus; dldsy, dorsal lobe of the dentary symphyseal surface; dsys, symphyseal surface of the dentary; dt, dentary tooth; ms, Meckelian sulcus; msy, mandibular symphysis; sp, splenial; spss, sutural surface for articulation with the splenial. Scale bar equals 20 mm.

FIGURE 21. Baru wickeni, bones from the maxillary rostrum of small juveniles. A-C: QM F31062, right premaxilla. A, ventral view. B, dorsal view. C, lateral view. D, E: QM F31063, posterior fragment of right maxilla. D, ventral view. E, lateral view. The dashed line in (D) represents margins of the ectopterygoid sutural scar. Abbreviations: jss, sutural surface for articulation with the jugal; ma, maxillary alveolus; mss, sutural surface for articulation with the maxilla; mt, maxillary tooth; nar, naris; nm, narial margin; pa, premaxillary alveolus; pdp, posterior dorsal process of the premaxilla; pml, posterior medial lamina of the maxilla; rp, reception pit; sofm, margin of the suborbital fenestra. Scale bar equals 20 mm.

FIGURE 22. Reconstruction of the posterior skull of Ultrastenos huberi. A: Composite reconstruction showing marked anterior narrowing congruent with the slen- der longirostrine morphotype. Redrawn from Stein et al. (2016). B, C: QM F42665, temporal fragment from the holotype of U. willisi, mirror imaged to create reconstructions. B, temporal fragment arranged to match the reconstruction in (A). C, Alternative arrangement of the temporal fragment to match the brevirostrine reconstruction of U. huberi supported here. Abbreviations: ju, jugal; oto, otoccipital; q, quadrate; qj, quadratojugal; sq, squamosal. Hatched areas represent broken bone surfaces. Scale bar equals 50 mm.

FIGURE 23. Curvature of the mandible of Ultrastenos huberi compared to Baru iylwenpeny Yates et al. A-C: Ultrastenos huberi, QM F42665, mandibular fragments. A, posterior fragment of the left mandibular ramus. B, right surangular. C, mirror image of the right surangular overlain onto the left mandibular ramus fragment (drawn in red). The left surangular in (A) is filled with white, the other bones are shaded grey. The rectangular bracket in (A) indicates a highly fractured and re-glued zone. Note the offset of the end anterior of the left ramus in (C), which has rotated medially relative to the right surangular (marked with red arrow). D: Baru iylwenpeny, NTM P2787, right mandibular ramus (image reversed). The triangle indicates an inflection point similar to that exhibited in QM F42665. Scale bar equals 50 mm.