Diplocynodon hantoniensis Brochu, 1999

Diplocynodon hantoniensis Brochu, 1999

Holotype: NHMUK OR 25166 (holotype of ‘ Alligator hantoniensis ’), anterior portion of a skull ( Fig. 1). Type locality: Hordwell Cliff, near Lymington, Hampshire, United Kingdom. Type horizon: Totland Bay Member, Headon Hill Formation; Priabonian (late Eocene) ( Edwards & Daley 1997; Daley, 1999).

Synonym: Crocodilus hastingsiae Owen, 1848 . Crocodilus hastingsiae ( Owen, 1850; Huxley, 1859; Woodward, 1885).

Holotype: NHMUK OR 30393 About NHMUK , from the same locality and horizon.

Referred material ( NHMUK): Skulls (R 1041, R 1042, OR 25167, OR 25170.a, OR 29694, OR 30287 partim, OR 30392, OR 30414 partim); rostrum fragment ( OR 30381 partim); premaxillae ( OR 30289 partim, OR 30369, OR 30370, OR 30414 partim); maxilla (R 5010 partim, OR 25168, OR 25169, OR 25170, OR 30289 partim); nasal ( OR 30289 partim); jugals [OR 25188 (with partial quadratojugal), OR 25219, OR 30289 partim]; quadrates ( OR 25187, OR 25193. OR 30372, OR 30382, OR 30414 partim); pterygoid [OR 30251 (with fragmentary ectopterygoids and palatine)]; ectopterygoids [OR 25216 (with pterygoid fragment), OR 25217]; basioccipital ( OR 36812); lower jaws [R 1043, R 1045 partim, R 5267 partim, OR 25178, OR 30274, OR 30381 partim, OR 30394, OR 30396 (two rami in connection), OR 30397]; dentaries (R 1044, R 5010 partim, R 5121, R 5212, R 5216, R 5217, OR 30318 partim, OR 30321, OR 30287 partim, OR 30289 partim, OR 30338, OR 30365, OR 30378, OR 30383); angulars (R 5217 partim, OR 30285, OR 30289 partim, OR 30307, OR 30308, OR 30318 partim, OR 30414 partim); articulars [OR 25192, OR 30282, OR 30283, OR 30284, OR 30388 partim, OR 30289 partim, OR 30363, OR 30363.a, OR 30388 partim (joined with 30289), OR 30414 partim]; surangular ( OR 30414 partim); toothbearing bone ( OR 25260); isolated teeth (R 1043, R 5009, R.6323, R 6852, OR 25165, OR 25598, OR 30317 partim); proatlas ( OR 30289 partim); odontoid process ( OR 25186); axes (two unnumbered specimens); vertebrae (R 1041, R 1045 partim, R 1046, R 1047, R 1048, R 1049, R 1050, R 1051, R 1052, R.1068 partim, R 5213, R 5267 partim, OR 25175, OR 25177, OR 25176, OR 25179, OR 25180, OR 25181, OR 25182, OR 25195, OR 25199, OR 25201, OR 25207, OR 25208, OR 25209, OR 25210, OR 25213, OR 25214, OR 25215, OR 25222, OR 25223, OR 25224, OR 30289 partim, OR 30402 partim, plus unnumbered specimens); ribs (R.1068 partim, OR 25191, OR 25248, OR 30402 partim, OR 30414 partim); osteoderms (R 1043, R 1045 partim, R 1068 partim, R 5214, R 5267 partim, OR 25174, OR 30318 partim, OR 30366, OR 30367, OR 30368, OR 30388 partim, OR 30414 partim, plus one unnumbered specimen); scapulae ( OR 30247 partim, OR 30414 partim); coracoids ( OR 25245, OR 30359); humeri ( OR 25237, OR 30206, OR 30206.a, OR 30219, OR 30247 partim); radii ( OR 25244 partim, OR 30401); ulnae ( OR 25239, OR 30236, OR 30237, OR 30247 partim, OR 30389); radiale (R.1054); ilia ( OR 25252, OR 25253, OR 30362, OR 30414 partim); ischia ( OR 30354 partim); femora (R 5215, R 5267 partim, OR 25231, OR 25232, OR 25238, OR 25244 partim, OR 30210, OR 30211, OR 30213, OR 30214, OR 30222, OR 30223, OR 30228, OR 30247 partim, OR 30399, OR 30414 partim); tibiae (R 5267 partim, OR 25236, OR 30215, OR 30216, OR 30217, OR 30242, partim, OR 30414 partim); fibulae ( OR 30233, OR 30234, OR 30235, OR 30241); metapodials (R 1053 partim, R 5267 partim, OR 25244 partim, OR 25254, OR 30240, OR 30242 partim, OR 30243, OR 30246, OR 30247 partim, OR 30336); phalanges (R 1053 partim, R.1054 partim, R 5267 partim, OR 25243). CAMSM: skulls ( TN 907 partim, TN 917, TN 918 partim); lower jaws ( TN 904; TN 907 partim); dentaries (C31041, C31042, C31043, plus 8 unnumbered specimens); angulars (C31044, 31045); cervical vertebrae (C31026–40); dorsal vertebrae (C31012–25); caudal vertebrae (C30999–31011); osteoderms (C31046–95; C31352–87).

Revised diagnosis: Diplocynodon hantoniensis can be diagnosedbythefollowinguniquecombinationofcharacters (autapomorphies marked by an asterisk): (1) premaxillary surface posterolateral to naris with a deep notch; (2) short anterior ectopterygoid process, which forms the medial wall of the posteriormost maxillary alveolus, before veering medially to the suborbital fenestra*; (3) ectopterygoid– pterygoid flexure retained throughout ontogeny*; (4) surangular–angular suture meets the articular dorsal to its ventral tip in the mandibular adductor chamber*; (5) broad preorbital ridge on the lacrimal; (6) prominent laminae and sulci either side of the choana.

Description: skull

General proportions, preservation and ornamentation: Several remains represent portions of the skull, including three nearly complete skulls: NHMUK OR 30392, NHMUK OR 30393 ( Figs 2–4) and CAMSM TN 907. All three of these skulls are large: NHMUK OR 30392 is 375 mm long from the anterior tip of the premaxillae to the posterior margin of the skull table (measured along the midline on the skull surface), with NHMUK OR 30393 and CAMSM TN 907 comparable in size ( Table 1). The preserved portion of NHMUK OR 25166 also indicates a similarly sized individual. At the other end of the spectrum, NHMUK OR 25170.a is a partial juvenile skull that is about 60 mm long from the anterior tip of the maxillae (the premaxillae are not preserved) to the posterior edge of the skull table.

Preservation varies significantly from specimen to specimen, with different degrees of completeness, but deformation is generally minor. For example, the nasals of NHMUK OR 30393 are slightly lowered in respect to the maxillae. Some partial skulls, including NHMUK R 1042, OR 25170a and OR 29694, are still partly embedded in matrix, whereas others have been internally reinforced withplasteroracement-likematerial, includingNHMUK OR 30392. The surfaces of the two best-preserved skulls have been partly covered by plaster in order to provide the impression of completeness.

The overall appearance of the skull is that of a generalized crocodylian, with the rostrum (the region anterior to the orbits) representing about 65% of the entire length of the skull. There is no medial dorsal boss on the rostrum. Canthi rostrali are clearly absent, but preorbital ridges are prominent in adult individuals (see description of the lacrimal). In posterior view, the skull table is approximately planar, except for a modest depression in mature specimens (e.g. NHMUK OR 30392 and 30393) ( Fig. 3D), at the dorsal exposure of the supraoccipital. The lateral sides of the skull table are convex in juvenile specimens ( NHMUK OR 25167 and OR 25170.a), and the paroccipital processes have short squamosal rami ( Figs 5, 6). However, the lateral sides are straight or slightly concave in adult specimens ( NHMUK OR 30392 and 30393), in which the squamosal rami are long.

Cranial fenestrae and openings: The dorsally facing naris is entirely hosted within the premaxillae ( Figs 1, 2). It is approximately circular and has a rounded, convex anterior rim, nearly straight lateral rims (especially in NHMUK OR 25166) and a concave posterior rim (as a result of an anterior projection of the medial edge of each premaxilla). The rim of the external naris is flush with the dorsal surface of the surrounding premaxillae, which is posterolaterally marked by a depression that is nearly as developed as the one that characterizes Alligator ( Figs 1B, 2B). Whereas the lateral walls of the naris are nearly vertical, the anterior walls are very shallowly inclined posteroventrally.

The dorsal margin of each orbit is composed of the frontal, the posterior margin by the postorbital, the posterolateral and lateral margins by the jugal, and the anterior margin is formed by the lacrimal and prefrontal. The orbits are deeply concave medially, but straight or slightly convex laterally, and are truncated by the postorbital bars posteriorly. The orbital rim is flush with the jugal surface (a hint of a crest is present in NHMUK OR 30392), but raised into a crest along the prefrontal edge. In the juvenile partial skull NHMUK OR 25170a, the posterolateral edge of the prefrontal is extremely high and forms a vertical laminar ridge delimiting the anteromedial surface of the orbit ( Fig. 5). It is not clear if palpebral bones were originally absent or are simply not preserved in any available specimen.

The supratemporal fenestrae are oval-shaped, with their long axes oriented anteroposteriorly. They are much smaller than either the orbits or infratemporal fenestrae in adult specimens, but comparatively large in the juvenile individual NHMUK OR 25170.a, which has a very broad supratemporal fossa. The medial rim of each supratemporal fenestra is slightly raised into a ridge in the largest specimens, with a depressed interfenestral bar ( Figs 3B, 4B). However, in smaller specimens (e.g. NHMUK OR 25167 and 25170a) the rim of the supratemporal fenestra is nearly flush with the parietal surface ( Figs 5, 6). Whereas the posterior and medial fenestral margins are curved, the anterolateral margins are straight and the fenestrae taper in width anteriorly to form an acute corner ( Fig. 2B). The rim of each supratemporal fenestra does not significantly overhang the supratemporal fossa. A shallow fossa occurs at the anterior corner of the supratemporal fenestra. The medial parietal wall of the supratemporal fenestra is imperforate.

The infratemporal fenestrae are triangular in shape and are approximately as long as the orbits, although their surface area is smaller overall than that of the orbits. The most completely preserved infratemporal fenestra is the right opening of NHMUK OR 30392, which is missing only a very small portion of the quadratojugal, close to the postorbital ( Fig. 3B). The jugal forms the ventral margin and anterior angle of the infratemporal fenestra, and the dorsal angle appears to be formed by the quadratojugal ( CAMSM TN 918). From the juvenile specimen NHMUK OR 25167 ( Fig. 6), it is clear that the quadratojugal formed the entire posterior corner of the infratemporal fenestra.

The external auditory meatus is clearly visible in NHMUK OR 30392 and 30393 ( Figs 3B, 4D) and has a cloverleaf shape. Whereas the dorsal margin of the external auditory meatus is composed of the squamosal, the quadrate forms the bowed posterior rim, as well as the ventral and anterior margins. The quadrate–squamosal suture extends dorsally along the posterior margin of this opening. The quadrate forms a narrow ridge on the floor of the external auditory meatus, extending medially into the ear canal. A large preotic foramen pierces the quadrate anterior to the external auditory meatus ( Fig. 4D).

The elliptical foramen magnum, delimited by the exoccipitals and the basioccipital, is equal in width to the occipital condyle ( Fig. 3D). Post-temporal fenestrae are rather small: they are delimited by the parietal and squamosal dorsally, and by the supraoccipital and exoccipital ventrally. The lateral carotid foramen opens lateral to the basisphenoid, and the small lateral Eustachian canals open dorsally to the large, medial canal.

The suborbital fenestrae are fully visible in NHMUK OR 30392 ( Fig. 2D) and CAMSM TN 907. Anteriorly, they reach the level of the ninth maxillary alveolus, but reach the tenth alveolus in NHMUK OR 25166 ( Fig. 1D). The lateral edge of the suborbital fenestra is straight, not bowed medially at the ectopterygoid– maxilla suture. Each suborbital fenestra has a posterior notch (best observed on the right opening of NHMUK OR 30392). The posterior corner of the suborbital fenestra is formed by the pterygoid, with the pterygoid–ectopterygoid suture situated on the posterolateral margin of the fenestra. Anteriorly, the suborbital fenestra tapers to an acute tip. The anterolateral margin of the fenestra is longer than the posterolateral margin.

The incisive foramen is relatively large and is located far from the tooth row, with the anterior rim corresponding to the third alveolus, and the posterior margin corresponding to the fourth or fifth alveolus. Whereas this foramen is circular in NHMUK OR 30392 and CAMSM TN 907, it is teardrop-shaped in NHMUK OR 25166, with an acute anterior margin, resulting from the intrusion of the occlusal pits for the first dentary teeth.

The internal choana is best preserved in CAMSM TN 907 and the juvenile specimen NHMUK OR 25167 ( Fig. 6D). The posteroventrally oriented choana is heart-shaped and situated entirely within the posterior region of the pterygoids, with its posterior rim close to the posterior edge of the pterygoids. The anterior rim of the choana is flush with the pterygoid surface, whereas the lateral rim develops a large lamina that separates the choana from a slightly depressed area. In addition, the midline of the anterior margin of the choana is interrupted by a posteriorly directed process of the pterygoid. This is observed in all specimens where preserved. In NHMUK OR 30392, this process extends far posteriorly into the choana, although it does not fully bisect it. The posterior rim of the choana, preserved in NHMUK OR 25167 and CAMSM TN 907, is smooth, lacking the sharp notch that characterizes Caiman, but there is a small concavity on the posterior edge. Although slightly incomplete in its median sector, these specimens show the presence of a sagittal bony septum recessed within the choana.

Premaxilla: The premaxillae completely encircle the external naris. They are characterized by five alveoli, even in adult specimens. Palatal laminae are flat, with no obvious ornamentation, except for a slightly rugose texture leading up to the alveolar necks (e.g. NHMUK OR 25166). Numerous small foramina pierce the premaxillary palate, adjacent to the alveoli. The dorsal process of the premaxilla is relatively short and wide, posteriorly reaching the level of the second or third maxillary alveolus. The premaxilla–maxilla suture is slightly irregular, rather than straight. One large reception pit for the first dentary tooth occurs lingually, in between the first and second premaxillary alveoli. Posteriorly, the premaxillae are separated along their midline by the anterior process of the nasal.

Maxilla: The well-preserved maxillary tooth rows of NHMUK OR 30392 host 17 alveoli on each side ( Fig. 2C, D). In ventral view, the tooth row has a slight lateral concavity at the level of the sixth and seventh alveoli, but it is nearly straight posteriorly. The fourth and fifth alveoli are about the same size. They are not fully confluent in any of the three best-preserved skulls and are separated by a thin (but complete) bony septum. Confluent alveoli, in which the septum is not complete, having a V-shaped fenestration, is observed in one small, isolated maxilla NHMUK OR 25168–9. In dorsal view, at the level of the fifth maxillary alveolus, and slightly medially inset, there are two protuberances. On the ventral surface of the skull, the maxilla terminates a significant distance anterior to the postorbital bar, forming a short edentulous process that is as long as the distance between the last two maxillary alveoli ( Fig. 2D). The penultimate alveolus is slightly less than twice the diameter of the last alveolus. The medial walls of the last two maxillary alveoli are formed by the ectopterygoid.

Nasal: An acute process of the nasal protrudes anteriorly between the premaxillae, but does not reach the external naris. In NHMUK OR 30392 ( Fig. 2), this nasal process terminates about 20 mm posterior to the external naris. The nasals broaden posteriorly along the maxilla–nasal suture, reaching a maximum width at their midlength. Whereas the nasal–prefrontal suture is concave, the nasal–lacrimal suture is linear.

Lacrimal: The lacrimal forms the anterolateral rim of the orbit. It is longer than the prefrontal ( Fig. 7A). NHMUK OR 30381 preserves the right lacrimal duct, which is large and opens immediately lateral to the lacrimal–prefrontal suture. The lacrimal and the nasal appear to be in broad contact, lacking any posterior process from the maxilla ( Fig. 7A), although the poor demarcation of sutures makes this morphology difficult to establish conclusively in the four largest skulls. Preorbital ridges are present in all three of the adult skulls preserving the lacrimals: they are more prominently developed in NHMUK OR 30393 and CAMSM TN 907 than in NHMUK OR 30392. A low ridge is present in the smaller specimen NHMUK OR 30381 and a clear boss is present in the juvenile skull NHMUK OR 25170.a, immediately anterior to the anterior orbital corner ( Fig. 5A).

Jugal: The jugal forms the ventral margin of the orbit and extends posteriorly past the level of the infratemporal fenestra ( Fig. 4D). The lateral and ventral margins of the jugal are heavily ornamented with pits, whereas the medial side is smooth. A very large medial jugal foramen is visible in the isolated juvenile jugal NHMUK 20219, the juvenile partial skull NHMUK R 1041 and in NHMUK OR 30393. Three of the four isolated jugals catalogued as NHMUK OR 30289 have large medial foramina, but the fourth one shows a foramen of moderate size. Two medial jugal foramina are visible in the isolated jugal arch NHMUK OR 25188 ( Fig. 7B) and in the left jugal of NHMUK OR 30392, although there is only one foramen on the right jugal.The ectopterygoid is sutured onto the anteromedial side of the jugal, ventral to the postorbital bar. The ventral portion of the postorbital bar that is formed by the jugal is inset from the dorsal edge of the jugal, with a moderately developed sulcus. At the level of the postorbital bar there is a small step in the dorsal outline of the jugal, reminiscent of the condition in Alligator and Caiman, and differing from the straight dorsal outline of Crocodylus . Posteriorly, the jugal terminates in an acute process that is sutured ventrolaterally to the quadratojugal. This process extends slightly further posteriorly than the level of the basioccipital tubera.

Prefrontal: The prefrontals are separated from one another along the midline by the frontal process and by the posterior tip of the nasals ( Fig. 4B). Around the orbit, the prefrontal surface is raised into a ridge ( Figs 4, 5). The prefrontal pillars are poorly preserved and/or partially obscured in all specimens, limiting anatomical observations. Ventrally, they form a suture with the dorsolateral margin of the palatines, forming two walls.

Frontal: There is no distinct step at the base of the frontal process in two of the largest skulls, NHMUK OR 30392 and NHMUK OR 30393, and is very weakly developed in CAMSM TN 907. However, a slight step is present in both the juvenile skull NHMUK OR 25167 ( Fig. 6) and the much larger, though not fully grown, individual represented by NHMUK OR 30381 ( Fig. 7A). A very steep slope is visible in the very small individual represented by NHMUK OR 25170.a ( Fig. 5). The step in both NHMUK OR 25167 and 30381 is highlighted by the ornamentation of the frontal: it is pitted posterior to the step (similar to that of the skull table), but smooth along the frontal process. The dorsal surface of the frontal is gently concave and its dorsolateral (orbital) margin is slightly upturned. The anterior process of the frontal extends beyond the orbital margin, to the level of the anterior tip of the prefrontal and the midlength of the lacrimal. It is difficult to discern the anterior extent of the frontal relative to the jugal in NHMUK OR 30392 and 30393 as a result of poorly defined sutures, as well as the plaster that covers the dorsal surface. However, based on NHMUK R 1042 and OR 29694, the anterodorsal tip of the jugal lies just posterior to the anterior extent of the frontal process. In the juvenile specimen NHMUK OR 25170.a, the anterior extent of the frontal and jugal appear to be approximately equal; however, the frontal process is not well preserved in this specimen. On the skull table, the linear frontoparietal suture enters so deeply into the supratemporal fenestrae that the parietal and postorbital are not in contact.

Often in crocodylians, the anterior frontal process bisects the nasals for a short distance. The morphology in this area is extremely difficult to discern in the largest specimens of Diplocynodon hantoniensis (i.e. NHMUK OR 30392, 30393 and 25166). In NHMUK OR 30381 ( Fig. 7A) (and possibly also NHMUK OR 29694), the frontal appears to invade the nasals anteriorly via two separate processes, between which the nares project posteriorly. However, in the juvenile NHMUK OR 25170.a, the usual condition is observed. In NHMUK OR 30381 and 29694, the elements may have been pulled apart at their sutures and filled with matrix. Consequently, this unusual morphology might merely be an artefact of preservation.

Postorbital: The postorbital bar is incomplete in most specimens and often reconstructed with plaster. However, it appears strongly inclined laterally and inset from the lateral surface of the skull based on NHMUK OR 30392 and CAMSM TN 918. The bar is slender and circular in cross-section, with a short and low process. A deep fossa excavates the lateral surface of the dorsal portion of the postorbital bar, pierced by a large foramen. The postorbital is in contact with a thin dorsal projection from the quadratojugal, but not with the quadrate (see NHMUK OR 30392, CAMSM TN 918) ( Fig. 3B).

Squamosal: The squamosals form the posterolateral corners of the cranial table and the posterolateral margins of the supratemporal fenestrae. Their dorsal surfaces are flat in both juvenile ( NHMUK OR 25167 and 25170.a) and fully grown individuals ( NHMUK OR 30392, 30393, CAMSM TN 907). A prominent squamosal groove is delimited by subparallel rims. A linear series of foramina pierce the groove, as well as the dorsal margin of the groove. Dorsal to the external auditory meatus, there is a slight ventral deflection along the ventral margin of the squamosal. The squamosal–quadrate suture is visible on the right side of NHMUK OR 30393 ( Fig. 4D), extending along the posterior margin of the external auditory meatus. In lateral view, the posterodorsal edge of the squamosal is curved gently ventrally. The squamosal rami do not reach the tip of the paroccipital processes.

Parietal: The parietal forms the medial margin of the supratemporal fossa and fenestra, a large portion of the post-temporal fossa and the roof of the braincase. Posterior to the supratemporal fenestra, the dorsal surface of the parietal is concave and the interfenestral bar has a sagittal sulcus ( Fig. 4). The medial wall of the supratemporal fossa is steeply inclined, facing laterally and slightly dorsally, and the parietal rim of the supratemporal fenestra only slightly overhangs the supratemporal fossa. On the medial wall of the supratemporal fenestra, the laterosphenoid– parietal suture is horizontal. The post-temporal fossa is prominent, forming a shelf that is principally composed of the parietal, anterior to the post-temporal fenestra. Laterally, the parietal approaches, but does not contact, the squamosal on the posterior wall of the supratemporal fenestra. The parietal–quadrate suture undulates strongly.

Quadrate: The quadrate condyle is characterized by a small, ventrally deflected medial hemicondyle, and by a dorsal notch (open medially) that hosts the foramen aëreum ( Fig. 3D). The latter opening is relatively small and placed on the dorsal surface of the quadrate, far from the mediodorsal edge. Whereas the articular surface of the medial hemicondyle faces posteriorly and ventrolaterally, the lateral hemicondyle faces posteroventrally. On the midline of the ventral surface of the quadrate ramus, there is a well-developed attachment scar for M. adductor mandibulae posterior ( Holliday et al., 2013) [scar ‘B’ of Iorndansky (1973)] ( Fig. 2D). This scar develops into a large sharp lamina, projecting approximately 10 mm from the ventral surface of the quadrate in NHMUK OR 30392 and NHMUK OR 30393. The scar continues ventrally along the posterolateral braincase wall, although it becomes less pronounced.

Quadratojugal: The quadratojugal is well preserved on the right side of NHMUK OR 30392 and CAMSM TN 918, in which it is represented by a small step situated far from the posterior corner of the infratemporal fenestra, immediately dorsal to the midpoint of the posterior fenestral margin ( Fig. 2B). As can be observed on NHMUK OR 25188 ( Fig. 7B), a moderately developed process extends along the medial surface of the lower temporal bar. The quadratojugal has a long, apically very thin process that reaches the dorsal corner of the infratemporal fenestra. Posteriorly, the quadratojugal extends to the level of the quadratic condyles, forming the lateral margin of the lateral hemicondyle. The ventral suture with the jugal is approximately parallel with the quadrate–quadratojugal suture.

Supraoccipital: The supraoccipital is well exposed on the posterior surface of the skull and delimits the ventromedial rim of the post-temporal fenestra. The dorsal exposure of the supraoccipital is clearly visible in the juvenile specimen NHMUK OR 25170.a ( Fig. 5), in which it forms a broad and dorsoventrally short triangle. This triangle becomes taller in mature specimens, in which the sutures delimiting the supraoccipital are not easily discernible. The posterior surface of the supraoccipital is excavated by two circular sulci that are separated by a vertical, sagittal crest. Two processes project posteriorly from the supraoccipital, immediately ventral to the slitlike post-temporal fenestrae, and are most clearly observable in NHMUK OR 30393 and NHMUK OR 25167 ( Fig. 6). These processes are not large enough to exceed the level of the posterior margin of the cranial table in the adult specimen NHMUK OR 30392, but they are clearly visible in dorsal view of the juvenile specimen NHMUK OR 25167.

Exoccipital: The exoccipitals have a smooth paroccipital surface devoid of any boss. They extend considerably lateral to the posterior opening of the cranioquadrate passage, but do not reach the basioccipital tubera ventrally ( Fig. 3D). Lateral to the post-temporal fenestra, the surface of the exoccipital is strongly concave. Overall, the occipital surface formed by the exoccipitals faces posterodorsally. The paroccipital process extends posterolaterally, sitting above the posterior quadrate rami. The ventral margin of the paroccipital process above the cranioquadrate canal is marked by a ventrally directed lamina. This lamina terminates immediately medial to the cranioquadrate canal, continuing towards the foramen magnum as a faint horizontal ridge. Neurovascular foramina pierce the exoccipitals and are well preserved in NHMUK OR 30392, 30393 and 25166. Four foramina are present on the right exoccipital of NHMUK OR 30392 ( Fig. 3D). Two foramina occur close together at the level of the foramen magnum for the hypoglossal ( XII) nerve. Slightly ventral and lateral to these foramina is the vagus (X) foramen. Further ventrally, close to the lateral margin the descending process of the exoccipital, approximately at the dorsoventral midpoint of the occipital condyle, is the much larger lateral carotid foramen. This foramen sits within a fossa and appears to be orientated posteroventrally and slightly laterally.

Basisphenoid and basioccipital: The basisphenoid forms the anterior margin of the median eustachian foramen and extends ventrally on to the posterior pterygoid processes. Ventral to the basioccipital, the basisphenoid is anteroposteriorly very thin, but dorsoventrally tall. The median eustachian foramen opens between the basioccipital plate and the basisphenoid. The lateral eustachian foramina are poorly preserved; however, in NHMUK OR 30392, the left foramen is visible, albeit filled with matrix ( Fig. 3D). It occurs roughly in line with the median eustachian foramen, but slightly posterior to it. The external surface of the basioccipital faces posteriorly, ventral to the spherical occipital condyle. A small vertical, sagittal crest extends along the basioccipital plate.

Braincase and orbitotemporal region: One unnumbered juvenile specimen in the CAMSM preserves some of the braincase anatomy. The epipterygoid appears to be absent in this specimen. There is little to no exposure of the prootic on the external braincase wall ventral to the foramen ovale. The anterior margins of the laterosphenoid capitate processes are oriented obliquely relative to the midline. The well-formed lateral laterosphenoid bridge is anteroposteriorly thick, fully enclosing the ophthalmic foramen medially. The lateral surface of the laterosphenoid bridge is excavated by a broad groove for the maxillary ramus of cranial nerve V, and there is no development of a caudal laterosphenoid bridge dorsal to the foramen ovale. The supraorbital foramen, which usually pierces the laterosphenoid–quadrate suture at this level, cannot be discerned. The pterygoid is broken away ventrally, but it is unlikely that any ascending process of the pterygoid contributed to the laterosphenoid bridge sensu Alligator . Although missing, the suture scar for the pterygoid can be traced on the quadrate, and this was most likely linear from the large, posterior, semilunate basisphenoid exposure to the foramen ovale. Only the posteriormost portion of the basisphenoid rostrum is preserved, which lacks a large lateral fossa, and is poorly exposed on the lateral braincase wall anterior to the foramen ovale. The foramina for the palatine ramus of cranial nerve VII are not preserved. Palatine: The palatines flare anteriorly, developing a lateral process projecting into the suborbital fenestra. The palatine–maxilla suture enters the suborbital fenestra at its anteromedial margin. The anterior tip of the palatine, truncated and with a slightly convex edge, reaches the level of the seventh interalveolar space (i.e. anterior to the suborbital fenestra), without tapering. The palatines do not significantly flare posteriorly, and thus do not form a palatine shelf, such as the one seen in Alligator ; instead, the lateral margins of the palatines are roughly parallel posteriorly. The palatine–pterygoid suture is well preserved in NHMUK OR 25166, 30392 ( Figs 1, 2) and CAMSM TN 907; in all specimens it is situated far anterior to the posterior corner of the suborbital fenestra, but varies in shape from being straight to prominently ‘W’ shaped.

Ectopterygoid: The relationship between the ectopterygoid and the maxilla was evaluated on the basis of NHMUK OR 30392, the only specimen preserving the posterior region of the maxillary tooth row. The anterior process of the ectopterygoid is pointed, reaching the level of the anterior margin of the 15 th maxillary alveolus ( Fig. 2D). The ectopterygoid forms the medial wall of the last maxillary alveolus only. Anterior to this alveolus, the maxilla is so prominently developed medially that the ectopterygoid does not about the maxillary tooth row. As commented upon by Brochu (1997), this condition is closer to other alligatoroids than to the plesiomorphic crocodylian condition. The posteroventral process is approximately twice the length of the anterior process. This posteroventral process does not reach the tip of the pterygoid at maturity; instead it terminates beyond midlength of the pterygoid wing. The ectopterygoid– pterygoid flexure is clearly visible in the juvenile skull NHMUK OR 25167, as well as the pterygoids and associated ectopterygoids ( NHMUK OR 30251) ( Fig. 8A) of a fully-grown individual that might belong with NHMUK OR 30393. Based on NHMUK OR 30251, the ventrolateral surface of the ectopterygoid, near the margin of the suborbital fenestra, is pierced by two foramina on the right side and three on the left. The ectopterygoid has a blunt posterior process on the medial surface of the jugal arch ( Fig. 7B), and sends an ascending branch along the medial surface of the postorbital bar. Immediately anterior to the sutural surface for the jugal, on the anterior branch of the left ectopterygoid, there is a large foramen.

Pterygoid: The pterygoid forms the posterior margin of the suborbital fenestra. The pterygoid– ectopterygoid suture, visible in lateral view, intersects the posterolateral/posterior edge of the suborbital fenestra. The pterygoid wings are large and robust. Their lateral margins are thickened and expanded dorsoventrally, with a rugose texture. A distinct sulcus runs parallel and dorsal to the suture with the ectopterygoid along the lateral surface of the pterygoid ( Fig. 7C). Two foramina are present within the fossa on the left side of NHMUK OR 30251: a large posterior foramen, filled with matrix, and a much smaller anterior foramen. An equivalent small anterior foramen is also present on the right side. Anteriorly, the pterygoid is sutured to the palatine. Dorsally, they form the dorsolateral walls of the nasopharyngeal passage. The two halves of the pterygoids rise dorsomedially, meeting at the midline to surround the airways, forming a dorsoventrally tall and transversely very thin wall, immediately below the level of the frontal. The posterior pterygoid processes project posterodorsally and slightly laterally. They are dorsoventrally tall and posteriorly prominent, and developed ventrally to the median eustachian foramen. The choanae open centrally within the pterygoids. In posterior view, the pterygoid wings extend posteroventrally at around 30° from the horizontal. The dorsal surface of each wing is strongly concave. This concavity results from the large rugose ‘buttress’ that defines the lateral edge of each pterygoid wing, the dorsomedially directed lamina that encloses the nasopharyngeal duct, as well as the posteromedial process. This concavity flattens out posteriorly towards where the pterygoid wing forms an acute process.

Mandible

Preservation: As with the crania, some lower jaws are completely isolated [ NHMUK OR 25188 ( Fig. 9), NHMUK OR 30394 ( Fig. 10), NHMUK OR 30397, NHMUK OR 30396, CAMSM TN 904], whereas others are preserved on a slab along with vertebrae, e.g. NHMUK R 1045. NHMUK OR 30396 is a fused pair of mandibles braced with a metal rod. Only one pair of mandibles is associated with a cranium – NHMUK OR 30393, but very few mandibular features can be discerned from this specimen. Across these specimens, nearly all elements of the mandible are preserved, with the exception of the coronoid. A list of mandibular measurements is presented in Table 2.

Mandibular fenestrae and major openings: The external mandibular fenestra is moderately large and oval-shaped, but the foramen intermandibularis caudalis is barely visible in lateral view. The presence of the foramen intermandibularis medius cannot be determined due to the absence of the coronoid in all specimens. The dentary–surangular suture intersects this fenestra slightly anterior to the posterodorsal angle of the opening (very close to the angle in NHMUK OR 25178) ( Fig. 9C, D), whereas the dentary–angular suture reaches about the midlength of the ventral rim of the fenestra. The angular– surangular suture intersects the fenestra ventral to the posterodorsal angle.

Dentary: The best-preserved dentaries (i.e. NHMUK OR 30396, 30397) each host 20 alveoli. The dentaries are gently curved between the fourth and 11 th alveoli. There is some variation in the posterior extent of the dentary symphysis: it extends to the middle of the fourth alveolus in NHMUK OR 25178 and 30394 ( Figs 9A, 10A), to the posterior rim of the fourth alveolus in NHMUK OR 30397 and to the middle of the fifth alveolus in NHMUK OR 30396 and R 1043. In lateral view, the dorsal edge of the dentary gently undulates: the concavities correspond to the second, seventh and 14 th alveoli, whereas the convexities correspond to the first alveolus, the third interalveolar space, and the 12 th alveolus. The jaw is also slightly festooned in dorsal view, with a main lateral convexity corresponding to the confluent third and fourth alveoli. Posteriorly, between the fourth and tenth alveoli, the jaw is slightly concave laterally. The third and fourth alveoli are the largest, with the latter slightly the larger of the two, and are confluent with one another. Posteriorly, alveoli five to nine are smaller and approximately equal in anteroposterior length. The next largest dentary alveolus is the 12 th. The seventh interalveolar space is at least a little larger than the surrounding ones in all specimens (for very clear examples, see NHMUK OR 30287 and 30321). The Meckelian groove begins at the posterior end of the dentary symphysis, at approximately mid-height. In NHMUK OR 30397, a foramen perforates the dentary along the Meckelian groove, at the level of the ninth dentary alveolus.

Splenial: The splenials are well preserved in at least NHMUK OR 25178, 30394 and 30396. They are not involved in the symphysis of the lower jaw and their anterior tip is located ventral to the Meckelian groove ( Fig. 9E, F). Given the lack of foramina in this region, the mandibular ramus of cranial nerve V can be inferred to have left the Meckelian groove at the anterior end of the splenial. There is a foramen at mid-height of the splenial, at the level of the 16 th dentary alveolus in NHMUK OR 30394 ( Fig. 10F) and possibly in NHMUK OR 25178 too. In NHMUK OR 30396, the splenial does not form the dorsomedial wall of the posteriormost dentary alveoli; however, in NHMUK OR 25178, the splenial approaches the medial wall of the final three alveoli, and formed the wall at least in the last two. The anterodorsal margin of the foramen intermandibularis caudalis is open in all specimens where preserved, indicating that the splenial formed the anterior margin of this foramen.

Angular: The angular of NHMUK OR 30394 ( Fig. 10) is well preserved. Towards its anterior end, the angular extends dorsally to the foramen intermandibularis caudalis, with a tip that is rather rounded and dorsoventrally tall ( Fig. 10F). The lateral surface of the angular is strongly ornamented with pits and furrows ( Figs 9C, 10C). However, they do not continue onto the ventral surface, nor do they extend onto the lateral surface of the retroarticular process. A longitudinal ridge occurs on the ventral margin of the angular, extending onto the lateral surface, and defines the boundary between the anterior ornamented surface of the angular from the posteriorly smooth surface, beneath the retroarticular process ( Fig. 11A). This ridge is very prominent in NHMUK OR 30396, NHMUK OR 30397 and CAMSM TN 904 but less well-developed in NHMUK OR 30394 and 25178. This feature probably marks the anterior limit of the insertion of M. pterygoideus ventralis, which inserts in a similar position in living caimans ( Bona & Desojo, 2011), imparting the same ridge ( Fig. 11B). This differs markedly to the condition in all Crocodylus ( Fig 11C) and Alligator species, which lack a sharp ridge and in which the ventral margin of the angular is very exposed in lateral view.There is a large longitudinal sulcus along the ventromedial surface of the angular. This sulcus begins posterior to the foramen intermandibularis caudalis, and terminates approximately at its anterior margin. A series of large foramina occur posterior to this sulcus in all specimens except for NHMUK OR 30397, which is poorly preserved in this region. Whereas there are two foramina in NHMUK OR 30394, there are three in NHMUK OR 25178 and NHMUK OR 30396. The anterior extent of the angular is difficult to fully discern; however, it appears to reach the level of the 17 th alveolus based on the articulated mandible NHMUK OR 30396 and the isolated left ramus NHMUK OR 25178. Posteriorly, the angular extends to the posterior end of the retroarticular process, along the ventrolateral margin, terminating in an acute process.

Surangular: The anterior processes of the surangular are best preserved in in NHMUK OR 30381 About NHMUK ( Fig. 12) and NHMUK OR 30396 About NHMUK . They are subequal in length, with the dorsal process extending slightly further anteriorly than the ventral one. This morphology is confirmed by NHMUK OR 25178 About NHMUK ( Fig. 9D), in which the dorsal process, despite being slightly incomplete anteriorly, is so thick and robust at its preserved anterior end that it had to be longer than the completely preserved ventral process. NHMUK OR 30394 About NHMUK preserves a broken ventral process of the surangular (whose development is implied by the scar it left on the accompanying dentary) and a very long, tapering, laminar dorsal process ( Fig. 10D). The surangular spur between the dentary and splenial is not completely preserved in any one specimen. In NHMUK OR 30394 About NHMUK , the spur is mostly preserved and visible sutured to the medial surface of the splenial ( Fig. 10B). The posteriormost alveoli are not preserved in this specimen, but the process most likely reached at least the 17 th interalveolar space. This can be confirmed in NHMUK OR 25178 About NHMUK and CAMSM TN 904 . In both specimens, the surangular spur is not completely preserved. However, the scar for the spur on the splenial is adjacent to the last three alveoli .

Just above the external mandibular fenestra and up to the glenoid fossa, the dorsal edge of the surangular is flat and gently laterally sloping area ( Fig. 9A, B). Posterior to this smooth area, the surangular approaches the dorsal tip of the lateral wall of the glenoid fossa, though does not completely cover it in lateral view. A very prominent sulcus occurs on the dorsolateral margin of the surangular, at the level of the glenoid fossa ( Fig. 9D). Posteriorly, the tip of the surangular does not reach the posterior tip of the retroarticular process in either of the mandibular rami of NHMUK OR 30396, but it is much more posteriorly developed in NHMUK OR 25178 ( Fig. 9D). The surangular–angular suture intersects the articular close, but dorsal to the ventral tip ( Fig. 9F).

The surangular–articular suture is very straight in the posterior wall of the adductor chamber, lacking laminae and not recessed in a sulcus. A lingual foramen for the articular artery, infilled with matrix, can be discerned in all specimens. In NHMUK OR 30396 and CAMSM TN 904, the lingual foramen is situated on the surangular–articular suture, whereas it is located solely on the articular in remaining specimens (i.e. NHMUK OR 25178, 30394 and 30397). Posteriorly, the surangular–articular suture is situated on the lateral edge of the glenoid fossa, and is not bowed within the fossa. It also extends onto the dorsolateral surface of the transverse ridge that separates the retroarticular process from the glenoid fossa. At its posteriormost extent, the suture forms the sharp lateral margin of the retroarticular process.

Articular: The retroarticular process is short and oriented posterodorsally. The foramen aëreum is large and clearly inset from the margin of the retroarticular process. A sharp, vertical lamina occurs for a short distance posterior to the foramen aëreum. On the medial surface, there is a prominent, wedgeshaped sulcus, dorsally defined by a ridge. This runs approximately parallel to the articular–angular suture, and is perpendicular to the anteromedial corner of the articular. This ridge was probably associated with the attachment of M. pterygoideus dorsalis, which is known to attach to the medial surface of the articular in living crocodylians, e.g. Alligator mississippiensis ( Holliday et al., 2013) . The medial edge of the retroarticular process is sharp, and overhangs the medial surface of the articular as a shelf, as does the medial edge of the hemifossa. At the level of the transverse ridge, there is a large medial notch dividing the retroarticular process from the glenoid fossa.

Pattern of occlusion: There are five alveoli in each premaxilla, 17 in each maxilla and 20 in each dentary. All of the alveoli are approximately circular in cross-section. None of the juvenile specimens preserve the premaxilla, meaning that it is not possible to directly assess the changes through ontogeny of the relationships between the fourth dentary tooth and the region corresponding to the premaxillo-maxillary suture. However, the four fully grown specimens show significant variation, suggesting that ontogenetically the primitive condition was the one with the tooth hosted in a pit, and that in some cases the outer wall of the pit could be progressively eroded, giving origin to a notch. In particular, NHMUK OR 25166 About NHMUK has a deep pit on the premaxilla between the last alveolus and the suture with the maxilla ( Fig. 1C, D). NHMUK OR 30392 About NHMUK , 30393 About NHMUK and CAMSM TN 907 all clearly show a notch corresponding to the premaxillo-maxillary suture ( Figs 2D, 3B). Such variability is also shown in isolated premaxillae NHMUK OR 30369 About NHMUK and 30370, with a pit in the former and a lateral notch in the latter. The left premaxilla of NHMUK OR 25166 About NHMUK (the type of Diplocynodon hantoniensis ) shows a very deep and large occlusal pit located lingual to the tooth row, close to the suture with the maxilla; a second pit, much smaller but as deep, is placed lingual to the first maxillary alveolus. The preservation on the right side of the same skull hinders the detection of the latter pit, but the one on the premaxilla is placed more in line with the alveoli than the corresponding pit on the left premaxilla .

The disposition of the occlusal pits on the maxillae indicates that the dentary teeth occluded lingual to the maxillary teeth, except for those occluding in line after the sixth and seventh maxillary teeth. This is clearly shown in the maxillae of NHMUK OR 25166 ( Fig. 1D), 25168–9 and 30392 ( Fig. 2D), and is confirmed by the dentaries of NHMUK R1043, 25178, 30394 and 30397, all of which have occlusal pits labial to interalveolar spaces 14 and 15 ( Figs 9B, 10B).

In the premaxilla, the third and fourth alveoli are the largest, with the latter slightly larger. The fifth alveolus is the smallest, and the first and second alveoli are equally intermediate in size. Two large occlusal pits (one on each side) occur within the premaxillary palate, between the first and second premaxillary alveoli, and are separated posteromedially from the incisive foramen by a thin straight wall. Only the left occlusal pit is visible in NHMUK OR 30392, as the right is infilled with matrix. In NHMUK OR 25166, the walls of the pit are very thin and worn.

In the maxilla, alveoli four and five are the largest, and subequal in size, corresponding to the first lateral and ventral convexity of the maxilla. Alveoli six to eight are smaller and subequal in size. Alveolus size increases posteriorly from alveolus eight, reaching a second maximum at position ten and 11, which are subequal. Posteriorly from alveolus 11, the alveoli gradually decrease in size and remain circular.

In the dentary, the first alveolus is largest and the second alveolus is considerably smaller. Alveoli three and four are subequal and confluent; they are larger than alveolus two, and slightly smaller than alveolus one. Alveoli five to ten are smaller than alveoli three and four.Alveoli 11 and 12 are larger than the preceding six alveoli. In some specimens, alveolus 13 is as large as 11 and 12, e.g. NHMUK OR 30396. Posteriorly from alveolus 13, the alveoli are subequal and smaller.

Dentition

NHMUK OR 25166 preserves a nearly continuous series of premaxillary and maxillary teeth in place. All premaxillary and anterior maxillary teeth are conical and lingually curved, with smooth carinae, and commonly preserve faint longitudinal striations ( Fig. 13A, B). Carinae occur on the medial and lateral edges of the anteriormost premaxillary teeth, but on the anterior and posterior edges of all other premaxillary and maxillary teeth ( Fig. 13D).

Posteriorly, the maxillary teeth become shorter, more globular and less lingually curved, though never becoming blunt as they retain modest carinae and pointed tips ( Fig. 13C). A small constriction separates the tooth crown from the root, as in D. remensis ( Martin et al., 2014) ( Fig. 13E, F).

Fewer dentary teeth are preserved in place (e.g. NHMUK OR 30396). However, the same pattern is observed, with anteriorly very conical teeth, with smooth carinae, becoming shorter and more globular posteriorly.