Taxonomic revision of Eoalligator (Crocodylia, Brevirostres) and the paleogeographic origins of the Chinese alligatoroids

Description: material originally assigned to Eoalligator chunyii

Seven of the fragments that were originally assigned by Young (1964) to Eoalligator chunyii can be identified as adult based on size and fusion of neurocentral sutures (Brochu, 1996): IVPP V 2716-1.1, IVPP V 2716-1.2, IVPP V 2716-2.1, IVPP V 2716-2.2, IVPP V 2716-2.3, IVPP V 2716-3, IVPP V 2716-6, IVPP V 2716-12, and IVPP V 2721.4.

Skull

Premaxilla—IVPP V 2716-2.1 (Figs. 2A and 2B) comprises the lateral portion of a left premaxilla, with a small fragment of the maxilla still attached. In lateral view, the ventral margin curves posterodorsally as in Crocodylus porosus (Schneider, 1801). In Alligator sinensis the margin is less strongly curved. Four alveoli are preserved, and damage to the anterior end of the bone has probably removed one additional alveolus. Given that crocodylian premaxillae consistently contain either four or five teeth, the preserved alveoli are probably the second through fifth. Broken tooth crowns are preserved within the second, third and fifth alveoli. The third alveolus is the largest in diameter, the second and fourth considerably smaller and subequal in size, and the fifth alveolus is the smallest. Immediately posterior to the fifth alveolus, the premaxilla-maxilla suture runs laterally across a ventrally facing fossa. As in extant Alligator, this fossa accommodated the fourth dentary tooth, which would thus have been partly concealed when the jaw was closed. The first maxillary alveolus is situated just posterior to the recess, and is equal in diameter to the fifth premaxillary alveolus.

Maxilla—IVPP V 2716-2.2 is a partial left maxilla with a piece of the lacrimal attached, whereas the partial skull IVPP V 2716-11 includes large portions of both maxillae. In IVPP V 2716-2.2 (Figs. 2C–2E), only the posterior part of the left maxilla is preserved. This bone contains four alveoli with damaged teeth, rather than the five described by Young (1964). The second and fourth preserved teeth have relatively intact crowns, which are blunt with smooth (unserrated) carinae (Fig. 2D). The teeth are clearly somewhat laterally compressed, demonstrating that E. chunyii lacks the extremely bulbous, almost ball-shaped tooth crowns seen in typical globidontans.

In IVPP V 2716-11 (Figs. 3A–3D), the dorsomedial portions of the two maxillae are preserved over a long interval representing the antorbital region of the skull. Each maxilla contacts the nasal medially and the lacrimal posteriorly. There is no maxillary process either intruding into the area occupied by the lacrimal or projecting between the lacrimal and nasal. Such a maxillary process is a derived character present in derived gavialoids, some Crocodylus species, most tomistomines and most alligatoroids (Brochu, 1999; Delfino & John, 2010; Langston, 1965). In alligatoroids, however, the location of the maxillary process is not always within the lacrimal. Poorly preserved sculpturing is visible on the dorsal surfaces of both maxillae. In ventral view, the nasal channel (NAC) is exposed anteriorly due to postmortem damage.

Lacrimal—Lacrimals are preserved in IVPP V 2716-2.2 and IVPP V 2716-11. In IVPP V 2716-2.2 (Figs. 2C–2E), the anterior part of the left lacrimal is present and contacts the maxilla anteriorly. In medial view, the nasolacrimal duct is apparent as a groove extending anteriorly towards the external naris (EN). The duct is “V”-shaped in cross section, with the apex pointing laterally.

In IVPP V 2716-11 (Figs. 3A and 3B), the anterior part of the left lacrimal is intact and contacts the maxilla anteriorly and the nasal medially. The posteromedial contact with the prefrontal may also be preserved, but the suture is not clearly apparent. Similarly, the right side of the skull retains part of the lacrimal and possibly a small part of the prefrontal, but sutural boundaries in this region are uncertain. However, the lacrimal certainly extends further anteriorly than the prefrontal, and contacts the nasal. Anterior extension of the lacrimal beyond the prefrontal is a plesiomorphic character, and the derived condition (shorter anterior extension of the lacrimal) is present in most alligatorines and nettosuchids (Brochu, 1999; Brochu, 2003).

Nasal—Both nasals are preserved in IVPP V 2716-11 (Figs. 3A–3D), although they are damaged anteriorly. Each nasal contacts its counterpart medially, the maxilla laterally, and the lacrimal posterolaterally, but the posterolateral contact with the prefrontal is not clearly preserved on either side of the skull. The lateral border of the nasal is straight, rather than concave as in A. sinensis.

Jugal—IVPP V 2716-12 comprises a fragment of a right jugal preserved with the ectopterygoid, and IVPP V 2716-14 includes a small piece of a right jugal. The portion of the bone preserved in IVPP V 2716-12 (Figs. 2E and 2F) appears to be a posterior fragment of the jugal, probably situated below the infratemporal fenestra (ITF). It is slightly disarticulated from the ectopterygoid. A recess with a sharp, shelf-like ventral margin is present on the lateral surface of the preserved part of the jugal. A recess in the equivalent position, approximately below the ITF, is absent in extant alligators and at least some Crocodylus species (C. porosus and C. niloticus) (Wang, pers. obs., 2016) (Laurenti, 1768). The lateral surface of the jugal is sculptured, and the contact with the quadratojugal is not preserved.

The jugal fragment in IVPP V 2716-14 (Figs. 2I and 2J) consists only of the base of the ascending process, which contributes to the postorbital bar, and the adjacent part of the jugal body. The preserved part of the postorbital bar is formed entirely by the jugal, implying that the descending process of the postorbital does not reach the base of the postorbital bar. There is no groove on the dorsal surface of the jugal between the base of the ascending process and the lateral margin of the bone.

Postorbital—IVPP V 2716-14 (Fig. 2K) includes a small postorbital that was originally attached by matrix to the partial right jugal. The two bones were separated during preparation, but the postorbital fragment reveals no important morphological information.

Ectopterygoid—A right ectopterygoid is preserved in IVPP V 2716-12, and both ectopterygoids are preserved in IVPP V 2716-11. In IVPP V 2716-12 (Figs. 2E and 2F), the body of the ectopterygoid is preserved but the maxillary and pterygoid processes are broken away. The anterior edge of the ectopterygoid, which forms the posterolateral margin of the suborbital fenestra (SOF), is strongly concave.

In IVPP V 2716-14 (Figs. 3A–3D), the medial part of the left ectopterygoid and a compressed portion of the right ectopterygoid are preserved. Each ectopterygoid contacts the maxilla laterally and the pterygoid medially. The narrow and deeply concave posterior apex of the left suborbital fenestra is enclosed by the pterygoid, but the ectopterygoid’s contribution to the margin of the fenestra begins only a short distance anterolateral to the apex.

Palatine—Both palatines are preserved in IVPP V 2716-11 (Figs. 3A–3D), but are damaged anteriorly. Each palatine contacts the pterygoid posteriorly. The posterior part of the lateral margin of the palatine runs posterolaterally. The palatine does not contribute to the concave posterior apex of the left SOF. The palatine-pterygoid suture extends transversely between the SOFs. Based on the position of the palatine-pterygoid suture, the choana must be surrounded entirely by the pterygoid, but the pterygoid is too poorly preserved for this to be confirmed by direct observation.

Pterygoid—The pterygoid is preserved in IVPP V 2716-1.1 and IVPP V 2716-11. The pterygoid of IVPP V 2716-1.1 (Figs. 4E and 4F) is represented only by a small fragment that remains in articulation with the braincase but has been deflected towards the right side of the skull. The fragment comprises the median part of the bone, and includes the small, damaged left and right posterior processes. The pterygoid is preserved in contact with the basioccipital dorsally and the quadrates dorsolaterally. The basisphenoid and laterosphenoid presumably also contact the preserved part of the pterygoid, but the boundaries of these bones are not clearly visible. Unfortunately, the orientation of the posterior process is uncertain due to distortion.

In IVPP 2716-11 (Figs. 3C and 3D), the anteromedial part of the pterygoid is preserved but the posterior part is severely damaged. The pterygoid contacts the palatine anteriorly and the ectopterygoid anterolaterally.

Quadratojugal—The posterolateral part of the right quadratojugal is preserved in IVPP V 2716-1.1 (Figs. 4A–4D, 5A), as a small piece of bone attached to the lateral margin of the quadrate.

IVPP V 2716-3 comprises the posterior end of a left mandible preserved in contact with the quadrate region of the skull. The posterior part of the quadratojugal is preserved, and contacts the quadrate medially (Figs. 6A and 6B). The quadratojugal extends to the posterior tip of the lateral hemicondyle of the quadrate, obscuring the hemicondyle in lateral view. Anteriorly, the root of the quadratojugal spine (QJS) remains intact (6A). The position of this structure suggests the spine would have protruded out into the infratemporal fenestra rather than following the superior margin of this opening, a plesiomorphic character shared with Gavialis gangeticus, Crocodylus and Osteolaemus. A mediodorsally placed quadratojugal spine is a derived feature shared by Trilophosuchus rackhami (Willis, 1993) and Crocodylus depressifrons (Delfino & Smith, 2009), and most alligatorines (Brochu, 1999; Brochu, 2011). Sculpturing is preserved on the lateral surface.

Quadrate—One or both quadrates are preserved in IVPP V 2716-1.1, IVPP V 2716-3 and IVPP V 2716-13. In IVPP V 2716-1.1 (Figs. 4A–4D, 5A and 5B), the right quadrate is relatively complete apart from damage to the margins and the absence of the condyles, but the left quadrate is less well-preserved. The quadrate contacts the squamosal dorsally, the parietal and the exoccipital medially, the pterygoid ventrally, the quadratojugal laterally and the laterosphenoid anteriorly. Anteriorly, the quadrate and laterosphenoid surround the dorsal part of the foramen ovale (FO), meeting at a nearly vertical suture that intersects the dorsal border of the FO. In the midline of the skull, a remnant of the basisphenoid contributes to the floor of the braincase and is visible in anterior view because the front part of the skull is missing. Below the posterior rim of the supratemporal fenestra (STF), the quadrate appears to contribute to the floor of the orbitotemporal foramen (OTF) (Figs. 4A, 4B and 5B), intruding between the parietal and squamosal. This character is shared with L.canadensis, Brachychampsa and Deinosuchus (Brochu, 1997; Gilmore, 1911; Norell, Clark & Hutchison, 1994). In the posteroventral view, there is a crest on the ventral side of quadrate for attachment of the adductor mandibulae posterior (AMP). An equivalent, but less well-developed, crest is present in A. sinensis.

In IVPP V 2716-3 (Fig. 6A) the body of the left quadrate is preserved, and contacts the exoccipital dorsomedially and the quadratojugal laterally. The lateral hemicondyle of the quadrate is covered by the quadratojugal laterally. As in other crocodylians, there is no thin crest on the dorsal surface of the quadrate. The medial hemicondyle of the quadrate is larger than the lateral hemicondyle, as in extant Crocodylus. No foramen aerum (FA) is evident on the dorsal surface. The dorsal surface above the medial hemicondyle is damaged, as is the nearby part of the medial side of the quadrate. Furthermore, the ventromedial corner of the quadrate has broken away from the rest of the bone along a vertical crack and has been displaced a short distance posteriorly. The FA may have originally been located either on the line of the crack or on the medial surface of the quadrate. The latter possibility seems more likely, given that the FA on the articular is located close to the medial edge of the dorsal surface (see below). A fragment of the paroccipital process is preserved in articulation with the quadrate. Ventromedial to this fragment, the cranioquadrate canal (CQC) is exposed by post mortem damage.

Fragments of the ventral process of the left quadrate are preserved in the damaged partial skull IVPP V 2716-13, and the crest for the AMP is visible as in IVPP V 2716-1.1. However, no other important morphological details are discernable.

Laterosphenoid—A remnant of the laterosphenoid is preserved in IVPP V 2716-1.1, near the foramen ovale (FO). It contacts the quadrate posteriorly, the parietal dorsally and the pterygoid ventrally, although the suture with the pterygoid is unclear.

Parietal—The parietal is preserved in IVPP V 2716-1.1 and IVPP V 2716-13. In IVPP V 2716-1.1 (Figs. 4A–4D, 5B), the parietal is nearly complete, except that the anterior part of the bone is damaged. The parietal contacts the squamosals laterally, the exoccipitals posterolaterally, and the supraoccipital posteriorly, and exhibits an inverted “Y” shape in dorsal view. The intertemporal bar bears a deep longitudinal sulcus that is absent in extant Alligator, and less distinct in adult Crocodylus niloticus than in IVPP V 2716-1.1. The rims of the STFs are elevated above the skull table. A similar, but less distinct, rim is present in “Crocodylus” depressifrons (Delfino & Smith, 2009). The sculpturing on the surface of the parietal is better-developed near the squamosal than elsewhere.

In IVPP V 2716-13 (Fig. 2H) the middle of the posterior portion of the parietal is preserved, but the surface is badly damaged. The parietal has also been displaced to the left relative to the occipital condyle and the rest of the braincase. The sulcus between the STFs is less well-developed than in IVPP V 2716-1.1, and the intertemporal bar is substantially broader. Both differences might reflect ontogenetic variation, as IVPP V 2716-13 is considerably smaller than IVPP V 2716-1.1.

Squamosal—The medial part of the left squamosal, and most of the right squamosal apart from the anterior process, are preserved in IVPP V 2716-1.1 (Figs. 4A–4D, 5A and 5B). The squamosal contacts the parietal medially, the quadrate ventrally, the supraoccipital posteromedially, and the exoccipital ventrally. The posterior process extends posterolaterally, rather than posteriorly as in extant alligators, and reaches the posterior tip of the exoccipital process. In occipital view, the squamosal-exoccipital suture extends horizontally, but its lateral portion has an irregular ventral concavity. In lateral view, the squamosal-quadrate suture runs alongside the posterior margin of the otic recess for a short distance before curving anteriorly to intersect the edge of the recess. The posterior margin of the otic recess forms a small posterior notch (Fig. 5A).

Supraoccipital—The supraoccipital is completely preserved in IVPP V 2716-1.1 and IVPP V 2716-13. The supraoccipital contacts the parietal anteriorly, the squamosal laterally and the exoccipital ventrolaterally. In IVPP V 2716-1.1 (Fig. 4), the portion of the supraoccipital that is exposed on the skull table is semicircular. The supraoccipital is excluded from the foramen magnum (FM) by both exoccipitals.

In IVPP V 2716-13 (Figs. 2G and 2H), the supraoccipital is less well-preserved and well-exposed. The supraoccipital appears morphologically similar to its counterpart in IVPP V 2716-1.1, but makes a smaller contribution to the skull table.

Exoccipital—One or both exoccipitals are preserved in IVPP V 2716-1.1, IVPP V 2716-3 and IVPP V 2716-13. In IVPP V 2716-1.1 (Fig. 4) the entire right exoccipital and the medial part of the left exoccipital are present. Each exoccipital contacts the supraoccipital dorsomedially, the squamosal dorsally, the quadrate ventrolaterally and the basioccipital ventromedially. The exoccipital forms the lateral margin of the foramen magnum (FM). Further laterally, the exoccipital bears a tapering ventral flange that extends to the same level as the ventral margin of occipital condyle. Two foramina are preserved lateral to the occipital condyle and medial to the suture with the quadrate: the dorsolaterally positioned jugular foramen transmitting the glossopharyngeal (IX), vagus (X), and accessory (XI) nerves, and the medioventrally positioned carotid foramen. Situated further dorsally, and lateral to the FM, is the hypoglossal canal for branches of the hypoglossal nerve (XII).

Only a small fragment of the exoccipital, preserved in contact with the quadrate, is present in IVPP V 2716-3. Similarly, only fragments of the right exoccipital are preserved in IVPP V 2716-13 (Figs. 2G and 2H). The sutural boundaries of the bone are unclear due to postmortem damage. However, the exoccipital has a ventral flange of about the same extent seen in IVPP V 2716-1.1.

Basioccipital—The basioccipital is complete in IVPP V 2716-1.1 and IVPP V 2716-13. In IVPP V 2716-1.1 (Figs. 4 and 5C) the basioccipital contacts the exoccipitals dorsolaterally and the pterygoid ventrally, and contributes to the ventral part of the FM. The occipital condyle is wide and suboval in occipital view, being somewhat dorsoventrally compressed, and projects posteriorly. A small basioccipital tuber appears to be present on the left side of the basioccipital, but there is no evidence of the midline crest below the occipital condyle that combines with the tubera to define a “W” shape in ventroccipital view (Fig. 4E) in extant alligators and C. niloticus.

Lower jaw

Dentary—In the partial mandible IVPP V 2716-1.2 (Figs. 6C and 6D), the middle part of the right dentary is in contact with the medially positioned splenial, and the entire preserved section of the mandible has been laterally compressed. A second mandibular fragment has been attached with plaster to the posterior end of the main fragment, and was tentatively described by Young (1964) as though this represented its natural position. However, the second fragment appears to represent a small portion of the opposite (left) mandible of either the same individual or another of similar size.

Anteriorly, the main fragment of IVPP V 2716-1.2 is preserved almost to the level of the symphysis. The upper margin of the dentary as seen in lateral view has a slight undulating curvature in most crocodylians, although this margin is straight in some taxa such as Gavialis gangeticus. In some alligatoroids the upper margin of the dentary is more strongly curved, and in these cases the lower margin tends to become convex. In IVPP V 2716-1.2 the lower margin of the preserved part of the dentary is straight, whereas the dorsal margin is relatively straight posteriorly, rises to a low eminence containing sockets for two relatively large teeth, and slopes gently anteroventrally towards the anterior end of the bone. The curvature of the mandible thus appears to match the gently curved condition of typical crocodylians.

The preserved series of alveoli extends along the entire preserved part of the main fragment and may represent the fourth to seventeenth dentary teeth, but the anteriormost and posteriormost alveoli are incomplete. Young’s (1964) estimate that twenty-three teeth would have been present in life included the alveoli present on the posterior fragment. The incomplete anteriormost alveolus on the main fragment, which we identify as the fourth, appears to be the largest in the series. The fifth through ninth alveoli are relatively small, whereas the tenth and particularly the eleventh are somewhat enlarged. The twelfth alveolus is considerably smaller than the eleventh. The more posterior alveoli are also relatively small, and appear to have suffered from lateral compression.

The posterior fragment included in IVPP V 2716-1.2 contains five alveoli that probably represent teeth 10-14 of a left mandible, based on their size and the presence of an anteriorly widening shelf medial to the tooth row. This fragment corresponds closely in morphology to the equivalent part of the main fragment. The medial surface of the posterior fragment bears a subdued area, bounded dorsally by the prominent medial edge of the shelf, which may represent part of the articular surface for the splenial. Below this is a longitudinal furrow that presumably corresponds in part to the Meckelian groove, but seems to have been enlarged by damage to the mandible.

IVPP V 2716-2.3 is the anterior part of a left dentary (Figs. 6E and 6F), whereas IVPP V 2721.5 is the anterior part of a right dentary (Figs. 6G and 6H). IVPP V 2771 is a poorly preserved left dentary that may not pertain to the same species as the other material (Figs. 6I–6K), because it shows no diagnostic characters of A. nanlingensis and is the only crocodylian specimen recovered from L2. An anteriorly tapering depression on the medial surface of IVPP V 2716-2.3 represents the anterior end of the Meckelian groove. The mandibular symphysis consistently extends posteriorly to the level of the fourth dentary tooth, but without reaching the tooth’s posterior margin. Five alveoli (for teeth 4–8) are preserved in IVPP 2716-2.3, whereas four alveoli (for teeth 4–7) are preserved in IVPP V 2721.5 and eight indeterminate alveoli are preserved in IVPP V 2771. The part of IVPP V 2771 immediately posterior to the symphysis is relatively intact, and may include the splenial, but more posteriorly the mandible is damaged, distorted and laterally compressed. The fourth tooth is large, although the apex is missing. About 2 cm posterior to the fourth tooth is preserved a small tooth with an acute, laterally compressed crown and smooth carinae (Fig. 6J). A tortoise pleural bone (TPB) is attached to the medial surface of the mandible, near the posterior end of the preserved portion.

Splenial—The main fragment of IVPP V 2716-1.2 (Figs. 6C and 6D) includes a preserved partial splenial, whose anterior tip is slightly broken but clearly would have closely approached the symphysis in the intact mandible (Figs. 6E and 6F). The splenial contacts the dentary laterally, and the medial surface of the former bone lacks any evident perforation. In IVPP V 2771 a fragment of the splenial may be present immediately caudal to the symphysis (Figs. 6I–6K). In IVPP V 2716-2.3, the Meckelian groove ends a short distance posterior to the mandibular symphysis, implying the splenial would not quite have entered the symphysis even if it covered the groove completely.

Surangular—The left surangular is preserved in both IVPP V 2716-3 and IVPP V 2721.1. In IVPP V 2716-3 (Figs. 6A and 6B) the surangular is nearly complete, although the ventral part of the bone is distorted. The surangular contacts the articular medially and the angular posteroventrally. The dorsal margin of the surangular is flush with the transverse ridge at the posterior edge of the glenoid fossa, so that the surangular fully conceals the glenoid fossa in lateral view. Immediately lateral to the glenoid fossa, there is a sulcus on the narrow dorsolateral margin of the surangular. This sulcus is a unique feature among crocodylians, seen only in specimens originally assigned to Eoalligator chunyii and Asiatosuchus nanlingensis, although a smaller, more posteriorly positioned sulcus is present on the surangular of the European alligatoroid Diplocynodon (Figs. 5 and 8 in Martin et al., 2014). The surangular extends posterodorsally, covering the anterior part of the lateral surface of the retroarticular process. However, it is uncertain whether the surangular reaches the distal end of the retroarticular process, which is damaged.

In IVPP V 2721.1 (Figs. 6L and 6M) the sulcus next to the glenoid fossa is better developed than in the considerably larger IVPP V 2716-3, which indicates that the sulcus becomes less distinct during ontogeny. The surangular-articular suture runs anteroposteriorly near the lateral edge of the glenoid fossa. Participation of the surangular in the glenoid fossa is feature shared with crocodyloids.

Angular—The left angular is preserved in IVPP V 2716-3 and IVPP V 2721.1 (Figs. 6A, 6B and 6M). The angular contacts the surangular anterodorsally and the articular medially, extending up the retroarticular process.

Articular—The left articular is preserved in IVPP V 2716-3 and IVPP V 2721.1. In IVPP V 2716-3 (Fig. 6B) the articular is nearly complete, although the posterodorsal tip of the retroarticular process is missing. This bone is covered laterally by the angular and surangular. In medial view, no lingual foramen is evident either on or near the articular-surangular suture. In contrast to the condition in many crocodylians, the articular does not form a lamina that projects forward to form an overlapping contact with the medial face of the surangular. In dorsal view, the foramen aerum (FA) is visible on the medial part of the articular. The presence of an FA on the articular is typical among crocodylians, but in alligatoroids the FA is displaced dorsolaterally, rather than remaining medially positioned as in IVPP V 2716-3 (Brochu, 1999).

In IVPP V 2721.1 (Figs. 6L and 6M), the left articular is nearly intact, although the posterodorsal tip is damaged. The articular is smaller than that of IVPP V 2716-1. The glenoid fossa contains a small longitudinal ridge separating the recesses that accommodate the two hemicondyles of the quadrate. The medial recess is much narrower than the lateral one.

Postcrania

Proatlas—A proatlas is preserved in IVPP V 2716-1.1 (Figs. 4A–4D and 5C), and is complete but displaced from its original position. The proatlas approximates the shape of an isosceles triangle in dorsal view. Anteriorly, it forms an obtuse point, similar to the apex of the proatlas in Alligator sinensis (Cong et al., 1998) but quite different from the prominent anterior process characterizing Diplocynodontinae (Brochu, 1999). The sagittal crest is damaged, but appears to extend anteroposteriorly. A small ventral tuberosity (Fig. 5C), representing the point of attachment of the atlantoccipital ligament (Cong et al., 1998), is situated halfway along the exposed left anterolateral surface of the proatlas. The posterior margin of the proatlas is linear instead of smoothly concave as in Crocodylus acutus (Cuvier, 1807).

Atlas—The left half of the neural arch of the atlas is preserved, surrounded by matrix, in IVPP V 2716-1.1 (Figs. 4A–4D). An embayment is present in the anterior margin of the neural arch, and the proatlas likely articulated with a tapering anterior prominence that is situated dorsal to the embayment. The atlas intercentrum is preserved separately, positioned to the right of the occipital condyle and close to the axis. The intercentrum is similar to the proatlas in size, and has a transversely broad ventral surface that would be tilted to face somewhat anteriorly if the skull and cervical vertebrae were in natural articulation (anterior surface of Brochu, 1999, Fig. 39). In contrast to the condition in extant Alligator (Brochu, 1999), the posterior margin of the ventral surface does not form a recess between the two posterolaterally facing facets for the atlantal ribs, and the intercentrum as a whole is not anteroposteriorly compressed.

Axis—The axis is almost completely preserved in IVPP V 2716-1.1 with the odontoid process (atlantal centrum) in place (Figs. 4A–4D), although only the base of the neural spine is present. The odontoid process is not fused to the axis centrum, and remains in contact with the atlantal intercentrum although this joint has undergone slight disarticulation. On the anterior part of the lateral surface of the centrum is a small longitudinal crest, dorsal and ventral to which are two shallow recesses. The hypapophysis on the ventral surface of the centrum is long and well-developed, occupying the anterior half of the centrum’s length. What appears to be the first two right cervical ribs are preserved in contact with the right side of the axis, although not in their natural positions (Fig. 5C). Further anteriorly, a displaced rib from one of the postaxial cervicals is preserved adjacent to the atlantal intercentrum and the left quadrate.

Caudal Vertebra—IVPP V 2721.4 is a disarticulated, procoelous caudal vertebra (Figs. 7A–7C). The prezygapophysis extends further laterally than the postzygapophysis. Remnants of the transverse processes indicate that these structures were originally wide and well-developed, extending for half the length of the centrum. A ventral groove (VG) is present on the ventral surface of the centrum near the posterior condyle. The presence of the VG suggests that this vertebra is from the anterior part of the tail. There is no visible neurocentral suture, indicating that this specimen probably represents a mature individual. However, it could still be a sub-adult, as the sutures close first in the caudal vertebrae during ontogeny (Brochu, 1997).

Femur—IVPP V 2716-5 is a left femoral head (Figs. 7D and 7E), resembling in gross morphology the equivalent structure in extant crocodylians. Judging from its size, the femur belongs to an immature individual.

Osteoderms—IVPP V 2716-7 consists of a cluster of six disarticulated osteoderms with dorsal keels (Figs. 7F and 7G). Two of the osteoderms are distinctly narrower in the direction perpendicular to the keel, whereas three are approximately square. The sixth osteoderm is incomplete and concealed by the others. The narrow osteoderms have keels that extend for almost their entire lengths, whereas the keel of the only well-preserved and well-exposed square osteoderm is separated from the embayed anterior margin by a small subdued area. The pits along the anterior margin of this osteoderm are small and transversely narrow in the vicinity of the keel, whereas the other visible pits are larger and more round. The sculpturing of the narrow osteoderms appears to consist entirely of round pits.

Description: material originally assigned to Asiatosuchus nanlingensis

The most informative component of the holotype of Asiatosuchus nanlingensis (IVPP V 2773) is a pair of very large incomplete mandibles (IVPP V 2773.1), each representing the articular region and a part of the ramus (Figs. 8A–8E). A small section of dentary containing four alveoli, with the corresponding piece of the splenial still in place medially (IVPP V 2773.5; Figs. 8F and 8G), may represent an anterior fragment of the right mandible of IVPP V 2773.1 as suggested by Young (1964). The size and proportions of the fragment seem compatible with those of the large mandibles, under the assumption that the fragment originally occupied an anterior position within the right mandible, and the alveoli in the isolated fragment are similar in diameter to those preserved in the left mandible. The dorsal margin of the fragment slopes gradually anteroventrally, a feature consistent with an anterior placement within the lower jaw. The ventral part of the external surface of the fragment is heavily sculptured, but the fragment provides little morphological information.

The left mandible of IVPP V 2773.1 preserves three incomplete teeth, lacking the crowns. There are two empty alveoli immediately posterior to the teeth, and a third immediately anterior to them. A single displaced tooth, bearing smooth carinae, is preserved in contact with the medial surface of the right mandible. Young (1964) considered the small size of the external mandibular fenestra (EMF) in both mandibles to be a diagnostic character, but this condition appears exaggerated by dorsoventral compression. The curvature of the dorsal and ventral margins of the EMF suggests, in both jaws, that this structure was originally larger. In particular, the ventral margin is distinctly concave, implying that the EMF was both larger and rounder than the slit-shaped opening seen in Planocrania hengdongensis (Li, 1984). However, it is difficult to be sure whether the intact EMF would have been very large, as in Alligator, or only moderately large as in Crocodylus niloticus. Based on this interpretation, the EMF is similar to that of Asiatosuchus grangeri (Mook, 1940). Most sutures on both mandibles are fused, presumably as a result of aging. There is no foramen aerum (FA) preserved on either articular. It is conceivable that the FA closed as the individual continued to develop ontogenetically, but there is no evidence to support closure of the FA with advancing age in any crocodylian. On the dorsolateral part of the surangular, IVPP V 2773.1 has a similar sulcus seen in IVPP V 2716-3 (Figs. 8A and 8B), although this sulcus is smaller in size. Inferring from the relative sizes between IVPP V 2773.1 and IVPP V 2716-3, the smaller sulcus in IVPP V 2773.1 may again reflect an ontogenetic variation, where the surangular sulcus degenerates as the crocodylian ages. Such sulcus is highly diagnostic at least within Crocodyloidea, which reflects the existence of taxonomic problems between A. nanlingensis and E. chunyii.

As originally designated by Young (1964), the holotype also includes four cervical and dorsal vertebrae (IVPP V 2773.2), an incomplete left coracoid (IVPP V 2773.3), and the distal end of a right femur (IVPP V 2773.4). Two of the vertebrae are much smaller than the others, and clearly do not belong to the same individual represented by either of the large vertebrae or by the large mandibles. All four vertebrae are procoelous, as expected in any eusuchian.

Two mandibular fragments (IVPP V 2772.1 and IVPP V 2772.2) from a site (L5) several km from the locality that produced the holotype (L4) were both referred to A. nanlingensis by Young (1964), but differ from one another in some important morphological features. The articular region is absent in both cases, limiting the potential for comparisons with the holotype mandibles. In IVPP V 2772.1 (Figs. 8H and 8I), the mandibular symphysis extends to what is almost certainly the fourth dentary tooth, based on its large diameter. This corresponds to the condition in specimens assigned to Eoalligator chunyii (IVPP V 2716-1.2), but distinguishes IVPP V 2772.1 from Asiatosuchus grangeri from the Eocene of Inner Mongolia, in which the symphysis extends back to the sixth dentary tooth (Mook, 1940). In IVPP V 2772.2, however, the symphysis extends to an alveolus that may be interpreted as the seventh, if the first large alveolus is identified as the fourth. Also, IVPP V 2772.2 (Figs. 8J and 8K) has a dorsoventrally compressed symphysis, unlike IVPP V 2772.1 and specimens assigned to Eoalligator chunyii but as in longirostral taxa such as Gavialis gangeticus (Gmelin, 1789). In IVPP V 2772.2, the fourth dentary alveolus is smaller than all three posterior alveoli seen in IVPP V 2773, indicating that IVPP V 2772.2 must represent a smaller individual. Based on the length and dorsoventral compression of the mandibular symphysis, IVPP V 2772.2 likely represents a different species from the holotype, in which the middle part of the left mandibular ramus is sufficiently deep to indicate that the skull is not particularly longirostral. IVPP V 2772.1 is similar to IVPP V 2716-1.2 in that the dentary symphysis extends back to the fourth dentary alveolus, and in that the tenth to twelfth dentary alveoli are enlarged. These morphological similarities and the provenance of IVPP V 2772.1 suggest that referral of this specimen to A. nanlingensis is likely valid, although some uncertainty remains given that IVPP V 2772.1 shares no highly distinctive synapomorphies with either the holotype of A. nanlingensis or specimens referred to E. chunyii. IVPP V 2772.2, by contrast, seems to represent an unidentified longirostral crocodylian different from A. nanlingensis.

IVPP V 2772 also includes 11 isolated teeth (V 2772.3), a number of coprolites and possible coprolites (V 2772.9), and several additional non-diagnostic cranial and postcranial fragments (Table 1). All of the material, except possibly some of the coprolites, does appear to be of crocodyliform origin, but the isolated teeth have serrated carinae and therefore represent a taxon distinct from A. nanlingensis. Their conical morphology implies that they are unlikely to belong to either species of Planocrania (Li, 1976; Li, 1984), as Planocrania has laterally compressed teeth. They are also unlikely to be conspecific with the longirostral form represented by IVPP V 2772.2, because no Paleocene longirostral crocodyliform is known to have a ziphodont dentition. The teeth are not necessarily from a crocodylian, but it is notable that five vertebrae included in the holotype collection (IVPP V 2772.4) are procoelous, implying that they are at least of eusuchian origin. These vertebrae, along with the other indeterminate cranial and postcranial fragments in the sample, could belong to any of the three species (the longirostral form, the ziphodont form and tentatively identified A. nanlingensis) present at L5.

IVPP V 2721a (Figs. 8L and 8M) is a small dentary fragment from a locality that has also produced specimens assigned to Eoalligator chunyii (L3). This specimen was listed by Young (1964) as belonging to A. nanlingensis. The fragment clearly belongs to a large individual, and bears some sculpturing on the lateral surface, but no diagnostic morphological features are preserved. Its provenance suggests it likely belongs to A. nanlingensis, assuming E. chunyii is indeed a junior synonym of this taxon.

IVPP V 2775, the only specimen in this study recovered from L6, is the posterior end of a large right crocodylian mandible referred to A. nanlingensis by Young (1964). This poorly preserved fragment shows no diagnostic features, and could conceivably belong to either A. nanlingensis or the unidentified longirostral taxon represented by IVPP V 2772.2.

Description

Protoalligator huiningensis was originally named Eoalligator huiningensis, the second nominal species of Eoalligator (Young, 1982). The holotype and only known specimen comprises a snout and the anterior parts of both mandibles (Figs. 9 and 10). Despite the fact that P. huiningensis and some specimens that we refer to Asiatosuchus nanlingensis were originally assigned to the same genus, few taxonomically significant features can be compared between the two species. There are two evident differences: (1) the apex of the fourth dentary tooth in P. huiningensis is occluded by a pit between the premaxilla and maxilla, while no such pit exists in A. nanlingensis; and (2) the mandibular symphysis of P. huiningensis extends to the fifth dentary tooth, while that of A. nanlingensis extends only to the fourth. Conversely, only limited and taxonomically unimportant morphological similarities exist between these two species. The following description of P. huiningensis is intended to supplement that of Young (1982).

Skull

Premaxilla—The premaxillae (Figs. 9 and 10) form the anterolateral margins of the naris, which appears oval in dorsal view. Each premaxilla contacts its counterpart medially and the maxilla posteriorly. The premaxilla bears four slender teeth (Fig. 10E), which increase in length posteriorly. The fourth is the largest in diameter, as in extant alligators and Crocodylus niloticus, whereas in Asiatosuchus nanlingensis the third premaxillary tooth is largest. Near the contact with the maxilla, the lateral margin of the premaxilla is recessed to form the anterior part of the pit for the fourth dentary tooth, which is exaggerated by compression. At the anterior margin of the external naris (EN), the two premaxillae combine to form a posterior process that protrudes a short distance into the EN but is clearly incomplete. In basal Alligatorinae, there is no such process, and the nasal bar is formed entirely by the nasals and fails to fully bisect the EN. In Alligator, which is characterized by a complete nasal bar that bisects the EN (Wermuth, 1953; Malone, 1980; Norell, 1988; Brochu, 1999), the anterior part of the bar is always formed by a process of the premaxillae. The presence of a posterior process of the premaxillae may therefore indicate that the nasal bar completely bisects the EN in the intact skull of P. huiningensis, although confirmation of this possibility will require better-preserved material.

Maxilla—The maxillae (Figs. 9 and 10) are complete, and form the main part of the snout. Each maxilla contacts the premaxilla anteriorly, the nasal medially, the lacrimal posteromedially and the jugal posteriorly. The anterior part of the lateral margin is embayed by a recess that is continuous with the similar feature on the premaxilla, forming the posterior part of the notch for the fourth dentary tooth. Posterodorsal to this notch is the more anterior of the two depressions that Young (1982) described on the lateral surface of the maxilla and regarded as a distinguishing feature of P. huiningensis. This depression is shallow, irregular and transversely elongate, extending from just above the alveolar margin to a point on the suture with the nasal. The depression is bounded posteriorly by a low elevation aligned with the large fifth maxillary tooth. The opposite side of the skull is unfortunately damaged in this region, and the snout is distorted by leftward skew. There is at least a slight anterior depression corresponding in position to the one on the right side, but it may not be equally large and well-developed. Even if the anterior depression is a genuine morphological feature rather than an artifact of distortion, it represents little more than an irregularity in the surface of the snout, and its taxonomic value is questionable.

A partial lacrimal is preserved on the right side of the skull. There is no discernable maxillary process either clasped by the lacrimal or projecting between the lacrimal and the nasal. The medial part of the maxilla-lacrimal suture is transverse, but the lateral part of the suture extends posterolaterally and is situated just within a second depression on the maxilla. This posterior depression has the shape of an irregular oval, and is separated from the alveolar margin by a prominent area of bone. The left side of the skull is again damaged in the corresponding region, in that the surface of the maxilla is worn away and the body of the maxilla is interrupted by a slit-like hole that is clearly artifactual. Young (1982) considered the posterior depression to represent a degenerate antorbital fenestra, but this interpretation seems untenable. As with the anterior depression, it is difficult to rule out the possibility that the posterior depression is simply a product of damage and/or deformation, particularly given that the sculpturing inside the depression appears eroded. Furthermore, the region of the crocodylian snout believed to correspond to the formerly open antorbital fenestra is situated mainly on the maxilla, with a small degree of lacrimal participation, but does not extend onto the jugal (Witmer, 1995). However, the right jugal of P. huiningensis does contribute to the posterior depression on the snout. Young compared P. huiningensis with a notosuchian, Uruguaysuchus aznarezi, but the antorbital fenestra in U. aznarezi has no jugal component (Soto, Pol & Perea, 2011). Moreover, members of Crocodylia consistently lack an antorbital fenestra, so the presence of even a degenerate version of this feature in the specimen would be extremely surprising. For these reasons, the posterior depression is clearly not a homologue of the antorbital fenestra. Like the anterior depression, it may represent either an artifact or a genuine topological feature, but seems unpersuasive as a taxonomic character.

The left maxilla bears nine teeth, while the right bears twelve, rather than the eleven reported by Young (1982). Combining information from the two sides of the skull shows that the first two maxillary teeth share the small and slender morphology of the first three premaxillary teeth, while the third is slightly larger. The fourth tooth is even larger than the third, and the fifth and sixth teeth are the largest in the maxilla. The seventh to ninth maxillary teeth are shorter and blunter than the fourth. The tenth to twelfth teeth are slightly larger than the fourth, but smaller than the fifth and sixth.

Lacrimal—Only an anterior remnant of the right lacrimal is preserved (Figs. 9A, 9B, 10A and 10B). A small portion of the left lacrimal may be present as well, but no suture is visible to separate this bone from the maxilla. The right lacrimal contacts the maxilla anteriorly, the nasal medially and the jugal ventrally. The fact that there is a preserved remnant of the lacrimal, but no preserved remnant of the prefrontal, indicates that the former bone extended further anteriorly in the intact snout.

Nasal—Both nasals are preserved (Figs. 9A, 9B, 10A and 10B), but are damaged in the region of the external naris (EN). Each nasal contacts its counterpart medially, the premaxilla anteriorly and the maxilla laterally, and presumably reached the frontal posteriorly. The nasals clearly extended into the EN, forming a long nasal bar, but most of this structure has been obliterated by a transverse gouge across the dorsal surface of the snout. The tips of the two nasal processes remain in place anterior to the gouge, isolated within the narial opening. The nasal-maxilla suture is linear in dorsal view. A very thin, meandering transverse line near the posterior margin of the preserved part of the snout may represent the nasofrontal suture. If this is the case, the contact between the nasal and frontal is broad, and a small remnant of the frontal remains in place.

Jugal—As preserved, the right jugal (Figs. 9A, 9B, 10A and 10B) extends posteriorly somewhat beyond the level of the postorbital bar. A small anterior part of the left jugal may be preserved in the vicinity of the orbit, but this is uncertain. The right jugal contacts the maxilla anteriorly, and the lacrimal dorsally. The base of the slightly inset postorbital bar is not separated by a depression from the lateral margin of the jugal, and bears a small longitudinal crest on its anterior surface.

Lower jaw

Dentary—The left dentary (Figs. 9 and 10) is broken at the level of the tenth maxillary tooth, with only the portion anterior to the break remaining in place. The right dentary is also broken away posteriorly, but remains intact to the level of the anterior part of the external mandibular fenestra (EMF). The dentary contacts the splenial medially and the surangular posterodorsally. The tooth-bearing portion of the dorsal margin of the dentary is similar to its equivalent in Asiatosuchus nanlingensis in undulating gently along its length, whereas the posterior part of the dorsal margin slopes posterodorsally. The ventral margin of the dentary is slightly convex. The dentary symphysis extends posteriorly as far as the level of the fifth dentary tooth. The dentary-surangular suture intersects the anterodorsal margin of the EMF.

The preserved part of the left dentary bears eleven teeth, rather than the seventeen described by Young (1982), while the right bears fourteen teeth. The posteriormost dentary tooth is displaced and rotated, resting in the space between the dentary and maxilla. However, it appears to belong to the dentary because of its lack of the consistently bulbous morphology seen in the posterior maxillary teeth. The anterior two dentary teeth point dorsally, rather than anterodorsally as in some other crocodylians (e.g., Osteolaemus tetraspis (Cope, 1861)). The fourth dentary tooth is the largest, and the third is the second-largest. The eleventh dentary tooth is also enlarged, having twice the diameter of the tenth.

Splenial—The right splenial is preserved, although its surface is mildly damaged (Figs. 9C, 9D, 10C and 10D). The shape of the anterior part of the splenial suggests this bone was likely excluded from the mandibular symphysis, although the anteriormost part of the thin splenial is slightly damaged. The splenial forms the anterior margin of a low, elongate opening in the medial surface of the mandible that may represent the foramen intermandibularis medialis (FIM). The splenial appears to contact anterior processes of the angular both ventral and dorsal to this opening, but the sutural contacts are difficult to follow with certainty, and the angular cannot be traced onto the lateral surface of the mandible. Another putative suture line dorsal to the opening may represent the contact between the splenial and a small exposed strip of the coronoid. There is no evident foramen intermandibularis oralis near the symphysis or on the medial surface of splenial body. A small anteroventral portion of the left spenial is exposed, but the anteriormost part of the bone is missing or damaged.

Surangular—Only the anterior part of the right surangular is preserved, contacting the dentary anteriorly. As in other alligatoroids, the surangular bears a forked process (Figs. 10A and 10B ) whose ventral branch is long, closely approaching the posteriormost preserved dentary teeth. The preserved portion of the surangular represents the tapering ventral process, which extends anteriorly near the dorsal margin of the lateral surface of the mandible. The posterior part of the surangular is damaged, and whether a sulcus is present on the dorsal portion of this bone as in A. nanlingensis cannot be determined.

Angular—Only the anterior part of the right angular is preserved (Figs. 9C, 9D, 10A and 10B), but this bone evidently contacts the dentary anteriorly and the surangular dorsally. The angular comprises the anteroventral part of the EMF, and possibly the ventral part of the FIM.