Monjurosuchus sp.

Matsumoto, Ryoko, Evans, Susan E. & Manabe, Makoto, 2007, The choristoderan reptile Monjurosuchus from the Early Cretaceous of Japan, Acta Palaeontologica Polonica 52 (2), pp. 329-350 : 331-340

Monjurosuchus sp.

Figs. 2–15 .

Material.—SBEI 1223 (dorsal vertebra), SBEI 1496 (left squamosal), SBEI 1792 (block bearing an association of bones), and SBEI 2230 (left squamosal); SBEG 045 (association of right squamosal, jugal, quadratojugal, and quadrate). Description.—SBEI 1792 is a block of dark greenish−grey mudstone preserving 25 choristoderan bones including: cervical, dorsal, and caudal vertebrae; right and left prefrontals; right and left dentaries (associated angular); an articulated left maxilla and the anterior part of a jugal; a left surangular; a left splenial; an articulated left squamosal, jugal and quadratojugal; a right quadratojugal; an associated right surangular, prearticular, articular, and angular; an articulated right parietal, the lateral part of the right squamosal, and the posterior part of the right frontal; and an articulated right postorbitofrontal, the anterior part of the right squamosal, the lateral part of the right frontal, and the posterior part of the right jugal. The smaller, more fragile cranial elements and most of the postcranium are not preserved. The bones on SBEI 1792 form an association within a small area of 110 mm × 140 mm × 20 mm ( Fig. 2 ). Most of the bones are at least partly articulated, but four skull elements, the left splenial, surangular, squamosal, and right dentary are isolated, as are the vertebrae. With one exception (see below) there are never more than two examples of each skull element on the block (allowing for the fact that some bones are broken and their parts attached to neighbouring bones), these are always left and right, and they are always of matching size. It is therefore highly probable that the bones on SBEI 1792 constitute the remains of a single individual. Many other associations of this type are known from the same deposit. However, an additional left squamosal, SBEI 2230, is also found on the block ( Fig. 2A ). It is morphologically identical to the other squamosals, but smaller, and clearly came from a second individual. The presence of this second individual renders SBEI 1792 problematic as a potential holotype, and although the Japanese Monjurosuchus is probably specifically distinct from the Chinese Monjurosuchus splendens , it is not formally named herein.

Skull ( Figs. 3–12 ).—In Choristodera , the upper surface of the skull is composed of eleven or twelve pairs of bones: nasals, prefrontals, premaxillae, maxillae, lachrymals, frontals, parietals, postorbitals, postfrontals (or combined postorbitofrontal), jugals, squamosals, quadratojugals, and quadrates. Only four elements, the lachrymal, premaxilla, nasal, and quadrate, are not identified in SBEI 1792, but parts of the quadrate are preserved on SBEG 045.

The skull is reconstructed on the basis of SBEI 1792 and isolated specimens (SBEI 1496, SBEG 045; Fig. 3 ). A skull of M. splendens (DR0003C) is shown for comparison ( Fig. 4 ). In SBEI 1792, the partly articulated postorbital elements are preserved, but some preorbital elements are not. As a result, the postorbital skull length can be reconstructed with some confidence, but the preorbital region is more difficult. However, SBEI 1792 preserves a nearly complete right dentary and a left dentary with an associated angular. In addition, a nearly complete right surangular is associated with the angular and a small part of the dentary. From these elements, the length of the mandible can be reconstructed as approximately 70 mm. As reconstructed, the skull preserved on SBEI 1792 is 80 mm in length and about 50 mm wide. The difference between jaw and skull length can be explained by the fact that the quadrate−articular joint lies forward of the posterior limit of the skull. The dermal bones are ornamented with a pattern of anteroposterior striae. As seen in dorsal view ( Fig. 3A ), the orbits are large and dorsally directed and the upper temporal fenestrae are anterolaterally expanded. In SBEI 1792, the squamosal lacks the process for the parietal so the length of parietal process was inferred from an isolated complete squamosal (SBEI 1496). The squamosal process on the parietal is complete, and suggests that the suture between the squamosal and parietal lay half way along the posteromedial border of the upper temporal fenestra. As reconstructed, the length of the upper temporal fenestra is similar to that of the orbit.

The squamosal, quadratojugal, jugal, and part of the postorbitofrontal lie vertically and form the lateral side of the skull ( Fig. 3B ). The quadrate is located behind the squamosal/postorbitofrontal suture so that the lower jaw is posteriorly elongated in comparison to that of Monjurosuchus splendens .

An incomplete left maxilla is preserved in SBEI 1792, missing the regions for articulation with the premaxilla and prefrontal but with part of the jugal in association ( Fig. 5 ). The maxilla is mediolaterally narrow and dorsoventrally flattened, with its lateral surface ornamented with a pattern of fine anteroposterior striae and sensory foramina ( Fig. 5A ). In dorsal view ( Fig. 5B ), an anteroposterior flange is seen to bear a medially inflected notch−like facet for the lachrymal. Posteriorly, the flange articulates with the orbital ramus of the jugal. At least seven tooth sockets are visible on the maxilla, but all teeth have been lost ( Fig. 5C ). The implantation is subthecodont and tooth sockets are ovoid.

The elongated right and left prefrontals are both preserved in SBEI 1792 ( Fig. 6 ). The right bone is nearly complete, but the ventral side is covered by matrix ( Fig. 6A ). The left bone is of equal size but has a crack in the middle, and is partly lacking the frontal process ( Fig. 6B ). The dorsal surface is horizontal and sculptured, with a well−developed orbital flange bearing weak tubercles ( Fig. 6 ). The straight medial edge bears a deep suture for the opposite prefrontal. The prefrontal tapers posteriorly, and its narrow frontal process enters the rim of the large, dorsolaterally directed orbits. Anterodorsally, there is a small recess for the nasals ( Fig. 6A ). This is limited to the anterior tip of the bone, and covers only one fifth of the entire prefrontal length. Laterally, there are anterior and posterior facets for the maxilla and lachrymal respectively ( Fig. 6A ). The maxillary facet is extremely reduced, and is limited by the anterior part of an elongated lachrymal facet that lies parallel to the interprefrontal suture. Below the lachrymal facet lies the palatine process ( Fig. 6B ). The anterior nasal cavity terminates in the middle of the palatine process, separated from the posterior nasal cavity by the medial edge of the palatine process ( Fig. 6B ). Anteroposteriorly, the medial edge forms the margin of a channel leading forward from the frontal into the posterior nasal cavity ( Fig. 6B ).

SBEI 1792 preserves the right parietal in association with parts of the frontal, and squamosal ( Fig. 7 ). It lacks the tip of the frontal facet and the neomorph process, but is otherwise nearly complete. The bone is anteroposteriorly elongate. As in all known choristoderes, the parietals are paired and lack a parietal foramen ( Fig. 7A ). A deep concavity runs parallel to the midline ( Fig. 7A ). Anterior and lateral processes met the frontal and postorbitofrontal respectively ( Fig. 7A, B ). Posterolaterally, the parietal is drawn into an expanded squamosal wing that has a strong overlapping facet with the squamosal ( Fig. 7A, C ).

As in all choristoderes except Champsosaurus and Cteniogenys , the postorbital and postfrontal are fused without trace of a suture ( Fig. 8 ). The resulting postorbitofrontal is divided into two parts. The main body of the bone contacts the squamosal and jugal and forms the posterior margin of the orbit and the anteroventral margin of the supratemporal fenestra ( Fig. 8A ). A dorsally expanded wing meets the frontal and parietal and contributes to the posterodorsal margin of the orbit and the anterior margin of the supratemporal fenestra ( Figs. 3 , 8A ). The external surface of the postorbitofrontal is sculptured and the dorsolateral edges are tuberculate ( Fig. 8A ). The jugal facet is anteroventrally expanded, and is longer than the squamosal facet in lateral view ( Fig. 8A ). As seen in medial view, the anterior orbital rim is strengthened by buttresses but behind this the bone becomes very thin ( Fig. 8B ).

The left and right jugals are preserved in SBEI 1792 ( Figs. 8 , 9), but are fragmented. The anterior part of the left jugal is associated with the maxilla and the posterior part is associated with the quadratojugal and squamosal, but the middle section with facets for the maxilla and postorbitofrontal is missing ( Figs. 5A , 9A). The right jugal is better preserved and is associated with the postorbitofrontal and squamosal, but its orbital flange and quadratojugal facet are missing ( Fig. 8 ). Overall, the jugal is anteroposteriorly elongated quadratojugal jugal with a strongly sculptured lateral surface ( Figs. 8A , 9A–C). The anterior process is dorsoventrally flattened with a triangular cross−section ( Fig. 5A, B ). It gives rise to a dorsally expanded flange that forms the curved orbital margin ( Fig. 8 ). In the postorbital region, the jugal is mediolaterally flattened and dorsally expanded, forming the vertical lateral skull margin. Its dorsal and ventral margins run almost in parallel, but the ventral edge is much thicker. The jugal bears a number of distinct facets. The ventromedial maxillary facet lies along the ventral margin, terminating just behind the level of the orbital rim ( Fig. 8B ). The medial ectopterygoid facet lies above the maxillary facet as a weak notch ( Fig. 8B ). Dorsolaterally, the jugal overlies the postorbitofrontal, while posteriorly it meets the squamosal and quadratojugal to close the lower temporal fenestra, the squamosal facet being roughly equal in length to the quadratojugal facet ( Fig. 9A, D). The quadrate facet is weakly striated and is located just above the junction of the jugal and quadratojugal ( Fig. 9C).

squamosal?rib jugal quadratojugal squamosal quadratojugal jugal quadrate facet squamosal facet?rib

The quadratojugals are preserved on SBEI 1792 ( Fig. 9) and SBEG 045 ( Fig. 10B ). On SBEI 1792, the left bone is complete and has an expanded medial surface ( Fig. 9A–C); the right bone is detached, and its medial side is covered with matrix ( Fig. 9D). The quadratojugal is reduced to a small triangular plate, ornamented with the characteristic sculpture pattern and a tuberculate ventral margin ( Fig. 9A, B). It articulates with the jugal, the quadrate, and the squamosal but does not contact the postorbitofrontal. In lateral view, the quadratojugal−jugal suture is a straight line, subequal in length to the quadratojugal−squamosal suture ( Fig. 9A). A medially expanded flange gives the bone an “L” shape in ventral view ( Fig. 9C). This flange meets the squamosal dorsally and the quadrate posteriorly, forming a deep quadrate recess ( Fig. 9C).

Both squamosals are preserved on SBEI 1792 ( Figs. 8 , 9A–C), although the right bone is split, with its anteromedial process attached to the parietal ( Fig. 7 ) and its lateral wing associated with the jugal and postorbitofrontal ( Fig. 8 ). The left squamosal is articulated with a jugal and a quadratojugal ( Fig. 9A, B). It lacks the dorsal margin and the parietal process. SBEI 2230, the smaller left bone on the same block, is isolated, and it is also missing the parietal process, but SBEI 1496 ( Fig. 10A ) and SBEG 045 ( Fig. 10B ) are almost complete. The squamosal is made up of lateral and medial plates. The dorsoventrally expanded lateral plate forms the side of the skull and the lateral margin of the supratemporal fenestra ( Figs. 9A, 10A 1 , A 2 , B 1 , B 2 ). From anterodorsal to posteroventral, the plate has sutures with the postorbitofrontal, jugal, and quadratojugal ( Fig. 10A 2 , B 1 , B 2 ). The medial plate forms the posterior rim of the supratemporal fenestra and extends to meet the parietal ( Fig. 10A 5 , B 3 , B 4 ). This parietal process is well−preserved in SBEI 1496, and is seen to be rather square in dorsal view, with an uneven surface ( Fig. 10A 3 ). Anteriorly, as shown in SBEG 045 ( Fig. 10B 3 –B 6 ), this parietal process bears a small concavity for the quadrate and the quadratojugal. The medial and lateral squamosal plates contact each other vertically ( Fig. 10B 5 ), although the medial plate is inclined posterodorsally at 45 degrees ( Fig. 9B, 10B 4 ). The squamosal has a strongly sculptured lateral surface, with spike−like tubercles ornamenting the dorsal and posterior rims ( Figs. 9A, 10 ).

The quadrate is not preserved in SBEI 1792, but supplementary information is available from SBEG 045, an association of the right quadrate, squamosal, quadratojugal, and posterior part of the jugal ( Fig. 10B 3 –B 6 ). SBEG 045 matches SBEI 1792 in the shape and the size of the component elements. The quadrate is triangular, with a narrow dorsal apex capped by a small cephalic condyle ( Fig. 10B 3 –B 6 ). The condyle is located near the lateral surface of the skull and has a smooth surface, but the surrounding facets indicate that no movement could have occurred. The bone is attached to the quadratojugal posterolaterally, the squamosal dorsally, and the jugal laterally ( Fig. 10B 3 –B 6 ). Its medial face is broad and bears a series of interlinked facets for the opisthotic, pterygoid, and neomorph, each separated by grooves ( Fig. 10B 3 , B 4 ). The large, posteroventral pterygoid facet extends about half way up the medial edge, with the deep, anteroventral neomorph facet running beside it ( Fig. 10B 3 , B 4 ). Above these two facets there is a shallower surface for the opisthotic.

postorbitofrontal facet

Lower jaw.—In choristoderes the lower jaw is made up of seven bones: dentary, splenial, coronoid, angular, surangular, prearticular, and articular. Of these, six (the dentary, splenial, angular, articular, prearticular, and surangular) have been identified in SBEI 1792.

SBEI 1792 preserves a nearly complete right dentary, and a partial left dentary articulated with an angular ( Fig. 11 ). The dentary is a slender element with the anterior end slightly upturned. The depth gradually increases posteriorly ( Fig. 11A ). The external surface is finely striated, with a line of sensory nerve foramina, each of which opens into a short groove ( Fig. 11A 1 ). In medial view, the symphysial surface is short by comparison with that of neochoristoderes and Lazarussuchus ( Fig. 11A 2 ). The Meckelian fossa is enclosed at the symphysis. Its dorsal and ventral margins run in parallel, but the subdental shelf is thicker than the lower edge ( Fig. 11A 2 ). This resembles the condition in the Jurassic Cteniogenys . Facets for the splenial are clearest on the lower border of Meckelian fossa and show that the splenial was excluded from the symphysis, leaving the anterior part of the fossa open ( Fig. 11A 2 ). The entry foramen of the inferior alveolar canal is not preserved. Towards the rear of the tooth row, the subdental ridge and the lower border of the Meckelian fossa narrow abruptly ( Fig. 11A 2 ). Here the splenial facets end and are replaced by the angular facet in the floor of the Meckelian fossa ( Fig. 11A 2 ). On the left dentary, part of the angular is associated ( Fig. 11B ). The tooth implantation is subthecodont as in all Choristodera . Most of the teeth have been lost, but four are preserved within the alveoli ( Fig. 11A 1 ). They are conical with a smooth base, as seen in Cteniogenys , Hyphalosaurus , and Lazarussuchus .

The left splenial is preserved, but is lacking its anterior and posterior parts ( Fig. 11C ). The bone is flat and thin with straight dorsal and ventral margins, and a smooth surface.

sensory foramina

Two lingual sensory foramina open anteroventrally ( Fig. 11C 2 ). The dorsal margin bears a slight posteromedial inflection, probably to accommodate the coronoid ( Fig. 11C ).

SBEI 1792 also preserves the left and right surangular. The right bone is nearly complete, and is associated with the dentary, angular, articular, and prearticular ( Fig. 12A ). The left bone is isolated, and is missing its anterior part ( Fig. 12B ). The surangular is a robust element that lies along the posterodorsal edge of the mandible. It is almost rectangular with parallel dorsal and ventral margins, and lateral sculpture ( Fig. 12A 1, B 1 ). The bone bears a deep anterodorsal coronoid facet and a broad, ventrolateral angular facet ( Fig. 12A 1, B 1 ). Posteriorly, the latter facet angles dorsally and is bordered by a raised, tuberculate rim. In medial view, the anterior adductor chamber is separated from the posterior articular region by a smoothly curved crest ( Fig. 12B 2 , B 3 ), the base of which is robust. The dorsal border of the adductor compartment is thick and posteriorly rounded, whereas the ventral border is incurved ( Fig. 12A 2, B 2 , B 3 ). There is a narrow groove facet for the prearticular. Just above it, posteriorly, is the facet for the articular ( Fig. 12B 2 ). The articular fossa is relatively shallow, occupying only the upper half of the surangular ( Fig. 12B 2 , B 3 ). This resembles the condition in Cteniogenys , Hyphalosaurus , and Lazarussuchus . In neochoristoderes, such as Champsosaurus , Ikechosaurus , Simoedosaurus , and Tchoiria , the articular fossa extends over a larger area and fills almost the entire depth of the surangular. There are no nerve foramina on the medial surface.

The right angular articulates with the surangular and prearticular ( Fig. 12A ); the left is more fragmentary, but its midsection is preserved in association with the dentary ( Fig. 11B ). The angular is a long, robust element that contributes to the posteroventral border of the mandible, covering the ventral margin of the surangular. It is therefore exposed both medially and laterally ( Fig. 12A ). Both surfaces are sculptured, and the posteroventral margin is also tuberculate. The angular runs parallel to the prearticular and narrows posteriorly ( Fig. 12A 2 ). Anteriorly it meets the dentary, extending medially into the Meckelian fossa ( Fig. 11B ).

The right prearticular is associated with the articular ( Fig. 12A 2 ). It runs above the angular and is anteroposteriorly elongate and slender, with a weakly sculptured medial surface ( Fig. 12A 2 ). The articular ( Fig. 12A 2 ) is robust and posteriorly expanded, lying above the posterior part of the prearticular, and mostly internal to both it and the surangular ( Fig. 12A 2 ).

Postcranial skeleton ( Figs. 13 , 14 ).—The only postcranial elements that can be attributed with confidence to the Japanese Monjurosuchus are vertebrae.

articular facet

A single cervical vertebra is preserved on SBEI 1792, with both the neural spine and anterior zygapophyses broken ( Fig. 13A ). The neural arch is fused to the centrum, with no trace of the neurocentral suture ( Fig. 13A 1 ). The posterior zygapophysis is a broad, rectangular plate ( Fig. 13A 2 ), and is orientated at 45 ° to the horizontal. Below it on the right side is a small expansion that may be equivalent to the spinous process of the dorsal vertebrae ( Fig. 13A 1 ). The transverse process is positioned above the centrum ( Fig. 13A 1 ). Its tip is vertically expanded and bears a posteriorly concave rib facet ( Fig. 13A 2 ). The parapophysis is shorter and is horizontal in orientation. The centrum has a strong midventral keel ( Fig. 13A 2 ), flanked on either side by lateral concavities.

Although two fragmentary dorsal vertebrae are preserved on SBEI 1792, by far the most complete specimen is SBEI 1223 ( Fig. 14 ). This is slightly larger than the vertebrae on SBEI 1792, but has the same morphology. The anterior and posterior zygapophyses are roughly equal in length ( Fig. 14A ), with the posterior zygapophysis orientated at 45 ° to the horizontal plane ( Fig. 14C ). The neural spine has an apex with a smooth surface. There is a small spinous process below the postzygapophyses ( Fig. 14A, C ), and it bears accessory facets. The transverse process is expanded and single−headed, positioned just above the level of the neurocentral suture, although this is closed without a trace ( Fig. 14A ). The centrum is platycoelous and is excavated by a shallow concavity on each side of the midline.

A partial anterior caudal vertebra is preserved on SBEI 1792 ( Fig. 13B ). It is longer than the cervical and dorsal vertebrae. The centrum is concave in lateral view ( Fig. 13B 1 ) and bears a midventral groove for caudal blood vessels ( Fig. 13B 2 ). Part of the caudal rib is fused to the centrum, just above the level of the neurocentral suture ( Fig. 13B 1 ). This rib is rectangular in cross−section and, as preserved, is long and robust although it is broken in the middle ( Fig. 13B 1 ).

Remarks.—SBEI 1792 presents the following choristoderan synapomorphies: median contact of prefrontals along entire length of elements; elongated postorbitofrontal; coarse external sculpture, striated on maxillae; mandibular foramina open into anteroposteriorly oriented grooves; flared posterior temporal region that extends posteriorly; parietal foramen absent; conical subthecodont teeth; slender dentary; and platycoelous vertebral centrum ( Evans and Klembara 2005). Furthermore, SBEI 1792 is distinguished from all neochoristoderes ( Champsosaurus , Ikechosaurus , Simoedosaurus , and Tchoiria ) and the Jurassic Cteniogenys , in the closure of the lower temporal fenestra and a posterior constriction of the prefrontal that is associated with medially expanded orbits. However, in these two characters SBEI 1792 shows similarities to the Oligocene to Miocene European Lazarussuchus ( Hecht 1992; Evans and Klembara 2005); the Early Cretaceous Chinese Hyphalosaurus ( Gao et al. 1999; RM personal observation); and the Early Cretaceous Chinese Monjurosuchus ( Gao et al. 2000; Evans and Klembara 2005; Gao and Fox 2005), and Philydrosaurus ( Gao and Fox 2005) . SBEI 1792 differs from Lazarussuchus in having the following characters states: amphiplatyan vertebral centrum with a closed notochordal canal; posteriorly constricted prefrontal with a much shorter maxillary facet; a shorter mandibular symphysis, limited to one or two tooth positions; and margin of lower jaw anteroposteriorly straight. SBEI 1792 differs from Hyphalosaurus in the following characters: the cervical neural arch is taller; the dorsal neurocentral sutures are closed; the squamosal is more slender in its posterior part; the dentary is more slender; and the prefrontal has a broader maxillary contact.

SBEI 1792 differs from the new Philydrosaurus in lacking distinct ridges on the prefrontal and postfrontal, in having distinct ornamentation on the squamosal, and in lacking the distinct supratemporal trough ( Gao and Fox 2005).

Apart from attributed jaws, no skull material is yet available for the Early Cretaceous Japanese Shokawa ( Evans and Manabe 1999) . However, SBEI 1792 can be distinguished from Shokawa based on characters of the postcranial skeleton: the cervical neural arch is taller; dorsal neurocentral sutures are closed (open in Shokawa with centra that are nearly equal in size); centra of caudal vertebra bear a shallow ventral groove for caudal blood vessels (deep ventral flanges flank grooves in Shokawa ); posterior zygapophyses expand in parallel with each side, as seen in a dorsal view of the cervical vertebra (laterally expanded in Shokawa ); and cervical diapophysis orientated vertically and above neurocentral suture (horizontally and just on the neurocentral suture in Shokawa ).

The new Japanese choristodere is very similar to Monjurosuchus splendens ( Fig. 4 ). The following comparison is based partly on the description of Monjurosuchus splendens by Gao et al. (2000) and Evans and Klembara (2005), but mainly on a study of new complete material in DR (DR0003C, Fig. 4 ), and IVPP (IVPP V 3673, 13273, 13279, 13866, 14269). The prefrontal of SBEI 1792 matches that of Monjurosuchus splendens , but not other choristoderes, in size and shape. Both share a prefrontal with a single facet for the maxilla that is limited to the anterior region between the nasal and lachrymal facets. The latter is extremely long and extends along the lateral margin of the prefrontal. At the anterior tip of the prefrontal, the interprefrontal suture is invaded by the nasal to about one quarter or one fifth of its length. The nasal facets are wedge−shaped. Posteriorly, the process for the frontal is markedly narrow and constricted. However, the prefrontal of SBEI 1792 differs slightly from that of M. splendens in the structure of the orbital margin, which bears a steep dorsally directed flange. In M. splendens , the orbital margin lacks this flange and is strongly tuberculate instead.

The parietal of SBEI 1792 resembles that of Cteniogenys ( Evans 1990) , Hyphalosaurus (RM personal observation), Lazarussuchus ( Hecht 1992; Evans and Klembara 2005), and M. splendens (RM personal observation), and differs from Philydrosaurus ( Gao and Fox 2005) in having an anterolateral expansion that forms most of the skull roof. The parietal of SBEI 1792 has a broad facet for the squamosal, matching the condition in M. splendens . However, there are several differences between the parietal of SBEI 1792 and that of M. splendens . In SBEI 1792, the parietal is longer anteroposteriorly and more slender, and the postorbitofrontal and frontal facets are borne on a dorsolateral flange that lies above the level of the interparietal suture. In contrast, the parietal of M. splendens is anteroposteriorly short, and the dorsal surface of the parietal is flatter, so that the postorbitofrontal and frontal facets lie on the same level as the interparietal suture.

The quadrates of the Japanese choristodere and Monjurosuchus splendens differ from those of neochoristoderans in that the lateral part of the quadrate mandibular condyle attaches to the skull margin. The quadrate of the Japanese reptile differs from that of Cteniogenys in the position of the neomorph and quadratojugal, having the facets parallel to each other. In Cteniogenys , the neomorph facet lies between the pterygoid and opisthotic, and the quadratojugal facet is limited to the posteroventral edge.

In the new Japanese choristodere, the lower temporal fenestra is closed by the elongated jugal and expanded postorbitofrontal and squamosal. Monjurosuchus splendens also had a closed lower temporal fenestra ( Gao et al. 2000). In both taxa, the compound postorbitofrontal met the frontal anteromedially and the parietal posteromedially. In lateral view, the postorbitofrontal contacts the squamosal anterior to the mid−point of the supratemporal fenestra. The squamosal and postorbitofrontal of both taxa are of similar shape, but the posterior part of the jugal and the quadratojugal of the Japanese choristodere differ from those of M. splendens in shape and length. In M. splendens , the length of the quadratojugal is roughly one half that of the jugal, whereas an elongated jugal meets the squamosal in the Japanese choristodere but not in M. splendens . The jugal meets the quadratojugal anterior to the squamosal/postorbitofrontal junction in M. splendens , but behind it in the Japanese choristodere. The squamosal of the Japanese choristodere matches that of M. splendens in its shape and in the anteroposterior expansion of the parietal process.

The mandible of Monjurosuchus splendens is partially preserved in IVPP V13279, and preserves a terminal symphysis, spanning the length of 1–2 tooth positions, as in SBEI 1792. This is shorter than that of Lazarussuchus (roughly six tooth positions; Hecht 1992; Evans and Klembara 2005). Furthermore, the posterior dentary of M. splendens (IVPP V13279) is dorsoventrally expanded, as in SBEI 1792. The surangular of SBEI 1792 resembles that of Cteniogenys , Hyphalosaurus , and Lazarussuchus in general shape. In these taxa, the dorsal and ventral margins of the surangular are subparallel, and the articular flange and symphysis lie on the same horizontal plane. In Neochoristodera, the surangular is posteroventrally deflected and the articular flange lies below the level of the symphysis. However, in SBEI 1792, the ventral margin of the surangular is robust and curves medially, unlike that of Cteniogenys , Hyphalosaurus , and Lazarussuchus .

The only postcranial elements known for the Japanese choristodere are vertebrae. The dorsal vertebrae have vertically expanded single−headed transverse processes, and platycoelous cervical, dorsal, and caudal centra with fully closed neurocentral sutures and no notochordal pit. This combination of characters matches only Monjurosuchus splendens . However, M. splendens and the Japanese choristodere differ in some characters. In the Japanese form, there is a small spinous process on the rear margin of the neural arch pedicel, just below the postzygapophyses. In M. splendens , this spinous process is absent ( Evans and Klembara 2005; RM personal observation).

In summary, Monjurosuchus splendens and the Japanese choristodere share a unique combination of character states: closure of the lower temporal fenestra and corresponding modification of the jugal, quadratojugal, squamosal, and postorbitofrontal (also closed in Hyphalosaurus , Lazarussuchus , and Philydrosaurus ); a short medial symphysis extending over 1–2 tooth positions; a posteriorly constricted prefrontal that contributes to large medially expanded orbits; steeply sloping, but strongly tuberculate or spike−like projections on the posterior margin of the squamosal (also Lazarussuchus ); an anteriorly elongated lachrymal; a prefrontal contacting the maxilla anteriorly through an extremely short facet; and closure of the vertebral neurocentral sutures in the adult. These similarities permit attribution of the Japanese choristodere to Monjurosuchus . However, the Japanese Monjurosuchus differs from M. splendens in the length and width of the parietal and the relative proportions of the postorbitofrontal and frontal facets; the level of the postorbitofrontal and frontal facets in relation to the midline of the skull; the length and ornamentation of the quadratojugal; the presence or absence of a jugal/squamosal contact, and the position of the jugal/quadratojugal contact in relation to the postorbitofrontal/squamosal suture; the level of ornamentation on the posterior edge of the squamosal; and the presence or absence of a facetted spinous process on the presacral vertebrae. These differences probably distinguish the Japanese Monjurosuchus from M. splendens at the species level but additional material and associated specimens (frontal, premaxilla and postcranial bones) would make the relationship clearer.

A life reconstruction of the Japanese Monjurosuchus is shown in Fig. 15 , using M. splendens ( Gao et al. 2000) to reconstruct the postcranial skeleton and soft tissue.

Figures

Fig. 2. Choristoderan reptile Monjurosuchus sp. (SBEI 1792), Lower Cretaceous Kuwajima Formation, Kuwajima, Ishikawa Prefecture, Japan. Photograph of main block of the holotype. A and B show alternate sides of the block.

Fig. 3. Choristoderan reptile Monjurosuchus sp., Lower Cretaceous Kuwajima Formation, Kuwajima, Ishikawa Prefecture, Japan. Partial reconstruction of the skull in dorsal (A) and lateral (B) views.

Fig. 4. Choristoderan reptile Monjurosuchus splendens (DR0003C), Yixian Formation, Liaoning Province, China. Skull in dorsal view. Circle and dash pattern indicates matrix.

Fig.5. Choristoderan reptile Monjurosuchus sp. (SBEI 1792), Lower Cretaceous Kuwajima Formation, Kuwajima, Ishikawa Prefecture, Japan. Left maxilla and jugal, in lateral (A), dorsal (B), and partial ventral (C) views.

Fig. 6. Choristoderan reptile Monjurosuchus sp. (SBEI 1792), Lower Cretaceous Kuwajima Formation, Kuwajima, Ishikawa Prefecture, Japan. A. Right prefrontal in dorsal view. B. Left prefrontal, in ventral view.

Fig. 7. Choristoderan reptile Monjurosuchus sp. (SBEI 1792), Lower Cretaceous Kuwajima Formation, Kuwajima, Ishikawa Prefecture, Japan. Right frontal, parietal, and squamosal, in dorsal (A), lateral (B), and ventral (C) views.

Fig. 8. Choristoderan reptile Monjurosuchus sp. (SBEI 1792), Lower Cretaceous Kuwajima Formation, Kuwajima, Ishikawa Prefecture, Japan. Right frontal, jugal, postorbitofrontal, and squamosal, in lateral (A) and medial (B) views. A1, B1, photographs; A2, B2, interpretive drawings.

Fig. 10. Choristoderan reptile Monjurosuchus sp. A. Lower Cretaceous Kuwajima Formation, Kuwajima, Ishikawa Prefecture, Japan. Left squamosal (SBEI 1496), in lateral (A1, A2), dorsal (A3), ventral (A4), and medial (A5) views. B. Okurodani Formation, Shokawa, Gifu Prefecture, Japan. Right squamosal, jugal, quadratojugal, and quadrate (SBEG 045), in lateral (B1, B2), medial (B3, B4), posterior (B5), and ventral (B6) views. A1, B1, B3, photographs; A2–A5, B2, B4–B6, interpretive drawings.

Fig. 1. Map of north−central Honshu, Japan, showing the “Kaseki−Kabe” locality (Kuwajima Formation) and the Shokawa locality (Okurodani Formation). The hatched area indicates the distribution of the Tetori Group (after Maeda 1961).

Fig. 11. Choristoderan reptile Monjurosuchus sp. (SBEI 1792), Lower Cretaceous Kuwajima Formation, Kuwajima, Ishikawa Prefecture, Japan. A. Right dentary in lateral (A1) and medial (A2) views. B. Left dentary in medial view. C. Left splenial in lateral view. C1, photograph; C2, interpretive drawing. Scale bars 5 mm.

Fig. 12. Choristoderan reptile Monjurosuchus sp. (SBEI 1792), Lower Cretaceous Kuwajima Formation, Kuwajima, Ishikawa Prefecture, Japan. A. Right surangular, angular, and prearticular, in lateral (A1) and medial (A2) views. B. Left surangular, in lateral (B1), medial (B2), and dorsal (B3) views. Circle and dash pattern in A1 indicates matrix.

Fig. 13. Choristoderan reptile Monjurosuchus sp. (SBEI 1792), Lower Cretaceous Kuwajima Formation, Kuwajima, Ishikawa Prefecture, Japan. A. Cervical vertebra, in left lateral (A1), and ventral (A2) views. B. Partial caudal vertebra, in lateral (B1) and ventral (B2) views. Scale bars 5 mm.

Fig. 14. Choristoderan reptile Monjurosuchus sp. (SBEI 1223), Lower Cretaceous Kuwajima Formation, Kuwajima, Ishikawa Prefecture, Japan. Dorsal vertebra, in left lateral (A), anterior (B), and posterior (C) views. A1, photograph, A2, interpretive drawing.

Fig. 15. Life reconstruction of the Japanese Monjurosuchus.