Cymbospondylus, 2006

Fröbisch, Nadia B., Sander, P. Martin & Rieppel, Olivier, 2006, A new species of Cymbospondylus (Diapsida, Ichthyosauria) from the Middle Triassic of Nevada and a re-evaluation of the skull osteology of the genus, Zoological Journal of the Linnean Society 147 (4), pp. 515-538 : 517-535

CYMBOSPONDYLUS NICHOLLSI SP. NOV.

Diagnosis: Medium-sized Cymbospondylus . Differs from Cymbospondylus petrinus in having a longer cheek region: the ratio between length posterior to nares and length of cheek region is 1.85 in C. nichollsi vs. 2.11 in C. petrinus . The parietal foramen is about 4.5 cm posterior to the fronto-parietal suture within the parietal, but is very close to the fronto-parietal suture in C. petrinus . Autapomorphies of C. nichollsi are: nasals have a long posterior process entering the upper temporal opening; the postorbital reaching far dorsally to the border of the temporal opening, whereas it is restricted to the medial portion of the cheek region in C. petrinus and C. buchseri ; the long, slender supratemporal extends beyond the back of the skull; the quadratojugal is short and restricted to the lateral side of the skull and does not make contact with the quadrate at the back of the skull; as a result, the squamosal takes part in the posterior border of the lateral embayment, in contrast to the condition seen in C. petrinus and most other ichthyosaurs.

The preserved postcranial parts are very similar to C. petrinus and C. buchseri , but the clavicle is more robust in C. nichollsi .

C. nichollsi possesses 8–10 cervical vertebrae in comparison with 12 in C. petrinus and six in C. buchseri .

Holotype and only specimen: Specimen FMNH PR2251 . Anterior half of an articulated skeleton, comprising the skull without the rostrum, articulated vertebral column with neural arches and ribs, extending to the middle dorsal region, and parts of the shoulder girdle.

Type locality: Augusta Mountains, Pershing County, Nevada (T25N, R39E; Section 14; USGS Cain Moutain 7.5′ quadrangle, 1990 provisional edition, exact locality on file at the FMNH).

Horizon and distribution: Fossil Hill Member, Favret Formation, Anisian, Middle Triassic.

Etymology: Named in memory and honour of Dr Elizabeth L. Nicholls, curator of marine reptiles at the Royal Tyrrell Museum, Drumheller, Canada, for her work on Triassic ichthyosaurs, which greatly contributed to the increased knowledge and understanding of the group.

MORPHOLOGICAL DESCRIPTION

SKULL

In dorsal view, the preserved skull has an overall trapezoidal shape, and quickly narrows towards the rostral region ( Fig. 3 ). The narrow end of the teardropshaped temporal opening points in posterior direction and a large and well-developed temporal terrace lies anteriorly to it. The parietal region between the temporal openings forms a parietal plateau, resulting in a clearly elevated skull roof, but it does not form a distinctive sagittal crest of the kind seen in C. petrinus specimen UCMP 9950. However, the second skull in the University of California collection described as C. petrinus, UCMP 9913, also shows a rather shallow and flat skull roof and lacks the high sagittal crest of the holotype. This skull did not receive much attention in the literature and it will be necessary to re-investigate it to clarify whether it indicates a range of morphological variation within C. petrinus or if it represents another specimen of C. nichollsi .

The orbit of FMNH PR2251 is rather small and eggshaped with the wider, slightly angular end lying posteriorly ( Fig. 4 ). In both orbits, the sclerotic rings are still preserved, although the left eyeball is turned anteromedially so that an exact count of the sclerotic plates is not possible on this side. On the right skull side the eyeball is in natural position, and the full sclerotic ring with 14 plates is visible. The sclerotic ring is oval in shape and appears small compared with the size of the orbit, with a likewise small inner opening of the sclerotic ring ( Table 1 ). The length/height ratio of the sclerotic ring is 1.55. However, even though the sclerotic plates are in articulation, some seem to be somewhat shifted from their original position, which might obscure the measured values slightly.

The postorbital region of the skull is rather long ( Table 1 ) and contains a large, rounded lateral embayment, bordered by the jugal, postorbital, quadratojugal, squamosal and quadrate.

Premaxilla: Only the posteriormost part of the premaxilla is preserved, and the posterior and ventral borders of this bone are broken off on both sides of the skull ( Fig. 4 ). On the right side, the premaxilla reaches back to the middle of the preserved external naris, while on the left side it reaches almost as far posteriorly as the posterior edge of the external naris. In the present state of preservation, the premaxilla does not contribute to the margin of the external nares, but lies slightly dorsally to it. Nevertheless, it is likely that the broken ventral edges originally did contribute to the dorsal border of the external nares. The preserved posterior end of the external nares suggests that they were oval in shape and rostrocaudally elongated.

Maxilla: The ventral and posteroventral margin of the preserved part of the external nares is formed by the maxilla, which is also truncated anteriorly by the fault plane. It has a straight, horizontal suture with the nasal, which runs from the posterior edge of the external nares posteriorly. The posterior part of the maxilla is drawn out into a short, triangular dorsal process, which makes contact with the lacrimal in a clearly visible suture, and an elongated ventral process. This process tapers posteriorly and extends back to one-third below the orbit, but is excluded from the ventral margin of the orbit by the lacrimal and the jugal. The ventralmost portion of this process is separated from the lacrimal by the slender anterior process of the jugal.

Lacrimal: The lacrimal is a thick bone, the posterior margin of which makes contact with the prefrontal in a long, clearly visible suture ( Fig. 4 ). The ventral process of the prefrontal excludes the lacrimal from the anterior and anterodorsal margin of the orbit, but the ventral part of the lacrimal takes part in the anteroventral border of the orbit. Parallel to the suture with the ventral process of the prefrontal lies the prominent antorbital crest of the lacrimal, which has a crescent-shaped outline.

Anteriorly, the lacrimal has two triangular processes, of which the smaller, dorsal one makes contact with the nasal and the maxilla, while the ventral one contacts only the maxilla. The posteroventral part of the lacrimal contacts the anterior process of the jugal. Apart from the posterior continuation of the horizontal suture with the nasal, which is poorly visible owing to fracturing of the bone in this area, the sutures of the lacrimal are clearly visible.

Pre- and postfrontal: Owing to some large cracks and breakage in the area dorsal to the orbit, the suture between pre- and postfrontal, as well as their suture with the nasal, is largely obscured, and thus the bones are discussed together. The narrow pre- and postfrontal extend along the entire dorsal margin of the orbit ( Fig. 4 ). There is a slightly roughened supraorbital crest along the anteromedial portion of the dorsal margin of the orbit. Anteriorly, the prefrontal has a well-defined, crescent-shaped suture with the lacrimal. The anterolateral process of the prefrontal runs parallel to the antorbital crest of the lacrimal and reaches into the orbit, excluding the lacrimal from the anterodorsal margin of the orbit.

The posterior boundary of the postfrontal is clearly visible as a serrated suture overlapping the postorbital, slightly posterior to the posterodorsal border of the orbit.

The medial contact of the postfrontal with the end of the posterior process of the nasal is visible as a low, but distinctive ridge between the two bones ( Fig. 3 ). Further anterior, this suture is not traceable because of the poor preservation of the bone in this area.

Nasal: A large portion of the dorsal skull region is built up by the large nasals, which also take part in the posterior margin of the external nares ( Figs 3 , 4 ). The medial suture, where the broad, anterior parts of the two nasals contact each other along the midline of the proximal rostrum, is well fused and is visible as a slight ridge. Dorsally, a clearly visible interdigitating suture with the broad anteromedial part of the frontals runs from side to side. The suture is infilled with calcite and therefore stands out as a white line against the dark brown bone material. Posterolateral to this interdigitating suture, the nasals quickly narrow to long, slender posterior processes, which extend all the way to the temporal openings. The median suture of each process runs along the lateral edge of the temporal terrace into the temporal opening, while the lateral border of the process runs from the posterior edge of the external nares in a nearly straight suture along the dorsolateral side of the skull. On the right side of the skull, the lateral contact with the maxilla and the lacrimal is clearly visible, whereas the suture with the prefrontal and the anterior part of the suture with the postfrontal are not because of fracturing of the bone in this area ( Fig. 4 ). Posterolaterally, the nasal process ends in a short, straight suture with the postorbital. The posterior nasal process includes a sharp ridge that borders the temporal terrace laterally. Therefore, the posterior process of the nasal takes part in the anterolateral border of the temporal opening and accounts for the lateral portion of the temporal terrace.

Jugal: The jugal is a long and slender bone, which makes up the entire ventral margin of the orbit. Anteriorly, it thins and loses contact with the margin of the orbit, wedging between the posteroventral parts of the lacrimal and the maxilla, and separating the maxilla from the border of the orbit. Posteriorly, the jugal widens slightly and shows a rounded posterior process that reaches back into the anteroventral part of the lateral embayment; this process may represent the remnants of a former lower temporal bar. The contact with the postorbital at the posterodorsal end of the jugal is clearly visible. However, this probably does not represent the exact surficial contact of these two skull bones because the overlapping postorbital possesses a slightly broken ventral margin on the right side of the skull. On the left side of the skull, the postorbital region is crushed, revealing that the jugal extends dorsally beneath the postorbital up to at least two-thirds of the height of the postorbital.

Postorbital: The posterior border of the orbit is almost entirely formed by the postorbital, which has an approximately rectangular shape and extends in dorsoventral direction ( Fig. 4 ). Its posteroventral margin also forms the anterior border of the lateral embayment. As described above, the overlapping suture of the postorbital with the jugal is slightly damaged, but it runs approximately from the anteromedial margin of the lateral embayment anteriorly to the posteroventral margin of the orbit. Anterodorsally, the postorbital makes contact with the postfrontal, whereas the latter overlaps the postorbital with a distinctive, serrated suture, which lies slightly posterior to the dorsal margin of the orbit. The suture with the posterior process of the nasal is only weakly visible; it runs dorsally in a straight line towards the margin of the upper temporal fenestra ( Fig. 4 ).

The posterior boundary of the postorbital lies in the continuation of the anterior margin of the lateral embayment and is clearly observable as the postorbital clearly overlaps the quadratojugal, squamosal and supratemporal. In the dorsal portion of the postorbital in particular, a fan-shaped arrangement of growth lines is visible, indicating a centre of ossification situated close to the suture with the postfrontal. The dorsal margin of the postorbital coincides with the sharp ridge that borders the upper temporal opening, and the bone therefore contributes to the lateral margin of the temporal opening.

Frontal: The medial portions of the frontals form a flat area in front of the parietal plateau, from which it is separated through a distinctive, interdigitating suture ( Fig. 3 ). The latter runs from side to side along the anterior end of the parietal plateau and stretches from the edges on each side of this plateau over a short distance anteriorly, approaching the interdigitating suture between frontals and nasals, before it runs as a straight suture on each side of the parietal plateau posteriorly along the midline of the temporal terraces into the temporal openings. Anteriorly, the medial, flat portion of the frontals is bordered by the interdigitating suture with the nasals. The suture continues posteriorly in a straight line, laterally paralleling the fronto-parietal suture on each side of the parietal plateau. Therefore, the frontals form slender processes posterolaterally, which lie medially to the slender posterior processes of the nasals. The frontal processes run into the sediment-filled temporal openings and contribute to the medial area of the temporal terraces.

The extreme posterior extension of the frontal, as well as the nasal, and the participation of these two bones in the border of the upper temporal fenestra, is a synapomorphy of Cymbospondylus and is not observable in any other diapsid clade.

Parietal: The parietals are prominent bones, which form the anterior part of the skull roof. They form a conspicuous elevation in the medial part of the skull, which slopes gently in rostral direction. This elevation forms a medially flattened parietal plateau that steepens laterally and forms the anteromedial walls of the temporal openings ( Fig. 3 ). Along the anterior border of the elevation runs the distinctive interdigitating suture with the frontal that stretches forward over a short distance from the lateral edges of the plateau. From there, the suture runs backwards along the centre of the temporal terrace into the temporal opening, where it is particularly clear.

The parietal plateau encloses the parietal foramen, which clearly lies within the parietals, about 4.5 cm posterior to the fronto-parietal suture. It is very large with a circular outline and its opening is anterodorsally orientated. Posterior to the parietal foramen are two more foramina formed by incomplete closure of the medial parietal sutures. The anterior one is quite small and round, whereas the posterior one is larger with an oval shape. Between the parietal foramen proper and the posterior of the two additional foramina, the calcite-filled suture between the two parietal bones is clearly visible. Anterior and posterior to this area, the suture between the parietals is fused and not apparent.

Postparietal?, neomorphic bone? Posterior to the second additional foramen, the parietal plateau widens distinctively, and medially opens into a V-shaped embayment. A short distance anterior to this embayment, an anteriorly convex suture runs along the lateral sides of the parietal plateau down into the temporal openings, with the posterior bone overlapping the parietals ( Fig. 3 ). The edges of this overlapping bone are broken off, but the general course of the border can be observed on both sides of the skull. Therefore, an extensive part of the posterior skull roof is formed by an additional paired skull bone posterior to the parietals. In this region, the lateral walls of the plateau slope more gently towards the temporal openings. The additional bone may be the postparietal, but the affinity and configuration of this skull bone are discussed below.

Supratemporal: The long and slender bone paralleling the posterolateral margin of the upper temporal opening is the supratemporal. It posteriorly tapers to a free pointed end that extends a short distance beyond the back of the skull. This feature is not an artefact of preservation and can be observed on both sides of the skull. Anteriorly, the supratemporal is overlapped by the postorbital, but it can be seen to extend beneath the thin postorbital to the posterior end of the posterior process of the nasal, where it probably makes contact with the nasal. The ventral margin of the supratemporal overlaps the squamosal with a short, straight suture, whereas further posteriorly it overlaps the lateral portion of the possible postparietal.

Squamosal: A broad area of the cheek region is formed by the large squamosal ( Fig. 4 ). It is excluded anteriorly from the border of the temporal opening by the supratemporal, which it contacts dorsally in a short, straight, clearly visible suture. Posteriorly, it is excluded from the upper temporal fenestra by the wedge-shaped anterior process of the possible postparietal/neomorphic bone. The anteromedial portion of the squamosal is overlapped by the postorbital in a straight, dorsoventrally extending suture, and the squamosal surrounds the dorsal and posterior part of the overlying quadratojugal. A small bony bridge of the squamosal posterior to the quadratojugal extends anteroventrally and contributes to a short stretch of the posterior margin of the lateral embayment. As a result, the squamosal contacts the quadrate posteriorly in a straight suture running in posterodorsal– anteroventral direction. Posteriorly, the squamosal tapers to a blunt point at the posterior margin of the skull ( Fig. 4 ). Growth lines are clearly visible on the surface of the squamosal and run with a fan-shaped outline from the posterior point anteriorly.

Quadratojugal: The outline of the quadratojugal is roughly rectangular with an anteroposterior extension ( Fig. 4 ). The bone possesses a short suture with the postorbital anteriorly and clearly overlaps the squamosal with its dorsal and posterior boundary. The ventral part of the quadratojugal forms the major part of the dorsal margin of the lateral embayment.

Quadrate: At the back of the skull together with the posterolateral part of the pterygoid, the quadrate forms a broad connection between the palate and the skull roof ( Fig. 5A ). The bones at the back of the skull are somewhat crushed, but the suture between quadrate and pterygoid is determinable in many parts, and it runs obliquely from the dorsomedial margin of the broad area at the back of the skull obliquely to the ventrolateral edge of the latter. Therefore, the quadrate forms the dorsolateral part of the back of the skull, while the pterygoid contributes to its ventromedial portion. Within the quadrate lies the large, elongate quadrate foramen, the greatest extension of which is in dorsoventral direction. The dorsal, medial and ventral part of the quadrate foramen is surrounded by the quadrate, whereas its lateral margin is probably formed by a narrow, flattened, posterior portion of the squamosal; owing to poor preservation lateral to the quadrate foramen, the sutures between the quadrate and the squamosal are not clearly identifiable.

In contrast to that of C. petrinus , the quadratojugal does not take part in the margin of the quadrate foramen; the posterior end of this bone clearly lies at the lateral side of the skull above the lateral embayment.

Cross-sectional view of the back of the skull: The posterior end of the temporal opening and dorsal portion of the back of the skull can be observed in cross-section in the fault plane at the back of the skull ( Fig. 5B ). In the right dorsolateral part of this cross-sectional view, the suture between the parietal and the additional posterior skull bone (possible postparietal/neomorphic bone) is visible, with the latter distinctly overlapping the parietal. The cross-section of the parietal stretches along the right medial side of the parietal plateau almost halfway down to the temporal opening, where it pinches out.

Unfortunately, the preservation of the right ventrolateral side of the posterior skull cross-section is not sufficient to determine where the sutures between the skull bones forming the posterior skull roof and the quadratum are.

Vomer: At the rostral end of the palatal region of the preserved portion of the skull lie the posterior winglike processes of the vomers ( Fig. 6 ). They possess a clearly visible suture with the palatines that stretches from the posterior edge of the wing-like processes of the vomers in a stepped manner anteriorly. The rostral continuation of the palatines as well as of the vomers is cut off by the fault plane that displaced the rostrum of the specimen.

Palatine: The medial suture of the palatines with the pterygoids is in large part covered by the hyoid bones, but can in part be observed on the right side of the skull, in the region where the palatine expands medially. There, the suture runs anteromedially towards the posterior edge of the wing-like process of the right vomer. Posteriorly, the palatines contact the pterygoids in an indistinct, convex, rounded suture.

Pterygoid: At their anterior border, the pterygoids make contact with the posterior wing-like portions of the vomers and rapidly narrow into a pointed rostral end that wedges between the two vomers ( Fig. 6 ). The interpterygoid vacuity, which is prominent in the posterior region of the palate, closes at this pointed end. Further posteriorly, the lateral margins of the pterygoids expand laterally in a smooth curve and make contact with the palatines, which form the lateral portions of the palate in this region. Behind the posterior end of the palatines, the pterygoids widen even more laterally to flat, smooth bone plates. Over a short distance, they cover the full width of the palate, apart from the distinctive interpterygoid vacuity that separates the two pterygoids nearly over their full length.

The pterygoids possess prominent posteromedial processes. These are slightly curled medially, forming channel-like bone flanges, and extend posteriorly beyond the basioccipital ( Figs 5 , 6 ).

In the occipital region of the skull, the pterygoids have flat, wing-like extensions, which contribute to the ventromedial area of the back of the skull and contact the quadrate ( Fig. 5 ).

Basisphenoid: A small portion of the basisphenoid is exposed in the V-shaped part of the interpterygoid vacuity between the two posterior processes of the pterygoids. The basisphenoid is approximately heartshaped with a pointed anterior end. The rounded posterior ends have roughened surfaces and make contact with the basioccipital. The preservation and the limits of preparation do not allow a more detailed description.

Basioccipital: The basioccipital is observable from its ventral and its left lateral side in the interpterygoid vacuity between the two posterior processes of the pterygoids, and it is still in full articulation with the atlas ( Fig. 7 ). The massive body of the basioccipital has a circular cross-section. At its anterior margin are two ventrolaterally positioned, roughened and slightly elevated articular facets for the articulation with the roughened posterior ends of the basisphenoid. Posteriorly, the basioccipital forms a sharp margin from which its posterior surface slopes obliquely towards the anterior articulation surface of the atlas ( Figs 5 , 7 ).

In the middle of the ventral surface of the basioccipital, the round external carotid foramen is clearly visible ( Fig. 7 ).

On the left lateral side, the articulation of basioccipital and atlas is clearly visible. Owing to tight articulation, the actual posterior articulation facet of the basioccipital is not visible. It nevertheless is clear that the basioccipital must have a concave articulation surface.

LOWER JAW

The lower jaw of specimen PR2251 is nicely preserved on the right side of the skull ( Fig. 4 ), but somewhat crushed and broken on the left side. The lower jaws are still in articulation with the skull and, like the latter, have been cut off by the fault plane at their anterior end so that the rostral portion is missing. The ventral border of the preserved lower jaw forms a distinctive arch and extends posteriorly beyond the back of the skull.

Dentary: The lateral face of the lower jaw is built up by the dentary, the surangular and the angular. Only the posterior portion of the dentary is preserved and overlaps the surangular with a clearly visible suture. The posterior margin of the dentary extends back to almost the middle of the orbit and has the form of a swallowtail with two distinct, pointed ends. Its ventral suture is straight and runs in anteroposterior direction.

Surangular: The surangular accounts for the largest part of the lateral side of the lower jaw. Ventrally, it contacts the angular in a clearly visible suture that stretches further dorsally in the posterior portion of the lower jaw and slopes smoothly ventrally towards the rostral end.

The surangular reaches into the ventral part of the lateral embayment with a horizontally flattened, shelf-like area ( Fig. 4 ). At the posterior end of the lateral embayment it contacts the articular. The suture between these two bones forms a small crest that runs slightly obliquely in anterodorsal–posteroventral direction along the posterodorsal part of the jaw ramus. The surangular thins towards the retroarticular process of the lower jaw and contributes to just a narrow portion of the latter.

Angular: The angular forms the ventral portion of the lateral side of the lower jaw and contacts the surangular in the long, sinuous suture described above. The angular wraps around the ventral part of the lower jaw and therefore also contributes to its ventromedial side.

Along the anteromedial half of the preserved lower jaw the angular contacts the splenial ( Fig. 6 ). On the posteromedial face of the jaw ramus, the straight suture between the angular and the prearticular is visible, shifting to a more ventral position towards the retroarticular process.

Articular: The massive articular contributes to a large part of the retroarticular process at the posterodorsal and the posteromedial portion of the lower jaw. At the medial side of the retroarticular process, it forms a broad channel with steepened lateral walls, the anterior one of which has a clearly visible contact with the ventral portion of the quadrate ( Fig. 5 ). Ventromedially, the articular shares a clear suture with the prearticular. The glenoid of the articular receives the bulbous anterolateral condyle of the quadrate ( Fig. 4 ).

Splenial: The splenial is a smooth, flat bone forming the anteromedial wall of the jaw ramus ( Fig. 6 ). The ventral and posterior edges seem to be slightly broken off, but it is unlikely that the splenial reached much further ventrally and posteriorly than preserved. It possesses the two-pronged posterior end that can be observed in many ichthyosaur groups ( Fig. 6 ).

Prearticular: Posterior to the broken edge of the splenial, the prearticular comes into view, forming the dorsomedial wall of the lower jaw ( Fig. 5 ). The anterior end of the prearticular is pointed. It widens slightly posteriorly before narrowing again in the region of the strongest ventral curvature of the lower jaw. Towards the retroarticular process, the bone significantly widens again and contacts the articular in a clearly visible suture, which lies perpendicular to the long axis of the ramus. The prearticular does not take part in the retroarticular process.

HYOID APPARATUS

Two hyoid bones with a round to oval cross-section are preserved nearly in situ lying on the palate. Although the anterior and posterior ends are broken off it is still visible that the bones thin towards their posterior end. In addition, the right hyoid bone seems to possess a slight undulation in its course, but it is not clear whether this is a natural feature of the bone or has resulted from deformation during fossilization.

POSTCRANIAL SKELETON

The postcranial axial skeleton is preserved to the middle dorsal region and consists of a continuous string of 28 vertebrae in articulation. Furthermore, atlas and axis each possess an intercentrum, and a number of ribs are present. The postcranial skeleton is preserved in two blocks that are separated from the skull and each other through fault planes, but can be tightly fitted together.

The oblique fault plane that runs through the back of the skull separated the major part of the cranium, together with the atlas, axis and third cervical, from the first postcranial block. The latter contains vertebrae 4–13 with neural arches and associated ribs, whereas the fault plane that separated the two postcranial blocks runs right through the 13th vertebra, separating the anterior half of the centrum and neural spine from its posterior half. Vertebrae 13–28 can be observed in the second postcranial block and are also associated with neural arches and ribs.

A distinctive torsion within the vertebral column leads to tilted neural spines between the fourth and the 17th vertebrae, which in this region can be seen from their right lateral side and cover the dorsal portion of their respective centra ( Fig. 1A, B ). Therefore, the direct articulation of the centra with the neural spines is not visible.

Vertebral column: Merriam (1908) established a count of 11 cervical vertebrae for C. petrinus , cervicals comprising all vertebrae anterior to the scapula and coracoid. This approach would suggest a count of eight cervical vertebrae in specimen FMNH PR2251. However, because the shoulder girdle may have shifted during fossilization, its position is not reliable for establishing the cervical count. The criterion that all vertebrae with parapophysis represent cervical vertebrae, as used by Sander (1989b), is used here to determine the number of cervicals. Vertebrae 9 and 10 are covered by the coracoids in FMNH PR2251, but vertebra 8 still possesses a small parapophyses, whereas vertebra 11 does not. Therefore, there are at least eight and at most ten cervical vertebrae present in this specimen, in comparison with six in C. buchseri and 12 in C. petrinus .

The atlas is still in articulation with the basioccipital, as well as with the axis, and it can be observed from its ventrolateral side. Its anterior, smooth and clearly convex surface makes contact with the oblique, roughened articular surface of the basioccipital ( Fig. 7 ). Owing to this tight articulation, the tip of the convex anterior surface of the atlas is not visible.

Along the narrow lateral edge of the atlas, an oval depression with a roughened surface is visible. No rib articulation facets are present on the centrum of the atlas.

Like the atlas, the massive centrum of the axis can be examined from its ventral and left lateral side ( Fig. 7 ). Slightly lateral from the ventral midline of the centrum are two large, oval and elevated parapophyses, which are orientated parallel to the anterior margin. More dorsally, the ventral portion of a strong diapophysis can be seen. It reaches from the anterior margin posteriorly to slightly behind the middle of the lateral side of the centrum. The dorsal continuation of the diapophysis is covered by sediment and the proximal portion of the short, double-headed first cervical rib. Along the ventral midline of the centrum of the axis, a slightly flattened and roughened area is developed.

There are two intercentra of different size preserved in specimen FMNH PR2251, one of which belongs to the atlas, the other one to the axis ( Figs 5 , 7 ). The anterior intercentrum is the larger one and has an elongated, oval and somewhat concave base. Its overall shape is that of a shallow cone with a medially positioned, pointed peak. It is nestled between the ventral surfaces of the basioccipital and the atlas.

The second intercentrum has an approximately rhombic base and is smaller and less elongated than the anterior one. It ends in a rounded peak above the centre of the rhombic base and is wedged in between the ventral surfaces of atlas and axis.

Slightly different from the situation in the axis, the parapophysis of the third cervical has a more anteroposterior extension along the ventrolateral portion of the centrum. It is truncated by the anterior margin and ends clearly before the posterior margin of the centrum. The third cervical also possesses a flattened and roughened ventral midline.

On the fault plane cutting through the back of the skull, the round posterior face of the third cervical vertebra is observable, 63 mm in height and 37 mm in length ( Table 1 ). The peripheral parts of its posterior face slope slightly and evenly towards the centre, whereas the central region is infilled by calcite. However, it appears that the slope of the posterior face steepens in its central region.

The fourth cervical vertebra is partially covered by the left clavicle, and only its round cross-section in the fault plane as well as its ventral parts are visible. The ventral side is smooth and round and does not show the flattened and roughened area seen in axis and third cervical.

Cervicals 5–8 are exposed from their ventrolateral side ( Fig. 8 ). The parapophyses of these vertebrae clearly decrease in size from anterior to posterior. Parallel with this decrease in size, the parapophyses move away from the anterior margin of the centrum, which still truncates the parapophyses of the fifth and sixth cervical, but does not contact those of the following vertebrae. The fifth to eighth cervical vertebrae also have strongly developed, elongated diapophyses on the lateral side of the centra. Their ventral end points anteriorly but does not contact the anterior margin of the centrum. The diapophyses reach far dorsally and are continuous with the articulation facets of the neural arch.

The ninth and tenth vertebrae are covered by the coracoids, and it thus remains unknown whether these two centra still possess parapophyses. However, because the parapophysis of the eighth cervical vertebra is reduced to just a small, round bump, it is likely that the ninth vertebra retains only a rudimentary parapophysis. The tenth vertebra probably does not have one at all, and therefore is the first dorsal.

Dorsal vertebrae 11–18 are only observable from their ventral side, as their dorsal and most of the lateral aspect are covered by the tilted neural arches and ribs and by sediment ( Fig. 1A, C ). Large parts of vertebrae 11 and 12 are also covered by elements of the pectoral girdle.

However, the ventrolateral view of vertebrae 15–18 reveals prominent, elongated diapophyses, which extend ventrally along the body of the centrum and are clearly truncated by the anterior margin of the centrum. Vertebra 13 is cut off by the fault plane separating the two postcranial blocks. The cross-section thus exposed is slightly more oval in shape than that of the third cervical vertebra ( Table 1 ).

Because only few rib parts are present in this hindmost preserved area the lateral sides of the centra can be studied in addition to their ventral sides. Although preservation is poor, the prominent elongate, anteriorly slanted diapophyses are clearly visible. Throughout the whole series of vertebrae 19–28, the diapophyses are distinctively truncated by the anterior margins of the centra and extend in a curve towards the neural arch. Although preservation is insufficient to determine this with certainty, the diapophyses seem to maintain contact with the articulation facet of the neural arch.

The ventral sides of vertebrae 13–28 are clearly exposed. In vertebrae 13–17, the ventral side is rounded and smooth, whereas the ventral sides of centra 18–26 are somewhat flattened. Vertebra 28, the last preserved, has a height/length ratio of 2.35 (vs. 1.70 of the third cervical) ( Table 1 ), and has a triangular outline. The increasingly tapered ventral parts and steeper lateral sides of vertebrae 25–28 indicate that a triangular outline of the centra is developed in this region.

Specimen FMNH PR2251 exhibits a continuous row of 26 neural arches, of which the first visible arch belongs to the third cervical vertebra and can be observed in cross-section in the posterior fault plane of the skull. The neural arches are numbered in conjunction with their respective centra. Neural arches 4–12 are flipped over and can be studied in right lateral view. From the 13th to the 20th neural arch they are rotated into a slightly more upright position, grading into a fully upright position in neural arches 20–28. Heavy weathering in this region allows little more than the general form of the neural arches to be inferred.

Throughout the preserved vertebral column, the neural arches posses a rather uniform shape with little differences between particular regions ( Fig. 8B ). The neural arches are robustly built with strong pre- and postzygapophyses that slope anteriorly at an angle of about 15°. In most areas, particularly clearly visible in neural arches 6–9, the pre- and postzygapophyses are tightly fused, as expressed in an area of rough, bulging and uneven bone between the zygapophyses ( Fig. 8 ). This fusion strongly stiffened the vertebral column in the preserved anterior dorsal area and restricted the lateral mobility of this region. However, given that specimen FMNH PR2251 is the so far only known specimen of Cymbospondylus nichollsi , it remains unknown whether this feature is characteristic for the taxon, or a sign of age and/or a pathology of this particular specimen.

The ventral bases for the articulation with the centrum are expanded. In addition, a small, flattened, transverse process occurs at the base of neural arches 10–15. It represents the dorsal portion of the diapophysis that extends from the centrum onto the neural spine.

Owing to the preservation of the anterior vertebral column and the insufficient preservation more posteriorly, no details of the articulation between the neural arches and the centra can be described for specimen FMNH PR2251.

The neural spines are fairly long and are all inclined posteriorly at an angle of about 30–35°. They have an oval cross-section with a sharp anterior edge and with a rounded posterior edge. The dorsal tip of the neural spines is slightly rounded.

The cross-section of the third neural arch at the posterior fault plane of the skull shows that there is a large, oval neural canal framed by the two stems of the neural arch and the dorsally flattened area of the centrum ( Fig. 5B ).

Ribs: A number of distally incomplete ribs are preserved in specimen FMNH PR2251. The anterior twothirds of the postcranial skeleton preserve numerous proximal rib parts, which are closely associated with the vertebral column, but no direct articulation with centra.

One cervical rib with a double articulation lies laterally to the atlas and axis, suggesting that it is the first cervical rib ( Figs 5A , 9A ). Although its distal end was cut off by the fault plane, not much seems to be missing.

The next preserved rib head lies on the right side of the body at the level of the fifth neural arch. It is single-headed, like the heads of all following ribs. The rib heads are expanded, anteroposteriorly flattened and clearly set off from the distal rib shaft ( Fig. 9B, C ). The articulation facet is elongated and has an oval outline with a narrow dorsal and a broader ventral part. Originating at the narrow portion of the articulation facet, a dorsal ridge extends along the rib shaft, resulting in grooves along its anterior and posterior side and in a figure-of-eight cross-section.

The transitional area between rib head and rib shaft is also the area of greatest curvature. The anterior ribs have a wider, smoother curvature than the more posterior ribs. In these, there is distinctive offset of head and shaft ( Fig. 9B, C ).

The large block with portions of ribs of the left body side between vertebrae 14 and 28 shows proximal to intermediate portions of ribs in ventral view ( Fig. 1C ). The shafts all possess the grooves described above, as well as the figure-of-eight cross-section.

Because no rib is preserved in full, total rib length remains unknown.

Appendicular skeleton: Only the coracoids and clavicles as well as a possible forelimb fragment are preserved of the appendicular skeleton.

The left coracoid is almost completely preserved and only the tip of the blade is missing. Although it is somewhat crushed and flattened, the original form can still be clearly discerned ( Fig. 10A ). Half of the blade of the right coracoid is missing, but the preserved part suffered less compaction than the left coracoid and therefore shows the original form nicely.

The coracoid has a thick, eye-shaped glenoid facet with a roughened surface. The latter forms the base of a short, thickened stem, which quickly expands and thins as the bone widens towards the blade. The anterior and medial margin of the coracoid blade is strongly convex, whereas the posterior margin forms a sharp, concave curve. The medial and posterior margins meet in an elongated, pointed end that is directed posteriorly.

A large lenticular coracoid foramen is seen anterolaterally close to the thickened area of the glenoid facet (see Table 1 for measurements of the shoulder girdle elements).

The left clavicle of the specimen is heavily crushed and weathered and parts are missing, but fortunately the right clavicle is almost uncrushed and completely preserved. The elongated, rounded anterior part of the clavicle is slightly twisted towards the medial body axis, and its tip consists of a conical, hat-like structure, which is set off from the slender neck of the clavicle ( Fig. 10B ). The conical structure is distinctively roughened with some deeper grooves running radially from the tip to its posterior edge, which might indicate that these roughened areas served as an attachment site for cartilage. The form of the thick anterior head suggests that the two clavicles did not articulate with each other in an interfingering fashion as seen in other ichthyosaurs; it is more likely that they solely articulated with a medially positioned interclavicle.

Posterolaterally, the slender neck of the clavicle widens into a thin, sail-like blade with a triangular outline that sits dorsally on the base of the clavicle. The latter is thickened and has a finished, rounded edge. Caudally, the clavicle possesses an elongated posterior process, which is more slender and slightly longer than the anterior neck of the clavicle. A fragment of a broken bony blade is still attached to this process ( Fig. 10B ). The fragment could represent remnants of the right scapula.

As in C. petrinus and in C. buchseri , the interclavicle is not known in specimen FMNH PR2251. This is surprising considering the good preservation and tight articulation of the preserved shoulder girdle in this new specimen.

COMPARISON OF THE THREE WELL-KNOWN CYMBOSPONDYLUS SPECIES

ESTIMATION OF BODY LENGTH

Specimen FMNH PR2251 is only preserved to its middle dorsal region, and therefore an estimate of total body length must depend on comparison with C. petrinus , which is known from more complete material (specimens UCMP 9950, UCMP 9947, etc.). Skull length posterior to the external nares in C. nichollsi is 443 mm, which is 84% of the same measurement of the skull length of C. petrinus UCMP 9950 ( Table 2 ). Merriam (1908: 105) estimated the body length of UCMP 9950 + UCMP 9947 as exceeding 9.1 m. The skull of this skeleton is 13% of its whole body length. However, this specimen represents a composite skeleton of which the anterior body half belongs to individual UCMP 9950, while the caudal region and posterior limb were added from specimen UCMP 9947 ( Merriam, 1908). Assuming that the overall body proportions of specimen FMNH PR2251 are similar to C. petrinus , the estimated body length for C. nichollsi is 7.6 m. It would therefore be intermediate in size between C. buchseri , with an estimated body length of 5 m ( Sander, 1989b), and C. petrinus . However, this value takes neither ontogenetic nor intraspecific variation into account, and therefore can only be regarded as a very rough estimate.

COMPARISON OF THE SKULL OSTEOLOGY

Shape and proportions: The skull of C. nichollsi seems to be less dorsoventrally flattened than that of C. petrinus and C. buchseri , and the general shape differs particularly from that of C. petrinus . The orbital shape of C. nichollsi and C. buchseri is alike and somewhat more rounded and enlarged than that of C. petrinus ( Fig. 11 ; Tables 2 , 3).

The ratio of orbit length/postorbital length, as a proxy for the relative length of the postorbital region, can only be given for C. petrinus (0.97) and C. nichollsi (0.83), as the postorbital region of C. buchseri is incompletely preserved. Sander (1989b) suggested the ratio of the orbit length and postdentary length of the lower jaw as a measure of the postorbital length. This ratio is 0.62 in C. petrinus , 0.77 in C. buchseri and 0.55 in C. nichollsi . However, the dentaries of C. buchseri are shorter than those of C. petrinus and only extend back two-thirds below the orbit. When comparing the ratio between the skull length posterior to the nares and the length of the cheek region in C. petrinus (2.11) and C. nichollsi (1.85) as a measure of relative length of the cheek region, it becomes apparent that the cheek region of the new species is slightly longer than that of C. petrinus . Moreover, the jaw articulation of C. petrinus is drawn out far posteriorly compared with C. nichollsi , resulting in an anteriorly slanted occiput in this taxon ( Fig. 11B , Merriam (1908: table 5).

Location of the supratemporal and re-evaluation of the skull osteology of the genus: New insights into the osteology of the skull of Cymbospondylus arise from comparison of the three species ( Fig. 11 ). Despite the diverse interpretations of the cheek region and skull roof of C. petrinus , the long and slender bone stretching along the dorsolateral portion of the cheek region has been interpreted as the postfrontal or the fused pre- and postfrontal by previous authors ( Merriam, 1908; von Huene, 1916; Romer, 1956; Camp, 1980; Motani, 1999; Maisch & Matzke, 2000, 2004). The long, splint-like bone entering the posterolateral margin of the temporal opening in C. nichollsi (here interpreted as the supratemporal) is most certainly homologous to this bone. We believe the bone in C. petrinus is the supratemporal, not the prefrontal.

Although a clear suture between pre- and postfrontal is not visible (due to a major crack) in specimen FMNH PR2251, the bone forming the posterior dorsal border of the orbit must be interpreted as the postfrontal, based on its location. In C. buchseri , the postfrontal can be observed in the same position and with almost identical dimensions.

The splint-like bone in the dorsal cheek region of C. nichollsi is very probably not the postfrontal because it is clearly overlapped by the postorbital, which in turn is overlapped by the bone forming the posterodorsal margin of the orbit. This then would have to be interpreted as the prefrontal. Although it can be seen that the splint-like bone extends further anteriorly beneath the postorbital, there is only a slight contact, if any, with the bone forming the posterodorsal border of the orbit. Therefore, this interpretation would result in the postfrontal having been displaced far posteriorly and not making contact with either the prefrontal or the frontal. Because this is highly unlikely, a new interpretation of the configuration of the skull bones of all three species becomes necessary.

The slender, splint-like bone can be observed in C. petrinus , C. nichollsi and, a little wider, but in the same position in C. buchseri . In C. buchseri it had originally been described by Sander (1989b) as squamosal, based on the suggestion of Romer and Camp (in Romer, 1968) and McGowan (1973) that a supratemporal is absent in ichthyosaurs. This interpretation of the absence of a supratemporal was revised by Massare & Callaway (1990) based on their study of the early ichthyosaur Grippia . Later studies unequivocally documented the presence of a supratemporal in various ichthyosaurs ( Nicholls & Brinkman, 1996; Callaway, 1997; Maisch, 1997; Motani, Minoura & Ando, 1998; Motani, 1999; Nicholls, Brinkman & Callaway, 1999).

Owing to its location at the posterolateral margin of the upper temporal opening, the splint-like bone is here interpreted as the supratemporal ( Fig. 11 ). The bone shelf forming the border of the orbit in C. petrinus consequently is formed by the pre- and postfrontal, with its posterior suture situated directly above the highest point of the orbit. The prefrontal is very small and makes contact with the postfrontal in a weakly visible suture, about halfway above the ascending anterior margin of the orbit ( Romer, 1956: fig. 85a; von Huene, 1916: fig. 79; own observations on specimen UCMP 9950). In C. petrinus , the supratemporal anteriorly makes contact with the long posterior process of the nasal as well as with the postfrontal and postorbital. In C. buchseri , it additionally contacts the prefrontal; in C. nichollsi , by contrast, the anterior portion of the supratemporal is overlapped by the postorbital, which reaches far dorsally towards the border of the temporal opening. However, in C. nichollsi the slender supratemporal also extends further anterior, but beneath the postorbital, towards the posterior margin of the postfrontal and the posterior process of the nasal. The supratemporal therefore has a similar extent and location in all three species.

A suture that is particularly clearly visible in C. petrinus specimen UCMP 9950 runs along the lateral walls of the parietal crest in dorsoventral direction into the upper temporal opening. Although the lateral walls of the parietal plateau are superficially fractured in C. nichollsi , an overlapping suture with somewhat broken, frayed edges can be seen on both sides of the skull in the same position as in C. petrinus . This observation implies that an additional skull bone posterior to the parietals forms a large part of the back of the skull in Cymbospondylus . This skull bone extends laterally around the posterior margin of the upper temporal opening, and contributes to the posterodorsal part of the cheek region, where it is separated from the squamosal by clearly visible sutures in both C. petrinus and C. nichollsi . This pattern can be inferred for C. buchseri as well. The right medial surface of the parietal plateau is visible in C. buchseri ( Sander, 1989b) , and in its posterior region an overlapping suture of the parietal with a posteriorly attaching skull bone is visible. Because the suture is exposed from the medial side in C. buchseri , the pattern of overlap of the two skull bones in question is opposite to that seen in C. petrinus and C. nichollsi .

The small strip of bone extending along the dorsal margin of the supratemporal in C. buchseri ( Fig. 11 ), interpreted as a possible posterolateral process of the frontal by Sander (1989b), could represent the anterolateral process of the additional skull bone as in C. petrinus and C. nichollsi .

What is the homology of the additional skull bone in Cymbospondylus ? One possibility is that it represents a neomorphic structure unique to the genus. Another interpretation, avoiding the assumption of a neomorphic bone, is its identification as the postparietal, which in primitive diapsids is located posterior to the parietal and medial to the supratemporal.

The presence of an unpaired postparietal bone positioned at the dorsal edge of the occipital plane has recently been suggested by Maisch & Matzke (2004).

The presence of a separate bone in this area in C. petrinus was not identified during our observations of specimens in the collection of the Paleontological Museum in Berkeley in 2002 and can therefore not be corroborated here. The paired bone discussed in this paper as a possible postparietal or neomorphic element is a different one from that identified as postparietal by Maisch & Matzke (2004), and is situated posterodorsally, forming the posterior skull table.

However, the find of the new species of Cymbospondylus described here as well as the recent different interpretations of the osteology of the skull of this taxon ( Maisch & Matzke, 2000: 64, 2004; Stöcker, 2003) clearly underline the peculiarity of the genus. Further preparation of the best-known specimen of C. petrinus, UCMP 9950, and further finds will be necessary for a better understanding of this taxon and the final resolution of its skull osteology.

Comparison of cheek region and occipital region: The posterior cheek region, consisting of the quadratojugal and squamosal, is missing in C. buchseri ( Fig. 11C ), but the shape of the postorbital is similar to that of C. petrinus . The jugal posteriorly encompasses the posteroventral portion of the postorbital in C. petrinus and makes contact with the quadratojugal, a pattern that cannot be observed in the other two species. In C. nichollsi , the jugal shows a rounded posterior extension that reaches slightly into the lateral embayment and may represent a remnant of a lower temporal bar. C. buchseri has a smooth posterior margin of the jugal without any such protuberance.

Additionally to the frontals, the nasals enter the upper temporal fenestra in C. nichollsi . This contrasts with the condition found in C. petrinus in which only the frontals reach posteriorly to the upper temporal opening.

The quadratojugal of C. nichollsi is very short and restricted to the lateral side of the skull. It does not contact the quadrate, so that a small bridge of the squamosal wedges in between the posterior end of the quadratojugal and the quadrate and contributes to the posterior border of the lateral embayment. This is also the reason why the quadratojugal does not contribute to the lateral margin of the quadrate foramen at the back of the skull. In C. petrinus , the long, slender quadratojugal extends posteriorly and clearly makes contact with the quadrate ( Fig. 11B ) and, together with the squamosal, contributes to the lateral border of the quadrate foramen ( Merriam, 1908: fig. 6). Where the back of the skull is exposed, i.e. in C. petrinus and in C. nichollsi , it presents very similarly in both species, although it is distinctly tilted forward in C. petrinus . The pterygoid has strong posterior processes and broad, wing-like portions that form the ventrolateral parts of the back of the skull. The quadrate reaches far dorsally and makes contact with the putative postparietal.

COMPARISON OF THE LOWER JAW

The shape of the lower jaw of C. nichollsi with its high posterior portion of the ramus resembles the lower jaws of C. petrinus and C. buchseri , although this upward bend is even slightly stronger in C. nichollsi and the posterior region of the jaw is more massive. However, the pattern of bones building up the lower jaw differs in a number of respects ( Fig. 11 ). The dentary is clearly shorter in C. nichollsi than in C. petrinus and reaches back only two-thirds below the orbit, whereas it reaches to the posterior edge of the orbit in C. petrinus . The dentary of C. buchseri is as short as in C. nichollsi , but as in C. petrinus it posteriorly tapers to a point instead of ending in a notched posterior margin.

The angular forms almost half of the lateral wall of the ramus in C. nichollsi , but contributes only to a small portion of the ventrolateral wall in C. petrinus and, as far as can be determined, in C. buchseri ( Fig. 11 ).

The jaw articulation is drawn far posteriorly in C. petrinus , resulting in the slanted occipital region, and lies far posteriorly to the posterior edge of the parietal crest ( Fig. 11B ). In C. nichollsi , the jaw articulation lies anterior to the posterior edge of the parietal region and has a broad contact between articular and quadrate ( Fig. 11A ). The articulation of lower jaw and skull is incompletely preserved in C. buchseri , but the position of the quadrate indicates a similar configuration as in C. nichollsi .

All three Cymbospondylus species possess a shelf in the angular bone of the lower jaw that forms the ventral border of the lateral embayment ( Fig. 11 ).

COMPARISON OF THE POSTCRANIAL SKELETON

Vertebral column and ribs: Comparison of the postcranial features of the three species strengthens the affinity of specimen FMNH PR2251 with the genus Cymbospondylus and its close relationship with the two recognized species, C. petrinus and C. buchseri . The occipital–cervical complex is unknown in C. buchseri , but is well known in C. petrinus and C. nichollsi . The ventrolateral side of the atlas in C. nichollsi is identical to that of C. petrinus with smooth, rounded edges and a strongly convex anterior surface that articulates with the concave surface of the basioccipital. Laterally, the atlas shows an elongated, smooth impression surface, which is also clearly developed in C. petrinus . The axis also shows a very similar form in both species, with strongly developed diapophyses and anteriorly and posteriorly bulging margins of the centrum.

C. nichollsi is intermediate with regard to the number of cervical vertebrae, which are here defined as the centra with parapophyses as used by Sander (1989b). C. buchseri has six cervical vertebrae, C. nichollsi has between eight and ten cervicals, while C. petrinus has 12.

A synapomorphy of Cymbospondylus is the ventrally elongated, anteriorly slanting diapophyses, which are truncated by the anterior margin of the centrum ( Sander, Rieppel & Bucher, 1994) ( Fig. 8A ). This pattern is clearly seen in C. nichollsi , even in the posteriorly preserved vertebrae, despite their heavily weathered surface. At variance with McGowan & Motani (2003), the morphology of the diapophysis is well suited to assign fragmentary specimens to this genus if the character is present in its entirety, and not only an anteriorly truncated diapophsis, which is plesiomorphic for ichthyosaurs ( Motani, 1999).

Although the holotype of the new Cymbospondylus species is only preserved to the middle dorsal region, an increase of the height of the centra and a change in their shape from circular to triangular can be ascertained in this specimen, as also reported for C. petrinus ( Merriam, 1908) and C. buchseri ( Sander, 1989b) . However, the ratio between height and length of the centra is higher than in C. buchseri , which has a ratio of 1.55 for the eighth and 1.74 for the 14th centrum. In C. nichollsi , the ratio for the fourth centrum is 1.65, and for the 28th it is 2.35. Merriam (1908) only gives measurements of the caudal vertebrae of C. petrinus .

The neural arches of C. nichollsi have an average height of 87 mm and do not vary much in height in the preserved row. The average height of the neural spines in the distinctively smaller C. buchseri is 64 mm, varying only little. The measurements of the neural spines of C. petrinus given by Merriam (1908: 117) are mainly for the posterior dorsal and caudal region and therefore cannot be compared directly. In all three species the zygapophyses are well developed and the neural spines are inclined posteriorly. In specimen FMNH PR2251 the zygapophyses of adjacent neural spines are tightly fused and show rough and bulging bone in the area of fusion, which would have greatly stiffened the vertebral column in this region ( Fig. 8 ). No such fusion is present in C. petrinus and C. buchseri and it remains unknown whether this feature represents an autapomorphy for C. nichollsi or is merely a sign of age or a pathology in that particular individual.

The general characteristics of the ribs and rib articulation are identical in the three species, with all of them showing the double rib articulation of the cervical ribs and the single, elongated rib head in the trunk ribs, which possess a distinctive figure-of-eight cross-section.

Appendicular skeleton: Only the coracoids and the clavicles are preserved of the shoulder girdle in the new specimen. The shape of the clavicle of C. nichollsi is very like that of C. petrinus ( Merriam, 1908: plate 11), although it is somewhat more robust, and the clavicle head is larger and more massive. Preservation of the clavicle of C. buchseri is insufficient for an interpretation of its original appearance, but it seems to have had a sail-like dorsal part as seen in the other two species.

The coracoids are almost identical in all three Cymbospondylus species. McGowan & Motani (2003) claimed that the holotype of C. buchseri is immature based on its small size compared with C. petrinus and the supposedly incomplete ossification of the humerus. We cannot agree with this latter observation and note that size cannot be used as a proxy for ontogenetic stage in the genus because the new specimen described here is also considerably smaller than typical C. petrinus (see above) but shows no signs of immaturity. However, the difference in humeral shape between C. petrinus and C. buchseri may have resulted from paedomorphosis in the evolution of the latter from a more plesiomorphic elongate humerus. This would indeed be in accordance with the smaller size of C. buschseri .

Figures

Figure 1. Holotype of Cymbospondylus nichollsi sp. nov., specimen FMNH PR2251 in lateral (A), dorsal (B) and ventral (C) view. C shows the upper surface of the specimen as found in the field. Note the sharp bend in the cervical vertebral column. Scale bar = 20 cm.

Figure 3. Drawing of the skull of Cymbospondylus nichollsi sp. nov. (FMNH PR2251) in dorsal view. See Appendix for explanation of abbreviations. Scale bar = 10 cm.

Figure 4. Drawing of the skull of Cymbospondylus nichollsi sp. nov. (FMNH PR2251) in lateral view. See Appendix for explanation of abbreviations. Scale bar = 10 cm.

Figure 5. Drawing of the skull of Cymbospondylus nichollsi sp. nov. (FMNH PR2251) in occipital view (A). Scale bar = 10 cm. B shows the detail view of the fault plane through the back of the skull. Scale bar = 3 cm. See Appendix for explanation of abbreviations.

Figure 6. Drawing of the skull of Cymbospondylus nichollsi sp. nov. (FMNH PR2251) in ventral view. See Appendix for explanation of abbreviations. Scale bar = 10 cm.

Figure 7. Close-up view of the occipital–cervical complex of specimen FMNH PR2251 in posteroventral view. Vertebrae are observed in ventrolateral view. See Appendix for explanation of abbreviations.

Figure 8. A, drawing of cervical vertebrae 5–8 and presacral vertebrae 26 and 27 as example for the vertebral structures in specimen FMNH PR2251. B, drawing of neural arches 9–12 of specimen FMNH PR2251. Anterior is to the right. Scale bar = 5 cm.

Figure 9. Drawings of ribs of specimen FMNH PR2251. A, double-headed first cervical rib. B, anterior dorsal rib. C, middle dorsal rib. Scale bar = 2 cm.

Figure 10. Drawings of preserved shoulder girdle elements of specimen FMNH PR2251. A, left coracoid. B, right clavicle; the broken bone plate attached to the posterolateral end of the clavicle may represent a remnant of the scapula (Sc?). See Appendix for explanation of abbreviations. Scale bar = 5 cm.

Figure 11. Comparison of the lateral views of the postrostral skull regions of Cymbospondylus. A, Cymbospondylus nichollsi sp. nov., based on the holotype FMNH PR2251. B, C. petrinus, based on UCMP 9950. C, C. buchseri, based on PIMUZ 4351, modified after Sander (1989b). B and C are based on left sides of the skulls and have been inverted.