Plesiopharos moelensis, Puértolas-Pascual & Marx & Mateus & Saleiro & Fernandes & Marinheiro & Tomás & Mateus, 2021

Puértolas-Pascual, Eduardo, Marx, Miguel, Mateus, Octávio, Saleiro, André, Fernandes, Alexandra E., Marinheiro, João, Tomás, Carla & Mateus, Simão, 2021, A new plesiosaur from the Lower Jurassic of Portugal and the early radiation of Plesiosauroidea, Acta Palaeontologica Polonica 66 (2), pp. 369-388 : 373-380

Plesiopharos moelensis sp. nov.

Etymology: After São Pedro de Moel, the nearest locality.

Holotype: ML2302 is a partial skeleton belonging to a single individual that consists of 22 vertebrae (one cervical, four pectorals, and 17 dorsal vertebrae), one cervical rib and at least 19 dorsal ribs, two gastralia, right humerus, radius, ischium, ilium, pubis, femur, and two phalanges. Type locality: Praia da Concha , north of São Pedro de Moel, municipality of Marinha Grande, district of Leiria, region Centro, Portugal. Coordinates: 39°46’03.9” N, 9°01’44.8” W.

Type horizon: Coimbra Formation, lower Sinemurian (Early Jurassic) .

Diagnosis.— Plesiopharos moelensis gen. et sp. nov. is a plesiosaur diagnosed by the following autapomorphies: iliac blade with rounded and convex anterior flank and less expanded than the posterior flank; ischial facet of the ilium highly developed and twice as long as the acetabular facet; first pectoral vertebra with the apex of the neural spine inclined posterodorsally and forming a small rounded tip at the posterodorsal margin. For the exclusive combination of synapomorphies see phylogenetic analysis section.

Description.— General preservation: Most of the elements are preserved in semi-articulation, namely, the bones are almost in contact with each other and approximately in their anatomical position, although most of them are not articulated since there is a slight post-mortem displacement. Most bones are well preserved, with no signs of deformation or crushing, preserving their three-dimensionality.

Vertebrae: A total of 22 vertebrae are associated with ML2302. The majority of the vertebrae are mostly well-preserved, although some breakage has affected the preservation in several vertebrae. The one preserved cervical vertebra is visibly eroded, missing the prezygapophyses and most of the neural spine. One dorsal vertebra is missing most of its neural spine, and a second dorsal vertebra completely lacks its neural spine, which has weathered away. Overall, the distal margins of the neural spines and transverse processes appear to have suffered the most damage in ML2302. The preserved vertebrae include one cervical, four pectoral, and seventeen dorsal vertebrae, with the last “dorsal” vertebra possibly being the first sacral. The location of the vertebrae within the vertebral column was determined by the location of the rib facet on the centrum, the morphology of the rib facet, the location of neurocentral suture relative to the rib facet, and the preserved semi-articulation of the vertebral column in ML2302 ( Brown 1981; Sachs et al. 2013). Six vertebrae have been completely removed during preparation from the rock matrix ( Fig. 3 ), while the rest remain in four blocks ( Fig. 2 ). The largest block contains eleven well-preserved dorsal and three pectoral vertebrae, and more than a dozen ribs and rib fragments (block 1A; Fig. 2 ). One of the recovered blocks connects directly with the large block and contains three posterior dorsal vertebrae and at least nine dorsal ribs and rib fragments (block 1B; Fig. 2 ). A third block includes three anterior dorsal vertebrae along with the fragment of a fourth vertebra (block 2C; Fig. 2 ). The last block includes a cervical and a pectoral vertebra (block 2D; Fig. 2 ).

All observable vertebrae are sub-platycoelous with a slight depression located in the center of the articular facets. Nearly all the centra are slightly excavated along the dorsal margin by the neural canal. The neural arches are all fully fused to the centra. The combined width of the prezygapophyses is slightly narrower than the centrum in all observable vertebrae of ML2302. The prezygapophyses of all observable vertebrae extend beyond the anterior articular facet of the centra, and the postzygapophyses extend to approximately the level of the posterior articular facet of the centrum or further posteriorly. The postzygapophyses of all the vertebrae are thickest at the articular facet and are oriented ventrolaterally.

One vertebra has been identified as a cervical vertebra Fig. 3A ), as it lacks transverse processes, exhibits rib facets on the ventrolateral surface of the centrum, and lacks contact between the neurocentral suture and the rib facets. This cervical vertebra most likely corresponds to the posterior-most position of the cervical column, as the centrum exhibits length, width, and height dimensions that are subequal to centrum dimensions in the pectoral vertebrae, and was also located in situ anterior to the first pectoral vertebra. The centrum of this cervical vertebra is wider than it is long and high, whereas the length is subequal to the height (W> L, H; L ≈ H). The anterior articular facet of the cervical centrum is damaged, however the posterior articular facet is preserved and presents a subcircular outline. The cervical vertebra exhibits two co-joined facets (diapophyses fused with parapophyses) to receive the heads of the cervical ribs. The rib facets protrude ventrolaterally in the cervical centrum. In lateral view, the ventral surface of the cervical centrum is slightly concave, and the neurocentral suture makes a high relief v-shape. Below the ventral-most extent of the neurocentral suture in the cervical vertebra is a low ridge that extends to the dorsal margin of the rib facet. The neural arch is sutured to the centrum of the cervical vertebra, so the specimen was apparently adult ( Vaughn 1955; Brown 1981; O’Keefe 2004b). Nevertheless, the neurocentral suture is still visible, so the specimen may be a young adult ( Storrs 1997; O’Keefe 2004b). In ventral view, the centrum is slightly hourglass shaped, and the median ventral surface of the centrum is slightly rounded and convex rather than having a sharp crest or a flat surface. Ellipsoidal foramina subcentralia straddle the median ventral ridge in the cervical centrum.

Four vertebrae of ML2302 have been identified as pectoral vertebrae ( Fig. 3B–E ) based on a dorsoventrally deep rib facet transected or contacted by a v-shaped neurocentral suture ( Seeley 1874; Brown 1981; Evans 2012; Sachs et al. 2013). The anterior-most pectoral vertebra was located in situ directly posterior to the last cervical vertebra ( Fig. 2A, B ), and is identified as a pectoral vertebra based on the contact between the neurocentral suture and the dorsal margin of the rib facet ( Evans 2012). The first pectoral vertebra ( Fig. 3B ) exhibits rib facets on the ventrolateral surface of the centrum that are single-headed. The rib facets of the anterior-most pectoral vertebra are also oriented slightly more laterally than the last cervical vertebra. In ventral view of the first pectoral vertebra, the median ventral convexity is less pronounced than that of the posterior cervical vertebra. The other centra of ML2302 exhibit a ventral surface without a median convexity. The neural spine of the first pectoral vertebra is completely preserved and is dorsoventrally shorter and anteroposteriorly longer than the neural spines of the other pectorals and dorsal vertebrae. The neural spine rises with a posterior inclination above the neural arch. In lateral view, the spine is 1.5 times higher dorsoventrally and almost as long anteroposteriorly as the centrum (SOM 2: table S1). The anterior and posterior margins of the first pectoral vertebra neural spine are straight and parallel to each other. The apex of the spine is inclined anteroventrally, with the posterior margin of the spine forming a rounded tip which is taller than the anterior margin. In anteroposterior view, the articular surfaces of the zygapophyses in the pectoral vertebrae, as in all the preserved vertebrae, are medioventrally inclined, and the distance between each pair of zygapophyses is slightly smaller than the width of the centrum, although they do not touch each other medially. Regarding pectoral vertebrae 2–4 ( Fig. 3C–E ), the rib facets are dorsoventrally deep, and situated below the neural canal. Both the pre- and postzygapophyses extend beyond the articular facets of the centra. In ventral view of the pectoral vertebrae, paired foramina subcentralia are evident and are not as widely separated as in the middle dorsal vertebrae. In lateral view, the neural spines of the pectoral vertebrae are posteriorly inclined. One of the mid-pectoral vertebrae ( Fig. 3C ) exhibits a damaged neural spine, while the neural spine from the mid-pectoral vertebra 3 ( Fig. 3D ) is better preserved, and exhibits a neural spine with a slightly convex anterior margin and a sub-straight posterior margin, thus, creating a nearly “shark-fin” shape. The last pectoral vertebra ( Fig. 3E ) exhibits a posteriorly inclined neural spine with straight and parallel anterior and posterior margins. In the last pectoral vertebra, the neurocentral suture makes light contact with the rib facet along the ventral margin.

The dorsal vertebrae of ML2302 ( Fig. 3F ) exhibit fully formed diapophyses with transverse processes that extend nearly straight horizontally from the neural arch, although the transverse processes of the posterior dorsal vertebrae are shorter than those of the anterior and middle dorsal vertebrae and are oriented ventrolaterally. Approximately 17 dorsal vertebrae are preserved in ML2302. The transverse processes of the anterior and middle dorsal vertebrae are dorsoventrally compressed with an ovate cross-section, and extend laterally beyond the body of the centra. The first two dorsal vertebrae exhibit tuberosities below the transverse processes on the lateral surface of the centra that are likely remnants of the parapophyses, as the preceding vertebrae are from the pectoral region ( Evans 2012). The diapophyses of the anterior and mid-dorsal vertebrae exhibit a swelling where the head of the dorsal ribs would have attached. In dorsoventral view, the transverse processes are oriented slightly posterolaterally. The lateral surfaces of the mid-dorsal centra bodies are noticeably constricted between the anterior and posterior articular facets. The neural spines of the mid-dorsal vertebrae rise straight above the neural arch and are not posteriorly inclined, while the anterior dorsal neural spines exhibit a slight posterior inclination. In lateral view of the dorsal vertebrae, the anterior and posterior margins of the neural spines are straight and parallel to each other. The apex of the dorsal neural spines is slightly convex in lateral view. The neural spine of each dorsal vertebra is taller than its respective centrum and is transversely compressed. The prezygapophyses are oriented anterodorsally and slightly medially. Articular facets of the prezygapophyses form concave depressions that face dorsomedially.

The two posterior-most dorsal vertebrae ( Fig. 3G ) indicate a transition to the pelvic region of ML2302. In these posterior dorsal vertebrae, the transverse processes are more ventral than in the other dorsal vertebrae and are partially involved in the vertebral centrum but are situated above the neurocentral suture. The transverse processes in the posterior dorsal vertebrae are more gracile and cylindrical in shape than the anterior and mid-dorsal vertebrae, and are oriented ventrolaterally. The diapophyses present a flat articular surface. The transverse processes are located immediately above the neurocentral sutures, which make a u-shape in lateral view. The centra of the posterior dorsal vertebrae, between the anterior and posterior articular facets, are slightly constricted, although less so than the mid-dorsal centra. The dimensions of the centra and neural spines of the posterior dorsal vertebrae are smaller than that of the anterior and mid-dorsal vertebrae. A single unpaired foramen is visible on the right ventrolateral surface of the posterior-most dorsal centra. Another foramen is visible on the left ventrolateral surface of the posterior dorsal centrum remaining in the unprepared block, although the right lateral surface of this vertebra is obscured by the rock matrix. In lateral view, the anterior and posterior margins of the neural spines are straight, parallel and slightly inclined posteriorly. It should be noted that the last dorsal vertebra ( Fig. 3G ) could be the first sacral vertebra. While the neurocentral suture appears below the facet for the rib, the facet overlies the dorsolateral surface of the centrum and is located below the neural canal. Without an associated dorsal or sacral rib, it cannot be known if the last dorsal is indeed a dorsal or the first sacral. However, the arrangement of the neurocentral suture below the transverse processes would indicate that it is probably the last dorsal.

Ribs and gastralia: One putative cervical rib, about 19 dorsal ribs, and two gastralia have been recovered. Most of the cervical rib is obscured by matrix and partially broken, preventing a detailed description. Only a flat rib facet and a pointed fragment that could correspond to one of the processes can be observed.

Most of the ribs are from the dorsal series ( Fig. 3H, I ), as determined by their elongated morphology and size relative to the associated vertebrae. The ribs are kinked at the neck and are compressed mediolaterally along the length of the body. The facets of the ribs in ML2302 are either flat to slightly concave with a circular cross-section, or slightly compressed anteroposteriorly. Distally, the ribs transition to an ovate cross-section, becoming more mediolaterally compressed. There is a smooth and subtle constriction distal to the head of the ribs on the surface of the neck. Subtle parallel ridges run along the proximal margin of the ribs.

Only two gastralia elements ( Fig. 3K, J ) are associated with ML2302. The gastralia are dorsoventrally compressed and spool-shaped, with distal ends that taper. The larger element is distinctly sigmoidal. Due to its asymmetric sigmoidal morphology, both gastralia probably correspond to the lateral gastral ossicles, which are different from the bilaterally-symmetrical midline ossicles.

Forelimb: humerus: The right humerus has been completely preserved ( Fig. 4B ), although it has some fractures and the proximal joint facet is slightly eroded. The humerus is a flat, robust, paddle-shaped bone, as long as the femur and slightly bent posteriorly. The proximodistal length, from the articular head to the distal end, is 150 mm, and the maximum anteroposterior width of the humerus, located in the distal region, is 72 mm. Consequently, its maximum proximodistal length is about twice its maximum anteroposterior width. The maximum anteroposterior width at the proximal end is 40 mm. The maximum dorsoventral height occurs at the proximal articular head and is about 40 mm. This thickness is influenced by the dorsal expansion of the tuberosity of the humerus. The minimum dorsoventral height of the humerus is 15 mm, and it occurs approximately at the midlength of the bone. The maximum dorsoventral height in the distal region is 20 mm. For the complete list of measurements see SOM 2: table S2.

In dorsoventral view, the humerus is slightly asymmetrical with a postaxial expansion in the distal region that makes the posterior (postaxial) outline concave while the anterior (preaxial) border remains straight without any preaxial expansion, a primitive character found in Liassic plesiosaurs ( Storrs 1997; Bardet et al. 1999b; O’Keefe 2001). This asymmetry differs from the morphology observed in the femur, which is almost symmetric with equal anteroposterior expansions in the distal region. The postaxial expansion of the distal region begins at approximately half of the total length of the humerus, orienting the articular facet of the ulna posterolaterally, while the facet of the radius lies along the axis of the humerus shaft and is oriented more laterally. The angle between both articular facets is approximately 120°. With the exception of the postaxial expansion, the anterior and posterior margins of the shaft are straight and parallel, maintaining a constant anteroposterior width of about 4 cm along the rest of the shaft. Consequently, there is also no distinctive postaxial process, only a smooth convexity in the postaxial margin near the proximal articulation. In dorsal view, the articular distal margin of the humerus is curved and convex, without any marked inflexion point between the facets for the radius and ulna. However, in ventral view, the facet of the radius is straighter while the facet of the ulna is markedly convex, therefore they can be slightly differentiable.

In distal view, the distal articular margin is groove-shaped for the ulna and the radius insertion, and although there is a small step that marks the boundary between these two articular facets, there is no observable extra facet for accessory ossicles. The presence of a groove-shaped insertion surface for the radius and the ulna could indicate maturity in the specimen. With the exception of the fine longitudinal striations running along the humerus, no grooves or crests are observed on the dorsal, ventral, anterior or posterior surfaces of the humerus. In ventral view, rather than a groove, there is a proximodistally short and anteroposteriorly long shallow depression between the two epipodial facets. A series of slightly more pronounced longitudinal rugosities are present around the first quarter of the proximal region of the shaft, and more abundant in the ventral region. In anteroposterior view, the proximal region of the shaft is dorsally expanded, while the ventral region remains almost straight along the entire body of the humerus. This expansion begins approximately in the middle region of the shaft and gradually increases proximally, producing the slightly concave dorsal contour of the humerus. This elevated surface on the proximal articular region coincides with the presence of the tuberosity of the humerus, however, the proximal articular face is slightly eroded. Thus, it is difficult to make inferences about this structure, or others such as the capitulum. Despite this erosion, in proximal view, an ovoidal cross-section of the proximal articulation can be inferred, and is slightly wider in the ventral facet (where the capitulum is located) and somewhat narrower in the dorsal area, where the humeral tuberosity is placed.

Radius: Only one epipodial bone has been preserved and has been identified as the right radius ( Fig. 4C ). As the specimen was not fully articulated, we cannot completely rule out that this bone is a tibia, since the tibia and the radius are usually very similar in shape and dimension. However, several reasons support its identification as a radius: although not in its exact anatomical position, the bone was found in situ next to the humerus ( Fig. 2D ); the bone fits and articulates perfectly with the radial facet of the humerus; and, usually, tibiae are somewhat more robust and anteroposteriorly wider with the distal and proximal margins having a similar anteroposterior development. With the exception of a small break notching into the preaxial margin, the bone is complete and well preserved. The radius is a flat, robust bone with a straight preaxial margin and concave postaxial border. It is longer than it is wide, with a maximum proximodistal length of 50 mm and a maximum anteroposterior width of 37 mm in the proximal area that articulates with the humerus. The distal margin that articulates with the radiale and the intermedium is narrower with an anteroposterior width of 26 mm. The bone is also quite flat, with a maximum dorsoventral height of 12 mm in the proximal and distal regions. In anteroposterior view, the cross-section of the radius has a slight hourglass shape, with a minimum dorsoventral height of 7 mm in the center and greater expansions, as mentioned, in the articular regions. For the complete list of measurements see SOM 2: table S2.

Both proximal and distal articulation facets are mostly concave in dorsoventral view. The anteroproximal and anterodistal corners of the radius are rounded without any prominent anterior flange. In dorsoventral view, there is a posterior expansion in the posteroproximal region of the radius that produces a concave postaxial margin. As a consequence of this concave margin, the presence of a proximodistally elliptic epipodial foramen between the radius and ulna can be inferred. Because the postaxial margin does not have an extremely marked concavity, the epipodial foramen would be slightly shorter than the epipodials. In proximal view, the outline of the articulation facet with the humerus is elliptical and anteroposteriorly elongated, with a flatter ventral margin and a wedged anterior margin. In distal view, the outline of the distal articulation facet is anteroposteriorly shorter than the proximal and it is ovoidal.

Pelvic girdle: All the bones from the right half of the pelvic girdle ( Fig. 4H ), including the right pubis ( Fig. 4F ), ilium ( Fig. 4G ), and ischium ( Fig. 4E ), have been preserved with little breakage or erosion. The surface of the bones is slightly striated. The general shape and dimensions of the pelvic girdle are here calculated by making a mirror image of the right side to establish bilateral symmetry. Therefore, the union of both pubes and ischia creates a heart-shaped contour in dorsoventral view. The total width of the complete pelvis has been estimated at 184 mm (distance between the lateral margins) and the total anteroposterior length is 188 mm (distance between the anterior-most margin of the pubis and the posterior tip of the medial blade of the ischium). The pelvic fenestration (also called puboischiatic fenestra by Dalla Vecchia (2017), Storrs (1997); or thyroid fenestra by Evans (2012) is elliptical and wider lateromedially (40 mm) than anteroposteriorly long (34 mm). All the articular surfaces of the pelvic girdle are slightly concave, well defined, pitted and irregular, indicating the presence of cartilage in life. While the lateral articulation between the pubis and the ischium is mediolaterally thick, its contact medial to the pelvic fenestra is thinner but seems to be present. Consequently, the pelvic bar would be complete. The laterally directed edges of the medial symphysis of the pubis, the posteromedial process of the pubis, and the anteromedial process of the ischium indicate that an anteroposteriorly elongated rhombic gap was located in the center of the pelvic bar.

Pubis: The right pubis ( Fig. 4F ) is a nearly complete flat plate of bone, having lost only a small fragment of the anterolateral margin, which still remains embedded in the rock matrix. The pubis is approximately as long anteroposteriorly as it is wide mediolaterally. It is a maximum of 94 mm in anteroposterior length (from the anterior-most margin to the posterior ischial facet), 92 mm in maximum mediolateral width (the distance between the medial and lateral margins without counting the anterolateral cornu), and about 8 mm in dorsoventral height (measured from the central region of the pubis). In lateromedial view, the cross-section of the pubis is flatter on the ventral surface and more convex on the dorsal surface. Both dorsal and ventral surfaces are covered by faint radial striations which are more apparent closer to the margins. For the complete list of measurements see SOM 2: table S3.

In dorsoventral view, the outline of the pubis, without counting the posterior concavity of the pelvic fenestra, exhibits a roughly hexagonal shape. The contour of the anterior half of the pubis consists of two distinct anteriorly convex margins divided by a notch. This notch is located on the anterolateral margin of the pubis, thus the medial anterior convexity is larger and more anteriorly placed than the lateral one. The presence of an anterior notch generally indicates greater ontogenetic development of the individual in contrast to the semi-lunate and rounded pubes observed in juvenile plesiosaurs ( Welles 1943; Brown 1981; Bardet et al. 2008a). The anterolateral edge seems rounded, although it is also slightly damaged and a small portion of bone remains in the rock matrix. However, the damaged part is minimal, so it can be inferred that the anterolateral cornu of the pubis was rounded and poorly developed. In dorsoventral view, the lateral margin of the pubis is straight and anteroposteriorly aligned. Posterior to this lateral margin is the acetabular articulation, which slightly faces lateroposteriorly. The angle between the lateral edge and the acetabular margin is about 155°, and the angle between the acetabular margin and the articular ischial facet is about 120°. In lateral view, the lateral edge is very thin dorsoventrally until it significantly expands, coinciding with the acetabular facet of the pubis, which is triangular in outline and tapered anteriorly. In dorsoventral view, the medial margin of the pubis is straight and faces medioposteriorly. Most of the medial edge corresponds with the pubic symphysis and the posterior-most end corresponds with the posteromedial process of the pubis, which is curved posterolaterally and pointied posteromedially. In medial view, the pubic symphysis is flat, rugose, lenticular in cross-section and tapered posteriorly until it reaches the posteromedial process of the pubis. This lenticular cross-section has its maximum convexity along the dorsal surface, while the ventral surface is almost straight. In dorsoventral view, the posterior edge of the pubis is strongly concave, forming a deep semi-circular notch that corresponds with the anterior margin of the pelvic foramen. The articular ischial facet of the pubis is straight and faces posteriorly, and is located lateral to the pelvic foramen. The pubis lacks any kind of articulation with the ilium. The posteromedial process of the pubis is located medial to the pelvic foramen. This process is thinner and more rounded than the articular ischial facet, and is laterally curved and tapered, with the tip pointing in a posterolateral direction. In posterior view, the margin of the pelvic foramen and the posteromedial process of the pubis are very thin dorsoventrally, while the articular ischial facet is dorsoventrally expanded with an elliptical cross-section.

Ischium: The right ischium is seemingly complete ( Fig. 4E ), although for reasons of maintaining fossil stability, the ventral surface remains unprepared. In dorsoventral view, the ischium presents the typical hatchet-shaped outline present in plesiosaurians. Most of the ischial body, with the exception of the articular head, is dorsoventrally flat and plate-like. It has a very similar anteroposterior length and lateromedial width to the pubis, but it is slightly anteroposteriorly shorter. Therefore, the ischium is short, approximately as long as it is wide, with a total length of 90 mm (from the tip of the anteromedial process to the posterior tip of the medial blade) and a width of 83 mm (from the medial edge to the lateral margin of the iliac articular head). For the complete list of measurements see SOM 2: table S3.

As a consequence of the pelvic fenestration, the anterior margin of the ischium is deeply concave and semi-circular. This anteriorly concave edge has the same shape and dimension as the complementary concavity observed on the posterior margin of the pubis. The articular head of the ischium is massive (maximum length⁄width is 39⁄ 23 mm) and is located lateral to the pelvic fenestra. In dorsoventral view, the articular head is laterally convex, semi-circular and bears three articular facets. In lateral view, the articular surface, including the three facets, is rough and elliptical in cross-section. The anterior facet corresponds to the articulation with the pubis. In dorsoventral view, this facet is mediolaterally wide and straight, and in anterior view is dorsoventrally thick with a triangular outline that tapers medially. Posteriorly, forming an angle of 130° with the pubic articular facet, is a lateral facet that corresponds to the acetabulum and the articulation with the femur. This lateral facet has a subcircular outline and is bigger and dorsoventrally thicker than the other two facets. The posterior-most facet of the ischial head corresponds with the iliac articulation facet. This articulation has a triangular outline that tapers posteromedially, is longer than the two anterior facets, and faces posterolaterally.

The articular head of the ischium transitions to the rest of the ischium via a constriction, which forms a lateromedially-directed shaft or neck. The shaft is relatively slender, lenticular in cross-section, and has an anteroposterior length of 24 mm and a dorsoventral height of 8 mm at its narrowest section. Medially, this shaft expands anteroposteriorly to form the anteromedial process of the ischium and the medial ischial blade. The posterior expansion is the longest and widest and corresponds to the medial ischial blade. The lateral margin of this blade is oriented posteromedially, while the medial margin, where the ischial symphysis is located, is straight anteroposteriorly. A portion of the medial margin has a flat and rough articular surface, indicating the presence of cartilage in the symphyseal region for attachment of the left ischium. The ischial symphysis and the anterior margin of the ischial neck form an angle of 90°. The posterior margin of the blade is lateromedially wide and rounded. In contrast, the anteromedial expansion of the ischium begins medial to the pelvic foramen, is anteroposteriorly shorter, lateromedially narrower and tapers gradually to a thinner, rounded tip. This anteromedial process corresponds to the connection with the pubis to form a complete pelvic bar.

Ilium: The right ilium is complete ( Fig. 4G ), with almost no signs of erosion. Only a small portion of the anterior and posterior margins of the iliac blade are partially worn (although not significantly), which does not affect the general shape. The ilium is an elongated bone with anteroposteriorly expanded proximal and distal ends. When it is in articulation with the ischium, the ilium is directed posterodorsally. The total proximodistal length of the ilium, from the acetabular facet to the distal end of the iliac blade, is 78 mm. The maximum anteroposterior expansion of the proximal region, from the anterior margin of the acetabular facet to the posterior margin of the ischial facet, is 34 mm, with a maximum proximodistal length of 17 mm. The lateromedial height of the shaft is 12 mm and its maximum anteroposterior width is 13 mm, giving the shaft a subcircular cross-section. The maximum anteroposterior expansion of the iliac blade is 25 mm. The iliac blade is transversally compressed with a lateromedial width of 8 mm. For the complete list of measurements see SOM 2: table S3.

The proximal end of the ilium is the thickest region of the bone and bears a posteroventral ischial facet and a ventral facet for the acetabulum. The surface of the ischial facet is flat and rough, and the acetabular facet, although partially covered by matrix, seems to be concave. In proximal view, both facets together have a teardrop-shaped outline, with the acetabular facet lateromedially thicker than the ischial facet, a flat lateral margin, and a medial margin more convex and rounded. The ischial facet is almost twice as long anteroposteriorly (30 mm) than the acetabulum facet (17 mm), and the angle formed by both articular facets is about 100°. The pelvic or proximal articular end is expanded anteriorly and posteriorly. While the posterior expansion is directly related with the extension of the ischial facet, the anterior expansion is related to the participation of both the ischial facet and the acetabular facet. As a consequence of the latter, the anterior expansion is slightly larger. The shaft of the ilium is relatively straight and smooth, without any ridges, tubercles, or knees. The anteroposterior expansion of the iliac blade is restricted to the dorsal one-third of ilium. Although these anterior and posterior expansions can be considered sub-equal, the posterior expansion is slightly larger. In lateromedial view, the outline of the posterior expansion is concave, the anterior expansion is more rounded and convex, and the dorsal/distal margin is rounded and convex. This anteroposterior expansion is approximately double the minimum width of the shaft. In distal view, the iliac blade has a lenticular cross-section. The expansions of the iliac blade and the proximal articular end are not oriented in the same plane, and are rotated about 45° from each other. Both lateral and medial faces of the iliac blade are flat, without any signs of a facet for a sacral rib. A series of very fine proximodistal striations can be observed on both the iliac blade and the proximal end.

Hindlimb: femur: Only the right femur is preserved Fig. 4A ), but it is complete and exhibits very little signs of erosion. The femur is a straight, long, and paddle-shaped bone with expanded proximal and distal ends. Its maximum proximodistal length (130 mm) is twice its maximum anteroposterior width (65 mm). The maximum dorsoventral height occurs at the proximal articular head and is about 37 mm. This thickness is influenced by the dorsal expansion of the trochanter and the ventral expansion of the capitulum. The maximum anteroposterior width at the proximal end is 34 mm. From the maximum proximal expansion, the shaft progressively constricts distally, until reaching an anteroposterior width and a dorsoventral height that are more or less constant until the distal expansion. The minimum anteroposterior width of the shaft is 28 mm, and occurs about 33 mm distal to the proximal end of the femur. The minimum height of the shaft is 13 mm, and occurs approximately at the midlength of the femur. The dorsoventral height of the distal end is 65 mm, and in this region the dorsoventral height is 18 mm. For the complete list of measurements see SOM 2: table S3.

The proximal articular head is strongly dorsoventrally expanded and slightly anteroposteriorly expanded. This articular end is composed of the capitulum and the trochanter. There is no well-developed or distinct separation between the capitulum and the trochanter. This feature, along with other previously mentioned characters related to the pubis and vertebrae, indicate that the specimen could be a young adult. The capitulum consists of a large hemispherical, convex, and pitted articulation that covers most of the proximal and ventral surfaces of the proximal end of the femur. Along the posterodorsal surface, the capitulum is separated from the trochanter by a concave depression. The dorsal trochanter exhibits as an elevation that begins and rises at the dorsal surface of the shaft until it reaches its maximum elevation at the articular proximal end of the femur. The articular surface of the trochanter is located more distally than that of the capitulum and is half its width. This surface is slightly concave, rugose and subcircular in outline. The entire surface of the shaft surrounding the proximal articular end is covered by small, irregular and proximodistally-oriented striae. Although the femur is a fairly symmetrical bone, in dorsoventral view, the posterior margin is slightly more curved and proximodistally concave due to the posterior expansion of the capitulum. In contrast, the anterior margin remains straight until the expansion of the distal end of the femur begins. The shaft (without counting the proximal and distal expansions) is straight, dorsoventrally compressed, with nearly-parallel margins, and elliptical in cross-section. Most of the surface of the shaft is smooth, however, on the proximal half of the ventral side there is a surface roughness corresponding to the adductor musculature insertions. Within this course region is a protuberance that connects with an irregular ridge, which is directed proximodistally. This ridge may correspond with the trochanteric crest, which marks the insertion of the M. puboischiofemoralis externus ( Bardet et al. 1999b). Posterior to the crest, on the ventral surface, there is an associated depression. In dorsoventral view, the shaft of the femur flares distally in a fanshaped anteroposterior expansion. Although this flared distal portion is quite symmetrical, the posterior expansion is slightly larger. This portion of the femur is also covered with fan-like longitudinal striations for muscle attachment. The flared distal end bears the tibial and fibular facets, although the boundary between them is not clear. The articular distal end is rugose and lenticular in cross-section.

Phalanges: Only two phalanges are associated with ML2302. The larger phalanx is derived from a more proximal position on the paddle, while the smaller phalange is from a more distal position. Little distortion or weathering is apparent on either element. The larger phalanx ( Fig. 4D ) measures 27 mm proximodistally, the width (measured along the anteroposterior axes on the articular facets of the phalanges) of the proximal end is 14 mm, and the width of the distal end is 11 mm. The smaller phalanx measures 25 mm proximodistally, with a width of 11 mm along the proximal facet and 11 mm for the distal facet. The phalanges are dorsoventrally compressed and constricted along the shaft. The distal ends of both are flatter, while the proximal ends are convex. No striae are apparent on the surface of the phalanges. The distal and proximal facets are rugose.

Remarks.—The 17 dorsal vertebrae span the mostly complete dorsal series and provide data on the morphological changes of the dorsal vertebrae along the vertebral column. The fully fused neural arches on the centra, as well as the articular facets on the humerus, femur, ischium, and pubis suggest a sub-adult to adult life stage for ML2302 (e.g., Brown 1981).

Geographic and stratigraphic range.— Type locality and horizon only.

Figures

Fig. 3. Axial bones of the plesisaur Plesiopharos moelensis gen. et sp. nov. (ML2302) from the Sinemurian (Lower Jurassic) of São Pedro de Moel (Leiria, Portugal). A. Last cervical vertebra (Fig. 2A: 1). B. First pectoral vertebra (Fig. 2A: 2). C. Second pectoral vertebra (Fig. 2A: 3). D. Third pectoral vertebra Fig.2A:4). E. Fourth pectoral vertebra (Fig.2A: 5). F. Dorsal vertebra (Fig.2A: 9). G. Last dorsal vertebra (Fig.2A: 22). In anterior, lateral, posterior, dorsal, and ventral views, respectively.H.Anterior or posterior dorsal rib in proximal (in H2), dorsal, anteroposterior, and ventral views, respectively. I. Middle dorsal rib in anteroposterior view. J, K. Lateral gastralia in anteroposterior and dorsoventral views, and cross section (in J2), respectively. Photographs A1–C1, D, E1–K1) and explanatory drawings (A2–C2, E2–K2); drawings by SM.Abbreviations: c, centrum; fs, foramina subcentralia; nc, neural canal; ncs, neurocentral suture; ns, neural spine; poz, postzygapophyses; prz, prezygapophyses; rf, rib facet; tp, transverse process; vmr, ventromedial ridge.

Fig. 2. Bone mapping and reconstruction of the skeleton of Plesiopharos moelensis gen. et sp. nov. (ML2302) from the Sinemurian (Lower Jurassic) of São Pedro de Moel (Leiria, Portugal). A. Bones distribution map of the three original extracted blocks (1, 2 and 3) containing ML2302.Arrows with question marks represent that the joining relationship between the blocks is unknown. B. Photographs of the five sub-blocks (1A, 1B, 2C, 2D, and 3E) after preparation works. C. Skeleton reconstruction with the preserved bones of ML2302. D. Archival photographs of how blocks 2 and 3 were found before their preparation. Drawings by SM.

Fig. 4. Limb bones and pelvic girdle of the plesiosaur Plesiopharos moelensis gen. et sp. nov. (ML2302) from the Sinemurian (Lower Jurassic) of São Pedro de Moel (Leiria, Portugal). Right femur (A), humerus (B), and radius (C), in proximal, dorsal, anterior, ventral, posterior, and distal views, respectively. D. Phalanx in dorsal, anteroporterior, and ventral views, respectively. Right ischium (E) and pubis (F), in ventral and dorsal views, respectively. G. Right ilium in distal, proximal, dorsal, posterior, ventral, and anterior views, respectively. The dashed lines in G2 highlight the curvature of the iliac blade. Photographs (A1–G1) and explanatory drawings (A2–G2). H. Anatomical reconstruction of the right side and mirrored left side of the pelvic girdle in dorsal view. Drawings by SM. Abbreviations: ace, acetabulum; adr, adductor rugosity; apis, anteromedial process of the ischium; cap, capitulum; cor, anterolateral cornu; epf, epipodial foramen; fif, fibular facet; gr, groove; huf, humeral facet; il, ilium; ilbl, iliac blade; ilf, iliac articular facet; ilsh, iliac shaft; is, ischium; isf, ischial articular facet; issh, ischial shaft; is sym, ischial symphysis; mebl, medial blade of the ischium; no, notch; pb, pelvic bar; pppu, posteromedial process of the pubis; pu, pubis; puf, pubic articular facet; pu sym, pubic symphysis; pvf, pelvic fenestra; raf, radial facet; radf, radiale facet; rug, rugosity; tif, tibial facet; tr, trochanter; trc, trochanteric crest; tub, tuberosity; ulf, ulnar facet.