Plesiosaurus dolichodeirus Conybeare, 1824

Vincent, Peggy & Taquet, Philippe, 2010, A plesiosaur specimen from the Lias of Lyme Regis: the second ever discovered plesiosaur by Mary Anning, Geodiversitas 32 (3), pp. 377-390 : 381-388

Plesiosaurus dolichodeirus Conybeare, 1824

MATERIAL EXAMINED. — Skeleton MNHN A. C. 8592.

GEOGRAPHICAL OCCURENCE. — Lyme Regis, Charmouth coastal region, Dorset, England.

STRATIGRAPHICAL OCCURRENCE. — No information is available for the stratigraphical occurence of the specimen MNHN A. C. 8592, but it is possibly the same as for other Plesiosaurus dolichodeirus specimens, that is uppermost Sinemurian (Lower Lias) Echioceras raricostatum zone, Black Ven Marl Formation ( Storrs 1997).

DESCRIPTION

Preservation and size ( Fig. 2 ; Table 1)

The specimen is preserved in one block of original matrix and exposes its ventrolateral aspect. It is articulated and comprises the almost complete postcranial skeleton. In its general aspect, the position of the specimen indicates that the skeleton was little displaced after the death of the animal. The vertebral column includes 56 vertebrae from the tail to the base of the neck. Most of the anterior part of the neck is lost. Of the pectoral girdle, only the dorsal aspects of the scapulae and the distal parts of the coracoids are visible (the main part is not preserved or is buried into the matrix). The pelvic one is partially preserved and includes the two ilia and pubes. The right forelimb and hindlimb are almost completely preserved, approximately in life position; the left ones are only partially preserved.

Vertebral column ( Figs 2 ; 3 ; Table 1)

Fifty-six vertebrae are preserved, including 8 basal cervicals, 4 pectorals, 17 dorsals and 27 caudals. No sacrals are preserved. Most of the vertebrae are preserved in lateral aspect. The articular facet of the centra is generally not visible with exception of the posterior articular facet of the anteriormost preserved cervical vertebra (C1), which is partially observable, and the anterior articular facet of one dorsal vertebra (D17) and two caudal vertebrae (CA8 and CA9).

Cervical vertebrae ( Figs 2 ; 3A, B ; Table 1)

With the exception of the two anterior-most preserved vertebrae, which are seen from the left, all cervicals present the right lateral side ( Fig. 3A ). It is diffi cult to envisage a taphonomical mechanism that may explain such an unusual position. It is therefore most likely that the two anterior-most cervicals were inaccurately repositioned during preparation of the specimen. Th e articular surface of the cervical centra can be partially observed thanks to the anterior-most cervical C1. It is a rounded oval in outline, gently concave and is surrounded by a sharply-defined, rounded border. A small pit is situated in the center of the articular facet, and likely marks the original position of the notochord.

The centrum of the anterior-most preserved cervical vertebrae is approximately as long as it is high (see Table 1). Th ough their precise height cannot be measured with the same degree of confidence, the observation of the lateral aspect of the other cervicals suggest that the centrum of these latter are also as long as or slightly longer than they are high (see Table 1).

The lateral face of the centra beneath the bases of the neural arch are almost flat. Only one half of the ventral surfaces of the cervical centra are observable. The ventral surfaces are concave in longitudinal direction and probably transversally too. No sign of a presence of a ventral longitudinal keel is observable. The prezygapophyses are oval in longitudinal direction and orientated almost vertically. Both the pre- and postzygapophyses project beyond the level of the articular surfaces of the centrum, the prezygapophyses more than the postzygapophyses. Only the anterior-most cervical (C1) preserves the entire neural arch, which is higher than long (height/ length ratio ≈ 2.5) and gently inclined posteriorly. The neural spine is sub-rectangular in outline with a rounded dorsal extremity. The remaining cervical vertebrae preserve only fragmentary neural arches. The cervical ribs are double-headed, however, this is not evident for the anterior-most cervical vetebrae (C1, C2 and C3), where the articular facets for the ribs are very closely spaced. Th e following cervical vertebrae present a distinct longitudinal groove between the rib facets, showing distinct tuberculum and capitulum. Towards the base of the neck, the rib facets are progressively located higher on the centra, and thus the width of the grooves between the double-headed rib facets increase. None of the cervical ribs is preserved.

Pectoral vertebrae ( Figs 2 ; 3C ; Table 1)

Four pectoral vertebrae are present, which represents the common number in plesiosaurs ( Brown 1981). Th ey are morphologically intermediate between cervicals and dorsals. Th e centra are sub-circular in cross section. Th e large oval rib facets are carried by both the centra and the neural arch.

Dorsal vertebrae ( Figs 2 ; 3D ; Table 1)

Seventeen dorsal vertebrae are preserved, which represent the standard count for plesiosaurs ( Brown 1981). However, Storrs (1997) indicated 21 dorsal vertebrae for P. dolichodeirus . In MNHN A. C. 8592, the absence of the sacrals indicate that the posterior part of the trunk is incomplete ( Fig. 2 ); it is therefore impossible to unambiguously estimate the original dorsal count in the specimen. Most of the dorsal vertebrae are preserved in right lateral view with the exception of the D17, which is exposed in anterior view. The dorsal centra are more rounded than those of the cervical vertebrae. Th e articular facets for the ribs are located on the neural arches, well rounded in outline and slightly concave. Th e ventral surface of the centra is smooth and evenly convex transversally. Th e nutritive foramina are well spaced and located on the lateral surfaces of the centra. Most of the neural arches of the dorsal vertebrae are obscured by overlying dorsal ribs or not preserved. Th e only observable neural arch is that of D17, which is slightly broken and shows its ante- rior margin. It consists of a narrow neural spine, which appears about 10% shorter than the neural spine of the anterior-most cervical vertebra (C1). Several dorsal ribs are preserved. Th ey are singleheaded, thick, and curved, with a flat and enlarged articular head. Th e dorsal ribs, being enlarged in their medial part, are probably pachyostotic s.s., a conclusion also reached by Storrs (1997) for the genus Plesiosaurus .

Caudal vertebrae ( Figs 2 ; 3E, F ; Table 1)

Twenty seven caudal vertebrae are preserved. Storrs (1997) count 28 caudal vertebrae for the species Plesiosaurus dolichodeirus , suggesting that this portion of the vertebral column is probably sub-complete in MNHN A. C. 8592. Th e caudal centra are quadrateshaped in anterior view and have almost platycoelous articular surfaces. All neural arches and spines of the caudals are only partially preserved, except for the caudal CA5. Most of the caudal vertebrae are preserved in lateral view with the exception of CA8 and CA9 (see Table 1 for mesurements). The caudal centra are wider than long and high. They possess well-developed rib facets, which are more or less pentangular in outline and are located on the centra. Towards the back of the neck, the rib facets are located more ventrally on the centra. The size of the centra decreases rapidly towards the posterior part of tail. Towards the posterior part of the tail, the preserved parts of the neural spines are increasingly inclined posteriorly. Haemal arch facets are located on both anteroventral and posteroventral edges of the centra, the posterior ones being generally larger than the anterior ones. Zygapophyses were probably present, but are not preserved on the specimen.

Girdles ( Figs 2 ; 4 ; Table 1)

The pectoral girlde of MNHN A. C. 8592 is only partially visible, possibly because some of its elements were not preserved or buried in the matrix. The left scapula and coracoid are almost complete and are observable in ventral view ( Fig. 4A ). The right portion of the preserved pectoral girdle and articulated humerus of the specimen MNHN A. C. 8592 was figured by Hulke (1883), based on its cast (BGS GSM 118412).

Scapula ( Figs 2 ; 4A )

The scapula is partially preserved, the dorsal ramus is almost totally missing or hidden by matrix. A small plate of unidentified bone near the glenoid fossa may represent a fragment of the dorsal ramus of the scapula. A strong ridge separates the ventral surface of the scapula from its dorsal ramus. The posterior ramus bears a posterolaterally-orientated facet that forms the anterior part of the glenoid fossa. The scapula contribution to the glenoid is less extensive than the coracoid one. Given the general outline of the left scapula, there was probably no middle contact between the two scapulae, as proposed by Storrs (1997) for its reconstruction of the pectoral girdle of P. dolichodeirus .

Coracoid ( Figs 2 ; 4A )

The coracoid is a large and broad plate of bone. Its medial margin is nearly straight. The posterior margin of the coracoid is irregular, suggesting that its posterior part is probably not complete and may thus have been originally more expanded posteriorly. Its lateral margin is concave. Anteriorly, it contacts the posterior ramus of the scapula. Its glenoid contribution is anterolaterally orientated. The medial suture between the coracoid and the anterior ramus of the scapula is marked medially by a notch and run posterolaterally to the pectoral fenestra. Th e glenoid fossa is shared by the scapula and coracoid and is oval in outline and concave.

Pectoral fenestration ( Figs 2 ; 4A )

The pectoral fenestra is elliptical, and diagonally orientated, with well-rounded margins. It is laterally formed by the scapula and posteromedially by the coracoid.

Ilium ( Figs 2 ; 4B, C )

Both ilia are preserved in ventral view, the distal portion of the right one being partially hidden by an indeterminate metatarsus. Th e ilium is expanded at both extremities, whereby the sacral extremity is more expanded than the acetabular. It is twisted and has a constricted shaft. The dorsal margin of the ilium, corresponding to the surface for the sacral ribs, is relatively broad. The

is.f

cr.i

anterior facet is reduced whereas the posterior one is well expanded and slightly concave. The acetabular end articulating with the pelvic plate is rod-like.

Pubis ( Figs 2 ; 4D, E )

Both pubic bones are partially preserved in ventral view. They are large and nearly pentagonal. Only the left one ( Fig. 4D ) presents a deep semicircular notch on its posterior border, forming the anterior margin of the pelvic aperture. Nevertheless, Cuvier (1825) figured a notch at this level in his original drawing ( Fig. 1 ), indicating that a piece of bone has been subsequently added on the mediodistal border of the right pubis. The anteromedial margins of the pubis is sligthly concave and its anterior edge is convex.

Forelimbs ( Figs 2 ; 5 ; Table 2),

humerus ( Figs 2 ; 5A, B )

Both humeri are well preserved. Th e left one is observable in ventral view and the right one in dorsal view. Th e humerus is markedly asymmetrical with its anterior margin being nearly straight, and its posterior margin being concave due to the distinctive enlargement of its posterodistal part. The proximal portion of the humerus is almost flat and bears rugosities on its ventral surface, which correspond to scars of the adductor muscles. Th e humeral head is surrounded by a lipped edge with tuberosities. A channel of finished periosteal bone between these tuberosities and the articular face of the glenohumeral condyle divides the proximal articular area into two. The deltopectoral crest, marking the insertion of the M. coracobrachialis, is more strongly developed than the dorsally situated tricipital crest, which marks the insertion of the M. subcoracoscapularis, M. subscapularis and M. subcoracoideus. A very pronounced projection is present near the proximal end and runs axialy down the posterior border of the humeral shaft. Sato et al. (2003) interpreted this projection as the attachment site for M. latissimus dorsi and/or other muscles that pull the humerus backward and upward for Bishanopliosaurus youngi author, date.

On the dorsal and ventral surfaces of the distal end of the humerus, there are extensive areas of shallow ornamentation. Th e distal end of the humerus possesses two well-developed epipodial facets, an anterior one for the radius and a posterior one for the ulna. Th e radial facet is nearly straight and the facet for the ulna is sligthly concave.

Radius and ulna ( Figs 2 ; 5A, B )

The left radius and ulna are preserved in ventral view, whereas the right radius and the right ulna are preserved in dorsal view. Th e left and right radii as well as the left ulna are complete; the right ulna preserves only its proximal half.

Both the radius and ulna are longer (proximally to distally) than broad (preaxially to postaxially). The proximal margin of the radius is nearly straight, while the distal margin is convex in outline and bears a large articular facet for the radial and a smaller one for the intermedium. The preaxial and postaxial margins of the radius are both concave, in a way that the radius has an hourglass outline.

The ulna is lunate, with a posteriorly convex margin. It has a small and sligthly convex proximal articular facet for the humerus and two distinct distal articular facets for the intermedium and ulnare. Both the postaxial margin of the radius and the preaxial margin of the ulna are concave. Together they enclose a narrow spatium interosseum.

Carpals, metacarpals and phalanges ( Figs 2 ; 5A, B ) The proximal rows of left carpals preserve (from the postaxial to the preaxial margin of the limb) the intermedium and the ulnare. On the right forelimb, only the intermedium (recognized by comparison with the shape and size of the proximal rows of left carpals) is preserved. Th e other elements of the right forelimb are missing.

The distal row of left carpals is only represented by one bone element, which probably corresponds to the distal carpal I according to its position with respect to other elements.

The metacarpals are phalangiform. The distalmost end of the paddle are missing. Th e digits one to four preserve three phalanges. The phalanges are hourglass-shaped.

Hindlimbs ( Figs 2 ; 5 ; Table 2),

femur ( Figs 2 ; 5C, D )

Both femora are preserved in ventral aspect. The femur is shorter and more slender than the humerus (see Table 1). The difference between propodial proportions is generally considered as an important taxonomic character ( Welles 1943; Brown 1981), when considered in adult forms. The proportion of propodial of MNHN A. C. 8592 is in accordance with the assumption correlating long-neck taxa with relatively longer humeri. In ventral view, the femur expands nearly symmetrically from its head to its distal part, its preaxial border being slightly more expanded than the postaxial one in the distal region. Th e head of the femur is moderately convex and rugose, which indicates the presence of a thick cap of cartilage in life. The trochanter is located along the ventral margin of the femoral shaft and projects distodorsally to the capitulum. The trochanter, which marks the insertion of the M. puboischiofemoralis externus, is separated from the head by a strip of finished periosteal bone, which divides the usually single cartilaginous area into two. In outline, the distal articular surface is convex, with shallow epipodial facets for the tibia and fibula.

Tibia and fibula ( Figs 2 ; 5C, D )

Both tibia are preserved in ventral view. In general shape and proportions, the tibia mirrors the radius. The tibia is longer than broad and presents a preaxially to postaxially constricted shaft in the medial region. It possesses a straight proximal margin and a slightly convex distal one.

Only the right fibula is preserved. Th e left one is depicted on the engraving of the specimen by Cuvier (1825) but is now lost (see Fig. 1 ). The preaxial edge of the fibula is deeply concave. Distally, the fibula bears two straight and distinct facets for the astragalus and calcaneum.

Tarsals, metatarsals and phalanges ( Figs 2 ; 5C ) Two rounded elements are preserved on the right hindlimb. One of them contacts the fibula into a straight facet and could therefore correspond to the calcaneum. Th e other one is hardly identifiable; considering its size, it could correspond to a metatarsal. One tarsal element of the left hindlimb is preserved. Given its position and general shape, it is diffi cult to identify it with confidence. Nevertheless, two tarsal elements were drawn by Cuvier (1825) on the left hindlimb on the original engraving of the specimen (see Fig. 1 ). Neither of these is positioned as the element currently present in the specimen, indicating that the latter as been displaced and that the other one has been lost. The remaining element, given its shape and original position on the figure of Cuvier (1825), probably corresponds to the calcaneum ( Fig. 1 ). Accordingly, the lost element could correspond to the astragalus. Another tarsal element is preserved and partially overlaps the left ilium ( Fig. 4B ). It is unfortunately impossible to assign it to one of the hindlimbs.

All metatarsals are missing. Judging from engraving of the specimen of Cuvier (1825), three left metatarsals were originally preserved.

Only one phalanx is preserved in the right hindlimb and nine phalanges for the left. Th ey are hourglass-shaped.Nevertheless, eight have been lost since Cuvier realised the drawing of the specimen.

COMPARISON

Specimen MNHN A. C. 8592 has been attribuated to P.dolichodeirus by Storrs (1997). The reconstruction of the pectoral girdle of Plesiosaurus dolichodeirus is partially based on MNHN A. C. 8592 ( Hulke 1883; Storrs 1997). According to the diagnosis proposed by Storrs (1997) for genus and species of P. dolichodeirus, MNHN A. C. 8592 and this species share: paired rib facets on the cervical vertebrae; a coracoid of moderate breadth; a humerus with prominent shaft curvature and marked posterodistal (postaxial) expansion but weak anterodistal (preaxial) corner; robust, pillar-like anterior epipodials offset to extend distally beyond posterior epipodials; broad crescentic posterior epipodials; a convex anterior margin of the pubis; an ilium with a little twist to shaft; and the forelimbs slightly longer than hindlimbs. Plesiosaurus dolichodeirus is known by 25 partial to sub-complete specimens. Th e recent revision and comprehensive description of the species by Storrs (1997) allow some detailed comparison between MNHN A. C. 8592 and other specimens of Plesiosaurus dolichodeirus .

The species Plesiosaurus dolichodeirus usually shows a centrum slightly longer than high ( Storrs 1997), as observed in MNHN A. C. 8592. Storrs (1997) reported 41 cervical vertebrae in BMNH 22656 (holotype), 42 cervical vertebrae in OXFUM J.10304 and 38 or 39 cervical vertebrae in BNMH 1313. He also noted the presence of 4 or 5 pectorals, about 21 dorsals, 3 sacrals and 28 caudals for BNMH 22656. A broadly similar number of pectoral, dorsal and caudal vertebrae is present in MNHN A. C. 8592. Because MNHN A. C. 8592 preserves 8 cervicals, 4 pectorals, 17 dorsals and 27 caudals, it can be estimated that 38 to 42 vertebrae are lacking in MNHN A. C. 8592 by comparison with others specimen of Plesiosaurus dolichodeirus , between 30 and 34 of them being cervical vertebrae. Additionally, Storrs (1997) describes the sacral end of the ilium as flattened in Plesiosaurus dolichodeirus , a condition also observed in MNHN A. C. 8592.

There are some notable differences between the specimen MNHN A. C. 8592 and the other specimens referred to Plesiosaurus dolichodeirus . The proximal portion of the humerus of MNHN A. C. 8592 bears a prominent projection, which is absent in other specimens of Plesiosaurus dolichodeirus ( Storrs 1997: 173) . Th e humeral projection observed in MNHN A. C. 8592 is unique among Plesiosauroidea, but has been reported in several Pliosauroidea. A similar humeral projection is observed in the same position in Bishanopliosaurus youngi Dong, 1980 ( Sato et al. 2003), Liopleurodon ferox Sauvage, 1873 ( Andrews 1913) and Eurycleidus arcatus (Owen, 1840) ( Andrews 1922) , and at the midway of the posterior edge of the shaft in Simolestes vorax Andrews, 1909 ( Andrews 1913) .

Size ratios of bone elements between BMNH 22656 and MNHN A. C. 8592 are also observable: in BMNH 22656 the humerus and femur are sligthly shorter but the caudal series is longer than in MNHN A. C. 8592, suggesting that MNHN A. C. 8592 could have been shorter than BMNH 22656 but more massive.

Figures

FIG. 2. — Skeleton of Plesiosaurus dolichodeirus Conybeare,1824 (MNHN A.C. 8592) in ventrolateral view,Lias,Lyme Regis,England. Abbreviations: C1, cervical vertebra number 1; CA1, caudal vertebra number 1; D1, dorsal vertebra number 1; D16, dorsal vertebra number 16; D17, dorsal vertebra number 17; P1, pectoral vertebra number 1. Scale bar: 50 cm.

FIG. 3. — Plesiosaurus dolichodeirus Conybeare, 1824 (MNHN A. C. 8592), Lias, Lyme Regis, England: A, cervical vertebra C1 in left lateral view; B, cervical vertebrae C4 and C5 in right lateral view; C, pectoral vertebrae P1 and P2 in right lateral view; D, dorsal vertebra D17 in anterior view; E, caudal vertebra CA9 in anterior view; F, caudal vertebrae CA3, CA4 and CA5 in right lateral view. Scale bar: 5 cm.

FIG. 4. — Plesiosaurus dolichodeirus Conybeare, 1824 (MNHN A.C.8592),Lias,Lyme Regis,England:A, left scapula and coracoid in ventral view, the dotted line represents the hypothetical limit between the scapula and the coracoid;B, left ilium in ventral view; C, right ilium in ventral view; D, left pubis in ventral view; E, right pubis in ventral view, the dotted line represents the hypothetical limit of the pelvic foramen. Abbreviations: c, coracoid; cr.i, crista ilii; dp, dorsal process of the scapula;gf, glenoid surface;is.f, facet for ischium. Scale bar: 10 cm.

FIG. 1. — Skeleton of Plesiosaurus dolichodeirus Conybeare,1824 (MNHN A.C.8592) in ventrolateral view,Lias, Lyme Regis, England. Drawing realized by Cuvier (1825), from the third edition of his “Discours sur les révolutions de la surface du globe”. Head and neck are from another specimen (see text).

FIG. 5. — Plesiosaurus dolichodeirus Conybeare, 1824 (MNHN A.C.8592),Lias, Lyme Regis,England:A, right forelimb in dorsal view; B, left forelimb in ventral view; C, right hindlimb in ventral view;D, left hindlimb in ventral view.Abbreviations:cal, calcaneum; dc I, distal carpal I; fem, femur; fib, fibula; h, humerus; il, ilium; im, intermedium; ph, phalanx; rad, radius; tib, tibia; ue, unlnare; ul, ulna. Scale bar: 10 cm.