Cervical and pectoral vertebrae

The Angolan specimen MGUAN PA248 (Fig. 3) has an anterior cervical centrum with the neural arch, but a comparable anterior centrum is seen in the New Zealand specimen MONZ R1526 (Fig. 6, Appendix 2). The cervical centrum can only be distinguished from the pectoral centrum due to the relative position of the rib facet, higher on the pectoral (see Sachs et al., Reference Sachs, Kear and Everhart2013). Although all centra present a shallow depression in the centre of the articular facets, the centra are effectively platycoelous. The vertebrae are wider than high and higher than long (W > H > L). The lateral sides are shallow and concave, and the ventral sides are flat. There is no evidence of a lateral keel. In anterior and posterior views, a shallow ventral and dorsal notch on the articular facets is reminiscent of the binocular-shaped vertebrae of other elasmosaurids (e.g. Kaiwhekea). The ellipsoidal foramina subcentralia are separated by a thick rounded ridge. The dorsal sides of the vertebrae have a concave medial region (which formed the floor of the neural canal) comprising about one-third of the dorsal area. This area is perforated by two ellipsoidal foramina that are separated by a sharp ridge. Laterally, the articular surface for the neural arch forms a crater-like D-shaped region, which is bounded by distinct edges. The rib facets are also single crater-like structures formed by ellipsoidal depressions with the long axis parallel to the anteroposterior axis of the centra. Two neural arches are preserved; one is missing one pedicle and the apex of the neural spine and the other comprises only the neural spine. The pedicle is thick at the base, becomes very thin in the middle and then expands to meet the neural spine. In ventral view, the articular surface with the centrum is roughened and oval. In the posterior part of the pedicle, at the junction with the neural spine, there is a small foramen. The prezygapophyses are proportionally small relative to the centra and form a rounded cup with no separation between the two. The prezygapophyses protrude at a high angle to the vertical plane (∼70°). The surface of the prezygapophyses is roughened. The dorsal edge of the prezygapophyses extends to the neural spine about one fifth its length in the form of a crest. The connection of the prezygapophyses to the neural spine is a smooth upwards curve. In anterior view, the base of the neural spine is pierced by two teardrop-shaped foramina. The postzygapophyses are in contact with each other, although at the intersection there is a shallow depression between them. The neural spine borders are straight and sub-parallel to each other. At mid-height, the anterior border is pierced by an elongated foramen, and the posterior border is excavated by an elongate slit.

Fig. 6.

MONZ R1526, previously referred to 'Plesiosaurus australis' by Hector (1874). A. Anterior cervical vertebral centrum in anterior (1), lateral (2), dorsal (3), posterior (4) and ventral (5) views. B. Dorsal vertebral centrum in dorsal (1), anterior or posterior (?) (2), ventral (3) and lateral (4) views. C. Pubis in ventral (1), acetabular (2) and dorsal (3) views. D. Mesopodial element: ventral or dorsal views (1 and 4), preaxial or postaxial view (2), proximal or distal views (3 and 5). E. Right ischium in posterior (1), medial (2), dorsal (3), acetabular (4) and anterior (5) views. F. Left ilium in anterior (1), medial (2) and posterior (3) views. G. Right femur in preaxial (1), distal (2), dorsal (3), proximal (4), postaxial (5) and ventral (6) views.

The cervicals preserved in MGUAN PA85 (Fig. 1B) represent a more posterior position than those of MONZ R1526 (Fig. 6A) and MGUAN PA248 (Fig. 3). The cervicals do not increase significantly in length posteriorly. Also, the vertebral width does not increase significantly relative to their length. The length of the centra is consistently less than is the height. The articular surfaces of the cervical centra are markedly elliptical with the short axis corresponding to the height. Cervical centra are concave laterally and slightly amphicoelous. The cervical centra have slightly concave articular facets in some vertebrae and do not present the binocular-shape as in other species (e.g. Kaiwhekea, Cruickshank & Fordyce, Reference Cruickshank and Fordyce2002). The vertebrae are from the posterior portion of the neck because they are flat (Sato, Reference Sato2005) and do not have a cylinder-like, gentle restriction on the adjacent vertebral facets (Carpenter, Reference Carpenter1996; Williston, Reference Williston1903). Furthermore, there are no lateral or ventral keels on the cervical vertebrae, indicative of a posterior position (Sato, Reference Sato2002). The articular facets for the ribs are sub-circular and single-headed. There is a well-defined protruding rim for the rib articulation. The pectoral centra can be identified by possessing functional rib facets transected by the neurocentral suture (Sachs et al., Reference Sachs, Kear and Everhart2013). In the pectoral centra, the areas of attachment for the neural spine form a lateral bump, and in dorsal view a D-shaped area of articulation (Fig. 1B). The area of attachment for the neural spine is not visible in the cervical centra. The foramina subcentralia are present in the cervical and pectoral vertebrae. These foramina are situated far apart from each other in the cervical centra. Some pectoral vertebrae bear a maximum of six foramina subcentralia. The widely separated foramina are shared by cryptocleidoids (O’Keefe & Street, Reference O’Keefe and Street2009), but are also visible in posterior cervical vertebrae of Dolichorhynchops osborni (Williston, Reference Williston1903). In addition to the ventral foramina, there are two elliptical dorsal foramina edged laterally and medially by two ridges; the median ridge only extends along the side of the foramina. Pedicles are thinnest halfway from the centrum to the neural spine. The neural spines are straight (i.e. not angled posteriorly), compressed and blade-like; as opposed to those seen in pliosaurids such as Macroplata (White, Reference White1940) or in the rhomaleosaurid Stratesaurus (Benson et al., Reference Benson, Evans and Druckenmiller2012). The apex of the neural spine is slightly rounded or flat and there is no swelling of the neural spine apex as in the elasmosaurid Futabasaurus (Sato et al., Reference Sato, Hasegawa and Manabe2006). The neural spine is slightly concave along the posterior border, producing a small space between neural spines for relative movement; the apex actually touches the adjacent neural spines. In the cervicals the prezygapophyses are conjoined yet distinct, but in the pectorals they are fully separated. The pre- and postzygapophyses are narrower than the centrum, and the zygapophyses are nearly horizontal. The zygapophyseal angle along the cervical vertebral series remains constant. The prezygapophysis dorsal surface is markedly concave whereas that of the postzygapophysis is flat. There is a slight ridge extending posteriorly from the prezygapophysis.

The dorsals preserved in MGUAN PA85 are in all respects very similar to those of MONZ R1526 (Fig. 6) and MML-PV5 (O’Gorman et al., Reference O’Gorman, Gasparini and Salgado2014). In MONZ R1526 (Fig. 6B) only the neural spines and the transverse processes are visible on the sequence of dorsal vertebrae (Fig. 6, central line drawing). In the dorsal vertebral centrum detached from the specimen block MGUAN PA85, as in MONZ R1526, the neural spines are short and thick lateromedially, bear straight or slightly concave anterior and posterior borders, and are well separated from the adjacent neural spines. The apex of the neural spines is smoothly convex in lateral view. The transverse processes arise from the anterior portion of the pedicles and are very stout and cylindrical; the rugose articular facet is flat or slightly convex. The dorsal centrum closely resembles that of MML-PV5 (Gasparini et al., Reference Gasparini, Salgado and Casadío2003; O’Gorman et al., Reference O’Gorman, Gasparini and Salgado2014) and GNS CD429 (Fig. 7C; Wiffen & Moisley, Reference Wiffen and Moisley1986); it is characteristically wide, giving an ellipsoidal shape to the sub-platycoelous anterior and posterior articular facets. The attachment area for the neural arch is sub-circular and expands slightly beyond the sides of the centrum. The neural canal carves a shallow gutter that is pierced by two anteroposteriorly elongated foramina, which in the preserved vertebrae are all in the same relative position in the centra.

Fig. 7.

GNS CD427–429, previously referred to 'juvenile' cf. Tuarangisaurus keyesi by Wiffen & Moisley (Reference Wiffen and Moisley1986). A. GNS CD428 coracoid and scapula in coracoid in medial (1), scapula in dorsal (2) and coracoid in lateral (3) views. B. GNS CD428 coracoid in lateral (1), anterior (2), ventral (3), medial (4) and dorsal (5) views. C. GNS CD427 dorsal vertebrae sequence in ventral (1), posterior (2), lateral (3) and dorsal (4) views. D. GNS CD427 paddle. E. GNS CD428 scapula in lateral (1), medial (2), anterior (3), dorsal (4) and glenoidal (5) views. F. GNS CD429 pubis in acetabular (1), dorsal (2) and anterior (3) views. G. GNS CD429 dorsal neural arch in ventral (1), anterior (2) and lateral (3) views. H. GNS CD429 ischium in anterior (1), glenoidal (2), ventral (3) and medial (4) views. I. GNS CD428 humerus in preaxial or postaxial (1), proximal (2), dorsal or ventral (3) and distal (4) views. J. GNS CD429 dorsal vertebral centrum in lateral (1), dorsal (2), anterior or posterior (?) (3) and ventral (4) views.

Ribs

In MGUAN PA85 (Fig. 1B) and MGUAN PA248, as well as in MONZ R1426 (Fig. 6, central line drawing), the dorsal ribs are gently curved and cylindrical. In MGUAN PA85 there are six ribs, which are most probably pectoral ribs due to the robustness and taphonomic location. However, there is an isolated dorsal rib overlying the vertebrae in the cervical-pectoral transition. Pectoral ribs have a capitulum and tuberculum, contrary to the dorsal ribs (see also Sachs et al., Reference Sachs, Kear and Everhart2013). The pectoral rib is sub-circular in cross-section, although a slight groove is present on the anterior and posterior sides. There is a sacral rib preserved in MGUAN PA120 (Fig. 2C). The medial heads of the sacral rib are heavily pitted and rugose, the lateral head is smooth. The medial head is roughly D-shaped, and it has two facets with two different orientations and the faint line that separates them is oriented anteroposteriorly. The dorsal border is slightly concave and the ventral border is more strongly concave. There are two processes: one distal on the ventral border and another anteriorly projecting. In dorsal view, the posterior border is sinusoidal and the anterior border is straight with the exception of the process.

Scapulae

Thus far a scapula has not yet been collected from the early Maastrichtian of Angola, but some material from New Zealand that is indistiguishable from Angolan material comprises overlapping material, and therefore we describe the New Zealand portions as a proxy for those not present in the Angolan material. A nearly complete scapula (AIM LH1521) has a broad base for the dorsal process and is very similar to GNS CD428, but the tip of the dorsal process is missing. The articular portion is slightly concave, highly rugose and pitted, bearing a coracoidal and glenoid facets. The glenoid in AIM LH1521 is slightly convex and without any articular facets, and differs from the condition of GNS CD428 (Fig. 7A and E), but may be due to ontogeny. The dorsal process has a broad straight base and tapers dorsally, being crushed dorsoventrally in GNS CD428 (Fig. 7A and E). The tip of the dorsal process of AIM LH1521 points slightly posteriorly. The median portion of the ventral plate of the scapula is missing. The preserved anteroposterior length of AIM LH1521 is 13.9 cm.

Coracoids

MGUAN PA250 (Fig. 5) preserves two coracoids with complete posterior borders, although the medial and lateral portions are partially damaged and compare well with GNS CD428 (Fig. 7A and E) and MML-PV5 (O’Gorman et al., Reference O’Gorman, Gasparini and Salgado2014). The coracoids are composed of the thickened and subrectangular glenoidal portion that expands into the posterior process. The anterior border is slightly concave. The glenoid border is straight and forms an angle with the lateral border of the posterior process, but there are no clearly defined glenoid and scapular facets. The posterior border is straight and nearly half the size of the anterior border. The intercoracoid vacuity is formed by a deep subcircular excavation of the medial border of the posterior process. The intercoracoid symphysis articulation is straight. The ventral buttress of the coracoid is small (∼1 cm) and semi-conical, arising from the midpoint of the intercoracoid symphysis (e.g. also known in Syxosaurus, Woolungasaurus or Wapuskanectes; Welles & Bump, Reference Welles and Bump1949; Sachs, Reference Sachs2004; Druckenmiller & Russell, Reference Druckenmiller and Russell2006). The dorsal and ventral surfaces of the coracoids are rugose, particularly near the posterior process, possibly as a result of muscle attachment. There is no evidence of the posterior cornua of the coracoids (i.e. lateral projection on the posterior border of the coracoids) or preglenoid processes (i.e. anterior projection on the medial intercoracoid symphysis). In MGUAN PA248 (Fig. 4F and G), the preserved coracoid portions comprise the intercoracoid symphysis. The ventral buttress is semicircular in medial view.

Humerus

The best preserved humeri are from MGUAN PA250 (Fig. 5A) and MGUAN PA248 (Fig. 4D), but very similar humeri are present in the New Zealand and Argentine collections (e.g. AIM LH1519, MML-Pv5). These elements are identified as humeri because in many plesiosaurs the humerus is typically more distally asymmetrical than the femur (Gasparini et al., Reference Gasparini, Salgado and Casadío2003). In these specimens the humerus possesses a well-marked postaxial angle between the articular facet and the postaxial edge. Additionally, the pre- and postaxial edges of the humerus tend to be more curved that in the femur (see also Gasparini et al., Reference Gasparini, Salgado and Casadío2003). The humeri in MGUAN PA250 and MGUAN PA248 are massive and short elements (W/L average ratio 0.76). The proximal epiphysis, although partially damaged in MGUAN PA250, is hemispherical and covered by small (0.2–0.5 cm) conical structures that occasionally anastomose with each other. These structures are probably vascular canals and not unfinished cancellous bone on the articular surfaces of the propodial. The distal end of the humerus is convex and there are no distinguishable epipodial facets for the radius and ulna. The distal end is oval in cross-section, with the dorsal border being slightly more convex than the ventral border. The dorsal and ventral sides of the humerus are heavily marked by serrations perpendicularly oriented to the proximodistal axis of the bone. These are apparently bite marks, probably caused by Squalicorax pristodontus due to the size, serration pattern and the presence of secondary lineations perpendicular to the main grooves (Schwimmer et al., Reference Schwimmer, Stewart and Williams1997; Shimada et al., Reference Shimada, Tsuihiji, Sato and Hasegawa2010). Squalicorax pristodontus is also the most common shark at the locality, represented by shed teeth associated with most amniote carcasses (Strganac et al., Reference Strganac, Jacobs, Polcyn, Mateus, Myers, Salminen, May, Araújo, Ferguson, Gonçalves, Morais, Schulp and Tavares2015a). The surface of the ventral and dorsal borders is striated on the distal end by shallow subparallel slits. The preaxial border is nearly straight whereas the postaxial border is concave, forming a distinct kink with the distal border. The whole extent of the preaxial border is covered with muscle scars. On the postaxial border there is a 1.5 cm foramen in the diaphysis but closer to the proximal end. In MGUAN PA550 (Fig. 8) the propodial element is very fragmentary; only the proximal portion is preserved and it is difficult to discern whether it is a femur or humerus. The head of the propodial is hemispherical, and, from what is preserved, is apparently a short, massive bone.

Fig. 8.

Aristonectinae indet. Bone histology of specimens MGUAN PA85 and MGUAN PA550, two isolated propodials. A–E. MGUAN PA85. A. The bone prior to osteohistological analysis. The areas from which the mid-diaphyseal and metaphyseo-diaphyseal sections were taken are marked with horizontal bars. B. Mid-diaphyseal section. C. Close-up of the area marked in B. Inset shows secondary osteons from the cortical region of the bone surrounded by remains of globular calcified cartilage. D. Metaphyseo-diaphyseal section. E. Close-up of the area marked in D. Inset shows secondary osteons from the cortical region of the bone surrounded by remains of globular calcified cartilage. F–H. MGUAN PA550. F. The bone prior to osteohistological analysis. The area from which the metaphyseo-diaphyseal section was taken is marked with a horizontal bar. G. Metaphyseo-diaphyseal section. H. Close-up of the area marked in G. Inset shows primary osteons from the cortical region of the bone surrounded by globular calcified cartilage. Scale bars represent 5 cm (A, F), 2 cm (B, D, G), 5 mm (C, E, H) and 500 µm (inset in C, E, H).

Autopodials

In both MGUAN PA120 (Fig. 2) and MGUAN PA248 (Fig. 3A, E and G), the autopodial elements are discoid and sub-circular with some degree of ellipsoidal eccentricity, thus belonging to the mesopodium or epipodium. They have a cap of periosteal bone on the dorsal and ventral sides bearing randomly placed foramina. On the remainder of the surface the trabecular structure of the bone is visible. The phalanx is hourglass-shaped; the proximal and distal borders are convex, and it is pierced by several irregularly located foramina. The proximal and distal facets are perforated by several equally-spaced conical structures.

Pubis

There is no pubis collected thus far from Angola, but the pubis in MONZ R1526 (Fig. 6C) is subcircular, flat and dorsoventrally thick as in MML-PV5 (Gasparini et al., Reference Gasparini, Salgado and Casadío2003; O’Gorman et al., Reference O’Gorman, Gasparini and Salgado2014) and GNS CD429 (Fig. 7F, Wiffen & Moisley, Reference Wiffen and Moisley1986). Likewise, the excavation for the pelvic fenestrum is gently concave. The ogival to ellipsoidal acetabular region is convex and slightly rugose. A minor anterior excavation forms the short constriction for the acetabulum. Although slightly damaged, the acetabular region is unfaceted.

Ischium

A single ischium from Angola was collected in MGUAN PA120 (Fig. 2B). It is comparable to MONZ R1526 (Fig. 6E), as in MML-PV5 (O’Gorman et al., Reference O’Gorman, Gasparini and Salgado2014), and GNS CD429 (Fig. 7H). The ischium is flat except on the thickened acetabular portion. The anterior and posterior borders are gently concave, whereas the medial border is convex. The shallow concavities of the pubic posterior border and the ischial anterior border form an ellipsoidal pelvic fenestra. Medially, the anterior angle is sharper than the rounded posterior angle of the ischium and bears several grooves in MGUAN PA120. The posterior projection of the ischium is short. The acetabular portion is oval and convex, bearing a rugose pattern, and anteriorly there is a small facet for the articulation of the pubis. The lateral head is ellipsoidal and fans laterally. The ischia have an anteromedial edge forming a right angle and the contour of the medial border is almost uniformly convex.

Ilium

No ilium was collected from Angola. In MONZ R1526 (Fig. 7F) and MML-PV5 (O’Gorman et al., Reference O’Gorman, Gasparini and Salgado2014) the ilium is conical but kinked with an elongated ovoid dorsal tip and hemispherical ventral articular surface. The dorsal articular surface is smoothly convex and the transition towards the more ventral portion of the ilium is smooth. The ventral articular shape is convex with a marked edge for the more dorsal portion of the ilium. The shape of the MONZ R1526 (Fig. 7F) ilium differs significantly from other Late Cretaceous elasmosaurid plesiosaur taxa (e.g. Thalassomedon, Terminonatator, Futabasaurus; Welles, Reference Welles1943; Sato, Reference Sato2003; Sato et al., Reference Sato, Hasegawa and Manabe2006) because of its simple conical but kinked shape. The ilia described here are not twisted.

Femur

The femora of MGUAN PA120 (Fig. 2A) and MGUAN PA85 overlap morphologically with MONZ R1526 (Fig. 6G), GNS CD427 (Fig. 7I) and MML-PV5 (O’Gorman et al., Reference O’Gorman, Gasparini and Salgado2014). The femur is robust, with an unfaceted, flattened distal epiphysis and the proximal end is sub-circular in articular view. The postaxial and preaxial borders are sub-parallel proximally and diverging distally, producing a slightly expanded epiphysis anteroposteriorly. The hemispheric proximal articular facet is faintly delineated. No trochanter is present. Both the proximal and distal epiphysis bear striated rugosities for muscular attachment that are proximodistally oriented, especially in MGUAN PA120 (Fig. 2A). In the distal epiphysis the striations cover about one fifth of the length of the femur. The striated zone can be divided into a proximal zone (mostly composed of pits that furrow proximally), an intermediate zone (composed of deep striations) and a distal zone (defined by the hints of either striae or pits). On the anterior and posterior sides there are extensive areas with muscle scars; one of the sides bears long sub-parallel grooves ridges. The proximal and distal epiphyses are intensely covered by small crater-like structures about 0.3 cm in diameter.

The elongation ratio (i.e. distal width divided by proximodistal length) is indicative of elasmosaurid affinities assuming isometric growth: 12.3 ÷ 17.7 = 0.69. The elongation ratio is 0.46 in the plesiosaurid Plesiosaurus dolichodeirus (Storrs, Reference Storrs1995), 0.46 in the rhomaleosaurid Rhomaleosaurus cramptoni (Smith, Reference Smith2007), 0.52 in the polycotylid Trinacromerum ?bentonianum (Albright et al., Reference Albright, Gillette and Titus2007), 0.53 in the polycotylid Eopolycotylus rankini (Albright et al., Reference Albright, Gillette and Titus2007), 0.60 in the elasmosaurid Terminonatator herschlensis (Sato, Reference Sato2003), 0.66 in the elasmosaurid Thalassomedon haningtoni (Welles, Reference Welles1943), 0.68 in the elasmosaurid Mauisaurus haasti (Hiller et al., Reference Hiller, Mannering, Jones and Cruickshank2005) and 0.63 in MGUAN PA85 (Fig. 1A). The femur is a massive bone lacking part of the proximal and distal epiphyses. The distal epiphysis does not have facets, the shaft is straight, the anterior and posterior facets are equally concave. The distal epiphysis seems to be ellipsoidal. Both the proximal and distal portions of the shaft have a high density of muscle scars.