Antarctopelta oliveroi, Salgado & Gasparini, 2006

Salgado, Leonardo & Gasparini, Zulma, 2006, Reappraisal of an ankylosaurian dinosaur from the Upper Cretaceous of James Ross Island (Antarctica), Geodiversitas 28 (1), pp. 119-135 : 119-135

Antarctopelta oliveroi n. sp. (Figs 2-9)

See also Gasparini et al. 1987: figs 1, 2; 1996: figs 1-5; Olivero et al. 1991: fig. 2; Ricqlès et al. 2001: fig. 2.

HOLOTYPE. — MLP 86 -X-28-1: partial skeleton consisting of a fragment of left dentary with an in situ tooth, three isolated teeth, a collection of fragmentary cranial ossifications, two cervical vertebrae and a latex cast prepared from a natural mould of three articulated cervical vertebrae, about eight fragments of dorsal ribs, two dorsal centra representing part of the presacral rod, a partial sacrum composed of three sacral centra, eight incomplete caudal vertebrae, the proximal (glenoid) portion of the left scapula, a fragment of the right ilium, a distal fragment of a left femur, five metapodials, two phalanges, and a collection of six different morphotypes of osteoderms.

Salgado L. & Gasparini Z.

TYPE LOCALITY AND HORIZON. — Santa Marta Cove, North James Ross Island ( Antarctica), locality D6-1 ( Gasparini et al. 1987; Olivero et al. 1991) (Fig. 1).

ETYMOLOGY. — Species named for Eduardo Olivero, an outstanding Argentine geologist and paleontologist specializing in Antarctica, who discovered the holotype.

STRATIGRAPHICAL PROVENANCE. — Lower part of the Gamma Member of the Santa Marta Formation (Marambio Group) (upper Campanian) ( Olivero et al. 1991; Olivero 1992).

DIAGNOSIS. — Medium-sized ankylosaur, estimated length of no more than 4 m; cervical centra short (centrum length about 70% of the centrum height), morphologically most similar to Silvisaurus Eaton, 1960 , with the anterior articular faces higher than the posterior ones, as in Edmontonia rugosidens Gilmore, 1930 ; anterior caudal vertebrae with relatively slender transverse processes; centra of the posteriormost caudals notably dorsoventrally depressed, with articular faces slightly anteriorly inclined and laterally expanded, transverse processes of the posterior caudals well developed (transverse processes length about 40% of the centrum width), dorsoventrally depressed, and positioned within the anterior half of the vertebral centrum; at least six morphotypes of postcranial osteoderms, including 1-narrow and spine-shaped, 2- ovoid plate-like with a rugose surface texture, 3-plate-like with a smooth surface texture, 4-polygonal with a rugose texture, 5-shield-shaped with a dorsal keel, and 6-small (less than 5 mm in diameter) button-like.

DESCRIPTION AND COMPARISONS

Teeth

The teeth of Antarctopelta oliveroi n. gen., n. sp. are leaf-like, with mesial and distal marginal denticles and a large apical denticle, slightly curved caudally (Fig. 2). The teeth have an asymmetrical cingulum (contra Gasparini et al. 1987; Olivero et al. 1991), similar to many nodosaurids ( Coombs & Maryaňska 1990). Of the four preserved teeth (designated I, II, III and IV), one remains in situ within the left dentary (tooth III). Tooth IV has lost most of its crown (only two denticles remain) but has a complete root.

A new ankylosaurian dinosaur from Antarctica

All four teeth demonstrate minor morphological differences. Tooth I (Fig. 2A, B; see also Gasparini et al. 1996: fig. 1C, D) has eight mesial denticles, a central apical denticle and five distal denticles (8+1+5).

Along the mesial margin of teeth I and II, the most basal denticle is partially enveloped by the cingulum (Fig. 2A, B, denticle 1 and Fig. 2C, D, denticle 1).

Tooth II (Fig. 2C, D; see also Gasparini et al. 1996: fig. 1A, B), the smallest of the preserved crowns, has a denticle formula of (7+1+5). In lingual view (Fig. 2C), there are vertical striations continuous across the crown and cingulum. In labial view (Fig. 2D), the crown of this tooth is relatively smooth, although thin vertical striations are present along the dorsal rim of the cingulum.

Coombs (1990) has observed that larger teeth tend to have more denticles, which could explain the existence of an additional denticle in tooth I.

Tooth III has minute denticles (three have been preserved on the anterior margin, Fig. 2E, F). Lingually (Fig. 2F), the cingulum is not as pronounced as in the other teeth. However, this sector is placed clearly below the area of the cingulum that belongs to the labial side.

Although very worn and incomplete, tooth IV does have a basal denticle partially ensnared by the cingulum. The root is well preserved and virtually complete.

Fourteen or more denticles have previously been reported for nodosaurid ankylosaurs (e.g., Galton [1980] reported a tooth from the Purbeck of England with 17 [8+1+8], and a large number of denticles is reported in replacement teeth of Sarcolestes leedsi Lydekker, 1893 [ Galton 1983]). A high denticle count is also known for some ankylosaurids (e.g., more than 14 in Euoplocephalus tutus Lambe, 1902 [Coombs 1990]; probably 14 in Ankylosaurus magniventris Brown, 1908 [Coombs 1990] and “polacanthids” [at least 16 in Gastonia burgei Kirkland, 1998 ]). Coombs (1990), however, cautions against the taxonomic significance of denticle count.

Lower jaw

The preserved portion of the lower jaw of Antarctopelta oliveroi n. gen., n. sp. (Fig. 2E) was described and figured by Gasparini et al. (1996: see fig. 1I, J). It consists of a mid-portion of a left dentary with nine tooth positions oriented longitudinally. In dorsal view, most of these alveoli resemble figure eights, suggesting that replacement teeth were converging on the erupted series. As characteristic for ankylosaurs, the tooth row is slightly curved in dorsal view.

Within the alveolar position fifth from the anterior end is a worn in situ tooth (tooth III, see above). On the lingual surface of the dentary is a shallow Meckelian canal (sulcus) (Fig. 2E) opposite to which, on the labial side, are at least four foramina subparallel to the alveolar border.

Skull ossifications

Gasparini et al. (1987) and Olivero et al. (1991) had referred to the existence of cranial elements, similar to the supraorbital projections of some ankylosaurs. Gasparini et al. (1996) countered that at least some of these elements may represent postcranial osteoderms. This reappraisal supports the original hypothesis with various fragments of this material interpreted as skull elements.

One of these ossifications (see Olivero et al. 1991: fig. 2C) (Fig. 3A, B) is a two-surfaced piece with one of its external surfaces, which is interpreted as latero-ventrally oriented, slightly concave, and another one, slightly convex, which is presumed to be latero-dorsally oriented (Fig. 3A). This convex surface is rugose in the medial part. The two surfaces form a sort of ridge (Fig. 3A, B, r). In our opinion, this bone is probably a supraorbital, with the concave ventral surface representing the dorsal surface of the orbital cavity. Unfortunately, the partial ossification impedes the recognition of articular surfaces through which their position in the skull may be accurately known.

Other two elements (Fig. 3C) also have two external surfaces, scarcely ornamented, and slightly convex, which form a dihedral angle of 45°, and an acute edge, strongly curved in lateral view. The curved flange is pointed at one of the ends, above which there is a protuberance (Fig. 3C). These elements are interpreted as quadratojugals or supraorbitals, similar to those of Edmontonia rugosidens ( Carpenter 1990) .

Another element, incompletely preserved, is rather flat with a relatively smooth surface (Fig. 3D). Toward one of its extremes (here interpreted as lateral), the bone thickens and ends in a protuberance. This bone is interpreted here as part of the skull roof, probably a right parietal.

Cervical vertebrae

Five cervical vertebrae are preserved, three of which are represented by a latex cast prepared from a natural mould by technicians of the Museo de La Plata (see Gasparini et al. 1996: fig. 2C) (Fig. 4A, B). Comparison with more complete material (e.g., Silvisaurus condrayi Eaton, 1960 ) suggests that these vertebrae represent the middle components of the cervical series ( Eaton 1960).

All the cervical centra are proportionally short, amphicoelus, and are transversely broader than anteroposteriorly long (Fig. 4). The cervical centra become progressively proportionally shorter, being the last of the cervical centra preserved proportionally shorter than in Ankylosaurus magniventris ( Coombs & Maryaňska 1990) ( Antarctopelta n. gen. centrum length/height ratio = 0,57, Ankylosaurus Brown, 1908 centrum length/height ratio = 0,78). This represents a notable difference with other ankylosaurs with relatively elongate cervical centra, such as Stegopelta landerensis Williston, 1905 (centrum length/height ratio = 1,14) ( Carpenter & Kirkland 1998), or Struthiosaurus austriacus Bunzel, 1871 (centrum length/height ratio = 1,35) (Pereda-Suberbiola & Galton 2001).

The posterior articular surface of the cervical vertebrae of Antarctopelta oliveroi n. gen., n. sp. is

A new ankylosaurian dinosaur from Antarctica lower than the anterior surface, as in Edmontonia rugosidens Gilmore, 1930 . The neural pedicels are relatively short, and more similar to those of Edmontonia Sternberg, 1928 , and unlike those of Panoplosaurus mirus Lambe, 1919 , which are relatively long ( Carpenter 1990). The neural canal is virtually circular in cross section. The parapophyses are situated immediately below the neural arch at approximately the same level in all the preserved cervicals of Antarctopelta oliveroi n. gen., n. sp. The

Salgado L. & Gasparini Z.

posteriormost cervical preserved is virtually complete (Fig. 4E-G). Compared to it, the prezygapophyses of the other cervical vertebrae are more widely spaced (see Gasparini et al. 1996: fig. 2A-C). In addition, the transverse processes of the anteriormost preserved cervical vertebrae are somewhat longer (Fig. 4A), although the degree of inclination of the transverse processes is constant throughout the series.

Synsacral vertebrae

The only two dorsal vertebrae preserved are those appearing to be incorporated into the presacral rod of the synsacrum (Fig. 5A, B). They were described by Gasparini et al. (1987, 1996: fig. 2F, G). The articular surface of the anterior vertebra is wider than high (Fig. 5B), and shows no traces of fusion with another vertebra, which suggests that it is the first vertebra of the presacral rod.

Two virtually complete sacral centra and the left portion of a third represent a portion of sacrum (Fig. 5C, D). In addition to the centra, these elements include the base of the neural pedicles and the proximal end (head) of the sacral ribs. All three sacral vertebrae are firmly fused together.

Similar to Silvisaurus , the ventral surface of the sacrum is notably flat, lacking the groove or paired ridges seen in other nodosaurids ( Carpenter & Kirkland 1998). In posterior view (Fig. 5C), the sacral centrum is dorsoventrally depressed. Indeed, the dorsoventral depression that characterizes the sacral vertebrae is also present in the dorsal vertebrae of the presacral rod, with a centrum width almost twice the centrum height (see Gasparini et al. 1996: fig. 2F, G, I, J). Among the sacral vertebrae, the width of the centra ranges from nearly twice the height anteriorly to more than three times the centrum height. The proximalmost portions of the sacral ribs are fused to each of the three sacral vertebrae with the ribs of the second and third centra being dorsoventrally deeper and more robust than the first.

Rib fragments

About eight fragments of rib exist that range in cross sectional morphology from “T” to “L”. One large and several smaller osteoderms have been preserved on one of these fragments. The shieldshaped osteoderms lie on the dorsal surface of the rib at only one of its ends (Fig. 9F). The ossicles instead, are distributed on the middle part of the dorsal surface ( Gasparini et al. 1987: lam. I3).

Caudal vertebrae

Eight fragmentary caudal vertebrae are preserved (Fig. 6). Comparison with other taxa (e.g., Sauropelta Ostrom, 1970 , Struthiosaurus Bunzel, 1871 ) suggests that four of them are relatively proximal in the series and four are relatively distal. The anteriormost is represented only by the ventral portion of the centrum. Ventrally, this surface is concave with pronounced facet for a chevron at the posteriormost edge. Another anterior caudal vertebra was illustrated in anterior view by Gasparini et al. (1996: fig. 2H). As oriented, the centrum appears to be triangular. However, this vertebra is incomplete, and only represents the right half of the centrum (Fig. 6A-E). On the dorsal surface of this vertebra, the base of the right neural pedicle can be seen. The anterior caudal vertebrae of Antarctopelta n. gen. are relatively wide (e.g., wider than in Struthiosaurus ; see Pereda-Suberbiola & Galton 2001: fig. 3), and would have had a heart-shaped anterior articular surface, although they are morphologically similar to those typical of various nodosaurids (e.g., Edmontonia , Niobrarasaurus coleii Mehl, 1936 , Sauropelta edwardsorum Ostrom, 1970 , Silvisaurus and possibly Struthiosaurus sp. (Pereda-Suberbiola & Galton 2001; see Pereda-Suberbiola 1999: 278, fig. 3E-G). In posterior view, the outline of the articular face of the centrum was apparently subcircular (Fig. 6D). The elliptically shaped base of the slender transverse process of this vertebra, is also preserved (Fig. 6E).

There is also a series of four more distally positioned caudal centra that have not previously been described. The neural arch is not preserved in any of these vertebrae. In relation to the anteriormost vertebrae, each centrum is elongated and dorsoventrally depressed (Fig. 6F-H) and resembles that described for some ankylosaurids ( Coombs 1978). The anteriormost of the distal caudal centra (Fig. 6F-H) is wider (55 mm) than long (50 mm); two others are somewhat longer (50 mm) than wide (45 and 48 mm). Although there is a noticeable decrease in width, all the vertebrae appear to have similar lengths. Unlike Struthiosaurus (see Pereda-Suberbiola & Galton 2001: fig. 3) and Sauropelta ( Carpenter, 1984) , these distal caudal vertebrae are more than 40% longer than the anterior caudals, where the posterior caudals are approximately equal in length to the anterior. This latter may be a plesiomorphic condition for the Thyreophora, since it is also present in stegosaurs ( Galton 1990).

In dorsal view (Fig. 6F), the posterolateral surfaces of the centra are concave anterior to which are the flat, horizontally oriented transverse processes. Mid-series and distalmost caudals of Struthiosaurus also have well developed transverse processes (Pereda-Suberbiola & Galton 2001), a feature that is otherwise uncommon amongst nodosaurids (e.g., Sauropelta and Nodosaurus textilis Marsh, 1889 ) (Pereda-Suberbiola & Galton 2001). The ventral surface of the distal vertebrae of Antarctopelta n. gen. is slightly concave and is crossed by in situ ossified tendons, similar to ankylosaurids.

A new ankylosaurian dinosaur from Antarctica

In lateral view, the distal caudals resemble a parallelogram with its articular surfaces offset from the long axis of the centrum. In contrast, nodosaurids such as Struthiosaurus (see Pereda-Suberbiola & Galton 2001: fig. 3) and Sauropelta (see Coombs & Maryaňska 1990: fig. 8C), have distal caudal vertebrae that are rectangular in profile.

Ossified tendons

These structures have been found associated with the caudal vertebral remains, arranged parallel to the long axis of the body. Numerous ossified tendons were preserved on both sides of an isolated neural spine. Other tendons arrange longitudinally on the ventral surface of the distal caudal vertebrae.

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Appendicular skeleton

A distal fragment of a left femur is here reported for the first time (Fig. 7A). It includes part of the medial condyle and the intercondylar furrow. The articular surface of the medial condyle is rugose, especially along its posterior surface. Reconstruction of the element based on similar material from Salitral Moreno, northwestern Patagonia ( Coria & Salgado 2001) yields a total femur length of approximately 30 cm.

The holotype of Antarctopelta n. gen. also includes five partial metapodials (Fig. 7B, C). These elements are relatively massive, with broad, distally expanded articular surfaces (see Gasparini et al. 1996: fig. 4A, B) (Fig. 7C). A proximal end of a right metapodial (probably the metatarsal IV) has recently been collected (Fig. 7B). The dorsal surface of this bone is slightly convex whereas the ventral and lateral surfaces are concave. The size of the metatarsals is consistent with the estimated size of the femur.

Two phalanges are preserved. One is cuboid, albeit slightly asymmetrical (see Gasparini et al. 1996: fig. 4C) (Fig. 7E); the other is wider and disc-like (see Gasparini et al. 1996: fig. 4D) (Fig. 7D). Gasparini et al. (1996) suggested that the cuboid one belonged to digit I (of hand or pes). The disc-like phalanx was interpreted by these authors as the second phalanx of digit II or III (of hand or pes). In dorsal view, the outline of the proximal end of the second phalanx of digit II of the pes of the juvenile Euoplocephalus Lambe, 1910 is slightly convex (see Coombs 1986: fig. 4A). The morphology of the disc-like phalanx of Antarctopelta n. gen. matches best with the first phalanx of digit IV in that ankylosaur; the size of this phalanx does not differ significantly from the transverse width of the distal end of the preserved metatarsal. The other phalanx is morphologically similar, although somewhat shorter, to the first phalanx of digit I of the manus of Edmontonia rugosidens (see Carpenter 1990: fig. 21.17 E, F).

A portion of the scapula (the glenoid region) is preserved, and shows no signs of being fused to the coracoid (see Gasparini et al. 1996: fig. 3A) (Fig. 8A). Other nodosaurids (e.g., Edmontonia ) and ankylosaurids (e.g., Euoplocephalus ) also show a similar feature ( Carpenter 1990; Penkalski 2001). The lack of fusion between coracoid and scapula in Antarctopelta n. gen. probably results from immaturity (see Discussion). The acromion process is missing.

A fragment of the right ilium, probably the midportion of the preacetabular process, is preserved (see Gasparini et al. 1996: fig. 3B) (Fig. 8B).

Osteoderms

There are six different morphotypes of osteoderms thus far recognized for Antarctopelta n. gen. (Fig. 9), including spine-like forms, plate-like forms and small button-like ossicles.

One fragment is interpreted as a base of spine ( Olivero et al. 1991: fig. 2d). Internally, on the fracture surface, it can be seen spongy tissue. Externally this element is very ornamented, especially on the border between ventral and lateral surfaces. This spine (described in Gasparini et al. 1987, and illustrated in Olivero et al. 1991: fig. 2d) was first interpreted as a tail club, and then by Gasparini et al. (1996) as a type of osteoderm (their “ type b” osteoderm). According to Gasparini et al. (1987), the supposed tail club had a convex external surface ornamented with ossicles, and a concave internal surface. Gasparini et al. (1996) described the osteoderms as “large, rough-surfaced plates [with the] dorsal surface […] sculptured with numerous small pits and rugosities. The ventral face is irregular and hollowed” ( Gasparini et al. 1996: 588). In this review the “internal surface” of the club ( Gasparini et al. 1987), or the “ventral surface” of the osteoderm ( Gasparini et al. 1996) is reconsidered as the internal surface of an eroded, probably hollow, spine. The external (anterior?) surface of the base of the spine actually belongs to what Gasparini et al. (1996) interpreted as the dorsal surface of their “ type b” osteoderm.

Ovoid elements, but with a straight side, flat, slightly curve in lateral view (Fig. 9A, B). Their dorsal surface has no keel, flange or prominence, but is externally ornamented with a rugose surface texture. Foramina, when present, occur only on the ventral surface. The position of these osteoderms is doubtful. It most closely resembles the lateral elements of the second cervical ring of Edmontonia rugosidens ( Carpenter 1990) . However, unlike these elements, the osteoderms of Antarctopelta n. gen. are more convex anteroposteriorly than transversely.

Osteoderms with sub-circular dorsal profile, large and flat, and smooth external surface (Fig. 9C, D). They belong to “ type c” of Gasparini et al. (1996: 588, fig. 5A, B). The margins are crenulated and demonstrate a few, scattered foramina. According to Gasparini et al. (1996), these osteoderms resemble those of the sacral region of Sauropelta , albeit without the central prominence.

Osteoderms relatively small and polygonal in dorsal view which were originally considered comparable with osteoderms of the previous type. However, these osteoderms differ in being smaller, with a rugose surface and relatively more foramina (Fig. 9C, E). They belong to “ type c” of Gasparini et al. (1996: 588, fig. 5C, D) and “ type D” of Coombs & Deméré (1996). These osteoderms may interdigitate along their edges with other osteoderms (Fig. 9C).

Although often intimately associated, the subcircular and polygonal osteoderms remain indi- vidually distinct (Fig. 9C). The mosaic of these elements is similar to the pattern noted for Sauropelta ( Carpenter 1984) . Probably the plates of these two types of osteoderms belong to what Blows (2001) identifies as “bosses” (elements incorporated to the pelvic-shield). These osteoderms may have contributed to a continuous armor, probably situated across the sacral area.

Shield-shaped osteoderms, with a smooth dorsal keel (Fig. 9F). These osteoderms include “ type d” osteoderms of Gasparini et al. (1996), and resemble those of “ type C” of Coombs & Deméré (1996) and the elements referred to as “scutes” by Blows (2001). These are ornamented and pierced by numerous foramina. Some have their ventral surfaces excavated whereas others have not. They are recognized by having both dorsolateral surfaces slightly concave. There is some variation with respect to the inclination of the keel. In the smallest osteoderms of this type, the keel is relatively short and the flanks less concave than in the biggest ones. Some of these osteoderms were preserved resting on ribs (Fig. 9F). Presumably, these osteoderms were arranged in one or more rows on the parasagittal surfaces of the body.

Small, ovoid to sub-rectangular osteoderms are often referred to as ossicles (e.g., Blows 2001; Ricqlès et al. 2001). They belong to “ type e” of Gasparini et al. (1996), and to the “ossicles” of Blows (2001). These ossicles are smaller than those of most ankylosaurs (for example, Minmi Molnar, 1980 : Molnar 1996, 2001). Numerous parallel and/or perpendicular fibrous-like elevations ornament their surfaces. The histological organization of these osteoderms has been studied by Ricqlès et al. (2001). The ossicles arrange on the external surface of the dorsal ribs. In one case they are associated with a large, flat osteoderm occupying mostly the middle part of the rib surface. In another case, the ossicles are larger occupying the entire surface of the rib.