Achillesaurus manazzonei, Martinelli & Vera, 2007

Martinelli, Agustín G. & Vera, Ezequiel I., 2007, Achillesaurus manazzonei, a new alvarezsaurid theropod (Dinosauria) from the Late Cretaceous Bajo de la Carpa Formation, Río Negro Province, Argentina, Zootaxa 1582 (1), pp. 1-17 : 3-12

Achillesaurus manazzonei gen. et. sp. nov.

Figures 2–4 , 6 , 8 , 10

Derivation of name. Manazzonei, in honor to Prof. Rafael Manazzone, an amateur paleontologist who provided valuable data about Patagonian fossil localities, and assisted to several paleontological field trips.

Holotype. MACN-PV-RN 1116, last sacral vertebra and the following two procoelous caudals with an articulated chevron, a partial proximal biconcave caudal (possibly the fourth), a portion of a distal caudal centrum, left ilium, proximal end of the left femur, distal end of the left tibia articulated with the astragalus, prox- imal portion of the left metatarsals II, III, and IV, and cast of the ilium and the vertebrae as originally found prior to disarticulation.

Occurrence. MACN-PV-RN 1116 was collected at Paso Córdova locality, Río Negro Province, Argentina ( Fig. 1 ). Bajo de la Carpa Formation, Río Colorado Subgroup, Neuquén Group; Santonian ( Bonaparte 1991; Leanza & Hugo 2001).

Diagnosis. Relatively large alvarezsaurid with the following autapomorphies: presence of a biconcave, possible fourth, caudal vertebra with the cranial surface 30 % larger in diameter than the caudal one. Achillesaurus differs from Alvarezsaurus calvoi in the presence of a lateral fossa in the proximal caudal centra, less developed supraacetabular crest, brevis shelf not reaching the base of the ischial pedicel, and lateral malleolus of the tibia at the same level of the medial one. Achillesaurus differs from Patagonykus and Mononykinae in the presence of an almost undeveloped supraacetabular crest, unfused astragalus and calcaneum, and from Mononykinae in the distally unreduced fibula and a non-arctometatarsalian pes.

Description and comparisons. Although none of the preserved elements of Achillesaurus manazzonei are totally complete, their preservation quality is rather good. The bones were found associated and partially articulated ( Fig. 2 ). For example, the left hind limb was originally placed in natural position in relation with the ilium ( Fig. 2 ). All the long bones (e.g. femur, tibia), originally hollow, are filled with matrix (sandstone).

Axial skeleton. All the vertebrae were found partially articulated ( Fig. 2 ). The neural arch is fused to the centrum in each vertebra without visible suture between both elements suggesting a sub-adult condition for MACN-PV-RN 1116.

Last sacral— This vertebra was found partially in contact with the neural arch of the first caudal ( Figs 3 a– b ). It preserves the centrum and an incomplete neural arch. The centrum slopes cranially, resulting in a ventral margin of the cranial articular facet more ventrally positioned than the caudal one. The lateral surfaces of the centrum are concave, but no distinct fossae are present. The cranial articular facet is gently concave and subtriangular in contour. Both lateral borders converge ventrally constituting a sharp ventral keel extended along the ventral surface ( Fig. 3 a ). This keel resembles the condition of the last sacral vertebra described for Alvarezsaurus ( Bonaparte 1991; Novas 1996). The caudal articular surface is slightly convex. This facet exhibits an oval-shaped contour in caudal view.

The base of the neural arch is proportionally very low, with the base of the transverse process close to the centrum. The right transverse process seems to be completely preserved, whereas only the base of the left one is available. The transverse process shows a square-shaped contour in dorsal view, and is laterally projected. The base of the right prezygapophysis is preserved, but the structure is too damaged and does not provide relevant data. The neural spine is square-shaped in lateral view and proportionally tall; in fact, its height is subequal to the height of the centrum.

A fragment of bone positioned cranially to this vertebra, is here identified as a portion of the centrum of the previous sacral vertebra.

First and second caudals— The proximal-most caudal vertebrae consist of a portion of neural arch of the first caudal and most of the second caudal with an articulated chevron ( Figs 3 a–b ). The neural arch of the first caudal consists of both postzygapophyses and the base of the neural spine and transverse processes ( Figs 3.A, 3.B ). The postzygapophyses are considerably shorter than the prezygapophyses of the following vertebra, and their bases are positioned at the level of the caudal tip of the neural spine. The caudal tips of the transverse processes are positioned slightly cranially to the end of the postzygapophyses, suggesting that the postzygapophyeses were not as strongly offset from the end of the centrum as occurs with the prezygapophyses of the second caudal vertebra.

The centrum of the second caudal vertebra is short ( Fig. 3 a ), with its total length (30 mm) subequal to its cranial height (29 mm). The cranial articular facet is deeply concave, whereas the caudal facet is strongly concave, with a ball-like structure. This morphology closely resembles the condition of the proximal caudal vertebrae of Patagonykus (Novas 1997) and the Mononykinae (e.g. Perle et al. 1994; Karkhu & Rautian 1996; Chiappe et al. 2002). The external surfaces of the centrum converge ventrally, in order to form a ventral keel, but it is not as pronounced as in the last sacral vertebra. On the proximal half of the centrum, a small and subcircular lateral depression is present. This fossa is in contrast with the larger and oval-shaped one present in Patagonykus . The neural arch is low, with poorly defined laminae connecting the arch with the centrum. In caudal view, the neural canal is oval, resembling the size and morphology of the proximal caudal vertebrae of Patagonykus . Only the base of the transverse processes is preserved, whereas both postzygapophyses are totally lost. The prezygapophyses are craniodorsally oriented, extended beyond the cranial level of the centrum. In dorsal view, the prezygapophyses are subtriangular, with a distinctive ridge that arises caudally from the base of the neural spine and is cranially extended up to the cranial tip of the prezygapophysis. This ridge separates the medial articular facet from the dorsal surface of the prezygapophysis. Although the neural spine is lost, the preserved base of this structure suggests a craniocaudally reduced spine, as occurs in Patagonykus (Novas 1997) .

Possible fourth caudal— A fragmentary vertebra, represented by a complete centrum and a right transverse process, exhibits the uncommon condition of possessing both concave cranial and caudal articular facets ( Figs 3 c–d ). According to its position on the matrix, it should correspond to the fourth caudal assuming that the third caudal is missing. We discard the possibility that this element could correspond to a cervical or dorsal vertebra because the proportion of the centrum for the first case and the absence of parapophyses for the second case. The cranial surface is the deepest and is 30 % larger in diameter than the caudal one. The borders of the cranial surface are sharp contrasting with the borders of the caudal surface that are thicker and rounded. The centrum does not have a visible external pneumatization. In ventral view, the centrum is convex with a gently transversal constriction and does not show clear facets for a chevron. Most of the neural arch is lost, only preserving a portion of the right transverse process. The neural arch is solidly fused to the centrum, and the contact between both structures is evidenced by a sharp ridge. The transverse process is thin and appar- ently projects laterocaudally. In lateral view, two laminae connect the base of the arch with the transverse process. The cranial lamina is wider than the distal one. Both laminae surround a moderately deep and subtriangular fossa ( Fig. 3 c ).

The differences in size of both articular surfaces suggest that this element represents a transitional vertebra in the centrum morphology of the preceding and following vertebrae and in the relative size of these cranial and caudal vertebrae. In light of these surfaces, the preceding caudal (apparently third caudal) should be procoelous (also accordingly with the preserved second caudal) and the following vertebra (apparently fifth caudal) should be ophistocoelous or biconvex. Proximal caudal vertebrae are incompletely known in Alvarezsaurus ( Bonaparte 1991) , Patagonykus (Novas 1997) , and Parvicursor (Karkhu & Rautian 1996) , and better preserved in Shuvuuia ( Chiappe et al. 2002; Suzuki et al. 2002). In none of these taxa a proximal biconcave caudal was recognized; in contrast, the available evidence indicate that the proximal caudal vertebrae are procoelous in these alvarezsaurids. We interpret that the differences in size of both articular surfaces of this caudal vertebra (possible fourth) and the biconcave centrum are autapomorphies of Achillesaurus .

Midcaudal— This element is extremely fragmentary and consists of the ventral portion of the centrum. We presume that it corresponds to a midcaudal. It preserves one articular surface concave and the other one is broken. The centrum is relatively short craniocaudally and strongly constricted at mid-length. In lateral view, the ventral contour is strongly concave. In ventral view, no keel is present but instead a shallow longitudinal sulcus runs along this surface, bordered laterally by a rounded ridge. The strongly concave contour of this centrum in lateral view is similar to that of the distal caudal vertebrae of Alvarezsaurus ( Bonaparte 1991) , but this latter taxon has very elongated centra.

Proximal chevron— A chevron is preserved, associated with the second caudal vertebra ( Fig. 3 a ). According to the morphology of the preserved proximal chevron of Alvarezsaurus , the element described here seems to lack most of its distal half. The available proximal portion shows a laminar structure. A cranial process is present at the proximal end of the chevron that articulates with the caudolateral corner of the second caudal.

Pelvic Girdle.

Ilium— Only the left ilium is preserved, lacking most of the dorsal border of the iliac blade and preacetabular process, and a portion of the pubic pedicel. As originally found, and similarly to other alvarezsaurids (e.g. Alvarezsaurus and Shuvuuia MGI 100/975 cast), the iliac blade is inclined dorsomedially with the dorsal edge meeting its counterpart over the sagittal midline of the sacrum. This condition contrasts that of several theropods which have the iliac blade in an almost vertical position (e.g. Deinonychus Ostrom ; Ostrom 1969). For descriptive purposes, however, the ilium is illustrated with the iliac blade in vertical position ( Fig. 4 ). Albeit incomplete, the overall shape of the ilium approaches that of Alvarezsaurus .

The preserved portion of the pubic pedicel reveals that it is craniocaudally smaller than the ischial pedicel, resembling the condition of other alvarezsaurids, but contrasting the typical tetanuran pattern (e.g. Allosaurus Marsh , Microvenator Ostrom , Deinonychus , and Unenlagia Novas and Puerta ; Ostrom 1969; Madsen 1976; Novas & Puerta 1997; Makovicky & Sues 1998). The iliac border of the acetabulum is well preserved, depicting the presence of an almost undeveloped supraacetabular crest ( Fig. 4 a ). This condition approaches that other maniraptorans (e.g. Microvenator , Unenlagia ; Makovicky and Sues 1998; Novas & Puerta 1997). In Alvarezsaurus , the supraacetabular crest is poorly developed but it is relatively more pronounced than in Achillesaurus ; otherwise, in more derived alvarezsaurids (e.g. Patagonykus , Mononykus , Shuvuuia ), this crest is more prominent ( Novas 1996; Chiappe et al. 2002).

The ischial pedicel is massive and distally rounded. It is well developed in both sagittal and transversal axes ( Fig. 4 ). This pedicel is medially conspicuous, when it is seen in ventral view. Despite its incompleteness, the contour at the base of the ischial pedicel in Alvarezsaurus seems to be narrower transversely and longer craniocaudally. On the lateral surface of the ischial pedicel the antitrochanter is partially eroded.

Caudal to the ischial pedicel, the postacetabular blade is well developed with a strongly concave ventral profile (as in Alvarezsaurus ), and ventrally expanded at its caudal tip, similar to other alvarezsaurids ( Fig. 5 ). The external surface of the postacetabular blade is straight, a difference with Alvarezsaurus where this portion is moderately concave.

The postacetabular blade flares lateroventrally forming a concave and wide brevis fossa with a weak medial shelf. In medial view, the brevis fossa is dorsally delimited by a sharp and conspicuous longitudinal crest. This crest does not arise from the caudal margin of the ischial pedicel, as occurs in more basal theropods. At the base of the medial longitudinal crest, a subtriangular and rugose surface is present, probably indicating the area for the attachment of the last sacral rib ( Fig. 4 b ).

Hindlimb Femur— Only a proximal portion of the left femur is available, lacking the anterior and greater trochanters ( Fig. 6 ). The head is dorsomedially protruding; in cranial view, the base of the articular surface of the head is preserved, although its caudomedial and medial portions are missing. The connection between the head and the shaft is represented by a sharp ridge that in cranial view has a concave margin. The cranial surface of the proximal end, lateral to the base of the head, is gently concave. In this region, there are two conspicuous elevations that represent the lateral and dorsal limits of the base of the anterior trochanter. The proximal development of this trochanter is unknown. In Patagonykus , the anterior trochanter reaches the level of the greater trochanter, separated from the latter by a deep cleft and from the base of the femoral head by a deep excavation (Novas 1997; Fig. 7 ). In Achillesaurus , this excavation is absent and, according to the preserved base of the anterior trochanter, the distalmost point of the cleft which separates it from the greater trochanter is much more distally placed in Achillesaurus than in Patagonykus . Achillesaurus seems to have possessed separate anterior and greater trochanters (as in Patagonykus ), but not a trochanteric crest as in Mononykinae . In Alvarezsaurus , the proximal portion of the anterior trochanter is broken off but its base is very well developed cranially ( Fig. 7 ), a feature not preserved in Achillesaurus .

The preserved dorsal surface of the femur is straight and it forms a right angle with the cranial surface. The lateral surface of the femur is strongly craniocaudally broad, convex, and smooth.

Tibia and astragalus— The distal end of the left tibia and an almost complete astragalus are preserved articulated ( Fig. 8 ). The broken shaft of the tibia reveals an oval cross-section. The distal end is strongly compressed craniocaudally, with an almost flat cranial surface for the reception of the ascending process of the astragalus. In caudal view, a gently concave longitudinal groove is present, separating both lateral and medial malleoli. The lateral malleolus exhibits a longitudinal caudal tuberosity. This structure also delimits a lateral facet for the fibular contact ( Fig. 8 b ). In caudal view, the medial malleolus is strongly convex whereas the craniomedial border supports the ascending process of the astragalus. At the distal-most end of the bone, both medial and lateral margins are almost parallel, while the tibial shaft gently tapers towards the mid-length of the bone. In caudal view, the distal edge of the tibia is straight with both malleoli at the same level as in Patagonykus ( Fig. 9 c ), but differing from Alvarezsaurus ( Fig. 9 b ).

Most of the astragalar body is complete, whereas the ascending process is almost completely lost ( Fig. 8 ). The astragalus is not fused with the calcaneum, despite the sub-adult condition of the specimen (based on the fusion between the centra and the neural arches in the available vertebrae). The overall morphology of the astragalar body of Achillesaurus closely resembles that of Alvarezsaurus and Patagonykus ( Fig. 9 ). The cranial surface of the astragalus is concave with a conspicuous medial condyle. In cranial view, below the base of the ascending process a deep pit for the attachment of ligaments exists, as in Alvarezsaurus and Patagonykus ( Bonaparte 1991; Novas 1997). The astragalar body is craniocaudally compressed in distal view, resulting in a strongly protruding medial condyle, similar to Patagonykus . The calcaneal facet of the astragalus is visible in lateral view. This facet is gently concave and extends from the distal tip of the body to the lateral base of the ascending process. There is a prominent craniolateral buttress that extends from the astragalar ascending process towards the facet for the calcaneum. A similar buttress is present in Patagonykus (Novas 1997; Fig. 9 c ). The ascending process of the astragalus arises immediately from the medial border of the bone. On the basis of the preserved ascending process, it seems to be strongly laminar, resembling the condition of other derived tetanurans (e.g. Deinonychus ; Ostrom 1969). The proximal development of the ascending process could not be observed, but the straight cranial surface of the tibia suggests it was proximally expanded as in Alvarezsaurus and Patagonykus ( Fig. 9 c ).

Metatarsus— The left metatarsals II, III, and IV are preserved ( Fig. 10 ). The proximal half of the second and third metatarsals is available, whereas the fourth metatarsal only consists of its proximal end. As it was mentioned above, the metatarsus was originally found in articulation with the astragalus. The proximal articular surfaces of the metatarsals are severely weathered; nevertheless, its overall contour could be discerned, indicating a non-arctometatarsal condition ( Fig. 10 ). The third metatarsal is the most slender element. Both metatarsals II and IV are proportionally wide bones, but the metatarsal II is stouter than the metatarsal IV. The cranial surface of each metatarsal is gently convex, whereas in caudal aspect both lateral and medial borders taper sharply, resulting in a less wide surface. At nearly half the length of the bones, they are subcircular in cross-section. The medial surface of the metatarsal IV is gently convex. A non-arctometatarsalian pes ( Fig. 10 ) is also reported for Alvarezsaurus and Patagonykus ( Bonaparte 1991; Novas 1996) and represents the primitive condition for the Alvarezsauridae , whereas the Mononykinae have a proximally reduced metatarsal III (arctometatarsalian condition; e.g. Chiappe et al. 2002).

Figures

FIGURE 1. Location map of Paso Córdova, Río Negro Province, Argentina, where the holotype of Achillesaurus was found.

FIGURE 2. Achillesaurus manazzonei. Portion of holotype skeleton as preserved before the isolation of each bone. Abbreviations: as, astragalus; c1, first caudal; c2, second caudal; c?4, possible fourth caudal; f, femur; il, ilium; mt II, second metatarsal; mt III, third metatarsal; s, last sacral; ti, tibia. Scale bar represents 50 mm. Grey area indicates matrix.

FIGURE 3. Achillesaurus manazzonei. Last sacral, and first and second caudals in articulation in lateral (A) and dorsal (B) views; and possible fourth biconcave sacral in lateral (C), ventral (D), cranial (E), and caudal (F) views. Abbreviations: f, fossa; ha, haemal arch; k, ventral keel; nc, neural canal; ns, neural spine; poz, postzygapophysis; prz, prezygapophysis; tp, transverse process. Scale bar represents 10 mm. Grey areas indicate broken bone, dotted indicates matrix.

FIGURE 4. Achillesaurus manazzonei. Left ilium in dorsolateral (A) and medioventral (B) views. Abbreviations: ar, attachment area for the ribs of the last sacral vertebra; bs, brevis shelf; ib, iliac blade; pi, ischial pedicel; pp, pubic pedicel. Scale bar represents 20 mm. Grey areas indicate broken bone, dotted indicates matrix.

FIGURE 5. Comparison of the ilium among Alvarezsauridae. A, Achillesaurus manazzonei (inverted); B, Alvarezsaurus calvoi (modified from Bonaparte 1991 and MUCPV 54); C, Patagonykus puertai (modified from Novas 1997); D, Shuvuuia deserti (modified from Chiappe et al. 2002). Not to scale.

FIGURE 6. Achillesaurus manazzonei. Proximal portion of left femur in cranial (A), medial (B), and caudal (C) views. Abbreviations: at, anterior throchanter; fh, femoral head; fn, femoral neck. Scale bar represents 20 mm. Grey areas indicate broken bone, dotted indicates matrix.

FIGURE 7. Comparison of the proximal portion of the femur among Alvarezsauridae in cranial and medial views. A, Achillesaurus manazzonei (inverted); B, Alvarezsaurus calvoi (modified from Bonaparte 1991 and MUCPV 54); C, Patagonykus puertai (modified from Novas 1997). Not to scale.

FIGURE 8. Achillesaurus manazzonei. Distal portion of left tibia and astragalus in cranial (A), lateral (B), caudal and distal (C), and medial (D) views. Abbreviations: apa, ascending process of astragalus; as, astragalus; clb, craniolateral buttress; fca, facet for the calcaneum; ti, tibia; tim, tibial malleolus. Scale bar represents 20 mm. Grey areas indicate broken bone, dotted indicates matrix.

FIGURE 9. Comparison of the distal portion of the tibia among Alvarezsauridae in cranial and caudal views. A, Achillesaurus manazzonei (inverted); B, Alvarezsaurus calvoi (modified from Bonaparte 1991 and MUCPV 54); C, Patagonykus puertai (modified from Novas 1997). Not to scale.

FIGURE 10. Achillesaurus manazzonei. Proximal portion of left metatarsal II, III, and IV in cranial (A) and caudal (B) views, with accompanying reconstruction of proximal view. Scale bar represents 20 mm. Grey areas indicate broken bone, dotted indicates matrix.