Lavocatisaurus agrioensis, Canudo & Carballido & Garrido & Salgado, 2018
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publication ID |
https://doi.org/ 10.4202/app.00524.2018 |
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persistent identifier |
https://treatment.plazi.org/id/6E67BB64-E351-C540-FC91-7F35FEA9DEF9 |
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treatment provided by |
Felipe |
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scientific name |
Lavocatisaurus agrioensis |
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sp. nov. |
Lavocatisaurus agrioensis sp. nov.
Figs. 2 View Fig , 3.
Etymology: In reference to the locality of Agrio del Medio, from which the new species was found.
Type material: Holotype: MOZ-Pv1232, partially articulated specimen: dentaries, left surangular, premaxillae and maxillae, left jugal, right squamosal, quadrates, 23 isolated teeth and two series of 8 and 9 maxillary teeth, hyoid bone, 11 cervical vertebrae (including atlas and axis), 28 caudal vertebrae, cervical ribs, 2 dorsal ribs, humerus, fragment of?radius. Paratypes, all from type locality: the juvenile specimens ( Salgado et al. 2012): MOZ-Pv 1248, posterior cervical centrum; MOZ-Pv 1249, cervical neural arch; MOZ-Pv 1251, dorsal neural arch; MOZ-Pv 1252, 1253, 1254, anterior caudal centra; MOZ-Pv 1255,
scapula; MOZ-Pv 1267, left radius; MOZ-Pv 1256, left ulna; MOZ-Pv 1257, right metatarsal I; MOZ-Pv 1258,?metatarsal V. An association of MOZ-Pv 1233, 1250, cervical centra; MOZ-Pv 1236, 1237, incomplete cervical neural arches; MOZ-Pv 1238, 1239, fragmentary neural arches; MOZ-Pv 1240, dorsal centrum; MOZ-Pv 1241, rib fragments; MOZ-Pv 1242, haemal arch; MOZ-Pv 1243, right ulna; MOZ-Pv 1244, left tibia; MOZ-Pv 1245, left fibula; MOZ-Pv 1246, end of metatarsal; MOZ-Pv 1247, indeterminate flat fragment.
Type locality: Agrio del Medio site, Neuquén Province, Argentina.
Type horizon: Aptian–lower Albian, Lower Cretaceous.
Diagnosis.—A middle-sized rebbachisaurid sauropod diagnosed by the following combination of characters (unique characters are marked with an *): extremely well-developed preantorbital fenestra (shared with Nigersaurus ); marked laterodorsal fossa in the dentary (shared with Demandasaurus and Nigersaurus ); ventrally expanded squamosal (shared with Limaysaurus ); dentary with pronounced ventral projection in the mesio-ventral corner; jugal long and contacting the squamosal (shared with Nigersaurus ) but without foramina as present in Nigersaurus ; *maxillary teeth significantly larger than the mandibular teeth; middle caudal vertebrae with anteriorly (nearly horizontally) projecting prezygapophysis
Remarks.— Lavocatisaurus agrioensis gen. et sp. nov. is represented by almost all the anatomical elements, with the exception of the neural arches of the dorsal vertebrae there is only one dorsal centrum, corresponding to one of the paratypes). Salgado et al. (2012) described in detail the bones of the juvenile specimens such as the radius, fibula, ulna, metacarpals, etc.; that description will not be repeated in the present work. Rather, we shall briefly describe the bones of the holotype, and any bones of the paratypes not described in the work cited above. Most of the adult material here presented is still under preparation, therefore a detailed description of it is out of the scope of the present work and will be presented once all the materials have been prepared. At the moment, and given the low number of elements in common between the juvenile specimens and the adult specimen, we couldn’t find any reliable ontogenetic difference.
Rebbachisaurids skull anatomy is mainly based on the reconstructions of Nigersaurus , the single taxon of this clade that preserved enough bones for allowing such reconstruction ( Sereno et al. 2007). The new taxon here described allowed us to present an almost complete skull reconstruction, which was through comparisons with Nigersaurus ( Sereno et al. 2007) and diplodocids (e.g., Diplodocus ; Whitlock et al. 2010). As reconstructed ( Fig. 3), the cranium of Lavocatisaurus was elongated, as in diplodocids. Indeed, the general shape of the skull resembles that of Diplodocus more than that of Nigersaurus (although sharing some new characters with the latter). The premaxilla is elongated, with space for 4 teeth, as in other sauropods. The maxilla is anteroposteriorly elongated, with a long ascending process, and pierced by a very large oval preantorbital fenestra ( Fig. 3A, B). A completely opened preantorbital fenestra is present in derived titanosaurs such as Tapuiasaurus and diplodocids such as Diplodocus ( Marsh 1884; Zaher et al. 2011). A similar opening was initially labelled in Nigersaurus as the antorbital fenestra ( Sereno et al. 1999: fig. 2D). Based on our personal observation on the skull elements of Nigersaurus we reinterpret this opening as the preantorbital fenestra, which is not reconstructed in following reconstructions published for this taxon (Sereno and Wilson 2006; Sereno et al. 2007). There are only 12 dental positions in the preserved maxillae, differing from the 25 dental positions of Nigersaurus ( Sereno et al. 2007) but having a similar number of maxillary teeth presents in diplodocids ( Whitlock et al. 2010; Tschopp and Mateus 2017). The maxillary teeth are large, decreasing in size laterally, as evidenced by the shape of the lateral plate of the maxillae ( Fig. 3B 2 View Fig ) and the two series of isolated functional teeth (see below). The nutritional foramina of the premaxilla and maxilla are small and circular as in most sauropods and differing from the extremely elongated nutritional foramina of Nigersaurus ( Sereno et al. 1999: fig. 2D). The jugal is elongated and fragile bone, lacking the foramina present in Nigersaurus . A novel articulation of the jugal with the squamosal excludes the postorbital from the infratemporal fenestra, resulting in an uncommon morphology not described for other sauropods. Nevertheless, a similar articulation and the resulting exclusion of the postorbital from the infratemporal fenestra can be detected in Limaysaurus ( Calvo and Salgado 1995: fig. 4) and Nigersaurus ( Sereno et al. 2007: fig. 1), indicating that this is a relatively widespread morphology amongst rebbachisaurids. The squamosal is ventrally expanded as in Limaysaurus , although in the latter the broad ventral expansion of the squamosal was interpreted as the quadratojugal by Calvo and Salgado (1995: fig. 4). The mandible has the typical rectangular outline, as in other diplodocids and derived titanosaurs, and presents 22 dental positions. The elevated number of dentary teeth in L. agrioensis gen. et sp. nov. is an uncommon character amongst Neosauropoda, differing from the 9 tooth positions of Demandasaurus ( Torcida Fernández-Baldor et al. 2011) but also from the 34 dentary teeth of Nigersaurus ( Sereno et al. 2007) . Although no functional teeth are preserved in any of the dentaries, several replacement teeth are observed, which seem to correspond to the first line of replacement teeth. These are much smaller and more fragile than those of the maxillae, an unusual discrepancy in size interpreted as an autapomorphy of the new taxon. A laterodorsal fossa is present in the dentary, as is a pronounced ventral projection in the mesio-ventral corner. The presence of an anteroventrally directed triangular process is a widespread morphology amongst Diplodocimorpha, with a reversion in more derived rebbachisaurids ( Demandasaurus and Nigersaurus ). Teeth are located exclusively in the anteriormost part of the maxilla and the mandibles ( Fig. 3). The teeth are elongated, with an slenderness index (sensu Upchurch 1998) of around 7–8, fine and pencil-like, as in all other diplodocimorphs known up to date (e.g., Nigersaurus , Diplodocus , Galeamopus ; Sereno et al. 2007; Whitlock et al. 2010; Tschoopp and Mateus 2017). The compression index (sensu Díez Díaz et al. 2013) cannot be evaluated at the moment as most of the teeth are still embedded in the matrix ( Fig. 3G 1, H 1 View Fig ). The enamel is asymmetrical, being around 5 times thicker on the labial surface than on the lingual one. Three series of functional teeth were recovered in what seems to be their natural position. One is composed by 9 teeth from the left maxilla Fig. 3G 1 View Fig ), the second has 10 teeth from the right maxilla Fig. 3H 1 View Fig ), and the third is composed by 4 teeth from the premaxilla. Given their size and in comparison with the alveoli of the upper and lower jaws and preserved dentary teeth, they are interpreted as upper teeth. As noted by Wiersma and Sander (2016), isolated tooth rows (ITRs) are not uncommon and have been reported for other sauropods such as Shunosaurus , Apatosaurus , Europasaurus , Giraffatitan , and Phuwiangosaurus (see Wiersma and Sander 2016, and references therein). As mentioned above, the dentary teeth are much smaller than the maxillary teeth and lack wear facets at least those preserved), whereas the maxillary teeth have a single, low-angle lingual wear facet ( Fig. 3H 1 View Fig ).
The spongy tissue of the bone is normal in all the bones of the skeleton. The cervical vertebrae are still embedded in the plaster with the right lateral side exposed. The cervical vertebrae are opisthocoelous and short ( Fig. 2 View Fig ). The eighth cervical is the longest (26 cm), and otherwise the fourth to the tenth vary between 20–22 cm in length. The cervical centra have pleurocoels,which are apparently undivided as in Nigersaurus , and are teardrop-shaped, with the more excavated side in the anterior part. From the third cervical vertebra on, there is a keel in the ventral part of the centrum. The neural spine of the axisisanteriorlyelongatedandV-shapedasin Demandasaurus . The neural spine of the rest of the cervical vertebrae is simple and slightly displaced anteriorly. The cervical neural spines of Lavocatisaurus are relatively higher than in Nigersaurus and Demandasaurus ( Sereno et al. 2007; Torcida Fernández-Baldor et al. 2011). The parapophysis is situated outside the centrum. The cervical vertebrae possess a well-developed system of laminae, as is characteristic of Eusauropoda ( Wilson 2002). These include the anterior centrodiapophyseal, posterior centrodiapophyseal, spinoprezygapophyseal, spinopostzygapophyseal, prezygodiapophyseal and postzygodiapophyseal laminae ( Fig. 2 View Fig ). The epipophyseal-prezygapophyseal lamina is well marked, although none of the cervical present a well-developed epipophysis, resembling the cervical vertebrae of other rebbachisaurids (e.g., Zapalsaurus, Limaysaurus ; Salgado et al. 2006: fig. 4; Calvo and Salgado 1995: fig. 8B) and differing from Nigersaurus ( Sereno et al. 2007: fig. 3B). In the axis, the centropostzygapophyseal laminae are united halfway along by a horizontal lamina, as in the third cervical of Zapalasaurus . The third cervical has postzygodiapophyseal and epipophyseal-prezygapophyseal laminae, as in Zapalasaurus ( Salgado et al. 2006) . In all the cervical vertebrae, the prezygapophyseal fossa is located between the prezygodiapophyseal and the centroprezygapophyseal laminae. The third cervical of Lavocatisaurus is similar in size to that of Zapalasaurus , but its laminae are more delicate, being in this character more similar to Nigersaurus than to Limaysaurus . The cervical rib has a well-marked anterior projection. The third and fourth cervical preserved their cervical ribs, which are as long as the centrum ( Fig. 2 View Fig ), as in other diplodocoids ( Wilson and Sereno 1998).
The centra of the caudal vertebrae are generally amphicoelous, although there are variations within the series. The anteriormost preserved vertebrae have centra with plano-convex anterior faces. The posterior face in these vertebrae is more concave than the anterior face. Both articular faces of the middle and posterior caudal vertebrae are concave. The distal caudal vertebrae have centra with convex articular faces. The caudal centra lack pleurocoels, with the exception of the most anterior one preserved, which has a small one. The anterior caudal vertebral centra have a ventral surface that is transversely concave, whereas in the middle caudal centra it is flat.
The neural spine in the caudal vertebrae is well developed, expanded anteroposteriorly, and slightly projecting posteriorly throughout the series. In the anterior ones, it is located in the middle part of the centrum, whereas in the middle and posterior ones it is located in the anterior part of the centrum as in other rebbachisaurids (e.g., Limaysaurus ) but not as anteriorly as is in titanosaurs ( Salgado et al. 1997; Wilson and Sereno 1998). The neural spine is rectangular in lateral view ( Fig. 2 View Fig ). The posteriormost part of the neural spine in the middle caudal vertebrae is in the same plane as the posterior articular face. The transverse processes are dorsally directed in the anteriormost caudal vertebrae,becoming perpendicular to the centrum in the more posterior of the anterior caudals. The pedicles are low beneath the prezygapophyses. In the middle caudal vertebrae, the prezygapophyses are oriented nearly horizontally, projecting anteriorly in such a way that they protrude beyond the anterior articular face of the vertebra. The prespinal lamina is present in the anterior caudal vertebrae, which lack a hyposphene ridge and a triangular process. The distal vertebral centra are distinctively elongated.
The scapula is racket-shaped ( Fig. 2 View Fig ). The scapular blade is perpendicular with respect to the orientation of the coracoid and shows a distal expansion. The scapular acromion is broad and well developed, and the dorsal face has a V-shaped concavity. The highest point of the dorsal margin of the scapular blade is higher than the dorsal margin of the proximal end. The ventral edge of the scapula does not show any sign of the ventral process observed in some diplodocids and titanosauriforms ( Tschopp et al. 2015; Mannion et al. 2017). The cross section of the scapular blade at its base is D-shaped.
The humerus is crushed and badly preserved, but some anatomical traits can be observed. Compared to other sauropods, the humerus of Lavocatisaurus is relatively gracile (medium sensu Wilson and Upchurch 2003). The lateral margin of the proximal half of the bone is almost straight. Its proximolateral corner is pronounced, with the dorsal surface almost flat. The midshaft cross-section is elliptical. The deltopectoral attachment is relatively narrow and has a low crest throughout its length. The articular surface of the condyles is restricted to the distal portion of the humerus.
Remarks.—The scapula of Lavocatisaurus agrioensis gen. et sp. nov. shows the typical racket shape, which is one of the diagnostic characters of Rebbachisauridae ( Bonaparte 1997;
Carballido et al. 2010; Wilson and Allain 2015). The bones of the new species are comparable to most of the rebbachisaurid specimens from Neuquén Basin. Among the taxa that are closest in geographical terms, it is notable that the scapula is reasonably similar to that of Rayososaurus ( Bonaparte 1997; Carballido et al. 2010), although it is distinguished by the development and orientation of the acromial process.
The deltopectoral crest of the humerus of Lavocatisaurus differs from Comahuesaurus in its position and lesser development. Particularly, the humeral articular head of the new species is more developed than in Comahuesaurus ( Carballido et al. 2012) .
Lavocatisaurus preserves the third cervical, as does Zapalasaurus . They are similar in size, but the one from Zapalasaurus is significantly more robust. The middle and posterior caudal vertebrae of Zapalasaurus have a neural spine that is anteroposteriorly elongated as in Lavocatisaurus . Nevertheless, the dorso-anterior corner of the neural spines of Zapalasaurus is located on a higher level than the posterior end, unlike in Lavocatisaurus , where the two corners are on the same level. The inclination of the prezygapophysis in the caudals of the new species is different from that of Zapalasaurus and is more similar to the more derived rebbachisaurids.
The holotype presented the cervical vertebrae and the first twenty vertebrae of the caudal series, articulated and arched, all of these together with the rest of the material scattered over an area of some 8 m 2 (map of site in Salgado et al. 2012). The paratypic material was differentiated on the basis of its size, as well as by the fact that the neural arch of the only dorsal vertebra preserved was unfused, unlike in the adult specimen. The state of preservation of the bones is good, without signs of abrasion or breakages prior to burial; nor do they show signs of scavenging. Accordingly, the presence of a disarticulated cranium, articulated vertebral series, the low dispersion of the bones, and a low-energy sedimentary environment suggest that the preserved specimens were conserved in an autochthonous mode of concentration ( Salgado et al. 2012).
Stratigraphic and geographic range.— Type locality and horizon only.
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