G Model PGEOLA-158; No. of Pages 10 Proceedings of the Geologists’ Association xxx (2011) xxx–xxx Contents lists available at ScienceDirect Proceedings of the Geologists’ Association jo urnal homepage: www.elsevier.com/locate/pgeola A robust sauropodomorph specimen from the Upper Triassic of Argentina and insights on the diversity of the Los Colorados Formation a,b, c,d Martin D. Ezcurra *, Cecilia Apaldetti a ´ Laboratorio de Anatomı´a Comparada y Evolucio´n de los Vertebrados, Museo Argentino de Ciencias Naturales ‘‘Bernardino Rivadavia’’, Av. Angel Gallardo 470 (C1405DJR), Buenos Aires, Argentina b Seccio´n Paleontologı´a de Vertebrados, Museo Argentino de Ciencias Naturales ‘‘Bernardino Rivadavia’’, Av. A´ngel Gallardo 470 (C1405DJR), Buenos Aires, Argentina c Instituto y Museo de Ciencias Naturales, Facultad de Ciencias Exactas, Fı´sicas y Naturales, Universidad Nacional de San Juan, San Juan, Argentina d CONICET, Argentina A R T I C L E I N F O A B S T R A C T Article history: The Late Triassic rocks document the first steps of the early dinosaur evolutionary radiation. Although Received 7 April 2011 the oldest dinosaurs were not abundant in their assemblages, sauropodomorphs achieved a wide Received in revised form 6 May 2011 taxonomic diversity and high abundance towards the Triassic–Jurassic boundary. In South America, this Accepted 7 May 2011 pattern is documented in the Ischigualasto-Villa Unio´ n Basin of northwestern Argentina, in which Available online xxx dinosaurs achieved a numerical dominance over other tetrapods during the deposition of the upper levels of the late Norian-Rhaetian Los Colorados Formation. In this contribution we enrich faunal list of Keywords: this assemblage with the description of a new medium-sized basal sauropod specimen with a very Dinosauria robust tibial morphology. This new specimen differs from the other known sauropodomorphs described Sauropodomorpha Sauropoda for the Los Colorados Formation and increases the alpha-diversity recorded for this group. A Triassic phylogenetic analysis recovered the new specimen at the base of Sauropoda and closely related to Argentina Lessemsaurus, Antetonitrus, and other basal sauropods. These results match with the high degree of robustness observed in the tibia of the specimen reported here, which closely approaches the morphology documented for other basal sauropods and departs from the morphospace occupied by non- sauropod sauropodomorphs. A two step pattern of tibia robustness increase is observed in the sauropodomorph phylogeny, a pattern that coincides and could be related with the acquisition of more habitual quadrupedal gait achieved by basal sauropods. ß 2011 The Geologists’ Association. Published by Elsevier Ltd. All rights reserved. 1. Introduction (ca. 231–226 Mya; Martinez and Alcober, 2009; Ezcurra, 2010; Martinez et al., 2011). Towards the downfall of the Triassic Period, Sauropodomorpha is one of the three major groups of dinosaurs sauropodomorphs become more abundant in late Norian and and includes the largest known terrestrial vertebrates in the history Rhaetian assemblages around the world (Galton and Upchurch, of life. This group appears in the Upper Triassic fossil record (Langer 2004; Upchurch et al., 2004). Thus, a better knowledge of the Triassic et al., 1999; Martinez and Alcober, 2009; Ezcurra, 2010) and evolution of sauropodomorphs is an essential issue in order to reach disappeared at the end of the Cretaceous (Galton and Upchurch, a more comprehensive understanding of the complex macroevolu- 2004). During this time span, sauropodomorphs achieved a global tionary scheme in which the early radiation of Dinosauria took part distribution and composed the most common elements of verte- (Brusatte et al., 2008, 2010; Langer et al., 2010). brate faunas in several Jurassic and Cretaceous continental In South America, the Los Colorados Formation (late Norian- assemblages. Indeed, the dawn of sauropodomorph history indi- Rhaetian; Bonaparte, 1972; Arcucci et al., 2004; Martinez et al., cates that, although does not abundant, the group was already quite 2011) has the highest sauropodomorph diversity, including taxonomically diverse in the late Carnian–early Norian of NW Riojasaurus (Bonaparte, 1972; Bonaparte and Pumares, 1995), Argentina, one of the oldest known dinosaur-bearing assemblages Coloradisaurus (Bonaparte, 1978), Lessemsaurus (Bonaparte, 1999; Pol and Powell, 2007), and an undescribed basal sauropodomorph (Martinez et al., 2004). These basal sauropodomorphs were found in the upper levels of the Los Colorados Formation, which * Corresponding author at: Museo Argentino de Ciencias Naturales ‘‘Bernardino represents a sequence of fluvial deposits with sand channels of Rivadavia’’, Laboratorio de Anatomı´a Comparada y Evolucio´ n de los Vertebrados, ´ moderate to high sinuosity and well-developed flood plains Av. Angel Gallardo 470, C1405DJR Buenos Aires, Argentina. E-mail address: [email protected] (M.D. Ezcurra). (Caselli et al., 2001). The presence of thick alluvian plain deposits, 0016-7878/$ – see front matter ß 2011 The Geologists’ Association. Published by Elsevier Ltd. All rights reserved. doi:10.1016/j.pgeola.2011.05.002 Please cite this article in press as: Ezcurra, M.D., Apaldetti, C., A robust sauropodomorph specimen from the Upper Triassic of Argentina and insights on the diversity of the Los Colorados Formation. Proc. Geol. Assoc. (2011), doi:10.1016/j.pgeola.2011.05.002 G Model PGEOLA-158; No. of Pages 10 2 M.D. Ezcurra, C. Apaldetti / Proceedings of the Geologists’ Association xxx (2011) xxx–xxx Table 1 Selected measurements (in mm) of the available bones of PULR 136. All the values are the maximum measurables. Error for vertebral measurements: 0.02 mm; error for tibial measurements 0.5 mm. Proximal caudal centrum Length 82.04 Anterior articular facet height 72.70 Anterior articular facet width 69.10 Posterior articular facet height 66.30 Posterior articular facet width 59.82 Mid-caudal centrum Length 76.88 a Anterior articular facet height 48.44 a Anterior articular facet width 53.22 Posterior articular facet height 54.56 a Posterior articular facet width 52.06 Left tibia Length 405 Anteroposterior depth of proximal end 200 Transverse width of proximal end 112 Anteroposterior depth at mid-shaft 75 Transverse width at mid-shaft 64 Anteroposterior depth of distal end 129 Transverse width of distal end 119 a Incomplete. Massopoda Yates, 2007 sensu Yates, 2007 Sauropoda Marsh, 1878 sensu Yates, 2007 Gen. et sp. indet. 2.1. Material Fig. 1. Simplified geologic map of the Agua de la Pen˜a Group showing the outcrops of the Ischigualasto (light grey) and Los Colorados Formations (dark grey). A PULR 136, a proximal caudal vertebral centrum, a middle caudal satellite image shows a close-up of the area in which PULR 136 was collected and vertebral centrum, a fragment of dorsal rib, and a complete left the white bone silhouette denotes the exact locality (Quebrada del Viento). The line tibia (Table 1 and Fig. 2). All these elements were discovered in the satellite image depicts the limit between the San Juan and La Rioja provinces closely associated with each other, are of matching size, and and the scale is 1 km. The geologic map was modified from Caselli et al. (2001). present a consistent morphology with that of a basal sauropod. Accordingly, we consider that these postcranial bones pertain to a representing episodic flows and high precipitation, suggest a single individual. humid or sub-humid environmental condition during the deposi- tion of the unit (Caselli et al., 2001; Arcucci et al., 2004). Here we 2.2. Locality and horizon describe from the upper levels of the Los Colorados Formation (Fig. 1) a new sauropodomorph specimen that has a distinct 0 00 0 00 Quebrada del Viento (29851 39 S, 68805 48 W), La Rioja morphology from that of previously known sauropodomorph taxa Province, NW Argentina (Fig. 1). Upper levels of the Los Colorados of this unit. Formation (late Norian-Rhaetian, Late Triassic; Martinez et al., Institutional abbreviations. BSP, Bayerische Staatssammlung fu¨ r 2011). Pala¨ontologie und Historische Geologie, Munich, Germany; ISI, Geological Studies Unit of the Indian Statistical Institute, Calcutta, 3. Description India; JM, Jura-Museum in Eichsta¨tt, Bayern, Germany; MB, Humboldt Museum fu¨ r Naturkunde, Berlin, Germany; MCP, Museo The middle caudal vertebral centrum of PULR 136 is weathered, de Ciencias e Tecnologı´a, Porto Alegre, Brazil; MHNM, Muse´um but the proximal caudal centrum, left tibia, and rib fragment are d’Histoire naturelle, Marrakech; NHMUK, Natural History Museum well-preserved (Fig. 2). The length of the tibia closely approaches of United Kingdom, London, UK; NM, National Museum, Bloemfon- to that of the holotype of the riojasaurid Eucnemesaurus fortis (TM tein, Free State, South Africa; OUMNH, Oxford University Museum of 119: body length of ca. 10 m; van Heerden, 1979) and is slightly Natural History, Oxford, England; PULR, Paleontologı´a, Universidad larger than the holotype of the basal sauropod Blikanasaurus (SAM Nacional de La Rioja, La Rioja, Argentina; PVL, Fundacio´ n ‘‘Miguel K403: body length of ca. 10 m; van Heerden, 1979; Galton and van Lillo’’, San Miguel de Tucuma´n, Argentina; PVSJ, Museo de Ciencias Heerden, 1985) (Table 2). Accordingly, PULR 136 would have been Naturales, Universidad Nacional de San Juan, San Juan Argentina; an animal of a total body length of approximately 10 m. QG, Zimbabwe Natural History Museum, Bulawayo, Zimbabwe; SAM; South African Museum, South Africa; SMNS, Staatliches 3.1. Caudal vertebrae Museum fu¨ r Naturkunde, Stuttgart, Germany; TM, Transvaal Museum, Pretoria; South Africa; YPM, Yale University, Peabody The centra of a posterior proximal and an anterior mid-caudal Museum of Natural History, New Haven, Connecticut, USA. vertebra are preserved (Fig. 2G–L, Table 1). The dorsolateral borders are damaged and, consequently, we cannot assess if the 2. Systematic palaeontology neurocentral sutures were closed or still open. The proximal caudal centrum is amphicoelous and approximately 1.13 times longer Dinosauria Owen, 1842 sensu Padian and May, 1993 than the height of its anterior articular facet.