The sauropodomorph specimen here described (MPEF-PV 10860) is housed at the MPEF collection. Pictures were taken using a camera with digital display Nikon SMZ 1000, assisted by a binocular microscope equipped with optic fiber point light. Images from the Scanning Electron Microscope (SEM) were taken in the industrial plant ALUAR Aluminio Argentino SAIC (Puerto Madryn, Chubut province), using a Jeol JSM-6460 with a secondary and backscattered electron detector (15 kV and 20 Pa). Tooth orientation follows Smith and Dodson (2003). Several clades are mentioned in the text, and its definition corresponds to the following authors: Sauropodomorpha (Sereno, 2007), Sauropodiforms (Sereno, 2007), Anchisauria (Galton and Upchurch, 2004), Gravisauria (Allain and Aquesbi, 2008), Sauropoda (Yates, 2007b), Eusauropoda (Upchurch et al., 2004) and Neosauropoda (Bonaparte, 1986). In order to test the phylogenetic affinities of MPEF-PV 10860, the specimen was included in a modified version of the data matrix of Carballido et al. (2015), adding new characters and taxa (see Supplementary Data). The phylogenetic analysis was carried out through a heuristic tree search with TNT 1.1 (Goloboff et al., 2008a and Goloboff et al., 2008b) starting from 1000 Wagner trees followed by branch swapping (TBR). The specimen MPEF-PV 10860 was included in the dataset compiled by Chure et al. (2010) to test its position in the crown shape morphospace as measured by the Slenderness Index (Chure et al., 2010 and Upchurch, 1998). Additionally, the broadening of the crown versus the root was analyzed following the same protocol. This is a highly variable feature that was traditionally used as a character in phylogenetic analyses given its variation along Sauropodomorpha (Upchurch, 1995, Upchurch et al., 2004, Wilson, 2002 and Wilson and Sereno, 1998). We measured the relation of the crown width against the root width, henceforth referred as Widening Index (WI), in several species of Sauropodomorpha. This ratio was plotted against the time in order to evaluate its change during the Mesozoic in a similar approach to the SI values in Sauropodomorpha (table at Supplementary Data). Plotts of SI and WI were performed using the PAST software version 2.10 (PAlaeontological STastics [Hammer et al., 2001]).

Dinosauria Owen, 1842

Sauropodomorpha Huene, 1932

Sauropodiforms Sereno, 2007gen. et sp. indet.

Material. MPEF-PV 10860, isolated tooth with complete crown and most of the root preserved.

Occurrence. The fossil material was collected at the Queso Rallado locality (Cañadón Asfalto Formation), located at 5.5 km to the northwest of the Cerro Cóndor scholar village, in Chubut Province, Argentina (e.g., Pol et al., 2011b and Rougier et al., 2007a). The associated fauna includes the mammals Asfaltomylos, Henosferus, Argentoconodon, and Condorodon (Gaetano and Rougier, 2012, Rauhut et al., 2002, Rougier et al., 2007a and Rougier et al., 2007b); the turtle Condorchelys (Sterli, 2008); the sphenodont Sphenocondor (Apesteguia et al., 2012); the heterodontosaurid ornithischian Manidens (Pol et al., 2011b); and isolated remains of anurans, crocodiles, and pterosaurs. A 10 cm-thick tuff bed located between the two lower basalt flows at the lower section of the Cañadón Asfalto Formation was recently dated in ca 178 Ma using radioisotopic methods on U – Pb from zircons (Cúneo et al., 2013). The Queso Rallado locality stratigraphically rests above the dated sediments, and although there is no accurate age for this locality, based on correlations the Queso Rallado sediments were regarded as close to the base of the formation (lower – middle Toarcian age; see Cúneo et al., 2013). Other localities from the Cañadón Asfalto Formation have yielded diverse sauropod remains and a recent study (Holwerda et al., 2015) identified different morphotypes of enamel-wrinkling among sauropod teeth from this unit. A detailed comparison with these morphotypes is provided after the description.

The overall morphology of the crown of specimen MPEF-PV 10860 is labiolingually compressed, mesiodistally expanded above the root, and with a basal constriction, characterizing a spatulate shape as in most Sauropodomorpha (e.g., Galton and Upchurch, 2004, Gauthier, 1986, Sander, 1997 and Upchurch et al., 2004). The crown is taller than wide but rather broad, its SI value (sensu Upchurch, 1998) is 1.31, close to Mussaurus (1.68; MPM-PV 1813/4) and Leonerasaurus (1.47; MPEF-PV 1663, fourth dentary tooth), but lower than Yunnanosaurus (1.95; Chure et al., 2010) and Melanorosaurus (2.13; Yates, 2007a). SI values lower than 3 are also present in the basal sauropods as Barapasaurus (1.6; Bandyopadhyay et al., 2010), Pulanesaura (1.97; measured from McPhee et al., 2015: fig. 2), Archaeodontosaurus (1.4; Chure et al., 2010), Amygdalodon (1.34–1.49; Carballido and Pol, 2010), Tazoudasaurus (approximately 1.4–1.6; measured from Allain and Aquesbi, 2008: fig. 7). Among tooth material known from the Cañadón Asfalto Formation, SI values lower than 3 are also measured in the spatulate-shaped crowns of Patagosaurus and other isolated sauropod crowns (1.1–1.9 and 2.6; Holwerda et al., 2015). The crown is wider than its root (Fig. 1A and B), a widespread character amongst basal sauropods (e.g., Upchurch et al., 2004) and more pronounced than in non-sauropod sauropodomorphs as Mussaurus (Pol and Powell, 2007), Plateosaurus (Prieto-Márquez and Norell, 2011), and Yunannosaurus (Barrett et al., 2007).

One of the carinae has fewer denticles and originates more apically, which allows identifying this margin as the mesial side of the tooth (Galton and Upchurch, 2004). In addition, one of the crown faces is slightly concave whereas the other is markedly convex in apicobasal direction, identified as lingual and labial faces respectively (Fig. 1C and D). The labial face of the crown is completely convex mesiodistally, but its lingual face is slightly sigmoidal (Fig. 1F) with a convex area close to the distal margin (due to presence of a low apicobasal eminence) and slightly concave close to the mesial margin (especially above the apical half). These features have been used to characterize a spoon-like crown and a D-shaped cross-section (Fig. 1E and F). This shape in cross-section differs from the condition of basal sauropodomorphs such as Plateosaurus (Prieto-Márquez and Norell, 2011), Thecodontosaurus (Benton et al., 2000) and at least in the middle-posterior teeth of Mussaurus and Anchisaurus (MPM-PV 1813/4 and YPM 1883, respectively). Conversely, this feature is described in Leonerasaurus (Pol et al., 2011a), Melanorosaurus (Yates, 2007a), Amygdalodon (Carballido and Pol, 2010), Pulanesaura (McPhee et al., 2015), and most sauropods (Wilson, 2002), including Patagosaurus and all other isolated eusauropod teeth from Cañadón Asfalto Formation (Holwerda et al., 2015 and Rauhut, 2003). The D-shaped cross-section is similarly developed in Chinshakiangosaurus, which combines a convex labial face in a mesiodistal direction with a lingual face bearing a slightly developed mesial concavity and a convexity along the distal region (Upchurch et al., 2007a). Thus, a marked concavity at the lingual face is clearly absent in MPEF-PV 10860 if compared with eusauropods, but shallow concavities appear as slightly developed in different non-eusauropod sauropodomorphs and may represent an early evolutionary stage of the conspicuous lingual concavities of eusauropods (Carballido and Pol, 2010, Upchurch, 1998, Upchurch et al., 2004 and Upchurch et al., 2007a). Details on the development of these concavities however are difficult to analyze, as its variation along the tooth row in many of the non-eusauropod sauropods is still unknown. The crown lacks distinct narrow grooves parallel to the crown margins on both the labial and lingual surfaces (e.g., Upchurch, 1998), and despite the fact that a median low eminence is identified on the lingual surface of MPEF-PV 10860, a conspicuous crest is absent on this face. The presence of grooves has been noted as an apomorphic feature for broad-crowned eusauropods and other broad-crowned sauropods (Barrett and Upchurch, 2005, Barrett and Upchurch, 2007, Carballido and Pol, 2010, Upchurch and Barrett, 2000 and Upchurch et al., 2004, Upchurch et al., 2007a), however their presence is variable in most eusauropod teeth from Cañadón Asfalto (Holwerda et al., 2015). Most of these teeth possess at least one well-developed groove (either on their labial or lingual face and either close to the distal and mesial margin), but the four types of grooves are rarely simultaneously present (e.g., MACN-CH 2008.2) and one isolated tooth (MACN-CH 2009) lacks all grooves. The latter, however, can be distinguished from MPEF-PV 10860 by considering its conspicuous lingual concavity, a V-shaped wear facet, and its distinct enamel-wrinkling (morphotype I, see below).

The crown margins bear seven (distal) and six (mesial) coarse denticles (with 1.69 and 1.82 denticles per mm in mesial and distal margins, respectively), a common feature among basal sauropodomorphs (Barrett and Upchurch, 2007 and Galton and Upchurch, 2004). The denticulated margins start at the mesiodistally widest section of the crown and occupy the apical two thirds of the crown, differing from the plesiomorphic condition of basal sauropodomorphs in which the denticles occupy most of the crown margin (e.g., Plateosaurus). The presence of denticles is also recorded in eusauropods (e.g., Omeisaurus, Shunosaurus, Barapasaurus), including specimens referred to Patagosaurus and other isolated teeth from the Cañadón Asfalto Formation, but they usually have a lower number denticles (between 2 and 4 denticles, with the exception of one tooth [MPEF-PV 3341] with 8 denticles at the mesial margin) which are proportionately smaller (if compared to the crown size) and restricted to the apical third of the crown (Rauhut, 2003), differing from MPEF-PV 10860 (below the apical half). The denticles of MPEF-PV 10860 are subconical rather than labiolingually compressed, increase in size towards the apex and are upwardly directed forming an angle of 45° respect to the margin of the tooth, the last being a feature shared with basal species of Sauropodomorpha (e.g., Galton and Upchurch, 2004, Upchurch et al., 2004, Upchurch et al., 2007b and Upchurch et al., 2007a). The orientation of denticles in eusauropods might vary depending on the species, however as in Patagosaurus, its size is proportionately smaller than in MPEF-PV 10860 and are only present above the apical half (Bandyopadhyay et al., 2010, He et al., 1988 and Zheng, 1996). Whereas in Patagosaurus denticles are regarded as apically oriented (Rauhut, 2003), other species show a more variable orientation along the carinae (Shunosaurus, Omeisaurus [He et al., 1988: fig. 16; Zheng, 1996: p. 40), or its orientation is poorly preserved in the available material (Barapasaurus [Bandyopadhyay et al., 2010: fig. 4]). The serrated mesial margin is slightly convex in lateral view and the distal margin is slightly concave, creating an asymmetrical profile of the crown in labial or lingual view (Fig. 1 and Fig. 2). In apical view, the denticulated mesial and distal margins form an S-shaped outline.

Both surfaces of the crown have their enamel wrinkled, covering the apical half and extending basal to the denticles close to the distal margin on both faces (Fig. 1 and Fig. 2). This wrinkling consists of enamel globular protuberances that are separated by broad anastomosed grooves varying in depth and randomly oriented along most of the surface of the crown, but with a slightly dominant apicobasal orientation along the mesial part of the lingual face (Fig. 2). The wrinkling progressively disappears toward the base of the crown and the base of the denticles (Fig. 2). The development of the enamel-wrinkling differs from the smooth enamel surface of non-anchisaurian sauropodomorphs, such as Thecodontosaurus, Yunannosaurus, and Plateosaurus (Barrett et al., 2007, Carballido and Pol, 2010 and Galton and Upchurch, 2004), and is far more conspicuous than the faintly wrinkling of Leonerasaurus (Pol et al., 2011a), Mussaurus (Pol and Powell, 2007), Anchisaurus (Yates, 2004 and Yates, 2010), and Melanorosaurus (Yates, 2007a). In addition, MPEF-PV 10860 enamel-wrinkling pattern also lacks of the evident apicobasal orientation of grooves and crests present in Anchisaurus and Melanorosaurus.

Regardless of the crown morphology, the enamel-wrinkling pattern of rounded enamel protuberances with shallow anastomosed grooves randomly oriented of MPEF-PV 10860 resembles the finely wrinkled enamel of Archaeodontosaurus (Buffetaut, 2005: fig. 2), Pulanesaura (McPhee et al., 2015: Fig. 3), and Tazoudasaurus (Allain and Aquesbi, 2008: fig. 2). However, there are some differences in the enamel-wrinkling of these species and that of MPEF-PV 10860: Tazoudasaurus bears coarse ridges on the lingual surface of the crown (Allain and Aquesbi, 2008) that are absent in MPEF-PV 10860; and enamel-wrinkling occupies the entire crown surface in Tazoudasaurus and Archaeodontosaurus, but it extends only slightly below the apical half in MPEF-PV 10860 and Pulanesaura (Allain and Aquesbi, 2008, Buffetaut, 2005 and McPhee et al., 2015). Enamel protuberances of MPEF-PV 10860 are also smaller in size and less conspicuous than in Chinshakiangosaurus (Upchurch et al., 2007a), Amygdalodon (Carballido and Pol, 2010), and the condition commonly found within Eusauropoda (Upchurch, 1998 and Wilson, 2005). In addition, MPEF-PV 10860 also differs from Chinshakiangosaurus in lacking of a reticulate pattern of wrinkling, and from Amygdalodon in having shallower and proportionally wider grooves in comparison with the enamel protuberances. Amygdalodon also possess enamel crests and grooves apicobasally disposed and interrupted by pits on the entire surface of the crown, autapomorphic features of this species (Carballido and Pol, 2010: figs. 4 and 5) that are clearly absent in MPEF-PV 10860. The enamel-wrinkling of MPEF-PV 10860 also differs from that of eusauropods, including the three morphotypes described for the Cañadón Asfalto Formation (see below).

The presence of small worn surfaces indicates the crown had already erupted. There are small worn areas at the crown apex and on the fifth and sixth denticles of the mesial margin (Fig. 2C). Furthermore, the apical half of the labial surface of the crown is uniformly worn but lacks a distinct wear facet, and the enamel-wrinkling is less notorious in this area (Fig. 2B, D and E). This incipiently developed wear of MPEF-PV 10860 is likely due to abrasion (tooth-to-food interaction), and there is no evidence of a facet produced by tooth-to-tooth occlusion. The type of wear is similar to that of weathered crowns described for Massospondylus, Plateosaurus, and Yunnanosaurus (e.g., Galton and Upchurch, 2004). This contrasts with the extensively developed V-shaped wear facets of Patagosaurus and other isolated teeth for the Cañadón Asfalto Formation, a feature also present in most gravisaurians (Carballido and Pol, 2010, Holwerda et al., 2015 and Wilson and Sereno, 1998). These differences, however, must be taken with caution, as it is still largely unknown how the development of wear facets varies along the tooth row in many taxa, including basal forms of Sauropoda.

The specimen MPEF-PV 10860 from the Cañadón Asfalto Formation (late Early Jurassic – early Middle Jurassic) increases the current knowledge on sauropodomorph dental diversity in this unit. The noted differences in tooth morphology, type of tooth wear, and tooth size indicate that MPEF-PV 10860 represents either a new species or the first dental record of Volkheimeria chubutensis, previously known from postcranial materials. Differences with all other eusauropod teeth from Cañadón Asfalto Formation may also be indicative of different feeding strategies and niche partitioning among sauropodomorphs from this unit. More complete and associated remains are clearly needed to achieve a more complete understanding of the paleoecological significance of the diverse association of megaherbivores from the Cañadón Asfalto Formation. Despite these uncertainties, the coexistence of multiple sauropodomorph lineages in the late Early to early Middle Jurassic places the Cañadón Asfalto Formation in an unparalleled position for understanding the dynamics of continental ecosystems in this key period of dinosaur evolution.