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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 of Argentina and

insights on the diversity of the

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 rocks document the first steps of the early evolutionary radiation. Although

Received 7 April 2011

the oldest 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– 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 - Los Colorados Formation. In this contribution we enrich faunal list of

Keywords:

this assemblage with the description of a new medium-sized 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 and closely related to

Argentina , , 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 (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 –early Norian of NW (Bonaparte, 1972; Bonaparte and Pumares, 1995),

Argentina, one of the oldest known dinosaur-bearing assemblages (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

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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, ; 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 fortis (TM

tein, Free State, ; 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 (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 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. The centrum is only

Saurischia Seeley, 1887 sensu Gauthier, 1986 slightly compressed transversely at mid-length. The anterior and

Sauropodomorpha von Huene, 1932 sensu Salgado et al., 1997 posterior articular facets are taller than wide, but this difference is

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

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Fig. 2. Left tibia of PULR 136 in lateral (A), anterior (B), medial (C), posterior (D), proximal (E), and ventral (F) views; proximal caudal (G–I) and middle caudal vertebrae (J–L) in

right lateral (G and J), ventral (H and K), anterior (I), and posterior (L) views. Abbreviations: alf, anterolateral fossa; cc, cnemial crest; fap, facet for the reception of the

ascending process of the astragalus; ls, lateral sulcus; plc, posterolateral condyle; plp, posterolateral process; pmc, posteromedial condyle; pmn, posteromedial notch. Scale

bars 10 cm for A–F and 2 cm for G–L.

greater in the posterior facet. The anterior articular facet is taller shallowly depressed but in a lesser degree than in the proximal

and wider than the posterior one. The ventral surface is convex, caudal vertebra.

lacking a ventral groove. The lateral surfaces of the centrum are

shallowly concave. The mid-caudal vertebral centrum is approxi- 3.2. Dorsal rib

mately 0.94 times anteroposteriorly shorter than the proximal one

but proportionally more elongated, contrasting with the propor- The preserved portion of the dorsal rib shaft is rod-like and with

tionally longer mid-caudal centra of several non-plateosaurian a circular cross-section.

sauropodomorphs, such as (Galton and Kermarck,

2010) and Nambalia (Novas et al., 2011). The articular facets are 3.3. Tibia

damaged, but the posterior one seems to have been roughly oval in

profile with a height 0.71 times the anteroposterior length of the The left tibia of PULR 136 is complete and well-preserved

centrum. In ventral view, the centrum is slightly compressed at (Fig. 2A–F and Table 1). The tibia has an overall robust morphology,

mid-length, resembling the condition of the proximal caudal with its length approximately twice the anteroposterior depth of

vertebra. The ventral surface of the element is convex and also the proximal end. This ratio resembles that of basal sauropods,

lacks a ventral groove. The lateral surface of the centrum is such as (MHNM To1-380: Allain and Aquesbi,

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

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Table 2

Selected measurements of tibiae (in millimeters) and ratios of non-neosauropod sauropodomorphs used in the bivariate plots. Abbreviations: L, total length; Mw,

anteroposterior depth at mid-length; Pw, proximal anteroposterior depth.

L Pw Mw Ratio L/Pw Ratio L/Mw Phylogenetic slice Source

PULR 136 405 200 75 2.02 5.4 New specimen This study

Adeopapposaurus 208.5 48.9 22.1 4.26 9.43 ‘‘Prosauropoda’’ PVSJ 569

Anchisaurus 145 47 19.8 3.08 7.32 ‘‘Prosauropoda’’ YPM 1883 (Galton, 1976: Table 3)

Barapasaurus 505 215.3 85.2 2.34 5.93 Sauropoda ISI R62 (Bandyopadhyay et al., 2010)

Blikanasaurus 376 180 64.5 2.08 5.83 Sauropoda Galton and van Heerden (1985: Table 1)

Cetiosaurus 945 390 – 2.42 – Sauropoda OUMNH J13621 (Upchurch and Martin, 2003: Table 3)

Chromogisaurus 175 41.8 17.3 4.19 10.12 Guaibasauridae PVSJ 845

Coloradisaurus 460 180 54.5 2.55 8.44 ‘‘Prosauropoda’’ PVL field #6

Efraasia 216 61 25.9 3.54 8.34 ‘‘Prosauropoda’’ SMNS 12667 (Galton, 1973: Table 2)

Efraasia 421.4 123.3 48.5 3.24 8.69 ‘‘Prosauropoda’’ SMNS 12220

Eucnemesaurus 397.1 176.5 73.5 2.25 5.4 ‘‘Prosauropoda’’ TM 119 (van Heerden, 1979: Fig. 20)

Gryponyx 447 163 55 2.74 8.13 ‘‘Prosauropoda’’ Broom (1911: plate XIV, Fig. 4)

Kotasaurus 730 370 135.2 1.97 5.4 Sauropoda 115/S1Y/76 (Yadagiri, 2001)

Lessemsaurus 460 165.5 76.4 2.78 6.02 Sauropoda PVL 4822-66

Melanorosaurus 499 217 59.4 2.3 8.4 ‘‘Prosauropoda’’ NM QR1551 (massive left tibia) Galton et al. (2005: Table 1.2)

Melanorosaurus 509 176 62.4 2.89 8.16 ‘‘Prosauropoda’’ NM QR1551 Galton et al. (2005: Table 1.2)

Panphagia 157 33 15.4 4.75 10.19 Guaibasauridae PVSJ 874 (Martinez and Alcober, 2009)

Pantydraco 72 13.5 5.5 5.33 13.09 ‘‘Prosauropoda’’ NHMUK P77/1 (Galton and Kermarck, 2010)

Patagosaurus 380.9 157.1 71.4 2.42 5.33 Sauropoda Bonaparte (1986)

Plateosauravus 372.2 161.1 52.8 2.31 7.05 ‘‘Prosauropoda’’ SAM 3341 (van Heerden, 1979: Fig. 17)

P. engelhardti 710 280 104 2.54 6.83 ‘‘Prosauropoda’’ BSP 1962 I 153, PQ XL (Moser, 2003)

P. engelhardti 154 46 21 3.35 7.33 ‘‘Prosauropoda’’ MB Skelett I (Moser, 2003: tafel 11, Fig. 1)

P. engelhardti 525 190 67.5 2.76 7.78 ‘‘Prosauropoda’’ JM Keu 2001/1 (Moser, 2003: tafel 27, Fig. 7)

P. engelhardti 527 155 57.5 3.4 9.16 ‘‘Prosauropoda’’ SMNS 13200 (Moser, 2003: tafel 36: Fig. 1)

P. gracilis 428.5 111.5 53.9 3.84 7.95 ‘‘Prosauropoda’’ SMNS 17928 (Galton, 1984: Table 1)

Riojasaurus 508.3 219.2 77.3 2.32 6.58 ‘‘Prosauropoda’’ PVL 3808

Saturnalia 158 41 12 3.85 13.17 Guaibasauridae MCP 3844-PV (Langer, 2003: Table 3)

Tazoudasaurus 250 109 44 2.29 5.68 Sauropoda MHNM To1-76 (left) (Allain and Aquesbi, 2008: Table 6)

Tazoudasaurus 710 360 143 1.97 4.96 Sauropoda MHNM To1-380 (right) (Allain and Aquesbi, 2008: Table 6)

Volkheimeria 365.4 180.8 80.8 2.02 4.52 Sauropoda Bonaparte (1986)

Vulcanodon 634 250 129 2.54 4.91 Sauropoda QG 42 (Raath, 1972: Table 5)

2008), (115/S1Y/76: Yadagiri, 2001), present in some of the most basal sauropodomorphs (e.g.

(Bonaparte, 1986), and Blikanasaurus (SAM K403: Galton and Saturnalia: Langer, 2003; : PVSJ 874; :

van Heerden, 1985), but is lower than those of non-sauropod Ezcurra, 2010; Coloradisaurus: PVL field #6). A distinct vertical

sauropodomorphs (e.g. Riojasaurus, PVL 3808; Coloradisaurus, PVL edge extends from the medial posterior condyle along the proximal

field #6: sensu Yates, 2004: Table 2) (Table 2). The proximal end of quarter of the posterior surface of the tibia and distally merges

the tibia of PULR 136 is well expanded anteroposteriorly and with the convex posterior surface of the shaft. The shaft of the bone

transversely with respect to the shaft, with a prominent cnemial is straight and sub-elliptical in cross-section, being anteroposter-

crest and protruding medial posterior condyle in lateral view. The iorly deeper than transversely wide at mid-length. However, the

cnemial crest is more anteriorly developed than in Lessemsaurus degree of assymetry of the tibial shaft at mid-length is lower than

(PVL 4822-66; Fig. 3A and B). The proximal articular surface of the in some basal sauropods, such as (Raath, 1972).

tibia is sub-triangular in outline, with a low and slightly laterally The distal end of the tibia is anteroposteriorly expanded in

curved cnemial crest. The tibia reaches its highest point at the comparison with the shaft, but is 0.64 times anteroposteriorly

anterior-most end of the cnemial crest and in consequence the shallower than the proximal end. In contrast, the proximal and

posterior condyles are situated below it. The proximal articular distal ends of the tibia are sub-equally expanded along their

surface of the bone is posteroproximally oriented, contrasting with transverse axis. This condition resembles the morphology of

the proximally oriented proximal surface of Lessemsaurus (PVL Lessemsaurus (Pol and Powell, 2007: Fig. 10), but differs from that

4822-66; Fig. 3A and B). of Riojasaurus (PVL 3808) and Coloradisaurus (PVL field #6), in

The posterior condyles are separated from each other by a which the transverse width of the distal end of the tibia is

shallow median depression on the proximal surface of the bone markedly narrower than that of the proximal end (Fig. 3). The

and a wide and shallow concavity at the posterior margin of the distal end of the tibia of PULR 136 is slightly anteroposteriorly

proximal end. The proximal surfaces of the posterior condyles are deeper than transversely wide, contrasting with most basal

slightly convex. The medial posterior condyle is more posteriorly sauropodomorphs, such as Coloradisaurus (PVL field #6), Rioja-

projected than the lateral one, as usually occurs in sauropodo- saurus (PVL 3808), Lessemsaurus (Pol and Powell, 2007), and

morph dinosaurs (Sereno, 1999), but contrasting with Antetonitrus (Yates and Kitching, 2003), in which it is transverse-

ingens (Galton, 1986), Jaklapallisaurus (Novas et al., 2011), and ly wider than anteroposteriorly deep (Fig. 3). In addition, the

Coloradisaurus (PVL field #6). The posterior condyles are proxi- distal end of tibia of Lessemsaurus (PVL 4822-66) is proportion-

mally projected at the same level, contrasting with Coloradisaurus ally more anteroposteriorly expanded than that of PULR 136

in which the lateral condyle is slightly ventrally deflected (PVL field (Fig. 3A and B).

#6). A moderately deep concavity, bounded by the cnemial crest The anteroposterior length of the medial and lateral margins of

and the lateral posterior condyle, extends on the anterolateral the distal end of the tibia is sub-equal. The facet for the reception of

corner of the proximal end of the tibia. This concavity probably the ascending process of the astragalus is well-developed, oblique,

participated in the proximal articulation of the tibia with the and sub-rectangular in distal view, resembling the condition of

fibula. Directly below the lateral posterior condyle the lateral other basal sauropodomorphs and theropods (Novas, 1989; Langer

surface of the bone is concave, without the vertical tuberosity and Benton, 2006). The posterolateral process of the distal end is

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

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Fig. 3. Comparison among the sauropodomorph species with known tibiae of the Los Colorados Formation. PULR 136 (A and E), Lessemsaurus sauropoides (B and F), Riojasaurus

incertus (C and G), and Coloradisaurus brevis (D and H) in lateral (A–D) and distal (E–H) views. Abbreviations: as in Fig. 2 and ln, lateral notch. Scale bars 10 cm.

anteroposteriorly thick and transversely short. Indeed, this process field #6) (Figs. 2C and F and 3). The medial border of the distal end

does not exceed laterally the level of the facet for the reception of of the tibia is slightly concave and the anteromedial corner forms

the ascending process of the astragalus. This condition is also an acute angle.

present in basal sauropods (e.g. Blikanasaurus: Galton and van

Heerden, 1985; Antetonitrus: Yates and Kitching, 2003) and has 4. Discussion

been considered a synapomorphy of Sauropoda (Yates, 2007). The

posterolateral process is lobe-shaped, vertically oriented, and 4.1. Phylogenetic affinities of PULR 136

distally extends well beyond the anterior articular surface of the

distal end of the tibia. The facets for the reception of the ascending The overall morphology of the tibia of PULR 136 is distinct from

process of the astragalus and the posterolateral process are those of the other known sauropodomorphs from the Los

separated laterally by a shallow concavity, which is developed on Colorados Formation (i.e. Riojasaurus, Coloradisaurus, and Lessem-

the lateral surface of the distal end of the bone as an incipient saurus) in its robustness, which is comparable with that of some

depression. The latter condition contrasts with the deep notch basal sauropod dinosaurs. We quantify the robustness of the tibia

present in plateosaurian sauropodomorphs, including Riojasaurus of PULR 136 and compare it with that of other non-neosauropod

(PVL 3808), Plateosaurus engelhardti (SMNS 13200), Adeopappo- sauropodomorphs in a plot showing the ratio between the length

saurus (PVSJ 569), and Coloradisaurus (PVL field #6) (Fig. 3). and the proximal anteroposterior depth of the tibia against the

However, the morphology observed in PULR 136 resembles in this ratio between the length and mid-shaft anteroposterior depth of

trait that present in basal sauropods (e.g. Blikanasaurus: Galton and the tibia (Table 2 and Fig. 4). The data set was separated into three

van Heerden, 1985; Antetonitrus: Yates and Kitching, 2003; groups: guaibasaurids (sensu Ezcurra, 2010), ‘‘core-prosauropods’’

Lessemsaurus: PVL 4822-66). The posteromedial corner of the (i.e. a paraphyletic grade including non-sauropod sauropodo-

distal end of the tibia of PULR 136 presents a concave notch, which morphs more derived than guaibasaurids; sensu Sereno, 2007; see

should have received the posteromedial dorsal expansion of the Langer et al., 2010), and sauropods. Convex hulls were recon-

astragalar body which is usually present in basal theropods (e.g. structed for the dispersion of the plotted data set of each group in

Zupaysaurus: PULR 076; Ezcurra and Novas, 2007) and sauropo- order to represent their morphospace. The bivariate plot and

domorphs (e.g. Riojasaurus: PVL 3845, 4364; Coloradisaurus: PVL convex hulls were constructed with PAST 2.03 (Hammer et al.,

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

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Fig. 4. Bivariate plot showing the ratio between the total length and anteroposterior depth of the proximal end of tibia (L/AP) with respect to the ratio between the total length

and anteroposterior depth at mid-length of tibia (L/AM). The convex hulls represent the morphospace generated by the following groups: Guaibasauridae (closed triangles),

‘‘Core-Prosauropoda’’ (closed circles), and Sauropoda (opened circles). PULR 136 is depicted with an opened triangle. Tibiae illustrated in the graphic are: I, Pantydraco

(NHMUK P77/1); II, Panphagia (PVSJ 874); III, Saturnalia (MCP 3844-PV); IV, (PVSJ 569); V, Coloradisaurus (PVL field #6); VI, Riojasaurus (PVL 3808); VII,

Lessemsaurus (PVL 4822-66); VIII, PULR 136.

2001). In this plot, PULR 136 falls into the morphospace occupied Coloradisaurus (PVL field #6), in which this process is well

by sauropods and outside the area occupied by both guaibasaurids laterally developed (Fig. 3E–H). The distal end of the tibia of

and ‘‘core-prosauropods’’ (Fig. 4). Riojasaurus and Coloradisaurus Lessemsaurus is strongly damaged and any statement on its

fall within the morphospace occupied by ‘‘core-prosauropods’’ and morphology must be considered tentative (PVL 4822-66), but Pol

Lessemsaurus was found at the lower limit of sauropod robustness and Powell (2007) described it as sub-rectangular with the major

but still below the morphospace occupied by ‘‘core-prosauropods’’ axis orientated lateromedially and being twice as long as its

(Fig. 4). The unique ‘‘core-prosauropod’’ which falls within the anteroposterior extension. The latter condition differs from that

sauropod morphospace is Eucnemesaurus. In this regard, the of PULR 136 in which the major axis is oriented anteroposteriorly

proportions of the tibia of PULR 136 approach more closely that of (Fig. 3E and F). In addition, the distal end of the tibia of

sauropods rather than those of Riojasaurus, Coloradisaurus, and Lessemsaurus is more anteroposteriorly expanded than that of

other non-sauropod sauropodomorphs. PULR 136 in lateral view (Fig. 3A and B).

Beyond its proportions, PULR 136 differs from the previously The basal massopodan Riojasaurus is the most abundant taxon

reported sauropodomorphs of the Los Colorados Formation in of the Los Colorados Formation and it is currently known by several

some key features. The proximal end of the tibia of PULR 136 specimens of different ontogenetic stages (Bonaparte, 1972).

differs from that of Coloradisaurus (PVL field #6), but resembles Bonaparte (1972) described that the tibiae of the juvenile

that of Riojasaurus (PVL 3808) and Lessemsaurus (Pol and Powell, specimens of Riojasaurus are lesser proximally expanded and

2007) in the presence of a medial posterior condyle distinctly more gracile than in adult individuals. Thus, with this ontogenetic

more posteriorly extended than the lateral one. Nevertheless, variation in mind, it could be assumed that PULR 136 would be a

PULR 136 differs from Lessemsaurus in the presence of a well late adult specimen of Riojasaurus. However, the total tibial length

anteriorly developed cnemial crest and a posteroproximally of PULR 136 is around 80% smaller than that of the holotype and

oriented proximal articular surface (Fig. 3A and B). In the distal adult specimen of Riojasaurus (PVL 3808), which is proportionally

end of the bone, PULR 136 lacks the widely open lateral notch more gracile than the taxon reported here (Table 2). Furthermore,

that separates the facet for the reception of the ascending process PULR 136 exhibits features only recognized among basal saur-

of the astragalus and the posterolateral process in Riojasaurus opods, indicating that the differences observed with Riojasaurus

(PVL 3808) and Coloradisaurus (PVL field #6) (Fig. 3). PULR 136 are not related with ontogeny.

shares with Lessemsaurus (Pol and Powell, 2007: Fig. 10), as well Based on the differences highlighted above, we consider that

as other basal sauropods (Yates, 2007), the presence of a short PULR 136 represents a distinct sauropodomorph taxon from that

posterolateral process completely medial to the lateral level of previously reported for the Los Colorados Formation. Unfortunate-

the facet for the ascending process of the astragalus, but ly, we do not have overlapping elements between PULR 136 and

this condition is not present in Riojasaurus (PVL 3808) and the sauropodomorph specimen preliminarily reported by Martinez

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

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et al. (2004) and any kind of comparisons are impossible. Due to treated as additive (ordered). As measures of tree support, Bremer

the fragmentary nature of the specimen we decided for the sake of supports were calculated and a bootstrap resampling analysis, set

taxonomic stability to not coin a new species for PULR 136. up with 10,000 pseudoreplicates, was performed. The Indian

Once determined the distinction of PULR 136 from the other specimen ISI R282 was pruned a priori because of its highly

known sauropodomorphs of this Argentinean assemblage, a unstable position among very basal saurischians (Novas et al.,

cladistic analysis was performed in order to test the phylogenetic 2011).

relationships of the new specimen. PULR 136 was codified in the The search recovered 90 MPTs of 1244 steps, with a consistency

data matrix of Novas et al. (2011) (see Appendix A), which is a index of 0.361, a retention index of 0.689, and the best score hit in

modified version of the data set originally published by Yates 27 of the 50 replications. The overall topology of the consensus

(2007) and subsequently modified by other authors (e.g., Smith tree agrees with that obtained by Novas et al. (2011) and only

and Pol, 2007; Ezcurra, 2010). Additionally, one character differs in the degree of resolution among very basal sauropods

(character 381; see Appendix A) was added to the data matrix after the inclusion of PULR 136. The specimen reported here was

of Novas et al. (2011). The data matrix was analysed under equally recovered in all the MPTs as a basal sauropod, being nested within

weighted parsimony using TNT 1.1 (Goloboff et al., 2008). A a polytomy also composed of Antetonitrus, Lessemsaurus, Blika-

heuristic search of 50 replications of Wagner trees (with random nasaurus, , , and a monophyletic

addition sequence) followed by TBR branch swapping algorithm including more derived sauropods (e.g. Tazoudasaurus and

(holding 10 trees per replicate) was performed. The best trees Vulcanodon) (Fig. 5A).

obtained at the end of the replicates were subjected to a final round The results of the phylogenetic analyses perfectly match with

of TBR branch swapping. Zero length branches among any of the the sauropod signal recovered for PULR 136 in the bivariate plots of

recovered MPTs were collapsed (rule 1 of Coddington and Scharff, tibia robustness. Indeed, the ratio between the total length and the

1994). Characters 8, 19, 23, 40, 57, 117, 121, 131, 147, 149, 150, anteroposterior depth at mid-shaft of the tibia (L/AM) was

167, 205, 207, 222, 227, 234, 242, 254, 267, 280, 299, 313, 349, 353, optimized as a continuous character in the strict-consensus tree

378, and 381 represent nested sets of homologies and/or entail using traditional parsimony (Goloboff et al., 2006). The result

presence and absence information and as a result they were showed a trend towards a more robust tibia progressing up the tree

Fig. 5. Strict consensus tree of 90 MPTs depicting the phylogenetic relationships among basal massopodan sauropodomorphs and the position of PULR 136 (A) and bivariate

plot showing the optimization of the ratio between total length and anteroposterior depth at mid-length of tibia (L/AM) in the phylogeny with respect to clade rank (patristic

distance) (B). Tibiae illustrated in the plot are: I, Pantydraco (NHMUK P77/1); II, Coloradisaurus (PVL field #6); III, PULR 136. In (A) the numbers above the nodes are decay

indexes greater than one. Abbreviations: EUSAU, ; PLATEO, .

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and insights on the diversity of the Los Colorados Formation. Proc. Geol. Assoc. (2011), doi:10.1016/j.pgeola.2011.05.002

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Table 3

in the sauropodomorph phylogeny (Fig. 5B). Two distinct points of

List of the tetrapod assemblage currently recorded in the La Esquina Fauna of the

tibia robustness increase were observed, depicting a two step

upper levels of the Los Colorados Formation (modified from Arcucci et al., 2004

pattern, one among basal non-plateosaurian sauropodomorphs

following Arcucci and Coria, 2003; Martinez et al., 2004; Ezcurra et al., in press).

and the other between the node including Melanorosaurus plus

Dinosauria (5–6 species: 43%)

more derived forms and the base of Sauropoda (Fig. 5B). The latter

Riojasaurus incertus

decrease in tibia robustness coincides and could be related with Coloradisaurus brevis

the acquisition of a habitual, if not obligate, quadrupedalism (Yates Lessemsaurus sauropoides

PULR 136 et al., 2010).

Unnamed sauropodomorph

PULR 136 is situated within Sauropodomorpha because of the

Zupaysaurus rougieri

presence of a proximal end of tibia with the posterior end of the Testudinata (1 species: 7%)

posterior lateral condyle anterior to the posterior margin of the Paleochersis talampayensis

Pseudosuchia (6 species: 43%)

posterior medial condyle. The new specimen has a synapomor-

Hemiprotosuchus leali

phy of the clade that includes Efraasia and more derived

Pseudhesperosuchus jachaleri

sauropodomorphs: length of middle caudal vertebra centra less

Neoaetosauroides engaeus

than twice the height of their anterior articular facets. In tennuisceps

addition, PULR 136 is nested within the clade including Fasolasuchus tenax

Unnamed poposauroid

Lessemsaurus, Antetonitrus, Blikanasaurus, Camelotia, Gongxiano-

Cynodontia (1 species: 7%)

saurus, and more derived sauropods because it presents a distal

Chaliminia musteloides

end of tibia with the lateral margin of the posteroventral process

set well back from the anterolateral corner and a ratio between

the length of the tibia and its anteroposterior depth at mid-

length equal or lower than 6.3 and greater than 5.5. However, upper Los Colorados Formation, the number of recognized

the absence of the tallest point of the cnemial crest about half- dinosaur species ranges between five and six (Table 3). This

way along the length of the crest, creating an anterodorsally alpha diversity represents, as a maximum, 43% of the number of

sloping proximal margin of the crest, and an anteromedial tetrapod species currently known from this faunal assemblage. It

corner of the distal articular surface of tibia forming a right must be noted that a small theropod specimen is also known from

angle are characters that prevent the allocation of PULR 136 this assemblage (Bonaparte, 1972), but further studies are needed

within the clade including Tazoudasaurus, Vulcanodon, Isano- in order to determine, if it can be distinguished from the basal

saurus, and eusauropod sauropodomorphs. It must be pointed theropod Zupaysaurus rougieri (Arcucci and Coria, 2003; Ezcurra

out that a strict consensus tree of the same topology to that of and Novas, 2007). Approximately the same number of species and

the first analysis is recovered if only the tibia of PULR 136 is relative diversity documented for dinosaurs is present for other

scored in the data matrix (i.e. considering the caudal vertebrae clade of archosaurs: Pseudosuchia (Table 3). This indicates that,

as belonging to a different specimen). despite of relegated in abundance of specimens, pseudosuchians

Decay indexes are minimal (i.e. 1) through most of the were still taxonomically diverse during the deposition of the

massopodan lineage, with a pair of values of 2 among nodes more upper levels of the Los Colorados Formation. Other conspicuous

derived than PULR 136. Low values have been also obtained for the representatives of the La Esquina Fauna are testudinatans and

bootstraping, in which all the nodes among massopodan probainognathian cynodonts (Bonaparte, 1972; Arcucci et al.,

sauropodomorphs present ratios lower than 50%. The enforce- 2004), but both lineages are represented by a single species at the

ment of one of the MPTs as a constrained topology leaving PULR moment (approximately 7%) of the total alpha taxonomic level

136 as a floating taxon depicted that only one extra step is richness of the assemblage (Table 3). This ratio is notably small

necessary to nest the new specimen as the sister-taxon of when compared with the 86% represented by all archosaur

Eucnemesaurus or Tazoudasaurus. Suboptimal topologies of 2 extra species. Nevertheless, within archosaurs, sauropodomorphs

steps found PULR 136 as the most basal plateosaurid or constitute almost the half of this diversity, a proportion that

massopodan or the sister-taxon of or Riojasaurus, seems to be even more conspicuous in other late Norian-Rhaetian

and 3 extra steps were necessary to recover the new specimen as beds of Gondwana (e.g. Lower Elliot and Upper Maleri formations;

the sister-taxon of Coloradisaurus or within or Yates, 2003; Novas et al., 2011). The report of the distinct tibial

Eusauropoda. The variable position of PULR 136 among plateo- morphology of PULR 136 from those previously known from the

saurians under slightly suboptimal topologies is not unexpected Los Colorados Formation contributes to a better understanding of

due to the very fragmentary nature of the specimen. Accordingly, the tetrapod diversity of this important latest Triassic South

although PULR 136 seems to be more closely related to American assemblage.

Lessemsaurus and other very basal sauropods with the evidence

currently available at hand, more complete specimens will be Acknowledgements

necessary in order to reach a more confident phylogenetic

relationship for this taxon. We acknowledge Emilio Vacari for the loan of PULR 136 to Julia

B. Desojo and we are indebted to her for allowing the study of the

4.2. Insights on the tetrapod diversity of the upper Los Colorados specimen. We thank Diego Pol for his comments and suggestions

Formation on the manuscript. Guillermo Rougier leaded the field trip in which

PULR 136 was collected and the project has been funded by NSF

The upper levels of the Los Colorados Formation (La Esquina DEB 0946430 and National Geographic Society ‘‘Cynodonts and

Fauna; Bonaparte, 1972) is recognized as one of the earliest known early mammals from the Mesozoic of Argentina’’. We thank

assemblages dominated by dinosaurs, in which the most Jonathan Kaluza who skillfully prepared in part the tibia of PULR

abundant form is the basal massopdan sauropodomorph Rioja- 136. The comments and suggestions of Stephen Brusatte and an

saurus incertus (Bonaparte, 1972, 1982; Arcucci et al., 2004). anonymous reviewer improved the overall quality of the manu-

Beyond the numerical dominance of dinosaurs in this assemblage, script. Fig. 1 was created in part using Google Earth. Access to the

Dinosauria is also one of the most taxonomically diverse . free version of TNT 1.1 was possible due to the Willi Henning

Indeed, with the addition of PULR 136 to the faunal list of the Society.

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

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Appendix A

Characters added here to the data matrix of Novas et al. (2011):

381) Ratio between the length of the tibia and its anteroposterior depth at mid-length:

>11.7 (0); =11.7 and >9.7 (1); =9.7 and >7.6 (2); =7.6 and >6.3 (3); =6.3 and >5.5

(4); =5.5 (5) ORDERED.

The discretization among the character-states employed here was made through a cluster analysis of the ratio values conducted in PAST 2.03

(Hammer et al., 2001). The cluster analysis recovered six main groupings which represent the six character-states employed here. The

boundaries between the character-states were obtained as the shortest distance between the most marginal values of each group. The ratios

employed to score this character are depicted in Table 2.

Scorings for character 381:

State (0) Euparkeria, Crurotarsi, Marasuchus, , , , , , Ornithischia,

Saturnalia, Silesaurus, Staurikosaurus, , Pantydraco, and Tawa.

State (1) Chromogisaurus and Panphagia.

State (2) Coloradisaurus, Efraasia, Melanorosaurus, and Plateosaurus gracilis.

State (3) Anchisaurus, Plateosauravus, and Riojasaurus.

State (4) , Blikanasaurus, and Lessemsaurus.

State (5) Eucnemesaurus, , , PULR 136, and Vulcanodon.

State (2/3) Plateosaurus engelhardti.

State (4/5) Tazoudasaurus.

Missing data: , Antetonitrus, Camelotia, , Gongxianosaurus, Plateosaurus ingens, , ,

Lufengosaurus, , , , , , , , , MACN-Pv

18649a, Jaklapallisaurus, ISI R277, ISI R282, Nambalia, and Pradhania.

Scorings for PULR 136 in the data matrix of Novas et al. (2011) (character 381 not included here) (4% of the 380 characters could be scored for PULR 136):

PULR 136 ??????????????????????????????????????????????????????????????????????????????? ??????????????????????????????????????????????????????????????????????????????? ???????????????????????????????11?????????????????????????????????????????????? ??????????????????????????????????????????????????????????????000000010111????

???????????????????????????????????????????????????????????0?????

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and insights on the diversity of the Los Colorados Formation. Proc. Geol. Assoc. (2011), doi:10.1016/j.pgeola.2011.05.002