Analyzed the data: CA RNM OAA DP. Contributed reagents/materials/analysis tools: CA RNM OAA DP. Wrote the paper: CA RNM OAA DP.
The authors have declared that no competing interests exist.
Argentinean basal sauropodomorphs are known by several specimens from different basins; Ischigualasto, El Tranquilo, and Mogna. The Argentinean record is diverse and includes some of the most primitive known sauropodomorphs such as
We describe here a new basal sauropodomorph,
Our phylogenetic results differ with respect to previous analyses by rejecting the massospondylid affinities of some taxa from the northern hemisphere (e.g.,
Basal sauropodomorphs are non-sauropod sauropodomorphs that diversified and spread to most continents from the Late Triassic through the Early Jurassic, and constitute the first global radiation of herbivorous dinosaurs
The assignation of the latter record—
Herein we report a partial skeleton of a new basal sauropodomorph from the upper levels of the Quebrada del Barro Formation that has implications for our understanding on the diversity of massospondylid sauropodomorphs in South America and for the supposed age of this stratigraphic unit.
The Quebrada del Barro Formation (Marayes-El Carrizal Basin) crops out 140 km southeast of San Juan City, northwestern Argentina (
. A: Location and geologic map; B: Section of the Marayes group at the type locality. The red circle indicates the site of the holotype of
The Esquina Colorada Formation is composed of approximately 500 meters of conglomerates, sandstones, diamictites and ash layers
The second unit is the El Carrizal Formation with 100–350 meters of sandstones, quartz conglomerates, siltstones and coal
The Quebrada del Barro Formation
The Instituto and Museo de Ciencias Naturales has the appropriate permits needed to unearth the fossil materials that have been described in this paper (PVL 706).
The new specimen is a disarticulated incomplete skeleton. The proximity of all elements in an area of one half square meters, as well as the lack of duplicated elements and their relative size, suggest this association belongs to a single individual.
The specimen is well preserved and all the bones of the partial skeleton are three dimensional, complete, and most preserve fine anatomical details. The incompleteness of the skeleton is likely attributable to pre-burial processes.
The white bones of the holotype were embedded in a red fine-grained sandstone matrix with clay cement. The bones were prepared using a pneumatic air scribe pin vice, and water immersion.
We employ traditional anatomical and directional terms over veterinarian alternatives
We used different sources for phylogenetic definitions of taxa within Dinosauria: Sauropodomorpha
The comparisons with other sauropodomorphs and some theropods made in the description of the new specimen were based on the literature and on personal observation of specific taxa detailed in
Taxon | Source(s) |
|
Martínez |
|
Yates |
|
Bonaparte |
|
Galton |
|
Sereno et al. |
|
Smith and Pol |
|
Sereno and Novas |
|
Knoll |
Zhang and Yang |
|
|
Bonaparte |
|
Young |
|
Cooper |
|
Yates |
|
Pol and Powell |
|
Martinez and Alcober |
|
Yates |
|
Galton |
|
Bonaparte |
|
Alcober and Martínez |
|
Rowe et al. |
|
Langer |
|
Sertich and Loewen |
|
Benton et al |
|
Leal et al. |
|
Bai et al. |
|
Young |
|
Lu et al. |
All specimens compared in the text were observed from their respective source listed in this table. Comparisons based on other specimens, or taken from additional references, are explicitly indicated in the text.
The electronic version of this document does not represent a published work according to the International Code of Zoological Nomenclature (ICZN), and hence the nomenclatural acts contained in the electronic version are not available under that Code from the electronic edition. Therefore, a separate edition of this document was produced by a method that assures numerous identical and durable copies, and those copies were simultaneously obtainable (from the publication date noted on the first page of this article) for the purpose of providing a public and permanent scientific record, in accordance with Article 8.1 of the Code. The separate print-only edition is available on request from PLoS by sending a request to PLoS ONE, 1160 Battery Street, Suite 100, San Francisco, CA 94111, USA along with a check for $10 (to cover printing and postage) payable to “Public Library of Science”.
In addition, this published work and the nomenclatural acts it contains have been registered in ZooBank, the proposed online registration system for the ICZN. The ZooBank LSIDs (Life Science Identifiers) can be resolved and the associated information viewed through any standard web browser by appending the LSID to the prefix “
Dinosauria
Saurischia
Sauropodomorpha
Massospondylidae
urn:lsid:zoobank.org:act:3772093A-E5CD-48CF-9163-54E55FEDA240
The generic name honors the Leyes family, inhabitants of the small town Balde de Leyes, who made the discovery and notified the paleontologists of the San Juan Museum.
urn:lsid:zoobank.org:act:F7CD3D40-C64C-463E-A063-9E3FECF9C50D
PVSJ 706, a partial skeleton including skull with articulated mandible, lacking both nasals, left prefrontal, middle section of the left maxilla, anterior half of the left lower jaw, supraoccipital, both exoccipitals, ophistotics, laterosphenoids and vomers; atlas-axis articulated with anterior cervical vertebrae (C3-C7); an anterior and a middle caudal vertebra; proximal region of the left scapula, coracoid and humerus; partial blade of the right pubis lacking distal and proximal ends; proximal region of both ischia; partial left pes that includs distal tarsals III and IV, metatarsal III lacking its distal end, complete metatarsals IV and V, first phalanx of digit I, second phalanx of digit II, and second phalanx of digit IV (
). Reconstruction only shows preserved bones. Modified from Martinez
The type specimen was found near the locality Balde de Leyes, Caucete Department of San Juan Province, Northwestern Argentina (
The type specimen was found in red silty mudstones with a low clay cementation in the uppermost level of the Quebrada del Barro Formation
The age of the Quebrada del Barro Formation was regarded as Norian by Bossi and Bonaparte
The lack of diagnostic characters in the PVL 4087 precludes its identification as
The close affinities of the new specimen reported here with the basal sauropodomorph
A basal sauropodomorph diagnosed by the following autapomorphies and combination of characters (asterisks indicate autapomorphies): sharply acute angle (50°) formed by the ascending process of the maxilla with the alveolar margin*; straight ascending process of the maxilla with a longitudinal ridge on its lateral surface; noticeably bulging labial side of the maxillary teeth; greatly elongated cervical vertebra (length/height ratio of sixth cervical centrum more than 5*); neural arches of the cervical vertebrae with sinuous dorsal margin of the neural spines and short epipophyses—extending two-third of the length of the postzygapophyses—; and proximal articular surface of metatarsal III shelf-like and medially deflected*.
The skull is relatively short and low (
Photograph of the skull (A) and interpretative drawing (B) in lateral view. Dark grey color represents matrix and light grey color represents foraminae.
Both premaxillae are preserved, but are lacking their dorsal processes (
Photograph of the skull (A) and interpretative drawing (B) in dorsal view. Dark grey color represents matrix and light grey color represents foramina.
The right maxilla is almost complete whereas the left element lacks its middle portion (
The ventral halves of the lacrimals are preserved (
Only the right prefrontal has been partially preserved (
The left frontal is complete and better preserved than the right element (
The parietals are not fully preserved but it is clear that they are not medially fused (
Both postorbitals of
The left jugal is complete but somewhat damaged and displaced from its natural position, whereas the right element is only partially exposed (
The quadratojugals are partially exposed on both sides of the skull. Only the anterior process of the left element is preserved, whereas the right quadratojugal consists of an incomplete main body and anterior process (
Both quadrates are preserved but they are somewhat distorted and displaced from their natural position (
The squamosal is a tetraradiate element that forms the posterolateral corner of the supratemporal fenestra and the posterodorsal corner of the infratemporal fenestra (
Most of the palatal elements are damaged, incompletely preserved, and overlap each other, precluding the observation of anatomical details.
The ventral side of both ectopterygoids is exposed in ventral view, but both are displaced from their natural position. The main body of the ectopterygoid has a typical flat triangular shape with a slender and strongly recurved anterolateral process. Its ventral surface is smooth and bears a shallow depression on its posteromedial area. The ectopterygoid of
Both pterygoids are partially exposed in ventral view. The main body of the pterygoid is anteroposteriorly longer than transversely wide, with an irregular ventral surface. Its posteromedial process is narrow and wraps around the basipterygoid process, as in other basal sauropodomorphs (
The occipital region has been strongly eroded and is missing most of its elements. Only the basioccipital and the posterior half of the basisphenoid-parasphenoid complex have been preserved.
The dorsal surface of the basioccipital is anteroposteriorly elongated (
The dorsal surface of the basisphenoid forms the anterior region of the floor of the braincase. This surface is concave and strongly rugose, and contacts the anterior edge of the basioccipital posteriorly. The anterior half of the basisphenoid of
Most of the right mandibular ramus is preserved and exposed in lateral view, whereas only the posterior half of the left ramus has been preserved (
The dentary is the largest bone of the lower jaw (
The angular, surangular, and articular are incomplete, partially exposed, and severely distorted (
A, photograph of the teeth of
The maxillary teeth are shorter than those of the premaxilla (
Labial surfaces of the maxillary and dentary crowns of
In many respects, the dentary teeth exhibit a similar morphology as the maxillary teeth, although the dentary teeth are more flattened labiolingually. Moreover, the dentary teeth have their maximum mesiodistal width closer to the base than the maxillary teeth. The anterior dentary teeth are slightly longer than the posterior dentary teeth, similar to most basal sauropodomorphs. The labial surface of the crowns is slightly convex, while the lingual surface is slightly convex to flat. Dentary teeth are slightly rotated giving an imbricated arrangement, similar to other basal sauropodomorphs such as
A pair of ceratobranchials is joined to the palatal elements, and they extend anteroposteriorly. The ceratobranchials are thin, elongate and gently curved along their length. They are subcircular in cross section and are almost constant in diameter, with blunt anterior and posterior ends.
The right proatlas lacks its posterior end (
A, right proatlas in medial view; B, odontoid in dorsal view; C, intercentrum in dorsal view; D, right atlantal neural arch in medial view; E–F, axis in dorsal (E) and lateral (F) view.
All elements of the atlas are disarticulated, and consist of the odontoid, intercentrum, and both incomplete neural arches (
The intercentrum is subrectangular U-shaped in anterior and posterior views and subtriangular in lateral view. The dorsal surface of the intercentrum bears two transversely oriented concavities for the articulation of the occipital condyle (anteriorly) and the odontoid (posteriorly) (
The right atlantal neural arch is almost complete whereas only the anterior half of the left neural arch has been preserved (
The length of the axial centrum is more than four times longer than high and anteroposteriorly shorter than any of the postaxial cervical (
The preserved postaxial cervical vertebrae are articulated from the third to the seventh vertebra (
A, neck vertebrae from C3–C7; B, sixth cervical vertebra in lateral view.
All cervical ribs of
Only two non-consecutive caudal vertebrae are preserved in
A–B, anterior caudal vertebra in lateral (A) and anterior (B) view; C–F, middle caudal vertebra in lateral (C), anterior (D), posterior (E), and ventral (F) view.
Only two isolated chevrons are preserved but they lack their distal ends. As in other basal sauropodomorphs, the shafts are rod-like with a forked proximal end, which produces the characteristic “Y” shape. In one of the chevrons the branches of the proximal end are fused and form an oval concave facet for articulation with the centrum, and likely indicate that it might belong to the anterior-mid caudal region. Its shape resembles to that of Adeopapposaurus and Pantydraco. The other chevron is more robust and its arms are proximally separated, suggesting that it belongs to a mid-posterior position in the caudal series. The proximal half of the posterior face of the larger chevron has a deep groove running between both arms which form the oval foramen for the caudal blood vessels.
The preserved elements of the pectoral girdle and forelimb are the proximal region of the left scapula, a damaged and dorsoventrally compressed left coracoid, and a poorly preserved proximal fragment of the left humerus (
A–B, left scapula, coracoid, and humerus in medial (A) and lateral (B) view.
Only the right pubic apron and the proximal portion of both ischia have been preserved in the type specimen of
A, right pubic apron in dorsal view; B–C, proximal portion of the left ischium in proximal (B) and lateral (C) view.
The pubic apron has several transverse fractures and lacks its proximal and distal ends (
The hindlimb of
A–B, left distal tarsal III in dorsal (A) and posterior (B) view; C–D, left distal tarsal IV in dorsal (C) and posterior (D) view; E–G, left metatarsal III (E), IV (F), and V (G) in proximal, dorsal and distal view; H–J, left pedal phalanges: first phalanx of digit I (H), second phalanx of digit II (I), and second phalanx of digit IV (J) in dorsal view.
A–B, photograph of the proximal half of metatarsal III in dorsal (A) and dorsomedial (B) view; C, interpretative drawing showing the medial shelf-like deflection of metatarlas III.
Distal tarsal III is proximodistally flat, has a subtriangular shape and its major axis is oriented anterolaterally (
Metatarsal III lacks its distal end (
The pedal phalanges of
1-
2-
The presence of both aforementioned features—lack of deflection of the ascending process with a ridge running along its entire lateral surface—gives a unique combination of characters that is only present in
3-
4-
5-
Moreover, the epipophyses of the anterior-mid cervical vertebrae of
6-
The new taxon
We performed an analysis using the modified dataset of 361 characters and 54 taxa usingTNT 1.1
Analysis was based on the dataset of Yates
The strict consensus tree shows a resolved placement of
The clade formed by
In this analysis, Massospondyidae and more derived sauropodomorphs share nine synapomorphies: presence of slot-shaped subnarial foramen (character 14.1); antorbital fossa on the ascending ramus of the maxilla weakly impessed and delimited by a rounded rim or a change in slope (character 31.1); crescentic antorbital fossa with strongly concave posterior margin that is roughly parallel to its anterior margin (character 32.0); length of the anterior ramus of the lacrimal less than half of the length of the ventral ramus (character 40.1); dorsal margin of the postorbital with a distinct embayment between the anterior and posterior dorsal process in lateral view (character 54.1); absence of a deep septum spanning the interbasipterygoid space (character 85.0); proximal width of the first metacarpal between 80–100 per cent of its length (character 227.2); transverse axis of the distal end of the first phalanx of manual digit one ventrolateral twisting 60 degrees relative to its proximal end (character 234.2); and presence of a notch separating the posteroventral end of the ischial obturator plate from the ischial shaft (character 268.0).
In contrast to the results presented by Sertich and Loewen
Similarly,
In order to test the affinities of these taxa in relation to massospondylids, we have run a constrained analysis with a
The new taxon described in the present work increases our knowledge of basal sauropodomorphs, in particular our understanding of the diversity of Massospondylidae. In addition, the new taxon helps refine the age estimate of the sedimentary unit where it was found.
The analysis performed here rejects the massospondylid affinities of some basal sauropodomorphs (e.g.
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We are indebted to Benito Leyes, a local goatherder who found the specimen in 2001 and collaborated with the field crew of the Instituto y Museo de Ciencias Naturales de San Juan (IMCN). D. Correa is thanked for her help on our fieldworks. We thank Diego Abelín for the preparation of the fossil materials and for his continuous cooperation. Photographs were made by Ivan Zabrodski. CA especially thanks Ronaldo Morilla for his help and support during the first steps of this study. S. Nesbitt and R. Irmis are thanked for their suggestions that improved the content of this paper. The comments and suggestions of Adam Yates and an anonymous reviewer greatly enhanced the quality of this work. We used the free version of TNT 1.1 available thanks to a subsidy of the Willi Henning Society.