[D] Leonerasaurus taquetrensis [sG] [T]
Pol, Garrido & Cerda 2011
Las Leoneras Formation, Canãdón Las Leoneras, Chubut Province, Central Patagonia, Argentina
The origin of sauropod dinosaurs is one of the major landmarks of dinosaur evolution but is still poorly understood. This drastic transformation involved major skeletal modifications, including a shift from the small and gracile condition of primitive sauropodomorphs to the gigantic and quadrupedal condition of sauropods. Recent findings in the Late Triassic–Early Jurassic of Gondwana provide critical evidence to understand he origin and early evolution of sauropods.
A new sauropodomorph dinosaur, Leonerasaurus taquetrensis gen. et sp. nov., is described from the Las Leoneras Formation of Central Patagonia (Argentina). The new taxon is diagnosed by the presence of anterior unserrated teeth with a low spoon-shaped crown, amphicoelous and acamerate vertebral centra, four sacral vertebrae, and humeral deltopectoral crest low and medially deflected along its distal half. The phylogenetic analysis depicts Leonerasaurus as one of the closest outgroups of Sauropoda, being the sister taxon of a clade of large bodied taxa composed of Melanorosaurus and Sauropoda.
The dental and postcranial anatomy of Leonerasaurus supports its close affinities with basal sauropods. Despite the small size and plesiomorphic skeletal anatomy of Leonerasaurus, the four vertebrae that compose its sacrum resemble that of the large-bodied primitive sauropods.
This shows that the appearance of the sauropod-type of sacrum predated the marked increase in body size that characterizes the origins of sauropods, rejecting a causal explanation and evolutionary linkage between this sacral configuration and body size. Alternative phylogenetic placements of Leonerasaurus as a basal anchisaurian imply a convergent acquisition of the sauropod-type sacrum in the new small-bodied taxon, also rejecting an evolutionary dependence of sacral configuration and body size in sauropodomorphs.
This and other recent discoveries are showing that the characteristic sauropod body plan evolved gradually, with a step-wise pattern of character appearance.
MPEF-PV 1663 (Museo Paleontolo´gico Egidio Feruglio, Trelew, Argentina); Anterior region of right dentary and isolated teeth, articulated series of cervical and anterior dorsal vertebrae, partially articulated posterior dorsal vertebrae, and articulated sacrum (preserved in natural contact with both ilia), right scapula and humerus, left and right ilia, right ischium, partially preserved femur, articulated metatarsal I and II, and pedal ungual.
All vertebrae, the scapula, humerus, and pelvis were found in natural position, as a partially articulated specimen. The dentary, teeth, femur, and pedal remains were found within a radius of one meter from the center of the articulated specimen. No other remains were found at this site and therefore we interpret all these elements as belonging to a single individual.
Leoneras, in reference to the lithostratigraphic unit where this taxon was found; saurus, lizard (Latinized Greek). The species name taquetrensis refers to the Sierras de Taquetre´n, where Las Leoneras Formation crops out in Central Patagonia.
Locality and Age
Canãdón Las Leoneras, south of Canãdón del Zaino (both of which are affluent of the left margin of the Chubut river), southeast of Sierra de Taquetrén, Chubut Province, Central Patagonia, Argentina. Precise locality information is deposited at the MPEF collection and can also be obtained from the first author upon request. The specimen was found approximately 42 m below the top of the Leoneras Formation, a unit considered as Lower Jurassic in age by Nakayama, and more specifically referred to the Pliensbachian–Toarcian or Upper Sinemurian–Toarcian, although no direct datings of these sediments are available.
The age of the Las Leoneras Formation is certainly constrained by the Middle Jurassic dating of the volcanic facies of the overlying Lonco Trapial Formation. Furthermore, the base of the Lonco Trapial Formation in this region contains sedimentary facies with a well preserved taphoflora that was originally regarded as Middle Jurassic in age, although new evidence suggests this taphoflora is Early Jurassic in age, based on comparisons with the flora from the Early Jurassic of northwestern Patagonia and the Antarctic peninsula.
Figari and Curtade interpreted the sequence of Las Leoneras Formation as initial rifting deposits, linked to the genesis of the Canãdón Asfalto Basin. It must be noted that similar rifting deposits of other regions of Patagonia have been linked to the initial break-up of southeastern Gondwana, in which small and narrow depocenters were formed by continental extension and strike-slip movements during the Upper Triassic and Lower Jurassic.
Therefore, the geological context of the area and the stratigraphic relationship with the Lonco Trapial Formation are consistent with a Lower Jurassic age for the Las Leoneras Formation. However, an Upper Triassic age cannot be completely ruled out at the moment, as there is not a well-defined lower constraint for the age of this unit.
Leonerasaurus is a small basal sauropodomorph diagnosed by a unique combination of characters including the following autapomorphies: anterior teeth with low, spoon-shaped crowns (SI = 1.3); dorsosacral rib attached to preacetabular process of ilium (paralleled in Lufengosaurus); neural arches of primordial sacrals positioned on the anterior half of the centrum; caudosacral rib directed anterolaterally; humeral deltopectoral crest low and medially deflected along its distal half; flattened ischial shafts (paralleled in Anchisaurus).
Leonerasaurus differs from most basal sauropodomorphs in the presence of the following characters: straight anterior region of the dentary; slightly procumbent teeth without marginal denticles and with convex labial surface and concave lingual surface; four sacral vertebrae, with two primordial sacrals bounded by a dorsosacral and a caudosacral; preacetabular process of ilium exceeding pubic peduncle and dorsoventrally low (except for Anchisaurus and Mussaurus).
Finally, several plesiomorphic features distinguish Leonerasaurus from basal sauropods: teeth lacking labial or lingual grooves; posterior teeth with large denticles oriented at 45 degrees from tooth’s margin and slightly developed wrinkling pattern; vertebral centra amphicoelous and acamerate; cervical vertebrae low and moderately elongated, without postzygodiapophyseal lamina, with elongated prezygapophyses; dorsal vertebrae with low neural arches and neural spines elliptical in cross section; absence of spinoprezygapophyseal laminae in all dorsals and of prezygodiapophyseal lamina in mid-dorsals; posterior dorsals with dorsoventrally low hyposphene-hypantrum; proximal metatarsal II hour-glass shaped in proximal view.