Alejandro Otero

A Reappraisal of the Late Cretaceous Argentinean Sauropod Dinosaur Argyrosaurus superbus, with a Description of a New Titanosaur Genus

ABSTRACT Argyrosaurus superbus is one of the earliest-named Argentinean dinosaurs. The holotype comprises a complete forelimb, probably from the upper member of the Bajo Barreal Formation (Late Cretaceous), Chubut Province. Numerous remains have been referred to Argyrosaurus from Argentina and Uruguay; however, the type specimen has not been adequately diagnosed and referrals have predominantly been based upon their large size. Here we redescribe Argyrosaurus, demonstrating it to be a valid titanosaur genus based on five autapomorphies, as well as an unique character combination. The exact placement of Argyrosaurus within Titanosauria is uncertain, although the probable presence of carpal bones, otherwise unknown in titanosaurs, may indicate a basal position. None of the referred remains can be attributed to Argyrosaurus, and most should be regarded as indeterminate titanosauriforms. The exception to this is a partial skeleton from the lower member of the Bajo Barreal Formation (early Late Cretaceous), Chubut Province, comprising dorsal and caudal vertebrae, as well as numerous appendicular elements. This specimen is distinct from Argyrosaurus and can also be differentiated from other sauropods based on an unusual character combination (including plesiomorphic tarsus), plus one autapomorphy. Elaltitan lilloi, gen. et sp. nov., displays numerous titanosaur characters and shares several features with derived taxa such as Neuquensaurus, Opisthocoelicaudia, Rapetosaurus, Saltasaurus, and Trigonosaurus. Elaltitan can be referred to Lithostrotia; however, its precise position within this clade must await future phylogenetic analysis. The revision and description of the titanosaurs Argyrosaurus and Elaltitan provides new information on this diverse but still poorly understood clade.

A new sauropod titanosaur from the Plottier Formation (Upper Cretaceous) of Patagonia (Argentina)

This paper presents a new titanosaur sauropod, collected from levels of reddish clays assigned to the Plottier Formation (Coniacian-Santonian). The holotype of Petrobrasaurus puestohernandezi gen. et. sp. nov. is a disarticulated specimen, from which teeth, cervical, dorsal and caudal vertebrae, sternal plates, metacarpals, femora, tibia, a fragment of ilium, pubis, haemal arches, and cervical and dorsal ribs have been preserved. This period is of particular interest because it saw the definitive isolation of the vertebrate faunas of Patagonia, with the separation of South America from the rest of Gondwana, a process that had begun during the Early Cretaceous. Although some of the characters observed in Petrobrasaurus gen. nov. suggest a relationship with the South American clade Lognkosauria, this new sauropod is regarded as Titanosauria incertae sedis until a more profound analysis of the Titanosauria that in which it is included is undertaken.

Postcranial anatomy and phylogenetic relationships of Mussaurus patagonicus (Dinosauria, Sauropodomorpha)

ABSTRACT— The transition from basal sauropodomorphs to sauropods is one of the most dramatic evolutionary transformations in the history of dinosaurs. Constituent taxa of this transition were recorded mainly in South Africa and South America, and to a lesser extent in North America. We describe here the postcranial anatomy of four specimens of basal sauropodomorphs from the Late Triassic of Patagonia, Argentina, and identify them as adult individuals of Mussaurus patagonicus. The material is composed of one subadult and three adult specimens and was originally identified as Plateosaurus. The completeness of the material provides more complete knowledge of this taxon and allows us to introduce aspects of basal sauropodomorph anatomy that were poorly understood until now, such as the configuration and arrangement of the distal carpal elements. The phylogenetic relationships of Mussaurus patagonicus are tested through a cladistic analysis of basal sauropodomorphs based on the anatomy of these specimens rather than on the post-hatchling and juvenile specimens previously known for this taxon. Mussaurus is recovered as a non-sauropod anchisaurian, being the sister group of Aardonyx plus more derived sauropodomorphs and is depicted outside the ‘quadrupedal clade,’ given the presence of plesiomorphic features such as a humerus/femur ratio <0.8, a curved femoral shaft in lateral view, and a nearly circular femoral midshaft cross-section. Mussaurus patagonicus adds new and valuable information that helps to clarify the core of the basal sauropodomorph-sauropod transition. SUPPLEMENTAL DATA—Supplemental materials are available for this article for free at www.tandfonline.com/UJVP

Paleobiology of Titanosaurs: Reproduction, Development, Histology, Pneumaticity, Locomotion and Neuroanatomy from the South American Fossil Record

Abstract. Much of the current paleobiological knowledge on titanosaur sauropods was attained in just the last fifteen years, in particular that related to reproductive and developmental biology. Recent years have also seen progress on other poorly explored topics, such as pneumaticity, muscle architecture and locomotion, and endocast reconstruction and associated structures. Some titanosaurs laid numerous, relatively small Megaloolithidae eggs (with diameters ranging from 12 to 14 cm) in nests dug In the ground and, as known from the South American records, probably eggs of the multispherulitic morphotype. During ontogeny, certain titanosaurs displayed some variations in cranial morphology, some of them likely associated with the differing feeding habits between hatchlings and adults. The bone tissue of some adult titanosaurs was rapidly and cyclically deposited and shows a greater degree of remodeling than in other sauropods. Saltasaurines in particular show evidence of postcranial skeletal pneumaticity in both axial and appendicular skeleton, providing clues about soft tissue anatomy and the structure of the respiratory system. Titanosaurs, like all sauropods, were characterized by being fully quadrupedal, although some appendicular features and putative trackways indicate that their stance was not as columnar as in other sauropods. These anatomical peculiarities are significantly developed In saltasaurines, a derived group of titanosaurs. Compared with other sauropods, some titanosaurs seem to have had very poor olfaction but would have been capable of capturing sounds In a relatively wide range of high frequencies, although not to the extent of living birds.

Sauropod haemal arches: morphotypes, new classification and phylogenetic aspects

Sauropod haemal arches are caudal bony structures that have been traditionally incorporated into two different types observed in different anatomical views: Y-shaped (anterior view) and forked (lateral view). This research proposes a classification combining information observed in anterior and lateral views. Four types of ‘Y-shaped’ and six types of ‘V-shaped’ chevrons were recognised. Complete chevron series in some eusauropods allows the comparison of topological equivalent structures along the tail and also among taxa. A basal titanosaur from Argentina exhibits mid-caudal chevron morphology in which more than one cranial and caudal process is present, arising from the distal blades as well as from the proximal rami, a condition not seen before in a sauropod dinosaur. The morphological variability seen in sauropod chevrons along the tail is in close relationship with the development and distribution of muscle. caudofemoralis longus, as seen in extant crocodiles and as previously proposed for non-avian theropods and for caudal centra and transverse processes of sauropod dinosaurs. Two new characters related to middle chevrons are proposed here, in which the transitional morphology is described.

Temporal and phylogenetic evolution of the sauropod dinosaur body plan

The colossal size and body plan of sauropod dinosaurs are unparalleled in terrestrial vertebrates. However, to date, there have been only limited attempts to examine temporal and phylogenetic patterns in the sauropod bauplan. Here, we combine three-dimensional computational models with phylogenetic reconstructions to quantify the evolution of whole-body shape and body segment properties across the sauropod radiation. Limitations associated with the absence of soft tissue preservation in fossils result in large error bars about mean absolute body shape predictions. However, applying any consistent skeleton : body volume ratio to all taxa does yield changes in body shape that appear concurrent with major macroevolutionary events in sauropod history. A caudad shift in centre-of-mass (CoM) in Middle Triassic Saurischia, associated with the evolution of bipedalism in various dinosaur lineages, was reversed in Late Triassic sauropodomorphs. A craniad CoM shift coincided with the evolution of quadrupedalism in the Late Triassic, followed by a more striking craniad shift in Late Jurassic–Cretaceous titanosauriforms, which included the largest sauropods. These craniad CoM shifts are strongly correlated with neck enlargement, a key innovation in sauropod evolution and pivotal to their gigantism. By creating a much larger feeding envelope, neck elongation is thought to have increased feeding efficiency and opened up trophic niches that were inaccessible to other herbivores. However, we find that relative neck size and CoM position are not strongly correlated with inferred feeding habits. Instead the craniad CoM positions of titanosauriforms appear closely linked with locomotion and environmental distributions, potentially contributing to the continued success of this group until the end-Cretaceous, with all other sauropods having gone extinct by the early Late Cretaceous.

Changes in vertebral laminae across the cervicodorsal transition of a well-preserved rebbachisaurid (Dinosauria, Sauropoda) from the Cenomanian of Patagonia, Argentina

ALEJANDRO HALUZA,1 JUAN I. CANALE,1,2 ALEJANDRO OTERO,*,2,3 LEANDRO M. PÉREZ,2,4 and CARLOS A. SCANFERLA2,5; 1Museo Municipal “Ernesto Bachmann,” Dr. Natali s/n, Villa El Chocón, Neuquén, Argentina, jujuyaspis@yahoo. com; juanignaciocanale@yahoo.com.ar; 2Consejo Nacional de Investigaciones Cientı́ficas y Técnicas; 3Departamento Cientı́fico Paleontologı́a de Vertebrados, Museo de La Plata, Paseo del Bosque s/n (1900), La Plata, Buenos Aires, Argentina, alexandros.otero@gmail.com; 4Cátedra de Rocas Sedimentarias, Facultad de Ciencias Naturales y Museo (UNLP), Av. 60 y 122 s/n, La Plata, Buenos Aires, Argentina, pilosaperez@gmail.com; 5Museo Argentino de Ciencias Naturales “Bernardino Rivadavia,” Av. Angel Gallardo 470, 1405, Buenos Aires, Argentina, agustin scanferla@yahoo.com.ar

Reassessment of Laplatasaurus araukanicus (Sauropoda: Titanosauria) from the Upper Cretaceous of Patagonia, Argentina

Abstract. The original material assigned to Laplatasaurus araukanicus Huene come from five different localities in northern Patagonia (Argentina) where the Upper Cretaceous (Campanian) lies exposed. This material includes several postcranial bones from multiple specimens and showing different conditions of preservation, often lacking anatomical overlap. The taxonomic status of the taxon is further obscured by the absence of quarry maps of the multiple localities (lacking the recognition of associated materials), and a proper designation of type material in its original description. After Huene, new material was assigned to this taxon, although none of it resolved the existing taxonomic issues. In 1979, the designation of a lectotype (one tibia and one fibula) was the first stage in the nomenclatural stabilization of the species. However, the assignment of the remaining material to L. araukanicus remained uncertain. Here we review all the material hitherto assigned to this taxon in order to clarify its taxonomic status. We also provide a re-description of the lectotype and discuss the taxonomic identification of material previously referred to this taxon. Lastly we include it for the first time in an updated phylogenetic data matrix. Laplatasaurus araukanicus is retained only for the lectotype. The material from Rancho de Ávila is referred to cf. Bonitasaura sp. because they share diagnostic features and are stratigraphically congruent. The remaining material is referred as Lithostrotia indet. A phylogenetic analysis nests Laplatasaurus within Titanosauria in a clade formed by ((Laplatasaurus + Uberabatitan) + (Bonitasaura + (Futalognkosaurus + Mendozasaurus))).

Forelimb muscle and joint actions in Archosauria: insights from Crocodylus johnstoni (Pseudosuchia) and Mussaurus patagonicus (Sauropodomorpha)

Many of the major locomotor transitions during the evolution of Archosauria, the lineage including crocodiles and birds as well as extinct Dinosauria, were shifts from quadrupedalism to bipedalism (and vice versa). Those occurred within a continuum between more sprawling and erect modes of locomotion and involved drastic changes of limb anatomy and function in several lineages, including sauropodomorph dinosaurs. We present biomechanical computer models of two locomotor extremes within Archosauria in an analysis of joint ranges of motion and the moment arms of the major forelimb muscles in order to quantify biomechanical differences between more sprawling, pseudosuchian (represented the crocodile Crocodylus johnstoni) and more erect, dinosaurian (represented by the sauropodomorph Mussaurus patagonicus) modes of forelimb function. We compare these two locomotor extremes in terms of the reconstructed musculoskeletal anatomy, ranges of motion of the forelimb joints and the moment arm patterns of muscles across those ranges of joint motion. We reconstructed the three-dimensional paths of 30 muscles acting around the shoulder, elbow and wrist joints. We explicitly evaluate how forelimb joint mobility and muscle actions may have changed with postural and anatomical alterations from basal archosaurs to early sauropodomorphs. We thus evaluate in which ways forelimb posture was correlated with muscle leverage, and how such differences fit into a broader evolutionary context (i.e. transition from sprawling quadrupedalism to erect bipedalism and then shifting to graviportal quadrupedalism). Our analysis reveals major differences of muscle actions between the more sprawling and erect models at the shoulder joint. These differences are related not only to the articular surfaces but also to the orientation of the scapula, in which extension/flexion movements in Crocodylus (e.g. protraction of the humerus) correspond to elevation/depression in Mussaurus. Muscle action is highly influenced by limb posture, more so than morphology. Habitual quadrupedalism in Mussaurus is not supported by our analysis of joint range of motion, which indicates that glenohumeral protraction was severely restricted. Additionally, some active pronation of the manus may have been possible in Mussaurus, allowing semi-pronation by a rearranging of the whole antebrachium (not the radius against the ulna, as previously thought) via long-axis rotation at the elbow joint. However, the muscles acting around this joint to actively pronate it may have been too weak to drive or maintain such orientations as opposed to a neutral position in between pronation and supination. Regardless, the origin of quadrupedalism in Sauropoda is not only linked to manus pronation but also to multiple shifts of forelimb morphology, allowing greater flexion movements of the glenohumeral joint and a more columnar forelimb posture.

Novel insight into the origin of the growth dynamics of sauropod dinosaurs

Sauropod dinosaurs include the largest terrestrial animals and are considered to have uninterrupted rapid rates of growth, which differs from their more basal relatives, which have a slower cyclical growth. Here we examine the bone microstructure of several sauropodomorph dinosaurs, including basal taxa, as well as the more derived sauropods. Although our results agree that the plesiomorphic condition for Sauropodomorpha is cyclical growth dynamics, we found that the hypothesized dichotomy between the growth patterns of basal and more derived sauropodomorphs is not supported. Here, we show that sauropod-like growth dynamics of uninterrupted rapid growth also occurred in some basal sauropodomorphs, and that some basal sauropods retained the plesiomorphic cyclical growth patterns. Among the sauropodomorpha it appears that the basal taxa exploited different growth strategies, but the more derived Eusauropoda successfully utilized rapid, uninterrupted growth strategies.