José L. Carballido

Small body size and extreme cortical bone remodeling indicate phyletic dwarfism in Magyarosaurus dacus (Sauropoda: Titanosauria)

Sauropods were the largest terrestrial tetrapods (>105 kg) in Earths history and grew at rates that rival those of extant mammals. Magyarosaurus dacus, a titanosaurian sauropod from the Upper Cretaceous (Maastrichtian) of Romania, is known exclusively from small individuals (<103 kg) and conflicts with the idea that all sauropods were massive. The diminutive M. dacus was a classical example of island dwarfism (phyletic nanism) in dinosaurs, but a recent study suggested that the small Romanian titanosaurs actually represent juveniles of a larger-bodied taxon. Here we present strong histological evidence that M. dacus was indeed a dwarf (phyletic nanoid). Bone histological analysis of an ontogenetic series of Magyarosaurus limb bones indicates that even the smallest Magyarosaurus specimens exhibit a bone microstructure identical to fully mature or old individuals of other sauropod taxa. Comparison of histologies with large-bodied sauropods suggests that Magyarosaurus had an extremely reduced growth rate, but had retained high basal metabolic rates typical for sauropods. The uniquely decreased growth rate and diminutive body size in Magyarosaurus were adaptations to life on a Cretaceous island and show that sauropod dinosaurs were not exempt from general ecological principles limiting body size.

Postcranial axial skeleton of Europasaurus holgeri (Dinosauria, Sauropoda) from the Upper Jurassic of Germany: implications for sauropod ontogeny and phylogenetic relationships of basal Macronaria

Neosauropods are well represented in the Late Jurassic fossil record, both in Laurasia and Gondwana. Among Macronaria, Europasaurus represents one of the most basal forms of this group. In addition to its systematic importance, Europasaurus is also the first unequivocal dwarf sauropod from which adult and juvenile material is available. Despite the abundance of sauropods in the fossil record, early juvenile specimens are rare, limiting knowledge about sauropod ontogeny. Therefore, the great amount of material of Europasaurus provides an excellent opportunity to improve our knowledge on the early evolution of Macronaria, as well as to shed light on some morphological changes through ontogeny. The postcranial axial skeleton of sauropods is extremely modified with respect to the anatomy observed in its ancestors, the ‘prosauropods’, proving to be one of the most informative regions of the body. Here we provide a detailed description of the axial skeleton of Europasaurus, including adult and juvenile elements, discussing its systematic and ontogenetic importance. We also analyse the phylogenetic position of Europasaurus through a cladistic analysis using TNT, which retrieves this taxon in a basal position among Camarasauromorpha. Additionally, the presence/absence of discrete characters and the comparison of juvenile elements with adult specimens allowed us to recognize different morphological ontogenetic stages (MOS). Whereas early stages lack derived characters (e.g. spinodiapophyseal lamina and prespinal lamina on dorsal vertebrae), all derived characters (including autapomorphies) are present in late immature specimens. Therefore, while late immature specimens provide the same phylogenetic signal as adult specimens of Europasaurus, more immature stages are recovered in a basal position among sauropods. Finally, we apply the MOS to other maturity criteria (e.g. neurocentral closure, sexual maturity) in a search for a wider definition of maturity.

The Osteology of Chubutisaurus insignis Del Corro, 1975 (Dinosauria: Neosauropoda) from the ‘Middle’ Cretaceous of Central Patagonia, Argentina

ABSTRACT Titanosauriform sauropods were one of the most widely distributed groups of dinosaurs during the Cretaceous. In contrast to most derived forms from the Upper Cretaceous, the most basal taxa of the group are poorly known. Thus, studies on these forms are of special interest for understanding the origin and early evolution of Titanosauria. Chubutisaurus insignis del Corro, 1975, is known from postcranial remains found in the Bayo Overo Member of the Cerro Barcino Formation (Chubut Group), which is commonly regarded as Aptian—Cenomanian in age. Phylogenetic analyses that include Chubutisaurus recovered this taxon as the sister group to Titanosauria. Nevertheless, most published studies have not included this taxon, probably due to its brief description and fragmentary remains. Fieldwork conducted in the quarry where the holotype was found yielded new materials that are regarded as part of the same specimen. These new materials, together with additional undescribed, and briefly described elements, originally collected by G. del Corro, are described here. The new information allows the recognition of a more extensive diagnosis of this taxon. The phylogenetic analysis presented here resolves Chubutisaurus as the most basal somphospondylian.

Modified Laminar Bone in Ampelosaurus atacis and Other Titanosaurs (Sauropoda): Implications for Life History and Physiology

Background Long bone histology of the most derived Sauropoda, the Titanosauria suggests that titanosaurian long bone histology differs from the uniform bone histology of basal Sauropoda. Here we describe the long bone histology of the titanosaur Ampelosaurus atacis and compare it to that of basal neosauropods and other titanosaurs to clarify if a special titanosaur bone histology exists. Methodology/Principal Findings Ampelosaurus retains the laminar vascular organization of basal Sauropoda, but throughout most of cortical growth, the scaffolding of the fibrolamellar bone, which usually is laid down as matrix of woven bone, is laid down as parallel-fibered or lamellar bone matrix instead. The remodeling process by secondary osteons is very extensive and overruns the periosteal bone deposition before skeletal maturity is reached. Thus, no EFS is identifiable. Compared to the atypical bone histology of Ampelosaurus, the large titanosaur Alamosaurus shows typical laminar fibrolamellar bone. The titanosaurs Phuwiangosaurus, Lirainosaurus, and Magyarosaurus, although differing in certain features, all show this same low amount or absence of woven bone from the scaffolding of the fibrolamellar bone, indicating a clear reduction in growth rate resulting in a higher bone tissue organization. To describe the peculiar primary cortical bone tissue of Phuwiangosaurus, Ampelosaurus, Lirainosaurus, and Magyarosaurus, we here introduce a new term, “modified laminar bone” (MLB). Conclusions/Significance Importantly, MLB is as yet not known from extant animals. At least in Lirainosaurus and Magyarosaurus the reduction of growth rate indicated by MLB is coupled with a drastic body size reduction and maybe also a reduction in metabolic rate, interpreted as a result of dwarfing on the European islands during the Late Cretaceous. Phuwiangosaurus and Ampelosaurus both show a similar reduction in growth rate but not in body size, possibly indicating also a reduced metabolic rate. The large titanosaur Alamosaurus, on the other hand, retained the plesiomorphic bone histology of basal neosauropods.

Braincase, neuroanatomy, and neck posture of Amargasaurus cazaui (Sauropoda, Dicraeosauridae) and its implications for understanding head posture in sauropods

ABSTRACT The braincase of Amargasaurus cazaui from the Lower Cretaceous of Argentina represents the only dicraeosaurid sauropod neurocranial material known from South America. It has been computer tomographically (CT) scanned and three-dimensional digital reconstructions of the endocranium and inner ear have been made. The cranial endocast is complete, with a volume of approximately 94–98 ml, excluding the dorsal sinuses. The labyrinth of the inner ear is dorsoventrally taller than the lagena, which is conical, and relatively short. The anterior semicircular canal is longer than the posterior and lateral semicircular canals, as in most non-titanosaurid sauropods. When the braincase is oriented with the lateral semicircular canal positioned horizontally, the occipital condyle is oriented posteroventrally, suggesting that the head was held with the muzzle pointing downward. The morphology of the atlas and axis, together with the reconstruction of the osteological neutral pose of the neck, supports this neck and head position, and also indicates the presence of the proatlas in this taxon. The evidence presented here for the skull and neck position of Amargasaurus fits with a midheight food-gathering strategy. The presence of titanosauriforms and rebbachisaurids, together with Amargasaurus, supports the niche partitioning hypothesis for the La Amarga Formation sauropods.

Cranial anatomy of the Late Jurassic dwarf sauropod Europasaurus holgeri (Dinosauria, Camarasauromorpha): ontogenetic changes and size dimorphism

Sauropods were the most successful herbivorous group of dinosaurs during the Mesozoic era. Despite their supremacy as reflected in the fossil record, sauropod skulls are very rare and current knowledge of skull anatomy is based on just a few taxa. Juvenile skull bones are even rarer than adult skulls; thus, our understanding of their morphology and ontogenetic changes is limited. The recent discovery of several adult and juvenile specimens of a Late Jurassic taxon from Germany, Europasaurus holgeri, extends our knowledge of sauropod skull anatomy. A total of 123 skull bones, representing at least 14 skulls, were examined, described and compared to other taxa. The skull material includes several individuals of various ontogenetic stages. Because size alone is not sufficient to determine the morphological ontogenetic stage (MOS), size-independent characters were used to stage the bone elements. Detailed studies of the skull bones proved that the material represents two morphotypes, independent of ontogenetic stage. Since the original description of Europasaurus, new skull material has been found, and an updated skull reconstruction of an adult individual is presented here. All the autapomorphic characters of Europasaurus recognized in the skull (i.e. anteroposteriorly long and lateromedially narrow frontal; presence of postparietal fenestra; large participation of the jugal to the ventral rim of the skull and the orbit; presence of a postparietal foramen and single optic foramen) are plesiomorphic characters of basal sauropodomorphs and/or present in embryos and juvenile sauropods. Therefore, we consider that in Europasaurus these characters evolved through paedomorphosis, which resulted in the dwarf condition of this taxon.

Evidence of Gregariousness in Rebbachisaurids (Dinosauria, Sauropoda, Diplodocoidea) from the Early Cretaceous of Neuquén (Rayoso Formation), Patagonia, Argentina

ABSTRACT For the first time an association of adult and juvenile rebbachisaurid sauropods is described. The material comes from the Early Cretaceous locality of Agrio del Medio (Neuquén, Argentina). The three specimens apparently formed a single group, and their death seems to have been almost simultaneous. The two juvenile specimens are represented by axial and appendicular bones. They show a close relationship with Zapalasaurus bonapartei, which comes from a different sector of the same basin, but which is approximately the same age. The discovery at Agrio del Medio suggests that rebbachisaurid sauropods displayed gregarious behavior. The paleoenvironments in which rebbachisaurids are normally recorded implies a greater tolerance toward extremely arid environments than that shown by macronarian sauropods.

A new Early Cretaceous brachiosaurid (Dinosauria, Neosauropoda) from northwestern Gondwana (Villa de Leiva, Colombia)

ABSTRACT Brachiosaurid sauropods achieved a broad distribution during the Late Jurassic, which has been considered to provide evidence of their origins during the Middle Jurassic, prior to the breakup of Pangea. In contrast to their broad geographic distribution during the Late Jurassic, formally named brachiosaurid species from the Cretaceous have so far been restricted to the Aptian—Albian of North America, which has been interpreted as a signal of differential extinction and/or a bias in the Early Cretaceous fossil record. Here we describe a new brachiosaurid titanosauriform taxon from the Early Cretaceous of Colombia, which is represented by axial elements. The material was recovered from marine sediments of the Paja Formation (Barremian), close to the locality of Villa de Leiva. The weakly laterally expanded and divided transverse processes of the anterior-most caudal vertebrae allows the recognition of a new sauropod taxon, Padillasaurus leivaensis, gen. et sp. nov. In order to test the phylogenetic relationships of the new taxon, we performed a cladistic analysis that recovered Padillasaurus as a brachiosaurid titanosauriform. This position is supported by a combination of characters, including the presence of blind fossae in anterior caudal vertebrae. Among titanosauriforms, the presence of blind fossae in anterior caudal vertebrae is an apomorphic character that is exclusive to Giraffatitan, Venenosaurus, Cedarosaurus, and Abydosaurus. Although more complete remains are needed to test more thoroughly the affinities of the new taxon, the available evidence indicates that brachiosaurids survived at lower latitudes in Gondwana until at least the Early Cretaceous.

A New Eilenodontine (Lepidosauria, Sphenodontidae) from the Lower Cretaceous of Central Patagonia

ABSTRACT A new species of eilenodontine sphenodontian, Priosphenodon minimus, is described from a locality 400 km south of those that yield the northern Patagonian eilenodontines (the largest known terrestrial sphenodontians). The new species, represented by subadult specimens, is both the smallest known eilenodontine and the southernmost record of the group. Pr. minimus is characterized by a nearly vertical lateral process of the premaxilla; anteriorly extended frontals that lack the naso-prefrontal projections; a rounded frontal anterior process; a prefrontal that posteriorly embraces the nasal; a coronoid process of the dentary that is tall and with an anterior border that is vertical in medial view; strong anteromedial flanges on all teeth; uncommonly dense dental packing; and an anteriorly extended angular that reaches the midlength of the lower jaw. Differences from juvenile specimens of Priosphenodon avelasi, such as the interruption of the prefrontaljugal contact, are marked. The holotype and associated material come from early Albian strata, and are therefore around 12 million years older than the northern Patagonian giant sphenodontians. The new species here described reveals that many of the characters used to diagnose Kaikaifilusaurus calvoi are widespread, so this taxon should be regarded as a nomen dubium. Therefore, the genus Priosphenodon is considered as valid herein, with the new species assigned to it. A phylogenetic analysis demonstrates the monophyly of Priosphenodon and suggests that the new taxon represents an incidence of dwarfing. This discovery increases the morphological disparity among these specialized herbivores, and provides new climatological and biostratigraphic information on the terrestrial ecosystems of Patagonia.

A Diplodocid Sauropod Dinosaur from the Late Jurassic Canadón Calcáreo Formation of Chubut, Argentina

ABSTRACT Late Jurassic dinosaur faunas from the Southern Hemisphere are still poorly known, and it thus remains unclear whether or not the famous Tendaguru fauna (Kimmeridgian—Tithonian, Tanzania) represents a typical Gondwanan dinosaur assemblage of that time. In South America, only the Oxfordian—Kimmeridgian Cañadón Calcáreo Formation of Chubut Province, Argentina, has yielded more than isolated Late Jurassic dinosaur remains so far. Here we report fragmentary remains of a dipolodocid sauropod from this unit, representing the first record of this family from the Late Jurassic of South America. Incorporating the basal macronarian Tehuelchesaurus, an unidentified brachiosaurid, the dicraeosaurid Brachytrachelopan, and the diplodocid described here, the taxonomic composition of the sauropod fauna from the Cañadón Calcáreo Formation is remarkably similar to that of the Tendaguru Formation, but also to roughly contemporaneous faunas in North America and Europe. The diverse non-neosauropodan sauropod fauna known from the early Middle Jurassic (Aalenian—Bajocian) of the same depositional basin within Chubut Province is congruent with the dominance of non-neosauropodan sauropods in continental faunas globally to at least the Bathonian. These assemblages suggest a rapid faunal turnover within sauropod faunas in the late Middle Jurassic-earliest Late Jurassic at least in western Pangea, through which basal eusauropods were replaced by diplodocoid and macronarian neosauropods. Taking paleogeographical reconstructions into account, this faunal replacement might have taken place in a surprisingly short time interval of maximally five million years close to the end of the Middle Jurassic.