Jaime E. Powell

Middle Eocene deformation and sedimentation in the Puna-Eastern Cordillera transition (23°-26°S): Control by preexisting heterogeneities on the pattern of initial Andean shortening

The Quebrada de los Colorados Formation, at the north end of Calchaqui Valley in Salta Province, northwest Argentina, preserves evidence of syndepositional deformation since the middle Eocene (ca. 40 Ma) that includes (1) an angular unconformity with the underlying Salta Group (Paleogene), and (2) internal unconformities and changes in vertical facies succession and provenance. Its fossil record [mammalian (notoungulates), middle Eocene] is correlatable to the Casa Grande Formation, which also unconformably overlies the Salta Group; both units record middle Eocene deformation along the eastern border of the Puna Plateau and outline previous fi rst-order mechanical heterogeneities related to the Cretaceous rift basin border. Along the western margin of the Puna, Eocene deformation coincides with thermal (magmatic arc) and mechanical (basin inversion) heterogeneities. Thus, the distribution of Eocene deformation followed an irregular pattern as a consequence of the heritage of preexisting heterogeneities.

New tertiary sebecosuchians (Crocodylomorpha) from South America: Phylogenetic implications

A new family, genus and species of a sebecosuchian crocodile from the late Paleocene of northwestern Argentina is described. This new taxon shows several apomorphic conditions such as the bent premaxillae, very high posterior part of the jugal, bent pterygoids, 3 pairs of procumbent teeth and a broad retroarticular process. The cladistic analysis carried out places this taxon as the sister group of Sebecus. The sebecosuchians are defined as a natural group restricted to South America. The baurusuchids come from several upper Cretaceous localities of southeastern Brazil, and northwest of Patagonia. The bretesuchids were found in the middle Paleocene of Brazil, late Paleocene and late Eocene of Argentina. The wider geographical and chronological distribution corresponds to the sebecids. They are registered from the early Paleocene in Patagonia and Bolivia to the middle Miocene in Colombia. The sebecosuchians were strong predators and probably more terrestrial in habits than other crocodilians. The lateral c...

Skull anatomy of Mussaurus patagonicus (Dinosauria: Sauropodomorpha) from the Late Triassic of Patagonia

The skull anatomy of Mussaurus patagonicus from the Upper Triassic Laguna Colorada Formation is described based on a revision of the type material and several recently found specimens. The studied material include two distinct size classes of individuals. The type material consists of extremely young individuals whereas the new specimens are interpreted as juvenile or subadult individuals. The latter are significantly larger, having a skull approximately three times longer than the type material. The skull anatomy of this taxon shows derived characters shared with some basal sauropodomorphs and eusauropods, which are absent in other basal sauropodomorphs (e.g. Thecodontosaurus, Plateosaurus). These include the presence of an extension of the infratemporal fenestra ventral to the orbit, dorsal and anterior rami of quadratojugal subperpendicular to each other, dorsoventral expansion of dentary at mandibular symphysis, slightly procumbent teeth with broad serrations restricted to the apical region (absent in some teeth). Differences among the studied specimens helps to understand the early ontogenetic changes occurring in this basal sauropodomorph, revealing major changes in the rostral and temporal regions.

Extreme postcranial pneumaticity in sauropod dinosaurs from South America

Birds are unique among living tetrapods in possessing pneumaticity of the postcranial skeleton, with invasion of bone by the lung and air-sac system. Postcranial skeletal pneumaticity (PSP) has been reported in numerous extinct archosaurs including pterosaurs and non-avian dinosaurs. Here we report a case of extreme PSP in a group of small-bodied, armored sauropod dinosaurs from the Upper Cretaceous of South America. Based on osteological data, we report an extensive invasion of pneumatic diverticula along the vertebral column, reaching the distal portion of the tail. Also, we provide evidence of pneumaticity in both pectoral and pelvic girdles. Our study reveals that the extreme PSP in archosaurs is not restricted to pterosaurs and theropod dinosaurs.KurzfassungVögel sind einzigartig innerhalb der lebenden Tetrapoden, da sie eine Pneumatisierung des Postkranialskeletts aufweisen, welche die Invasion von Knochen durch die Lunge und Luftsack-Systeme einschließt. Diese postkraniale Skelettpneumatisierung (PSP) ist bereits in zahlreichen ausgestorbenen Archosauriern, einschließlich Pterosauriern und Dinosauriern, die nicht der Vogellinie angehören, beschrieben worden. Hier berichten wir über einen Fall von extremer PSP in einer Gruppe von kleinwüchsigen, gepanzerten sauropoden Dinosauriern aus der Oberkreide von Südamerika. Basierend auf osteologischen Daten lässt sich eine umfangreiche Invasion von pneumatischen Divertikeln entlang der Wirbelsäule nachweisen, welche sich bis in die distalen Bereiche des Schwanzes erstreckt. Darüber hinaus zeigen sich Hinweise auf Pneumatisierung in beiden Brust- und Beckengürteln. Unsere Studie zeigt, dass diese extreme Form von PSP in Archosauriern nicht auf Flugsaurier und theropode Dinosaurier beschränkt ist.

Dermal armor histology of Saltasaurus loricatus, an Upper Cretaceous sauropod dinosaur from Northwest Argentina

The first unambiguous evidence of the presence of osteoderms in sauropod dinosaurs came from the discovery of Saltasaurus loricatus, a titanosaur from the Upper Cretaceous of Argentina. The dermal armor of Saltasaurus is composed of bony plates and small dermal ossicles. Here, we analyze the bone microstructure of these elements and provide information regarding its origin and development. The bony plates are composed almost entirely of reconstructed cancellous bone. Remains of primary bone consist of coarse bundles of mineralized collagenous fibers towards the external surface. Also, woven fibered bone tissue appears in the basal and lateral regions. Dermal ossicles lack secondary remodeling, and their matrix is formed by three orthogonal systems of collagenous fiber bundles. Growth lines are present in both bony plates and ossicles. Bone histology reveals that osteoderms mainly originated through direct mineralization (metaplasia) of the dermis, although other mechanisms are also involved (at least in the origin of dermal plates). The common features of development and integumental location of the osteoderms of Saltasaurus and other non-related vertebrates (e.g., lepidosaurs, crocodylomorphs) are linked to the intrinsic skeletogenic properties of the dermis.

Reassessment of the Vertebral Laminae in Some South American Titanosaurian Sauropods

ABSTRACT The neural spines of the sauropod presacral vertebrae contain a series of osseous laminae of unquestionable phylogenetic significance. However, the lack of articulated presacral series has impeded identification of these structures in many taxa. Titanosaur sauropods present a pattern of neural laminae that is somewhat different from other groups, especially to diplodocids. Understanding these differences is important in order to score adequately characters related to vertebral anatomy, in phylogenetic analyses of the Sauropoda. Particularly, many titanosaurs present two spinodiapophyseal laminae in the posterior dorsal vertebrae. Additionally, these taxa show the apparent disappearing of the postzygodiapophyseal lamina in those vertebrae. Other titanosaurs have a single posterior dorsal spinodiapophyseal lamina, but this probably corresponds to one (probably the anterior) of the two spinodiapophyseal laminae of the posterior dorsals of the other titanosaurs.

Vascularised endosteal bone tissue in armoured sauropod dinosaurs

The presence of well-vascularised, endosteal bone in the medullary region of long bones of nonavian dinosaurs has been invoked as being homologous to medullary bone, a specialised bone tissue formed during ovulation in birds. However, similar bone tissues can result as a pathological response in modern birds and in nonavian dinosaurs, and has also been reported in an immature nonavian dinosaur. Here we report on the occurrence of well-vascularised endosteally formed bone tissue in three skeletal elements of armoured titanosaur sauropods from the Upper Cretaceous of Argentina: i) within the medullary cavity of a metatarsal, ii) inside a pneumatic cavity of a posterior caudal vertebra, iii) in intra-trabecular spaces in an osteoderm. We show that considering the criteria of location, origin (or development), and histology, these endosteally derived tissues in the saltasaurine titanosaurs could be described as either medullary bone or pathological bone. Furthermore, we show that similar endosteally formed well-vascularised bone tissue is fairly widely distributed among nondinosaurian Archosauriformes, and are not restricted to long bones, but can occur in the axial, and dermal skeleton. We propose that independent evidence is required to verify whether vascularised endosteal bone tissues in extinct archosaurs are pathological or reproductive in nature.

Griphotherion peiranoi, gen. et sp. nov., A New Eocene Notoungulata (Mammalia, Meridiungulata) from Northwestern Argentina

ABSTRACT Griphotherion peiranoi, a new genus and species of notoungulate from northwestern Argentina, is presented here. The material studied consists of a nearly complete skull and part of the postcranial skeleton, recovered in the Lumbrera Formation (Eocene) at the locality of El Simbolar (Salta Province, northwestern Argentina). The specimen was evaluated in terms of a matrix of 28 craniodental characters and compared with 11 typotherian genera. Additionally, characters of the new taxon were compared to those included in a previously published data matrix that includes several native South American ungulates. The particular features present in the specimen indicate unequivocally that this is a new notoungulate taxon; nevertheless, due to its distinctiveness, we opted not to place the novel taxon in any of the known families of Notoungulata. The results of the analyses performed indicate that G. peiranoi is closely related to Mesotheriidae, Hegetotheriidae, and Archaeohyracidae. These families represent the ‘rodent-like’ notoungulates that evolved during the Late Paleogene and the Neogene. This new record contributes to our knowledge of the Eocene notoungulate fauna of northwerstern Argentina. It also supports the broader perspective that this region played an important role as an area of origin and diversification for several lineages of this remarkable order of endemic South American ungulates.

Morphology, Microanatomy, and Histology of Titanosaur (Dinosauria, Sauropoda) Osteoderms from the Upper Cretaceous of Patagonia

ABSTRACT Titanosaurs are the only group of sauropodomorph dinosaurs that possesses osteoderms. The Anacleto and Allen formations (Upper Cretaceous) from northern Patagonia (Argentina) have provided an abundance of these elements, isolated or associated with more or less complete skeletons. Here, we study the morphology, microanatomy, and histology of titanosaur osteoderms found in these stratigraphic units. The size and gross anatomy of the osteoderms are strongly variable, as well as their microanatomy, which ranges from compact structures to those with strong development of cancellous bone. The primary bone tissue is composed of structural fiber bundles that are ossified by dermal metaplasia. Bone pathologies were identified in at least two osteoderms. Bone histology suggests that the osteoderms were entirely imbedded in the stratum compactum of the dermis. Titanosaur osteoderms were probably employed for multiple functions, including mineral storage and defense.

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.