Rubén D. Martínez

An articulated specimen of the basal titanosaurian (Dinosauria: Sauropoda) Epachthosaurus sciuttoi from the early Late Cretaceous Bajo Barreal Formation of Chubut Province, Argentina

Abstract We describe an articulated specimen of the titanosaurian sauropod Epachthosaurus sciuttoi from the early Late Cretaceous Bajo Barreal Formation of Chubut Province, central Patagonia, Argentina. The skeleton was found in tuffaceous sandstone, with its hindlimbs flexed and its forelimbs widely extended. It is slightly deformed on its left side. The skull, neck, four or five cranial dorsal vertebrae, and several distal caudals are missing. Epachthosaurus is diagnosed by the following autapomorphies: middle and caudal dorsal vertebrae with accessory articular processes extending ventrolaterally from the hyposphene, a strongly developed intraprezygapophyseal lamina, and aliform processes projecting laterally from the dorsal portion of the spinodiapophyseal lamina; hyposphene-hypantrum articulations in caudals 1–14; and a pedal phalangeal formula of 2-2-3-2-0. The genus shares the following apomorphies with various titanosaurians: caudal dorsal vertebrae with ventrally expanded posterior centrodiapophyseal laminae; six sacral vertebrae; an ossified ligament or tendon dorsal to the sacral neural spines; procoelous proximal, middle, and distal caudal centra with well-developed distal articular condyles; semilunar sternal plates with cranioventral ridges; humeri with squared proximolateral margins and proximolateral processes; unossified carpals; strongly reduced manual phalanges; craniolaterally expanded, nearly horizontal iliac preacetabular processes; pubes proximodistally longer than ischia; and transversely expanded ischia. Epachthosaurus is considered the most basal titanosaurian known with procoelous caudal vertebrae.

A definitive abelisaurid theropod dinosaur from the early Late Cretaceous of Patagonia

Abstract A nearly complete, well-preserved maxilla of an abelisaurid theropod from the early Late Cretaceous (middle Cenomanian-Turonian) Lower Member of the Bajo Barreal Formation of Chubut, Argentina represents the first definitive member of the abelisaurid clade from pre-Senonian (Coniacian–Maastrichtian) deposits. The new maxilla shares derived characters with the maxillae of Carnotaurus and Majungatholus, and with AMNH 1955, a maxilla previously referred to Indosuchus, suggesting that it pertains to the abelisaurid subclade Carnotaurinae. Abelisaurus shares apomorphic characters with Carnotaurinae, but many of these characters are also found in the carcharodontosaurid allosauroid Giganotosaurus. As it is known only from cranial material lacking carnotaurine synapomorphies, Abelisaurus may represent a late-surviving carcharodontosaurid derivative. The presence of the Bajo Barreal predator in the early Late Cretaceous indicates that the origin of Abelisauridae had occurred by then. The occurrence of the new maxilla is nearly concurrent with the accepted interval of tectonic divergence between South America and Africa. Its discovery thus weakens support for the recent hypothesis that the abelisaurid clade could not have penetrated Africa. The known occurrence of Abelisauridae may reflect a former pan-Gondwanan distribution, and is thus of limited utility in the support of Late Cretaceous paleogeographic hypotheses.

A Gigantic, Exceptionally Complete Titanosaurian Sauropod Dinosaur from Southern Patagonia, Argentina

Titanosaurian sauropod dinosaurs were the most diverse and abundant large-bodied herbivores in the southern continents during the final 30 million years of the Mesozoic Era. Several titanosaur species are regarded as the most massive land-living animals yet discovered; nevertheless, nearly all of these giant titanosaurs are known only from very incomplete fossils, hindering a detailed understanding of their anatomy. Here we describe a new and gigantic titanosaur, Dreadnoughtus schrani, from Upper Cretaceous sediments in southern Patagonia, Argentina. Represented by approximately 70% of the postcranial skeleton, plus craniodental remains, Dreadnoughtus is the most complete giant titanosaur yet discovered, and provides new insight into the morphology and evolutionary history of these colossal animals. Furthermore, despite its estimated mass of about 59.3 metric tons, the bone histology of the Dreadnoughtus type specimen reveals that this individual was still growing at the time of death.

A Medium-Sized Ornithopod (Dinosauria: Ornithischia) from the Upper Cretaceous Bajo Barreal Formation of Lago Colhué Huapi, Southern Chubut Province, Argentina

ABSTRACT An associated partial postcranial skeleton is described as the third definitive ornithopod dinosaur record from the Upper Cretaceous Bajo Barreal Formation of central Patagonia, Argentina. Specifically, the specimen was recovered from the uppermost Cretaceous (Campanian—?Maastrichtian) Upper Member of the Bajo Barreal exposed on an ephemeral island in the southeastern portion of Lago Colhué Huapi in southern Chubut Province. Identifiable elements of the skeleton include four incomplete dorsal vertebrae, three partial anterior caudal vertebrae, a middle caudal neural arch, an incomplete posterior caudal vertebra, a dorsal rib fragment, the right calcaneum, and portions of the left metatarsal III and right metatarsal IV. Comparisons with corresponding elements in other ornithischians indicate that the material pertains to a medium-sized, non-hadrosaurid ornithopod. In particular, the morphology of the calcaneum is characteristic of ornithopods of this “grade”. The new discovery augments our understanding of the latest Cretaceous terrestrial vertebrate assemblage of central Patagonia and adds to the generally meager record of ornithischians in the Late Cretaceous of the Southern Hemisphere.

Aniksosaurus darwini gen. et sp. nov., a new coelurosaurian theropod from the Early Late Cretaceous of Central Patagonia, Argentina

The theropod dinosaur Aniksosaurus darwini gen. et sp. nov. has been recovered from the Upper Cretaceous, Bajo Barreal Formation, of Central Patagonia. Aniksosaurus darwini gen. et sp. nov. was a small tetanurine, approximately 2 meters long. Aniksosaurus exhibits several unique traits (e.g., cranial cervical vertebrae with dorsoventrally deep neural arches, provided with a pair of cavities at their cranial surfaces; neural canal wide; cranial caudals with ventral sagittal keel, and transverse processes triangular-shaped in dorsal view; manual ungual phalanges robust; ilium with extremely expanded brevis shelf; femur with deep notch for M. Iliotrochantericus; metatarsal and digit IV of pes transversely narrow). Available postcranial bones of Aniksosaurus exhibit derived features of Coelurosauria (e.g., ilium with well developed cuppedicus fossa; femur with anterior trochanter proximally projected, almost reaching the level of the articular head; greater trochanter craniocaudally expanded; femoral head rectangular-shaped in cranial aspect; and fibular shaft craniocaudally narrow), as well as characteristics suggesting that the new Patagonian taxon is more derived than some basal coelurosaurians such as compsognathids, Ornitholestes, and coelurids. Comparisons with maniraptoriforms (a clade including Ornithomimosauria, Tyrannosauridae, Oviraptorosauria, Alvarezsauridae and Paraves) support that Aniksosaurus is less derived than these theropods. In sum, Aniksosaurus is here considered as a Late Cretaceous survivor of a basal coelurosaurian radiation.

A Basal Lithostrotian Titanosaur (Dinosauria: Sauropoda) with a Complete Skull: Implications for the Evolution and Paleobiology of Titanosauria

We describe Sarmientosaurus musacchioi gen. et sp. nov., a titanosaurian sauropod dinosaur from the Upper Cretaceous (Cenomanian—Turonian) Lower Member of the Bajo Barreal Formation of southern Chubut Province in central Patagonia, Argentina. The holotypic and only known specimen consists of an articulated, virtually complete skull and part of the cranial and middle cervical series. Sarmientosaurus exhibits the following distinctive features that we interpret as autapomorphies: (1) maximum diameter of orbit nearly 40% rostrocaudal length of cranium; (2) complex maxilla—lacrimal articulation, in which the lacrimal clasps the ascending ramus of the maxilla; (3) medial edge of caudal sector of maxillary ascending ramus bordering bony nasal aperture with low but distinct ridge; (4) ‘tongue-like’ ventral process of quadratojugal that overlaps quadrate caudally; (5) separate foramina for all three branches of the trigeminal nerve; (6) absence of median venous canal connecting infundibular region to ventral part of brainstem; (7) subvertical premaxillary, procumbent maxillary, and recumbent dentary teeth; (8) cervical vertebrae with ‘strut-like’ centroprezygapophyseal laminae; (9) extremely elongate and slender ossified tendon positioned ventrolateral to cervical vertebrae and ribs. The cranial endocast of Sarmientosaurus preserves some of the most complete information obtained to date regarding the brain and sensory systems of sauropods. Phylogenetic analysis recovers the new taxon as a basal member of Lithostrotia, as the most plesiomorphic titanosaurian to be preserved with a complete skull. Sarmientosaurus provides a wealth of new cranial evidence that reaffirms the close relationship of titanosaurs to Brachiosauridae. Moreover, the presence of the relatively derived lithostrotian Tapuiasaurus in Aptian deposits indicates that the new Patagonian genus represents a ‘ghost lineage’ with a comparatively plesiomorphic craniodental form, the evolutionary history of which is missing for at least 13 million years of the Cretaceous. The skull anatomy of Sarmientosaurus suggests that multiple titanosaurian species with dissimilar cranial structures coexisted in the early Late Cretaceous of southern South America. Furthermore, the new taxon possesses a number of distinctive morphologies—such as the ossified cervical tendon, extremely pneumatized cervical vertebrae, and a habitually downward-facing snout—that have rarely, if ever, been documented in other titanosaurs, thus broadening our understanding of the anatomical diversity of this remarkable sauropod clade. The latter two features were convergently acquired by at least one penecontemporaneous diplodocoid, and may represent mutual specializations for consuming low-growing vegetation.

Katepensaurus goicoecheai, Gen. Et Sp. Nov., a Late Cretaceous Rebbachisaurid (Sauropoda, Diplodocoidea) from Central Patagonia, Argentina

ABSTRACT We describe Katepensaurus goicoecheai, gen. et sp. nov., a diplodocoid sauropod dinosaur from the Bajo Barreal Formation (Upper Cretaceous: Cenomanian—Turonian) of south-central Chubut Province, central Patagonia, Argentina. The holotypic specimen is a closely associated partial axial skeleton that includes cervical, dorsal, and caudal vertebrae. The dorsal vertebrae of Katepensaurus exhibit the following distinctive characters that we interpret as autapomorphies: (1) internal lamina divides lateral pneumatic fossa of centrum; (2) vertical ridges or crests present on lateral surface of vertebra, overlying neurocentral junction; (3) pair of laminae in parapophyseal centrodiapophyseal fossa; (4) transverse processes perforated by elliptical fenestrae; and (5) well-defined, rounded fossae on lateral aspect of postzygapophyses. Based on the results of previous phylogenetic analyses, we regard the new taxon as a member of Rebbachisauridae; more specifically, it may pertain to Limaysaurinae, a rebbachisaurid subclade that, to date, is definitively known only from southern South America. As currently understood, the rebbachisaurid fossil record suggests that the clade achieved its greatest taxonomic diversity within a few million years of its extinction during the early Late Cretaceous.

The Behavioral Implications of a Multi-Individual Bonebed of a Small Theropod Dinosaur

Background Central Patagonia, Argentina, preserves an abundant and rich fossil record. Among vertebrate fossils from the Upper Cretaceous Bajo Barreal Formation of Patagonia, five individuals of the small, non-avian theropod dinosaur Aniksosaurus darwini were recovered. Group behavior is an important aspect of dinosaur paleoecology, but it is not well-documented and is poorly understood among non-avian Theropoda. Methods/Principal Findings The taphonomic association of individuals from the Bajo Barreal Formation and aspects of their bone histology suggest gregarious behavior for Aniksosaurus, during at least a portion of the life history of this species. Histology indicates that the specimens were juvenile to sub-adult individuals. In addition, morphological differences between individuals, particularly proportions of the appendicular bones, are probably related to body-size dimorphism rather than ontogenetic stage. Conclusions/Significance Gregarious behaviour may have conferred a selective advantage on Aniksosaurus individuals, contributing to their successful exploitation of the Cretaceous paleoenvironment preserved in the Bajo Barreal Formation. The monospecific assemblage of Aniksosaurus specimens constitutes only the second body fossil association of small, coelurosaurian theropods in South America and adds valuable information about the paleoecologies of non-avian theropod dinosaurs, particularly in the early Late Cretaceous of Patagonia.

Histological evidence for a supraspinous ligament in sauropod dinosaurs.

Supraspinous ossified rods have been reported in the sacra of some derived sauropod dinosaurs. Although different hypotheses have been proposed to explain the origin of this structure, histological evidence has never been provided to support or reject any of them. In order to establish its origin, we analyse and characterize the microstructure of the supraspinous rod of two sauropod dinosaurs from the Upper Cretaceous of Argentina. The supraspinous ossified rod is almost entirely formed by dense Haversian bone. Remains of primary bone consist entirely of an avascular tissue composed of two types of fibre-like structures, which are coarse and longitudinally (parallel to the main axis of the element) oriented. These structures are differentiated on the basis of their optical properties under polarized light. Very thin fibrous strands are also observed in some regions. These small fibres are all oriented parallel to one another but perpendicular to the element main axis. Histological features of the primary bone tissue indicate that the sacral supraspinous rod corresponds to an ossified supraspinous ligament. The formation of this structure appears to have been a non-pathological metaplastic ossification, possibly induced by the continuous tensile forces applied to the element.

TAFONOMÍA DEL DINOSAURIO TERÓPODO ANIKSOSAURUS DARWINI, FORMACIÓN BAJO BARREAL, CRETÁCICO TARDÍO DE PATAGONIA (ARGENTINA)

Abstract. TAPHONOMY OF THE THEROPOD DINOSAUR ANIKSOSAURUS DARWINI, BAJO BARREAL FORMATION, LATE CRETACEOUS OF PATAGONIA (ARGENTINA). Aniksosaurus darwini is a coelurosaurian theropod from the Bajo Barreal Formation (Upper Cretaceous), south of Chubut Province, Patagonia, Argentina. The fossils include postcranial remains of 5 individuals and they were preserved in a overbank mantle deposit associated to a low-sinuosity fluvial system. A detailed analysis on the biostratinomic, fossildiagenetic, and paleoenvironmental aspects of the site of occurrence of Aniksosaurus allows us to recognize two meteorization stages and their possible association with the origin of accumulation of the skeletons. Moreover, tooth marks in one of the femora are preliminarily assigned to a small crocodyliform. The taphonomic study adds new anatomic evidence related to soft tissues. The holotype, MDT-Pv 1/48, is represented by an articulated right hindlimb including, femur, tibia, metatarsals and phalanges. The subaerial weathering and abrasion of the femur due to transport strongly contrast with the preservation of the rest of the hindlimb bones, suggesting the presence of a protecting soft tissue resembling the podoteca seen in extant birds. The presence of this soft tissue in Aniksosaurus, could support the early occurrence of this feature in basal coelurosaurians.