Abstract The Snow Hill Island Formation (SHIF; late Campanian – early Maastrichtian) crops out in the northeast of the Antarctic Peninsula and constitutes the basal part of the late Campanian-early Maastrichtian sedimentary succession of the James Ross Basin (NG Sequence). Its major exposures occur at the James Ross and Vega islands. Several fossil-bearing localities have been identified in the SHIF providing a valuable fauna of invertebrates and vertebrates, and flora. Our study focuses on the vertebrate fauna recovered at Gamma and Cape Lamb members of the SHIF. The marine vertebrate assemblages include chondrichthyans, actinopterygians, and marine reptiles (elasmosaurid plesiosaurs and mosasaurs). A diverse terrestrial vertebrate assemblage has been reported being characterized by dinosaurs (sauropod, elasmarian ornithopods, nodosaurid ankylosaur, and a paravian theropod), pterosaurs and birds. Most SHIF dinosaurs share close affinities with penecontemporaneous taxa from southern South America, indicating that at least some continental vertebrates could disperse between southern South America and Antarctica during the Late Cretaceous. The Snow Hill Island Formation provides the most diverse Late Cretaceous marine and continental faunas from Antarctica. The present study summarizes previous and new vertebrate findings with the best actualized stratigraphical framework, providing a more complete fauna association and analyzing further perspectives.

Titanosaur embryonic remains are described from megaloolithid eggs collected in the Anacleto Formation (Late Cretaceous) at Auca Mahuevo in Neuquén province (Argentina). These remains include complete articulate and disarticulate skulls, and numerous cranial and appendicular isolated elements. The embryonic skull is short, high, and presents numerous fenestrae, most of which are known in adult titanosaurs. However, the morphology of the maxilla and jugal is very different from that of adult titanosaurs. These elements show a series of processes that arise from their central body, which seem to have become reduced and transformed during ontogeny. The dental formula (Pm4 M7-8 /D10?) is similar to that of known adult titanosaurs. Detailed osteological study of these remains shows features of taxonomic, phylogenetic, and evolutionary significance. Several of the characters, including the snout with "stepped" anterior margin and the infratemporal fenestra extending ventrally to the orbit support the identification of the embryos as eusauropods. Other characters, such as the absence of crown denticles and the presence of a preantorbital fenestra are considered to be synapomorphies of Neosauropoda. Finally, the posteriorly wide skull and a maxillary notch at the ventral margin of the skull are regarded as synapomorphies of Titanosauria. Characters such as relatively large orbit, short rostral portion of the skull, long alveolar rows, "egg tooth", tooth crowns lacking wear facets, frontal occupying practically the entire dorsal margin of the orbit, and nonossified postcranial axial skeleton are typical characters of sauropod embryonic and juvenile stages.

Se describen restos embrionarios de dinosaurios saurópodos hallados en huevos Megaloolithidae recolectados de la Formación Anacleto (Cretácico Superior), en la localidad de Auca Mahuevo, en la provincia de Neuquén, Argentina. Estos materiales incluyen cráneos completos articulados y desarticulados, así como numerosos elementos aislados craneanos y postcraneanos. El cráneo embrionario es corto y alto, y presenta una serie de fenestras, la mayoría de las cuales está también en el cráneo de los titanosaurios adultos. Sin embargo, el yugal y el maxilar son muy diferentes con respecto a la morfología exhibida en los especímenes adultos. Estos elementos presentan una serie de procesos que radian de su cuerpo central, los cuales parecen modificarse durante la ontogenia. La fórmula dental de los embriones de Auca Mahuevo es Pm4 M7-8 /D10?, básicamente la misma que en los titanosaurios adultos conocidos. Varios de los caracteres, tales como margen rostral del hocico escalonado y fenestra infratemporal extendida ventralmente a la órbita, permiten asignarlos a los Eusauropoda, mientras que otros, como la ausencia de dentículos en las coronas dentales y la presencia de una fenestra preantorbital, los relacionan con los Neosauropoda. Por último, el cráneo posteriormente ancho y la existencia de una pronunciada escotadura maxilar en el margen ventral del cráneo son reconocidos como sinapomorfías de los Titanosauria. Otros caracteres como órbita relativamente grande, porción rostral del cráneo corta, hileras alveolares largas, ausencia de facetas de desgaste en los dientes, la presencia de un "diente de huevo" y el esqueleto axial no osificado son rasgos propios de los estadíos embrionarios y juveniles de los saurópodos.

We present new remains from the Cam-panian-Maastrichtian beds of the Alien Formation, in Salitral Ojo de Agua (Río Negro, Argentina), which are assignable to Alvarezsauridae indet. This clade of small coelurosaurian theropods is known from strata of similar age in Mongolia, and from older sediments (Turonian and Coniacian in age) in Argentina. Thus, the material presented here extends the temporal record of the alvarezsaurids from the Southern Hemisphere up to the Latest Cretaceous. The fossil bones reported were associated to eggshells of the oofamily Elongatoolithidae, which are actually attributed to Theropoda.

Se presentan nuevos restos provenientes de estratos campaniano-maastrichtianos de la Formación Alien, en Salitral Ojo de Agua (Río Negro, Argentina), los cuales son asignables a Alvarezsauridae indet. Este ciado de pequeños terópodos celurosaurios es conocido de estratos de edad similar en Mongolia y de sedimentitas más antiguas (de edad turoniana y coniaciana) en Argentina. De este modo, el material que aquí se informa extiende el registro temporal de los alvarezsáuridos del Hemisferio Sur hasta el Cretácico Tardío alto. Los huesos reportados estaban asociados a cascaras de huevo de la oofamilia Elongatoolithidae, los cuales son actualmente atribuidos a Theropoda.