A window to the Cretaceous of Patagonia

SEBASTIÁN APESTEGUÍA

Buenos Aires

Simposio; Simposio de jóvenes investigadores, VIII Jornadas Multidisciplinarias de la Sociedad Argentina de Biología; 2006

Instituto de Biología y Medicina Experimental

The large dinosaurs from Argentina are now well known in the entire world not only because of their scientific importance but also for their importance in the public knowledge. However, during the Mesozoic Era a great variety of small to medium sized animals were sharing the habitat, but they have lesser chances to be preserved by means of the fossilization process. “La Buitrera” is a locality discovered in 1999 that belongs to the Candeleros Formation (about 90-95 millon years ago) located over the NW side of the “Planicie de Rentería” (Río Negro Province). The “La Buitrera” fauna allows us to study poorly known aspects of the animals that lived under the shadow of the largest dinosaurs. From that place, an increasing number of semi-articulated and tridimensionally preserved remains are being currently obtained, including: lungfishes, chelid turtles, sphenodontid lepidosaurs, limbed snakes, araripesuchid terrestrial crocodiles, carcharodontosaurids, abelisauroids and dromaeosaurid theropods, diplodocimorph and titanosauriforms sauropods, and dryolestoid mammals. These fossils are well preserved and the disarticulation degree is low, with evidences of some aerial exposure and associated to paleosols. Abundant ichnofossils show bitten and scratched bones. When a large dinosaur died, its enormeous carcass remains visible for a long time, accessible to predators and exogenous agents. However, a small dead animal ties its fate to the scavenger’s skills, provided of a delicate smell and powerful jaws that reduce a small skeleton to almost nothing. However, at “La Buitrera”, the fine sands carried by the local river buried fast the animals smaller than two meters, probably during sudden floodings. This way, only the small animals of the area were adequately preserved, whereas remains of the larger ones are rather scarce. From the lower level, a very fine sandstone freshwater turtles have been found, and in the upper level, non-articulated remains of dinosaurs and lungfish tooth plates (the oldest from Argentina) were discovered. The vertebrates from “La Buitrera” lived a peculiar moment in the evolutionary history of dinosaurs, a moment commonly known as the “Mid Cretaceous” or the “Era of the giants”. At that time several lineages of long-necked sauropods lived at Patagonia, whereas they became completely extinct in Europe and North America. Among the carnivorous dinosaurs or theropods, several lineages were present, such as the carcharodontosaurids (e.g., Giganotosaurus), the top mega-predators, exceeding 15 meters in length; whereas the abelisauroids (like Carnotaurus), that would be some day the most important predators, still occupied a secondary place. The maniraptorans, small to medium-sized carnivorous, started to become abundant, including several lineages like the dromaeosaurids, probably distributed in the Southern hemisphere since Jurassic times. Most of them were feathered and shared several features with birds. The ornithischians, herbivorous dinosaurs, were not as abundant as in the Northern hemisphere, and rather scarce when compared to sauropods, or perhaps they lived in low-preservation potential areas. Although they are beginning to be known, as the basal ornithopods Gasparinisaura and Talenkauen, with protective bonny side plates, they’ve never been found at “La Buitrera”. The South American crocodiles include several primitive lineages known as mesoeucrocodiles. Among them we can find the notosuchians, of canine aspect, the araripesuchids, rather fox-like, and the peirosaurids, large terrestrial hunters of serrated caniniforms. At “La Buitrera”, the most abundant were the araripesuchids by far, carrying only two lines of scutes and long legs. Both juvenile and adult specimens have been found. Eilenodontine sphenodontids like Priosphenodon are the most abundant reptiles at “La Buitrera”. Over 200 specimens depict an unusual abundance or a natural concentration of remains by natural processes. Their presence in the Late Cretaceous of Patagonia extended the known survival of the group, up to that moment restricted to the Late Jurassic and Early Cretaceous of North America. The “La Buitrera” snakes are a different problem. The plesiomorphical features in their skull, as well as their hip and robust posterior legs placed them as the basalmost known fossil snakes. Although during the last decades, limbed fossil snakes have been considered as irrefutable evidences of primitivism and a basal position in the lineage, the “relictual” members in extant and almost all fossil snakes, indicates that this character should not be hardly taken into account. The finding of several marine limbed snakes in North Africa, Israel and Eastern Europe, conducted the "mariniste" researchers                 to consider them as basal forms, proof of a marine origin for the group. On the other side, the “terrestriste” researchers propose that snakes originated from a lizard with no legs, probably related to dibamids or amphisbaenians. Nevertheless, the latter hypothesis had bad fossil support. The finding of Najash at "La Buitrera", a fossil snake with a sacrum, pelvis and strong and functional hindlimbs positioned outside the ribcage, provenant from a clear terrestrial environment, interestingly contributes to an old debate. Actually, the occipital skull region, a wide stapedial base and low neural arches, suggest subterranean habits. Additionally, the phylogenetic analysis shows a basal position respect to all other known snakes, including the scolecophidian fosorial forms and also confirms marine pachyophid snaks as an early radiation of macrostomatan snakes. The turtles that inhabited the calm waters and small lakes of "La Buitrera" are chelids, today represented by Acanthochelys, aquatic ichtyophagous turtles from small rivers and lakes. The only fishes found at the moment are the dipnoi, fresh water lungfishes, belonging to the Ceratodontiformes, which left only tooth plates as testimony of their existence. Some few million years after the depositation of the typical layers of "La Buitrera" by the big local river, a volcanic eruption took place. Because of the great distance, only dense clouds of ashes arrived to the area, being accumulated as layers of several meters in thickness. The geological compaction reduced the enormous layer of ashes to only two meters, forming a wide white belt in the upper part of the canyon, in the first meters of the Huincul Formation. This eruption was part of the events that would culminate with the elevation of the Andes. The radiometric datation of the rock formed by the compacted ash, resulted in 88 million years. This way, the “La Buitrera" fauna provide us the very rare opportunity to contemplate the medium size faunal components of the Late Cretaceous of Patagonia, better known by their giants. The continuity of the work in the area remains implicit in the new yearly discoveries, like the pterosaurs found at 2005, now in preparation.