Out of time: Gigantic South American theropods in latest Cretaceous rocks of Bolivia.

APESTEGUÍA, SEBASTIÁN; MOLINA PÉREZ, RUBÉN; MEDINA RAMÍREZ, OMAR; GROVER MARQUINA

lima

Congreso; IX Congreso Latinoamericano de Paleontología; 2016

sociedad paleontologica del peru

Whereas the Age of Gigantic sauropods can be considered to be the Upper Jurassic in North America and the early Late Cretaceous in South America, the Age of Gigantic theropods could be considered the early Late Cretaceous in South America (e.g.: Giganotosaurus) and the latest Cretaceous in North America (e.g.: Tyrannosaurus). Actually, the study of the evidence, both bones and tracks, demonstrated that latest Cretaceous theropod dinosaurs were modest in South America, no longer than 9 m, in agreement with small sauropods and mid-sized hadrosaurids. A visit to outcrops of the El Molino Formation (Maastrichtian) exposed in the Maragua region, 65 km north from Sucre city, Bolivia, resulted in the finding of two unusually large theropod tracks. The first one was discovered on February 2016, and consisted of two contiguous tracks that permitted to take additional measures. The better preserved shows a length of 104 cm. In July 2016 one of the authors (GM) found a second, isolated but well-preserved and longer track, with a length of 114 cm. A comprehensive comparative analysis on development by one of the authors (RMP) suggests that both trackways belong to the same taxon under different preservative conditions. They both share long fingers and an asymmetrical disposition of digit IV which is longer than digit II. The digit II results closer than the distal part than digit IV, thus resembling abelisaurid and spinosaurid structure. However, spinosaurids were already absent for that time and furileusaurian abelisaurids commonly let symmetrical tracks, with a symmetrical heel and digits II and IV rather equal in length. Other possibilities to consider for that time and age are an overgrown neovenatorid species or immigrant tyrannosaurids, both hypotheses not supported by bones yet. The former group left tracks whose digit IV result closer than the distal part of the track whereas in tyrannosaurids there are species with digit IV closer than the distal part (Albertosaurus, Alioramus, Tarbosaurus), or dígit II (Gorgosaurus, Daspletosaurus, Tyrannosaurus). Among the members of the clade, Chilantaisaurus shows digit II and IV at the same distance from the distal part of the track. A size estimation based on the track alone suggests the specimen, weighting 7 tons, reached around 14 meters in length, being the world largest known theropod track (though other records are close to the same size, showing a structural limit for these dinosaurs), and the largest record for latest Cretaceous theropods in southern continents, showing that gigantic south American theropods, consisting on carcharodontosaurids at 100 to 90 mya, were perhaps replaced by gigantic neovenatorids by latest Cretaceous, 70 mya.