INVESTIGADORES
MELCHOR Ricardo Nestor
congresos y reuniones científicas
Título:
Early Cretaceous theropod footprints from the Lagarcito Formation, Sierra de las Quijadas National Park, San Luis province, Argentina
Autor/es:
RICARDO NESTOR MELCHOR; DAVID RIVAROLA; MAGDALENA NALÍN MOYANO; MARIANO PEREZ; ALDO MARTÍN UMAZANO
Lugar:
Colonia del Sacramento
Reunión:
Simposio; Tercer Simposio Latinoamericano de Icnología (SLIC2015); 2015
Institución organizadora:
Facultad de Ciencias, Universidad de la República
Resumen:
This contribution reports the discovery of abundant medium-sized theropod footprints from the Albian Lagarcito Formation at Quebrada de Hualtarán (32° 29? 36? S; 66° 59? 12? W), within the Sierra de Las Quijadas National Park. The fossil content of the formation includes body fossils (pterosaurs, actinopterygian fishes, conchostracans, ostracods, and angiosperms) and trace fossils (Treptichnus, Guilielmites and trails), essentially recovered from lacustrine facies at the nearby ?Loma del Pterodaustro?. The section is about 34 m thick and rest on the 109.4 - 107.4 Ma old Hualtarán basalts. This section includes, from bottom to top: minor sandstone and laminated fossiliferous claystone and siltstone (nearshore to offshore lacustrine), thick cross-bedded sets of fine-grained sandstone (eolian dune), laminated fine-grained sandstone with gypsum laminae (damp interdune), alternating thin beds of parallel to cross-laminated sandstones and massive or laminated mudstone (unconfined fluvial deposits), well-laminated fossiliferous claystone and siltstone (lacustrine), and massive red siltstones (mudflat deposits).The theropod footprints occur at the base of two thin beds of sandstone from the unconfined fluvial deposits. The most productive level is the upper one (12.6 m from base of section), which was quarried to recover several footprint-bearing slabs. This level is a 5-10 cm thick, trough cross-bedded fine-grained sandstone bed with current-rippled top and incipient mudcracks in the bottom. The underlying level yielded a few incomplete footprints. The footprints are preserved as convex hyporeliefs ranging from deep undertracks to shallow true tracks. Most are isolated, although two cases of consecutive footprints and a single bipedal trackway were identified. Pace length is 0.59 m, the stride length is 1.17 m, and the pace angulation is 175°. The footprints are tridactyl, mesaxonic, and some show metatarsal pad and digit I impressions. Digit imprints are thick with large and well-defined phalangeal pads and robust claw marks. Approximately 30 footprints were recovered whose average length is 205 mm (range= 159-252 mm), average footprint width is 120 mm (range= 93-150 mm), and the average footprint length / width ratio is 1.75. Digit III is the longest and projects anteriorly from the lateral digits about 35% of the footprint length. Divarication of digits II-IV averages 32° (range 42-26°) and the divarication of lateral digits is similar, although usually that of digits II-III is larger. The footprints can be compared with the ichnogenus Grallator, although are also similar to Eubrontes from the Early Cretaceous Sousa basin of Brazil. Associated invertebrate trace fossils are abundant Skolithos linearis and rarer Taenidium barretti. The producer of the footprints was a small- to medium-sized non avian theropod. The finding is significant because similar tracks are only known in South America from the Sousa basin and because the skeletal record of Early Cretaceous theropods from the region is fairly fragmentary.