THE OLDEST ORNITHUROMORPHA EGG FROM THE LATE CRETACEOUS OF LA RIOJA, NW ARGENTINA

FIORELLI, LUCAS E.; HECHENLEITNER, E. MARTÍN; LEA LEUZINGER; ROCHER, SEBASTIÁN; TABORDA, JEREMÍAS R. A.

Córdoba

Jornada; XXXIII JAPV; 2019

The rise of Ornithuromorpha, the clade that includes the modern birds (Neornithes, the crown avian clade), is a current hot topic that has steadily grown over the last decade, especially in the light of several recent Lower Cretaceous findings from China. Although these discoveries have improved our knowledge on the origin and diversification of Ornithuromorpha, many aspects on their palaeobiology and evolution remain unknown. For example, skeletal remains and soft tissues represent the only fossil record of Cretaceous ornithuromorphs, and reproductive traits such as eggs and evidence of nesting behaviorare limited to their sister taxon Enantiornithines. Here we report the first and oldest ornithuromorph egg (CRILAR-Pv 531) from the Late Cretaceous Los Llanos Formation based on a nearly complete and three-dimensionally preserved specimen. This egg comes from the pedogenized sandstones that characterize the uppermost section of the unit at the extraordinary titanosaur-nesting colony in the Sanagasta paleohydrothermal site, La Rioja province, northwestern Argentina. We infer a maximum length of L= 101 mm and a maximum breadth of B= 70.3 mm, and a 500 um thick three layered eggshell. Theegg is larger than other avian Cretaceous eggs ?i.e., enantiornithines? and the unique combination of character-state (ovoid shape, as well as eggshell microcharacterizations) matches with other Cenozoic and extant bird eggs. The Sanagasta ornithuromorpha egg is the first bird remain from the Mesozoic of La Rioja, and sheds light on the successive transitional oological stages from the non-avian dinosaurs, through early avialan, and to the modern avian condition.