A new enantiornithine specimen from the Lower Cretaceous of Las Hoyas: avifaunal diversity and life-history of a wetland Mesozoic bird

: The Lower Cretaceous fossil site of Las Hoyas (Cuenca, Spain) has yielded the richest Cretaceous avifauna of the European continent. We describe a new fossil (MUPA-LH-33333) of an enantiornithine bird from this locality. This specimen consists of a partially articulated skeleton preserving portions of the vertebral column, both girdles and limbs, ribs and sternum; it also preserves patches of soft tissues including remigial feathers and integumentary structures belonging to the postpatagium. MUPA-LH-33333 shares dimensions and some anatomical features with the holotype of Concornis lacustris , a species previously described from Las Hoyas. However, the new specimen shows differences especially in the coracoid and the sternum, suggesting the presence of a different morpho-type closely related to C. lacustris . Nevertheless, the poor preservation prevents asserting that it represents a new species. Histological evidence from its long bones indicates that is subadult or adult and that early fast rates followed by slower and protracted cyclical phases took place during its growth, a previously unnoticed pattern in Lower Cretaceous enantiornithines. This new finding supports the hypothesis that enantiornithines regularly inhabited the Las Hoyas wetland, making this site a hotspot for enhancing our understanding of the evolution and life history of these Cretaceous birds.