Bicentenaria

Discovery and naming

In 1998, falling water levels at the Ezequiel Ramos Mexía Reservoir revealed rocks from the Candeleros Formation. While on a fishing trip, Rauel Spedale and two friends spotted bones and claws protruding from the ground, which turned out to be multiple Bicentenaria specimens, including the holotype. Spedale dug out the fossils with a pocket knife and his bare hands, initially thinking they belonged to a machairodontine. He went back to the site in 1999 and 2000, using a hammer and chisel to uncover more bones from a patch just 30 by 40 centimeters wide and no deeper than half a meter.

He kept the finds in a display case at home. Later, rising water made further digging impossible. Spedale, excited about his discovery, always carried photos of it and showed them to a local school teacher. The teacher realized the find was special and connected him with paleontologist Raúl Ortiz.

Ortiz identified it as an unknown dinosaur and arranged for a team from the Museo Argentino de Ciencias Naturales to study the material. The fossils were recognized as a new genus, and the name Bicentenaria argentina first appeared in news articles in June 2012. The formal scientific description by Novas and colleagues was published in August 2012. The holotype, MPCA 865, includes the back of a skull still attached to the rear of the lower jaws.

The paratype, MPCA 866, is a set of loose bones from at least four individuals, containing parts of the postcranial skeleton.

Description

Bicentenaria was a small, two-legged meat-eating dinosaur, estimated to have been between 2.5 and 3 meters long. Its describers noted several unique traits. The teeth in the front of the upper jaw only had serrations near the base of the crown. The quadratojugal bone had a forward branch twice as long as its upward branch.

The quadratum bone featured a lower outer joint bump much larger than the inner one. In the lower jaw, the surangular had a raised, trapezoidal upper edge when viewed from the side. The backward-pointing projection at the rear of the lower jaw was flat, wide, and spoon-shaped. The top of the upper arm bone was flattened front-to-back, and its lower end had a deep groove on the front side.

The claws on the outer fingers had overhanging ridges on their rear upper surfaces. The overall skull shape is unclear, but the preserved fragments are sturdy. The premaxilla held at least four round teeth. Most teeth lacked serrations, except for one right tooth that had serrations on the upper half of its front edge.

The maxilla had a smooth outer surface with a horizontal row of large vein openings. The only known maxillary tooth was very flat, with serrations on both edges that ran all the way to the tip; those on the back edge were coarser. The cheekbone had a forked rear branch and a small side facet at its bottom that connected with the lower jaw when the mouth shut. The quadratojugal had a thin front branch much longer than its upward branch.

The quadratum was robust, lacked air pockets, and had a small, high-placed opening. The inner joint bump was greatly reduced, only half as wide as the outer one. From the braincase, only the left basipterygoid process of the basisphenoid is known, which had a deep, wide air-filled hollow at its bottom. The ectopterygoid touched the pterygoid on the inside and had a solid, hook-shaped projection on the outside for connecting to the skull wall.