The first record of a thunnosaurian ichthyosaur from the upper Albian of South America

This study describes the first thunnosaurian ichthyosaur specimen discovered in the upper Albian beds of the Hilo Formation at Guayabal de Síquima (Cundinamarca, Colombia), representing the first record of an ichthyosaur from the Albian of Colombia and South America. The specimen comprises a segment of the axial skeleton, including 31 articulated and 2 disarticulated vertebral centra, several ribs, gastralia fragments, and disarticulated appendicular elements. We assign the specimen to Thunnosauria based on the presence of ribs with 8-shaped cross-section. Nearly all Cretaceous thunnosaurians are ophthalmosaurids, with the exception of Malawania anachronus. The specimen is comparable in size to some ophthalmosaurids and is larger than M. anachronus. However, no diagnostic features support its inclusion within Ophthalmosauridae. Therefore, despite its Albian age and size consistency with Ophthalmosauridae, we conservatively limit the systematic position of the new specimen to Thunnosauria. The abundance of organic matter, pyrite and high articulation of the specimen, suggest deposition in anoxic waters with high organic matter content, which limited benthic fauna and prevented disarticulation by scavengers. This interpretation aligns with previous analyses of the Hilo Formation's depositional environment.