Colossus of the sea: gigantism in Mosasauroidea (Reptilia: Squamata)

• Applicant: Dr. Paulina Jimenez-Huidobro, Ph.D.
• Subject Area: Palaeontology
• Term: since 2021
• Project identifier: Deutsche Forschungsgemeinschaft (DFG) - Project number 464349130

Mosasauroidea is a group of squamate lizards that radiated into aquatic environments during the Late Cretaceous. Their record is known from the Cenomanian to the end of the Maastrichtian. Mosasaurs represent the most highly evolved and specialized group within the clade Squamata. They colonized the oceans of the world, reaching a cosmopolitan distribution as a group, in both hemispheres. The first records from the Cenomanian and Turonian are mostly represented by small-bodied facultatively aquatic aigialosaurs from the Mediterranean Tethys and North America; while the Campanian and Maastrichtian faunas are represented by large to gigantic-bodied obligatorily aquatic taxa around the globe. The transition from plesiopelvic-plesiopedal (facultatively aquatic forms) to hydropelvic-hydropedal (obligatorily aquatic forms) conditions constitutes one of the more dramatic cases of squamates radiation into aquatic environments during the mid-Late Cretaceous, and it has been studied mostly from the perspectives of morphological changes at the level of limbs and pelvic girdle. However, the group faced another dramatic transition in terms of body sizes, achieving gigantic body sizes while switching to streamlined body shapes by the end of the Late Cretaceous. Current phylogenetic hypotheses suggest that two lineages of mosasauroids evolved the hydropelvic/hydropedal conditions independently: Mosasaurinae and Russellosaurina (Tylosaurinae + Plioplatecarpinae). Convergently, the same two lineages evolved to gigantic-bodied taxa during the second half of the Late Cretaceous: tylosaurines reached gigantic body sized during the late Santonian and Campanian, while mosasaurines did so during the late Campanian and Maastrichtian. The main objective of this research project is to explore the evolution of gigantism reached by late taxa of Mosasauroidea in a phylogenetic framework. The present project proposes an extensive study of the body size increase seen in mosasauroids at the end of the Late Cretaceous, including morphological, ontogenetic, and phylogenetic assessment of primitive small-bodied aigialosaurs, as well as derived gigantic-bodied mosasaurids. The project will link the increase of body size to aspects of the biology of mosasaurs, such as their growth rate, their changes of sensorial structures, and how viviparity would be a key adaptation in the success of gigantic fully-aquatic taxa.

• DFG Programme: Research Grants