One of the most significant ecological events in evolution was the water-to-land transition in vertebrates which was supported by the evolution of sturdy limbs from the delicate fins of fish, allowing tetrapods to diversify into the myriad of forms we see today. Nonetheless, several groups of land-dwelling tetrapods, such as fossil reptiles (e.g. ichthyosaurs, mosasaurs), cetaceans (whales, dolphins, and porpoises) and sirenians (manatees and dugongs), underwent a reverse transition back to an aquatic habitat. Such transitions had major repercussions for vertebrate evolution and aquatic ecosystems. These major ecological have led to drastic reorganization of the tetrapod limb-based locomotory mode through the evolution of numerous transitional forms.
While most marine reptiles are extinct, more than 150 living species of mammals are secondarily adapted to aquatic environments, representing at least a dozen of independent transitions during mammalian evolution. These species exhibit different degrees of aquatic adaptation from four-limbed amphibious species (such as muskrats, beavers, and otters), to fish-like fully aquatic species (cetaceans and sirenians), making them an ideal group for investigating the impact of the land-to-water transitions on vertebrate evolution. In the most aquatic species, the land-to-water transition involved a shift from a limb-based mode of locomotion to a body-based mode relying on backbone oscillations. Despite the central role of the vertebral column in body-powered locomotion, previous works have mostly focused on limbs modifications. Few studies have focused on backbone morphology and rigorous functional validation is still sorely lacking leading to a significant gap in our understanding of vertebral adaptations along the land-to-water transitions both in extant and extinct species.
The Back2Sea project leverages the current and past diversity of secondarily aquatic mammals to illuminate the nature and evolutionary importance of morpho-functional modifications of the vertebral column during repeated land-to-water transitions. Given that land-to-water transition occurred multiple times in mammals, the project provides a strong empirical study for understanding fundamental drivers of major ecological transitions. Overall, the Back2Sea project asks how secondary invasion of the aquatic realm affected evolutionary patterns in the mammalian backbone by investigating the impact of land-to-water transitions on (i) the vertebral function in mammals, (ii) the regionalisation of the mammalian backbone, and (iii) the mode and tempo of axial evolution.
