Periodic Reporting for period 2 - Back2Sea (Back to the sea: Axial evolution in secondarily aquatic mammals)
Periodo di rendicontazione: 2023-10-01 al 2024-09-30
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.
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.
To tackle the questions outlined above, the project used a multidisciplinary approach relying on a variety of innovative methods to collect and analyse morphological and functional data in an evolutionary context. Morphological data quantifying the vertebral shape have been collected in natural history museums on more than 100 extant species of aquatic, semi-aquatic, and closely-related terrestrial mammals. Similar morphological data were also obtained for several key aquatic and semi-aquatic fossils. Finally, functional data on intervertebral flexibility along the backbone have been experimentally obtained for eight model species representing different degrees of aquatic adaptation. The combination of these morphological and functional dataset with cutting-edge statistical analyses highlight the re-organization of the mammalian backbone along the land-to-water transition. While semi-aquatic species retain vertebral shape and function similar to terrestrial species, fully aquatic species such a cetaceans and sirenians concentrate most of their variability in vertebral shape and flexibility in the caudal region of the skeleton, consistent with a tail-powered mode of locomotion in the aquatic realm.
The backbone is a defining feature of vertebrates and play a central functional role in body support and/or locomotion. Yet, vertebral adaptations across the land-to-water transitions have been overlooked as most studies have focused on adaptations of the head and limbs. Incorporating knowledge on the shape, function and evolution of the backbone will allow us to improve our understanding of the impacts of such major ecological transitions on the evolutionary history of life. Due to the numerous elements (vertebrae) constituting the axial column, such studies have only made possible recently thanks to the development of cutting-edge statistical methods and more powerful computational methods, some of which will be developed as part of the project. The Back2Sea project should allow to set a multidisciplinary framework for future studies aiming at investigating backbone evolution in other groups of vertebrates that will contribute to expand our comprehension of vertebral shape and function at a broader comparative level. Numerous independent secondarily adaptations to aquatic environments occurred during tetrapod evolutionary history. Beyond investigating the impact of these transitions in mammals, Back2Sea will provide a strong morphological and functional background to extrapolate its finding to other groups of aquatic tetrapods such as extinct marine reptiles (e.g. ichthyosaurs, mosasaurs).