Innovation and Speciation: cetacean spinal changes (Evolution)

by David Turell , Tuesday, September 10, 2024, 15:54 (53 days ago) @ David Turell

Not the same as terrestrial animals:

https://communities.springernature.com/posts/the-land-to-water-transition-led-to-a-repa...

"Cetaceans, the clade comprising whales, dolphins, and porpoises, represents one of the most emblematic group of living mammals. Besides their status of "ambassadors of the seas", it is quite remarkable that about 53 million years ago their terrestrial ancestors started a major ecological transition back into the aquatic realm, later giving rise to the most diverse group of extant fully aquatic mammals. This land-to-water transition involved a transition from a limb-based mode of locomotion on land to an axial-powered locomotion relying on oscillations of the body and underlying backbone. This was accompanied by deep modifications of their body plan such as the reduction of hindlimbs, the acquisition of pectoral, dorsal, and caudal fins, and migration of the nares on top of their skull, leading to a fish-like body shape. (my bold)

"The land-to-water transition also had drastic impacts on the vertebral column as the cetacean backbone seems more homogenous in shape compared to the vertebral column of terrestrial mammals which is composed of several well-defined regions (cervical, pectoral, anterior and posterior thoracic, lumbar, sacral, caudal), notably due to the loss of a well-defined sacral region. In addition to this apparent “de-regionalization” of the backbone, the vertebral counts of cetaceans vary broadly across species – ranging from 42 to 97 vertebrae – compared to terrestrial species, making the transposition of traditional mammalian vertebral regions to the cetacean backbone challenging. Because of this, the pattern of regionalization (or lack thereof) of the cetacean backbone has been a long-standing issue limiting our ability to compare their vertebral features with those of terrestrial mammals.

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"Contrary to the common belief – and to our expectations – the cetacean backbone is still highly regionalized as we found between six and nine post-cervical regions. Thanks to the spectral clustering, we could group these regions into six different modules homologous across cetaceans: anterior thoracic, thoraco-lumbar, posterior lumbar (only present in some dolphins and porpoises), caudal, peduncle, and fluke. These modules and regions do not match the regions found in terrestrial mammals, indicating that the cetacean backbone has been repatterned during the land-to-water transition. For instance, we did not find a distinct sacral region but we found numerous regions in the tail, which are most likely associated to their axial-powered aquatic locomotion. Each of the modules can be attributed to either the precaudal segment of the backbone (i.e., all the vertebrae anterior to the tail) or the caudal segment (vertebrae in the tail). We therefore named our regionalization model the “Nested Regions” hypothesis as the backbone can be divided into a precaudal and caudal segment, each of which divided into several modules, which can be further divided into regions.

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"We found that offshore dolphins and porpoises with numerous vertebrae tend to have more regions and can reach higher swimming speeds than riverine species which have fewer vertebrae. This suggests that increased vertebral counts allows for the differentiation into more numerous regions which could allow to better restrict swimming movements to specific parts of the backbone and improve hydrodynamics in comparison to riverine species which are slower but require increased maneuverability in more shallow and complex environments."

Comment: this shows the amazing number of modifications required for the mammalian spine to support an aquatic lifestyle, all in a 53-million-year period. All of these changes point to purposeful changes in short periods, a strong argument for design.