Abstract
Recent finds of early tetrapods have established that the most primitive form, Acanthostega , retained internal gills and other fish-like features; this has led to the conclusion that it was a primarily aquatic animal. Other Late Devonian tetrapods, such as Ichthyostega and Tulerpeton , provide no evidence of internal gills, but have also been interpreted as inhabiting an aquatic environment. The probable aquatic habits of a diversity of Devonian tetrapods has led to the suggestion that the entire early tetrapod radiation may have been an aquatic one, with terrestriality having evolved in later forms. However, consideration of the physiology of living amphibious vertebrates suggests that this scenario is unlikely. The use of the gills for the excretion of carbon dioxide and ammonia appears to be a fundamental feature of all primarily aquatic vertebrates. No living fish loses its internal gills, even if it excretes a significant portion of its nitrogenous waste as urea via the kidney in the water. Gills are simply too valuable to be lost by an aquatic animal, even in those air-breathing fishes that no longer use the gills for oxygen uptake. We suggest that the apparent loss of the gills in tetrapods more derived thanAcanthostega signals their descent from a more terrestrial phase in tetrapod evolution, following the primary assumption by the kidney of the excretion of nitrogenous wastes. Without this new role of the kidney, loss of the gills would have been impossible. With this new kidney role, loss of the gills may have been advantageous in reducing desiccation on land.
Talk to us
Join us for a 30 min session where you can share your feedback and ask us any queries you have
Disclaimer: All third-party content on this website/platform is and will remain the property of their respective owners and is provided on "as is" basis without any warranties, express or implied. Use of third-party content does not indicate any affiliation, sponsorship with or endorsement by them. Any references to third-party content is to identify the corresponding services and shall be considered fair use under The CopyrightLaw.