Investigating tides as an environmental driver during the Fish-Tetrapod transition.
[sciencythoughts.blogspot.com]
Only once in Earth’s history did vertebrates make the transition from an aquatic to terrestrial environment; trackway evidence indicates this occurred approximately 393 million years ago, although the earliest definite Tetrapod body fossils are approximately 20 million years younger. By contrast, there have been multiple adaptive radiations of Vertebrates from land back to the ocean, e.g. separate groups of semi-aquatic Mammals becoming the earliest Cetaceans and Sirenians at around 50 million years ago. The origin of Tetrapods was itself part of the rapid early diversification of Bony Fish (Osteichthyes); shortly after their origin, the Osteichthyes split into Ray-finned Fish (Actinopterygii, the predominant Fish group today) and Lobe-finned Fish (Sarcopterygii), the latter giving rise to Tetrapods. The earliest known crown-group Usteichthyans come from the Late Silurian (425 million years ago) of South China, suggesting that the whole process took little more than 30 million years. Most of the terrestrial adaptations, including the modification of the pectoral and pelvic fins into weightbearing limbs, were acquired during the origin of Tetrapods. However, one key component, the lungs, is older and can be traced back to the origin of the Osteichthyes, where they evidently evolved for use as supplementary respiratory organs in an aquatic environment before being co-opted to support terrestrial life. The crown-group Osteichthyes most probably originated in South China, as the earliest known members are found there, and the Late Silurian to Early Devonian (starting 425 million years ago) faunas of the region contain a diversity of Osteichthyans that cannot be matched elsewhere. The origin of tetrapods is more difficult to pinpoint, but the two earliest known trackway localities are situated in present day Europe, which at the time was part of the ancient supercontinent Laurussia; the earliest body fossils are also Laurussian. Although the drivers behind the evolution of Osteichthyans and Tetrapods are as yet poorly understood and many hypotheses have been suggested to be behind these evolutionary events, it is known that the palaeoenvironment was rapidly transforming due to the emergence of macroscopic Plant communities on land and a period of overall marine regression occurring from the Late Silurian to Middle Devonian.