The fish-to-tetrapod transition—followed later by terrestrialization—represented a major step in vertebrate evolution that gave rise to a successful clade that today contains more than 30,000 tetrapod species. The early tetrapod Ichthyostega was discovered in 1929 in the Devonian Old Red Sandstone sediments of East Greenland (dated to approximately 365 million years ago). Since then, our understanding of the fish-to-tetrapod transition has increased considerably, owing to the discovery of additional Devonian taxa that represent early tetrapods or groups evolutionarily close to them. However, the aquatic environment of early tetrapods and the vertebrate fauna associated with them has remained elusive and highly debated. Here we use a multi-stable isotope approach (δ13C, δ18O and δ34S) to show that some Devonian vertebrates, including early tetrapods, were euryhaline and inhabited transitional aquatic environments subject to high-magnitude, rapid changes in salinity, such as estuaries or deltas. Euryhalinity may have predisposed the early tetrapod clade to be able to survive Late Devonian biotic crises and then successfully colonize terrestrial environments.An approach using multiple stable isotopes reveals that early tetrapods of the Devonian period were euryhaline animals that inhabited aquatic environments of highly variable salinity.

中文翻译：

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稳定同位素揭示早期四足动物的广盐生态学

从鱼到四足动物的转变——随后是陆地化——代表了脊椎动物进化的重要一步，它产生了今天包含 30,000 多种四足动物物种的成功进化枝。1929 年，在东格陵兰的泥盆纪古红砂岩沉积物中发现了早期四足动物鱼螈（距今约 3.65 亿年前）。从那时起，由于发现了代表早期四足动物或进化上接近它们的群体的更多泥盆纪分类群，我们对鱼到四足动物过渡的理解大大增加。然而，早期四足动物的水生环境和与之相关的脊椎动物群仍然难以捉摸并引起高度争议。在这里，我们使用多稳定同位素方法（δ13C、δ18O 和 δ34S）来表明一些泥盆纪脊椎动物，包括早期的四足动物在内，是广盐性和有人居住的过渡性水生环境，受盐度的高幅度快速变化影响，例如河口或三角洲。广盐性可能使早期四足动物进化枝能够在晚泥盆世生物危机中幸存下来，然后成功地在陆地环境中定居。一种使用多种稳定同位素的方法表明，泥盆纪早期四足动物是广盐性动物，生活在盐度变化很大的水生环境中。