Euryhaline ecology of early tetrapods revealed by stable isotopes

Abstract

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 (δC, δO and δS) 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.

A fish and tetrapod fauna from Romer's Gap preserved in Scottish Tournaisian floodplain deposits

“…; Goedert et al . ) may support the hypothesis of a cosmopolitan post‐extinction fauna in the Tournaisian (Friedman & Sallan ).…”

“…A brackish habit was recently confirmed for the Famennian East Greenland tetrapods, long considered to have been part of a freshwater fauna (Goedert et al . ). The microfossil content of the three samples is different from that identified from two dolostone beds from the Ballagan Formation on the Isle of Bute, which contain actinopterygians, sarcopterygians, dipnoans, elasmobranchs and non‐gyracanth acanthodians (Carpenter et al .…”

Combined oxygen and sulphur isotope analysis—a new tool to unravel vertebrate (paleo)-ecology

“…For oxygen, the isotopic composition of water was calculated using published isotopic fractionation equations for different groups of vertebrates (Online Information 3). For sulphur, the isotopic composition of water is very close to that recorded in bone apatite (i.e., almost no isotopic fractionation) and was calculated using published values of sulphur isotope composition of bone apatite and associated environmental water measured in present-day vertebrates (Goedert et al (2018); Online Information 4). Each data point represents a biologically independent animal (n = 64) and corresponds to the average value of five and three repeated measurements for oxygen and sulphur isotope analysis, respectively (see "Material and Methods").…”

Land Invasion by the Mudskipper, Periophthalmodon septemradiatus, in Fresh and Saline Waters of the Mekong River

“…The idea that vertebrates abandoned drying-up freshwater bodies in drought to seek larger remaining ponds 7 had been widely accepted in the 20 th century, but has become questioned by more recent researchers 8 , particularly after the finding of trace fossils, which dated older than the first known body fossils of vertebrates, and was initially thought to indicate coastal transition to land 9 . In addition, the new analysis of stable isotopes of the bones of early tetrapods has lent support for the euryhalinity of these animals, which supposedly helped rapid global distribution and colonisation of different land masses 10 . Even though these palaeontological studies provide the most direct evidence for the process of land invasion by early vertebrates, the fossil records are inherently fragmentary, and some aspects of their ecology remain difficult to reconstitute from these materials.…”