Habitat of early stegocephalians (Chordata, Vertebrata, Sarcopterygii): a little saltier than most paleontologists like?

Abstract

A controversy on the degree of marine influence in the paleoenvironments represented by many Paleozoic stegocephalian-bearing fossiliferous localities has persisted for decades. Many authors have equated the absence of a typical stenohaline marine fauna with freshwater environments, but this ignores continental salt lakes and the many transitional environments (deltaic, estuarine, lagoonal, and some epicontinental seas that receive much freshwater influx, like the Baltic Sea) that separate typical marine environments from freshwater environments. This is problematic because it seems plausible that many of the late Paleozoic sediments that have been preserved were deposited on coasts in deltas and estuaries. The author had compiled a dataset of paleoenvironmental interpretations of Devonian to Early Permian stegocephalian (“tetrapod”)-bearing fossiliferous localities in 2010. How have these interpretations withstood the test of time, especially in the face of new results from different kinds of evidence? An updated dataset and a new literature review show that the case for a marine origin of stegocephalians has strengthened, especially through additional discoveries or reinterpretations of fossils that suggest marine influence in various classical vertebrate-bearing Permo-Carboniferous localities traditionally interpreted as freshwater, and a recent analysis of stable isotopes in Late Devonian localities.