University of Vienna, Institute for Zoology, Department of Theoretical Zoology, Althanstrasse 14, A-1090 Vienna, AustriaReceived May 2006; accepted for publication December 2006The habitat of the earliest vertebrates (craniates) is still being debated. Marine as well as freshwater habitats andanadromous behaviour have been proposed. In contrast, an estuarine origin of vertebrates is suggested here, basedon ontogenetic, comparative anatomical and functional data. This approach should resolve inconsistencies betweenthe probable existence of glomeruli in the vertebrate ancestors and the marine habitat of all related extant groups(e.g. urochordates and cephalochordates). The kidney, as the main osmoregulatory organ, must have been developedaccording to the environmental prerequisites even in stem vertebrates. In the absence of fossil evidence only deduc-tions from contemporary animals are possible. These data indicate that ancestral stem vertebrates probably hadwell-developed glomeruli, and were capable of at least some ion-exchange between urine and the body. However, theywere probably unable to cope with a strong osmotic gradient with respect to their environment. The conclusion isthat these animals were osmoconformers at around 300–350 mOsm and therefore were restricted to brackishwater. © 2007 The Linnean Society of London, Zoological Journal of the Linnean Society, 2007, 150, 435–441.