THE HABITAT OF THE EARLY VERTEBRATES

Abstract

SUMMARY On geochemical grounds it seems that the early Ordovician seas were very similar in salinity and ionic composition to seas at the present time. Physiological speculations based on the assumption of dilute sea water in the Ordovician oceans, especially those which relate the level of salts in vertebrate blood to that of these seas, are invalid. The first marine and freshwater vertebrates were probably subjected to the same physicochemical environmental stresses which affect present‐day fishes and the lower marine chordates. The earliest vertebrate remains are from the Ordovician and include problematical fish‐jaws (from Missouri, U.S.A.), plates and scales of heterostracans and coelolepids (from Colorado, U.S.A.), and teeth of acanthodian fish (from the neighbourhood of St Petersburg, Russia). All these remains are from marine sediments, associated with typical marine invertebrates in the first two cases. Ostracoderms and placoderms have been found in Silurian strata (including the Downtonian) in eight countries of Europe and in North America. The fossils comprise heterostracans, coelolepids, anaspids, osteostracans, acanthodians and arthrodires. Associated marine invertebrates are found with the vertebrate fossils in the vast majority of the records, and in no case is there any evidence that the ostracoderms and fishes lived otherwise than in the sea. Ostracoderms and bony fishes are represented in both marine and freshwater deposits of the Devonian period. A marine origin for the vertebrates is supported by the fact that the three protochordate groups, Hemichordata, Urochordata, and Cephalochordata, are marine. The members of the marine order Myxinoidea of the Cyclostomata have an internal medium of high salt concentration, equivalent to that of the surrounding sea water, a feature found in the lower chordates and in marine invertebrates. This may well be a primary feature, derived directly from marine chordate ancestors. Consideration of the glomerular kidneys of the marine myxinoids and elasmo‐branchs, forms with blood isosmotic with sea water, and of the kidneys of other fishes, leads to a rejection of the view that such kidneys are a freshwater adaptation which has arisen in some freshwater protovertebrate. Glomerular kidneys probably existed in marine protovertebrates, and subsequently became a useful preadaptation for life in fresh water. The arguments deemed to support a freshwater origin of vertebrates have been considered and rejected on the grounds that some are erroneous and others improbable. Palaeontological and physiological evidence leads to the conclusion that the vertebrates were originally a marine group.