Abstract
ABSTRACTThe hypothesis that tetrapods evolved from elpistostegids during the Frasnian, in a predominantly aquatic context, has been challenged by the discovery of Middle Devonian tetrapod trackways predating the earliest body fossils of both elpistostegids and tetrapods. Here I present a new hypothesis based on an overview of the trace fossil and body fossil evidence. The trace fossils demonstrate that tetrapods were capable of performing subaerial lateral sequence walks before the end of the Middle Devonian. The derived morphological characters of elpistostegids and Devonian tetrapods are related to substrate locomotion, weight support and aerial vision, and thus to terrestrial competence, but the retention of lateral-line canals, gills and fin rays shows that they remained closely tied to the water. Elpistostegids and tetrapods both evolved no later than the beginning of the Middle Devonian. The earliest tetrapod records come from inland river basins, sabkha plains and ephemeral coastal lakes that preserve few, if any, body fossils; contemporary elpistostegids occur in deltas and the lower reaches of permanent rivers where body fossils are preserved. During the Frasnian, elpistostegids disappear and these riverine-deltaic environments are colonised by tetrapods. This replacement has, in the past, been misinterpreted as the origin of tetrapods.
Highlights
During the past three decades it has been my privilege to work alongside Professor Emerita Jennifer A
The eight-digit feet, were readily accepted as compatible with its phylogenetic position as a very deep-branching tetrapod (Coates & Clack 1990; Gould 1993; Coates et al 2002). This contrasted with Ichthyostega, where the puzzles bequeathed by Jarvik (1952, 1980, 1996) included an incomprehensible otoccipital morphology, a humerus with muscle attachments seemingly quite different from those of other early tetrapods and an ankle morphology so peculiar that Carroll (1988) dismissed it in favour of his own more conventional reconstruction, created without direct reference to the fossil material
Many Devonian tetrapods were no doubt primarily fish-eaters, but as we have seen with the example of the pigeon-catching catfish, a jaw apparatus optimised for aquatic prey capture can still allow for remarkable behavioural flexibility
Summary
The discipline of Devonian tetrapod studies was born in 1932 with the description of the genus Ichthyostega from Famennian deposits in East Greenland (Save-Soderbergh 1932). The eight-digit feet, were readily accepted as compatible with its phylogenetic position as a very deep-branching tetrapod (Coates & Clack 1990; Gould 1993; Coates et al 2002) This contrasted with Ichthyostega, where the puzzles bequeathed by Jarvik (1952, 1980, 1996) included an incomprehensible otoccipital morphology, a humerus with muscle attachments seemingly quite different from those of other early tetrapods and an ankle morphology so peculiar that Carroll (1988) dismissed it in favour of his own more conventional reconstruction, created without direct reference to the fossil material.
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Schedule a callMore From: Earth and Environmental Science Transactions of the Royal Society of Edinburgh
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