When fish learned to walk

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The process by which fish took to the land occurred in several steps.

Gaining Ground: The Origin and Evolution of Tetrapods

Indiana University Press: 2002. 369 pp. $49.95, £37.95
Landing gear: the early tetrapod Acanthostega had limbs with digits but still lived in the water. Credit: JANICE MCCAFFERTY

The evolutionary transition from fish to air-breathing, four-legged land vertebrates, or tetrapods, is a fascinating segment of vertebrate evolution. It is increasingly well documented by fossils dating from the Late Devonian and Early Carboniferous periods, from 370 million to 320 million years ago. During the past ten years, a generation of young palaeontologists has discovered far more remains of these earliest tetrapods than ever before, and Jennifer Clack's earlier researches have been pivotal in this quest for the 'four-legged fish'. Her new book, Gaining Ground, presents data that have barely been dealt with before, even in the most recent textbooks on early vertebrates.

A major turning point in Clack's work occurred in the mid-1980s, with her incidental discovery of new skeletons of the Late Devonian tetrapod Acanthostega. These had been collected by geologists at the same Greenland site as Ichthyostega, long considered to be the key to the fish–tetrapod transition. Clack was also blessed with an outstanding preparator and several brilliant disciples, and her small team soon made the University of Cambridge a leading research centre in this field.

Thanks to their thorough study of Acanthostega, Clack and her collaborators discovered several apparently trivial anatomical features that turned out to be diagnostic for early tetrapods. This triggered a large-scale re-examination of new and old material from various Late Devonian localities in northern Europe. Some of these finds had been dismissed as “undetermined fish fragments” but turned out to be tetrapod remains, yet older and presumably more primitive than Ichthyostega and Acanthostega. The discovery of the key for recognizing tetrapod remains pushed back the earliest tetrapod evidence from 365 million to about 370 million years ago. By reviewing this new material and discussing its evolutionary significance, Gaining Ground is a particularly important update.

People often think that the most important event in the rise of land vertebrates occurred in the Late Devonian, when fins evolved into limbs. The main message of this book is that the fish–tetrapod transition and the conquest of land were quite separate evolutionary steps. Clack shows here that all Devonian tetrapods were essentially aquatic, with fish-like gills, and were merely lobe-finned fishes with strange, paddle-like paired fins elongated by new structures: the digits. The transformation of the paired fins into limbs does not imply walking or a terrestrial life, which probably occurred later, in the Carboniferous. As Clack puts it: “The problem actually consists of three parts: origin of limbs with digits, origin of walking, and origin of terrestriality.”

Gaining Ground begins with an introduction to fish and tetrapod anatomy, the diversity and relationships of living and fossil osteichthyans (bony fishes and tetrapods), and the Late Devonian environments that prevailed during the fish–tetrapod transition. The description of the Devonian tetrapods known to date is followed by a review of all the anatomical changes that occurred at the fish–tetrapod transition, either documented by fossils or inferred from indirect evidence, and a consideration of the processes that may have been involved in this drastic change. In particular, Clack examines the latest data from developmental genetics of limbs and fins, which now appear to be relatively consistent with the story told by the fossils.

The second part of the book focuses on the Carboniferous tetrapods, a subject that has also been enriched by the discovery of many new fossils during the past ten years. Until recently, hardly any tetrapods were known from the Early Carboniferous. This 20-million-year gap, known as Romer's gap, is now being filled by some recent discoveries (see Nature 418, 72–76; 2002). These show that mobile wrist and ankle joints had appeared by the Early Carboniferous and that these tetrapods, although still confined to water, were capable of walking. This review of the Carboniferous tetrapods ends with a section on the relationships of the Palaeozoic tetrapods to the modern amphibians and amniotes. This section, which is limited to brief comments about the most recent cladistic analyses, is perhaps too succinct. It would have been interesting to know, for example, why the phylogeny proposed by Laurin and Reisz in Amniote Origins: Completing the Transition to Land (eds S. Sumida & K. L. M. Martin, Academic Press, 1997) is so radically at odds with all the others.

Gaining Ground ends with a review of all the character changes involved in the rise of terrestriality proper, in particular those in the ear, braincase, vertebral column and limbs. The various states of each major structure are presented in association with a cladogram, which aids their understanding.

Gaining Ground is an outstanding update of early tetrapod anatomy, phylogeny and systematics, with many interesting insights into the processes involved in the fish–tetrapod transition. Clack's pedagogic style and the numerous illustrations (which are generally clear and informative) mean that it will be extremely useful to students and lecturers in palaeontology, geology, zoology and general biology, and is a 'must' for researchers in the field.

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Janvier, P. When fish learned to walk. Nature 419, 339–340 (2002) doi:10.1038/419339a

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