Vol 440| 6 April 2006 NEWS & VIEWS 747 PALAEONTOLOGY A firm step from water to land Per Erik Ahlberg and Jennifer A. Clack A project designed to discover fossils that illuminate the transition between fishes and land vertebrates has delivered the goods. At a stroke, our picture of that transition is greatly improved. The concept of ‘missing links’ has a powerful grasp on the imagination: the rare transitional fossils that apparently capture the origins of major groups of organisms are uniquely evoca- tive. But the concept has become freighted with unfounded notions of evolutionary ‘progress’ and with a mistaken emphasis on the single intermediate fossil as the key to understanding evolutionary transitions. Much of the impor- tance of transitional fossils actually lies in how they resemble and differ from their nearest neighbours in the phylogenetic tree, and in the picture of change that emerges from this pattern. We raise these points because on pages 757 and 764 of this issue 1,2 are reports of just such an intermediate: Tiktaalik roseae, a link between fishes and land vertebrates that might in time become as much of an evolutionary icon as the proto-bird Archaeopteryx. Several specimens have been found in Late Devonian river sediments on Ellesmere Island in Nunavut, Arctic Canada. They show a flat- tened, superficially crocodile-like animal, with a skull some 20 centimetres in length. The body is covered in rhombic bony scales, and the pectoral fins are almost-but-not-quite forelimbs; these contain robust internal skele- tons, but are fringed with fin rays rather than digits. Tiktaalik goes a long way — but not quite the whole way — towards filling a major gap in the picture of the vertebrate transition from water to land. It has long been clear that limbed vertebrates (tetrapods) evolved from osteolepiform lobe- finned fishes 3 , but until recently the morpho- logical gap between the two groups remained frustratingly wide. The gap was bounded at the top by primitive Devonian tetrapods such as Ichthyostega and Acanthostega from Green- land, and at the bottom by Panderichthys, a tetrapod-like predatory fish from the latest Middle Devonian of Latvia (Fig. 1). Ichthyo- stega 4 and Acanthostega 5 retain true fish tails with fin rays but are nevertheless unambigu- ous tetrapods with limbs that bear digits 6 . Panderichthys 7 is vaguely crocodile-shaped and, unlike the rather conventional osteolepi- form fishes farther down the tree, looks like a fish–tetrapod transitional form. The shape of the pectoral fin skeleton and shoulder girdle are intermediate between those of osteolepi- forms and tetrapods, suggesting that Pander- ichthys was beginning to ‘walk’, but perhaps in shallow water rather than on land 8 . Panderichthys lived about 385 million years ago at the end of the Middle Devonian; Ichthyo- stega and Acanthostega lived about 365 million years ago during the Late Devonian. How- ever, the earliest fragmentary tetrapods from Scotland 9,10 and Latvia 9 date back to perhaps 376 million years ago, so the morphological gap between fish and tetrapod corresponds to a time gap of under 10 million years. Into this gap drops Tiktaalik. The fossils are earliest Late Devonian in age, making them at most 2 million or 3 million years younger than Panderichthys. With its crocodile-shaped skull, and paired fins with fin rays but strong inter- nal limb skeletons, Tiktaalik also resembles Panderichthys quite closely. The closest match, Ichthyostega Acanthostega Tiktaalik Panderichthys Eusthenopteron Figure 1 | Tiktaalik in context. The lineage leading to modern tetrapods includes several fossil animals that form a morphological bridge between fishes and tetrapods. Five of the most completely known are the osteolepiform Eusthenopteron 16 ; the transitional forms Panderichthys 17 and Tiktaalik 1 ; and the primitive tetrapods Acanthostega and Ichthyostega. The vertebral column of Panderichthys is poorly known and not shown. The skull roofs (left) show the loss of the gill cover (blue), reduction in size of the postparietal bones (green) and gradual reshaping of the skull. The transitional zone (red) bounded by Panderichthys and Tiktaalik can now be characterized in detail. These drawings are not to scale, but all animals are between 75 cm and 1.5 m in length. They are all Middle–Late Devonian in age, ranging from 385 million years (Panderichthys) to 365 million years (Acanthostega, Ichthyostega). The Devonian–Carboniferous boundary is dated to 359 million years ago 18 . Nature Publishing Group ©2006