118 0169-5347/00/$ – see front matter © 2000 Elsevier Science Ltd. All rights reserved. PII: S0169-5347(99)01780-2 TREE vol. 15, no. 3 March 2000 A few decades ago, Devonian stego- cephalians (Boxes 1 and 2) were known from only two taxa from East Greenland: Ichthyostega and Acantho- stega 1 . The closest known relatives of these two taxa and of more recent stego- cephalians were the panderichthyids, a clade of sarcopterygians that shares many derived features with stegocepha- lians, but that retains paired fins. How- ever, recent studies of fragmentary remains, previously interpreted as ‘osteo- lepiforms’ 2 , revealed that many of these taxa (Metaxygnathus, Obruchevichthys, Elginerpeton and Ventastega) are more closely related to tetrapods than to pan- derichthyids 3,4 . No limb extremity (auto- pod; Box 2) is preserved in any of these taxa, but the fact that panderichthyids are our closest relatives known to have possessed paired fins prompted some authors to call these taxa ‘tetrapods’ 3 . However, the position of these taxa does not enable us to determine whether or not these taxa possessed digits; both hypotheses are equally parsimonious (Fig. 1). An additional genus (Hynerpeton) claimed to be an early tetrapod, repre- sented by recently discovered fragmen- tary remains, seems to be more closely related to extant tetrapods than to Acan- thostega (a taxon known to have had dig- its) 5 ; if this interpretation is correct, the parsimony criterion suggests that this taxon had digits (Fig. 1). When is a vertebrate with four feet not a tetrapod? A controversy in tetrapod taxonomy was recently triggered by the use of phylo- genetic definitions of taxon names. This is part of a larger controversy between practitioners of Linnean taxonomy (who advocate using taxa diagnosed by char- acters) and practitioners of phylogenetic taxonomy (who use the phylogeny to de- fine taxon names). For example, the name ‘Tetrapoda’ has usually been defined as the taxon that includes all vertebrates that bear digits (including those that have lost them, such as snakes). However, an alternative phylogenetic definition of Tetrapoda is ‘the most recent common ancestor of extant lissamphibians and amniotes and all of its descendants’ 6 (Box 1). These two concepts of Tetrapoda do not coincide (Fig. 2), because the phylo- genetic definition of Tetrapoda actually excludes some digit-bearing vertebrates. A taxon that includes all vertebrates pos- sessing digits is therefore needed, thus the old taxon name Stegocephali was given a phylogenetic definition to fill this taxonomic gap (Boxes 1 and 2; Fig. 1). Here, we use the phylogenetic definitions of the relevant taxon names, as defined by Laurin or Gauthier and colleagues (Box 1; Figs 1 and 2). Paleontological data on the origin of digits Paleontological data do not solve the problem of homology (or lack thereof) between the radials of early sarcoptery- gian fins and the digits of the autopod. Until recently, the fins most readily com- pared with a tetrapod limb were those of Eusthenopteron, which consist of a hu- merus (we discuss only the pectoral limb, but a similar argument could be made for the hind limb), radius, ulna, ulnare, inter- medium (the homology of the last two elements is not well established) and a PERSPECTIVES Early tetrapod evolution Michel Laurin, Marc Girondot and Armand de Ricqlès Tetrapods include the only fully terrestrial vertebrates, but they also include many amphibious, aquatic and flying groups. They occupy the highest levels of the food chain on land and in aquatic environments. Tetrapod evolution has generated great interest, but the earliest phases of their history are poorly understood. Recent studies have questioned long-accepted hypotheses about the origin of the pentadactyl limb, the phylogeny of tetrapods and the environment in which the first tetrapods lived. Michel Laurin, Marc Girondot and Armand de Ricqlès are at the Équipe ‘Formations squelettiques’, UMR CNRS 8570 ‘Evolution et adaptation des systèmes ostéomusculaires’, Case 7077, Université Paris 7-Denis Diderot, 2 Place Jussieu, F-75251 Paris cedex 05, France (laurin@ccr.jussieu.fr; mgi@ccr.jussieu.fr; ricqles@ccr.jussieu.fr).