CHARACTER DIAGNOSIS, FOSSILS AND THE ORIGIN OF TETRAPODS

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Abstract
SUMMARY: I. The traditional view of the origin of tetrapod vertebrates is that they are descendants of fossil osteolepiform fish, of whichEusthenopteronis best known. In recent years both that conclusion and the methodology by which it has been reached have been challenged by practitioners of cladistic analysis. Particularly a recent review by Rosenet al.(1981) claims that Dipnoi (lungfish) are the sister‐group of the Tetrapoda, that Osteolepiformes is a non‐taxon and thatEusthenopteronis more distant from tetrapods than are Dipnoi, coelacanths and probably the fossil Porolepiformes. We attempt to refute all these concludions by use of the same cladistic technique.2. We accept that all the above‐mentioned groups, together with some less well‐known taxa, can be united as Sarcopterygii by means of shared derived (apomorph) characters. We also agree that Porolepiformes and Actinistia (coelacanths) can be characterized as valid taxa. The primitive and enigmatic fossil fishPowichthysis accepted as representing the plesiomorph sister‐group of true porolepiforms.3. Only two apomorph features, the course of the jaw adductor muscles and the position of incurrent and excurrent nostrils, appear to unite all the fish, living and fossil, currently regarded as Dipnoi. The characteristic tooth plates and the presence of petrodentine both exclude important primitive fossil forms.4. Contrary to the opinion of Rosenet al., Osteolepiformes can be characterized — by the arrangement of bones forming the cheek plate, the presence of basal scutes to the fins and by the unjointed radials of the median fins. However, if these are true autapomorphies they exclude any osteolepiform from direct tetrapod ancestry.5. Tetrapoda is a monophyletic group characterized by ten or more autapomorphies, including the bones of the cheek plate, a stapes and fenestra ovalis, and a series of characters of the appendicular skeleton.6. Tetrapods have a true choana (internal nostril). We accept that the posterior (excurrent) nostril of Dipnoi is the homologue of the tetrapod choana. However, we assert that the posterior nostril of all bony fish is the homologue of the choana. This assertion would be refuted if any fish showed separate posterior nostril and choana. We reject the claim that this ‘three nostril condition’ occurred in porolepiforms and osteolepiforms. The evidence for a choana in porolepiforms is inadequate. Osteolepiforms had a true choana, characterized as in tetrapods by its relationship to the bones of the palate, but no third nostril. Dipnoans are not choanate.7. Following cladistic practice, the relationship of the extant taxa is established first. Dipnoi are thus shown to be thelivingsister‐group of tetrapods, but only on ‘soft anatomy’ characters unavailable in fossils. Coelacanths are the living sister‐group of the taxon so formed.8. The relationship of the fossil taxa to the extant sarcopterygians is then considered. The synapomorphy scheme proposed by Rosenet al.is discussed at length. Virtually all the characters they use to exclude close relationship ofEusthenopteron(and hence all osteolepiforms) to tetrapods, in favour of coelacanths and dipnoans, are invalid.9. A series of synapomorphies uniting osteolepiforms and tetrapods is proposed, including a true choana (hence the taxon Choanata), the histology of the teeth, and a number of characters of the humerus. The recently discovered fossilYoungolepis, which lacks a choana, represents the sister‐group of the Choanata, and is not uniquely close toPowichthys.The latter, as a porolepiform (s.l.) is a member of the sister‐group to Choanata plusYoungolepis.10. Our cladistic analysis suggests that all the extinct taxa considered are more closely related to tetrapods than are the Dipnoi. Moreover fossil evidence suggests that Dipnoi, considered as an extant taxon, may not even be the living sister‐group of Tetrapoda. Early fossil dipnoans appear to have been marine fish without specific adaptations for air breathing. If so the apparent synapomorphies of Dipnoi and Tetrapoda may be homoplastic — the insistence on grouping extant taxa first would then have yielded an invalid inference.