Character Diagnosis, Fossils and the Origin of Tetrapods

Abstract

SUMMARY I. The traditional view of the origin of tetrapod vertebrates is that they are descendants of fossil osteolepiform fish, of which Eusthenopteron is 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 Rosen et al. (1981) claims that Dipnoi (lungfish) are the sister‐group of the Tetrapoda, that Osteolepiformes is a non‐taxon and that Eusthenopteron is 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 fish Powichthys is 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 Rosen et 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 the living sister‐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 Rosen et al. is discussed at length. Virtually all the characters t...

Behavioral evidence for the evolution of walking and bounding before terrestriality in sarcopterygian fishes

“…One essential adaptation in the evolution of tetrapods was the ability to locomote on land, a trait that required significant morphological and functional changes in the appendicular system. Many of these morphological changes can be observed in the fossil record (1)(2)(3)(4)(5)(6)(7)(8)(9)(10)(11)(12)(13)(14). Work on basal tetrapod fossils has revealed that even those with digited limbs can be aquatic (5,7).…”

“…Lungfishes, including P. annectens, are the sister group to living tetrapods (2,4,15), and as such are important for understanding the evolution of movement patterns and limb function in this group. P. annectens is clearly specialized relative to other sarcopterygians in that its fins are longer and more slender than all other known examples (16,17).…”

Molecular Phylogenetic Information on the Identity of the Closest Living Relative(s) of Land Vertebrates

“…However, now they are reduced to only three relict genera that are found in Australia, Africa, and South America (Neoceratodus, Protopterus, and Lepidosiren;Forey 1987;Cloutier and Forey 1991). Despite much research there is still no general agreement regarding the phylogenetic relationships among Actinistia, Dipnoi, and Rhipidistia (including Tetrapoda; Ahlberg 1991; Cloutier and Ahlberg 1996;Forey 1988Long 1995;Maisey 1996;Marshall and Schultze 1992;Meyer 1995;Nelson 1994;Panchen and Smithson 1987;Rosen et al 1981;Schultze 1994). This controversy will continue until new relevant fossils of intermediate forms connecting the three groups are discovered, and agreement among paleontologists about the homology of some characters (e.g., the choanae) is achieved (Forey 1988;Schultze 1994;Rosen et al 1981).…”

“…Panchen and Smithson 1987). This major evolutionary transition involved many sweeping morphological (e.g., evolution of limbs and air breathing), physiological, and behavioral changes, but nonetheless it seems to have taken place within an astonishingly short period of time of only 10-20 million years (Carroll 1988;Maisey 1996;Cloutier and Ahlberg 1996).…”

Molecules, fossils, and the origin of tetrapods

“…Previously, Panchen and Smithson (1987) discussed many of the points made here. (1) Cladistic methodology is necessary for a solution to this problem.…”