Gaining Ground, Second Edition: The Origin and Evolution of Tetrapods

3 Relationships and Relatives: The Lobe-Fin Family

59 3 Relationships and Relatives: The Lobe-Fin Family This chapter introduces the tetrapods’ closest relatives, explains how tetrapods fit into the scheme of relationships with other lobe-fins, and explores how ideas about the ancestry of tetrapods have evolved with changing perspectives. There are a few characteristics of lobe-fins that distinguish them as a group, but the most conspicuous is the eponymous lobed fin, described more fully in Chapters 1 and 2, in which the paired fins are anchored to the respective limb girdle by a single bone. The structure appears to be a true shared, derived character of the group. Another shared feature is the possession of two dorsal fins, instead of the single one found in early ray-fins, and a third is the occurrence in the tail of a second series of fin rays growing above the body portion. This has enabled lobe-fins to evolve symmetrical tails without too much modification of the existing pattern, and they have done so in a number of separate lineages. The latter two features may after all be primitive for gnathostomes, but this is not clear. In addition, most early members of the lobe-finned group show an intracranial joint or hinge line. A hinge linked the front and back parts of the skull roof just behind the eyes, reflected in a matching hinge across the underlying braincase (see Chapter 2). The hinge line occurs at the point where the otoccipital portion of the braincase meets the ethmosphenoid dorsally, and the ventral otic fissure separates them ventrally (see Figs. 2.1, 2.3, 2.11, 2.13), an important landmark in building the vertebrate skull. The hinge itself is not seen in ray-finned fishes, and it is also lost from several of the lobe-finned groups independently, including tetrapods. Most early lobe-finned “fish” were moderately sized animals. Although a few grew to enormous size, few small species are known, so it does not appear that there were lobe-finned equivalents of anchovies, for example. It is not at all clear why this should be the case. Early lobe-fins lived in both marine and freshwater conditions, but almost always in relatively shallow water. Today there are few lobe-finned “fishes” remaining, as distinct from tetrapods (also lobe-fins). The surviving “fish” species are the coelacanth Latimeria and three genera of lungfish. In the Paleozoic, however, lobefins were dominant fishes for many millions of years, both in number of species and in size. Several lineages died out at the end of the Devonian , though some survived into the Early Permian. By the end of the Introducing the Lobe-Fins Gaining Ground 60 Paleozoic, only the lungfishes, coelacanths, and tetrapods remained, and the ray-finned fishes took over dominance of the water. Figure 3.1 gives one idea of the relationships of lobe-finned fish groups (Cloutier and Ahlberg 1997), including tetrapods, to one another, and that is the scheme that will be followed in this book. Only with an explicit phylogeny behind it can any ideas about what occurred in evolution have any validity. The phylogeny must take precedence in drawing up scenarios about what went on in the distant past; otherwise, any possible story can be put forward, but it will not be a scientific one. However, readers must bear in mind that not all paleontologists agree with this particular phylogeny, and it is subject to change as new finds and ideas emerge. This implies that should a different phylogeny emerge for the relationships of tetrapods, some of the conclusions drawn throughout this book might be refuted. Figure 3.2 puts the animals on a timescale of the Devonian period. Note the subdivisions or stages into which the Devonian is divided. 3.2. Timescale of the Devonian period with a phylogeny of lobe-finned vertebrates superimposed to show the times of origin and relationships of the main groups. Dates from Gradstein et al. (2004). Relationships and Relatives 61 Survivors Coelacanths Until about 60 years ago, the coelacanths (the name means “hollow spine” and the group is formally known as Actinistia) were considered to have become extinct during the Cretaceous period, probably at the end of it, along with the dinosaurs. In 1938, however, the first modern coelacanth was recognized (Smith 1956). It was named Latimeria, after Marjorie Latimer, who worked at the museum in East London (South Africa) and was the first to recognize its importance. Since then, knowledge of it has increased greatly from the...