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27 2 Skulls and Skeletons in Transition This chapter is an introduction to the skeletal anatomy of animals that exemplify the fish–tetrapod transition. The first part examines how the skulls and skeletons of lobe-finned fishes and tetrapods were built, and will introduce the terminology used for the bones. Unfortunately, many of the terms will be unfamiliar to the nonspecialist reader, but at least some of them need to be assimilated because in most cases, there simply are no other words available to describe them. It is probably a good idea to refer constantly to the diagram of skull structure (Fig. 2.1). One particularly important set of terms is that referring to the orientation of diagrams and their relationship to the whole animal that they describe: top, bottom, front, back, and so forth. These everyday terms are not accurate or precise enough for explaining anatomical structures. Figure 2.2 shows some of these terms. One thing to note is that in humans , because of their upright stance, words describing front and back mean different things from those that mean nose to tail. Human and veterinary anatomists have therefore coined separate words for them. For human anatomy, the terms anterior and posterior are ambiguous, so front to back is described by the terms ventral to dorsal, whereas nose to tail is described by rostral (toward the top of the head or snout) and caudal (toward the tail). However, dealing exclusively with four-legged tetrapods or fishes, there is no such distinction. In this case, front to back is the same as nose to tail; anterior to posterior are used for these, and the terms ventral to dorsal are used for top to bottom. Throughout the book, reference will also be made to the embryonic origins of certain tissues, so the second part of the chapter sets out the basics of how a vertebrate embryo forms, where various tissues come from, and what structures each becomes. A section on recent work in developmental genetics, especially on Hox genes, is necessary for understanding certain aspects of tetrapod evolution, where work on living animals has had an impact on paleontological studies. Once the basic anatomy of a fish, such as the Devonian lobe-finned fish Eusthenopteron, and a tetrapod such as the Devonian Acanthostega or the Carboniferous temnospondyl Dendrerpeton is understood, the differences that came about at the fish–tetrapod transition can be approached. These are outlined in the last part of the chapter. However, the features that fish and tetrapods share must be dealt with before those that separate them can be appreciated. Many of these features can be found in modern vertebrates as well as in very early ones. Gaining Ground 28 The Skull Dermal Skull Roof Figure 2.1 shows the skull of the Devonian lobe-finned fish Eusthenopteron , which will be used as an exemplar to show fish skull anatomy. Early bony fishes, early tetrapods, and some modern ray-finned fishes share some common features of skull construction that are considered to be primitive. In these animals, the skull is built of an outer covering of bony armor called the dermal skull roof, while the braincase is a separate, relatively small box containing the brain proper, the ear capsules, and the roots of the nerves serving the other sense organs. Hinged to the back of the dermal skull roof is the lower jaw, while the palate forms the roof of the mouth. In most modern fishes and tetrapods, much of the dermal skeleton found in Paleozoic members has been lost, which is why their skulls look rather different from those of these more ancient creatures. The dermal skull roof can be divided into several regions. At the top and back lies a flat, more or less rectangular lid called the skull table. It is made up of paired bones, sometimes given different names in fish and tetrapods. Reaching out from the front of this and running between the eye sockets, or orbits, runs a series of paired bones forming the interorbital region. In Eusthenopteron, the main bones in this region are the parietals, surrounding the parietal foramen, sometimes called the pineal foramen. The parietals attach at the front to the snout, which has the external nostrils on each side of it. In Eusthenopteron, the snout region is made up of a mosaic of small bones (Fig. 2.1A, B). Understanding Skeletal Structure 2.2. Diagrams explaining anatomical terms of orientation. Medial means looking toward the midline of the...

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