Gaining Ground: The Origin and Evolution of Tetrapods (2nd Edition)

Gaining Ground: The Origin and Evolution of Tetrapods (2nd Edition)

Jennifer A. Clack

Language: English

Pages: 527


Format: PDF / Kindle (mobi) / ePub

Around 370 million years ago, a distant relative of a modern lungfish began a most extraordinary adventure-emerging from the water and laying claim to the land. Over the next 70 million years, this tentative beachhead had developed into a worldwide colonization by ever-increasing varieties of four-limbed creatures known as tetrapods, the ancestors of all vertebrate life on land. This new edition of Jennifer A. Clack's groundbreaking book tells the complex story of their emergence and evolution. Beginning with their closest relatives, the lobe-fin fishes such as lungfishes and coelacanths, Clack defines what a tetrapod is, describes their anatomy, and explains how they are related to other vertebrates. She looks at the Devonian environment in which they evolved, describes the known and newly discovered species, and explores the order and timing of anatomical changes that occurred during the fish-to-tetrapod transition.

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reason, it may be a mistake to think of early tetrapods as being like frogs that produce spawn in their reproduction strategies. As for metamorphosis, it has become clear that an abrupt change from aquatic to terrestrial lifestyles with concomitant changes in skull morphology and degrees of ossification incorporates a suite of features that have evolved within temnospondyl amphibians and that are not found in more primitive members of the group (e.g., Schoch 2002b, 2004; Schoch and Milner 2004).

flattening of the tibia, fibula, and phalanges characteristic of Whatcheeria. The inference from this is that Whatcheeria was more aquatically adapted than Pederpes. The fossils show that at least five digits were present on the limbs, but none of the specimens is complete, and it is possible that more were present. SIGOURNEA MULTIDENTATA As well as Whatcheeria, the Delta locality in Iowa has yielded partial remains of several tetrapods, some of which may represent new taxa. One of these

(see Chapter 9 for more details; this is also mentioned in Chapter 7). Silvanerpeton shows this feature. A detailed description of Silvanerpeton, based on recently collected material, was published in 2006, showing new details (Ruta and Clack 2006) (Fig. 8.12). Unfortunately, some of the other key features remain lacking or are not preserved well enough to be sure about. Therefore, the position of Silvanerpeton with respect to anthracosaurs is a bit uncertain. In some phylogenies, it appears as a

vertebrae, sacrum: Acanthostega; in evolution of vertebrate terrestriality; in fish-tetrapod transition; neural arches and zygapophyses and; Proterogyrinus, salamanders: Acanthostega vs. Balanerpeton vs. brachial and sacral plexuses among; as branchiosaur descendants; circulatory system among; cladistic classification of; color patterns in Chinese fossil; discosauriscids vs. larvae of; earliest-known; early tetrapods vs. evolution of; eyes of; fish-tetrapod transition and; frogs and caecilians

fin, and two dorsal fins. Dorsal and anal fins were additionally supported by a series of radials, the pattern of which varied according to the family. In tetrapods, almost all the fin webbing disappeared, along with all traces of dorsal and anal fins. The only remnants are seen in the two earliest known tetrapods, in which some tail fin web was still retained, and in a single specimen from an embolomere (see Chapter 9) in which supraneural spines have been found. 3.1. Cladogram of lobe-fins

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