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387 10 Gaining Ground: The Evolution of Terrestriality This extended survey of the anatomy and lifestyles of tetrapods throughout the Paleozoic has explored the evidence and speculation bearing on the advent of tetrapods onto land. This final chapter now goes on to consider the evolution of several key aspects of their biology and how they became truly adapted to terrestriality. The solutions that these early tetrapods arrived at laid the foundations for terrestrial living in a huge group of vertebrates that have ultimately become a highly conspicuous part of the fauna of the planet. How these changes were achieved over that time has influenced the anatomy, morphology, and evolutionary pathways of all subsequent tetrapods and is still reflected in our own anatomy. The origin of terrestrial tetrapods is one of the few faunal events in the evolution of vertebrates that was evidently not some kind of faunal replacement: there were no vertebrate competitors preceding them. It is also clear that there was no single primary driver, but that a combination of circumstances and evolutionary responses to them fits with a model of correlated progression (Kemp 2006, 2007) in which gradual changes to many parts of the anatomy occurred in concert. This has been seen already in the evolution of the elpistostegalian fish and the earliest limbed tetrapods. Many of the other changes involved also took place gradually and can be followed from the early fossil lobe-finned fishes, through the earliest tetrapods, into the explosion of terrestrial forms in the late Paleozoic. Some of the aspects that can be followed in most detail include those related to the interconnected mechanisms of feeding, hearing, breathing, and locomotion, and this final chapter draws together threads from new discoveries in paleontology and in the biology of living animals. The story of skeletal evolution is continued in this chapter from Acanthostega and its Devonian contemporaries, and it shows how parts of the skeleton were gradually adapted by Carboniferous forms for terrestrial living, how changes to one part affected those to another, and how different groups made these changes in different ways and at different times. The origin of tetrapods is demonstrably separate from the origin of terrestriality, in that tetrapods as a group appear in the fossil record in the Late Devonian, whereas the ensuing adaptations for terrestriality are first seen in the latter part of the Early Carboniferous, appearing bit by bit throughout the rest of the late Paleozoic. Steps toward Terrestriality Gaining Ground 388 Changes to the Skull Roof What is known of the skulls of Devonian tetrapods shows them to be essentially conservative. There are a number of consistent features marking them out as different from their fish relatives, such as the reduced number of snout bones, the pattern of the cheek and skull table, and the subtle characters of the lower jaw, but in other ways they show little variation. The Devonian forms all seem to be rather flat headed, with spade-shaped snouts and eyes placed about the midpoint in the skull, looking more or less dorsally. Only Ichthyostega shows much in the way of elaboration of the inside of the skull roof, in that it has produced elaborate flanges that attach to the braincase at the back and sides. In Acanthostega, there are no such flanges, and the internal surface of the skull roof is essentially smooth. The only parts of its skull roof to show specially thickened bone are two ridges running up the frontals and prefrontals above the eye. The skull roof of Ventastega is much the same. Among Carboniferous forms, even those from the earliest parts, some striking modifications can be seen to the skull architecture. Several early forms such as Pederpes and Whatcheeria have skulls that are more deep than broad. Whatcheeria has specially thickened ridges inside the skull anterior to the eyes, and Crassigyrinus (Fig. 7.15) has taken both these modifications to extremes, with a particularly deep skull, elongated posteriorly , and with greatly thickened ridges running up the snout, between the eyes, and inside the prefrontal. Its eyes are relatively far forward, and furthermore the orbits are rhomboidal. Even in more conventionally shaped skulls such as those of baphetids, the eye sockets acquired forwardly extending embayments (Fig. 9.5). Other examples of extreme modifications can be seen among some of the lepospondyls, in which the skull roof pattern of early tetrapods is scarcely recognizable. Many of the skull roof bones are lost, in some forms to be replaced by small platelets and...

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