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3 The Origin and Radiation of Therapsids Tom S. Kemp The earliest fossils of amniotes—the clade that now consists of the reptiles birds and mammals—occur in 320 million-year-old rocks of the Late Carboniferous (sensu Laurin 2004; Laurin and Reisz 1995; Voigt and Ganzelewski 2010). They are characterized by several modifications that indicate an increased independence of freestanding bodies of water. For example, the aquatic sensory system of lateral lines is no longer present, the skull and jaws are strengthened for the biting action that terrestrial animals tend to rely on, and the limbs and girdles are robust. Whether they possessed the single, most characteristic of all amniote characters, an amniotic egg capable of developing on dry land, is unknown but seems probable (Packard and Seymour 1997). Surprisingly perhaps, the divergence between the two major living amniote sister groups was in existence from the very beginning of the fossil record of amniotes (Laurin and Reisz 1995). The Sauropsida consists of the modern reptiles and birds, plus the great range of dinosaurs, pterosaurs, and aquatic reptiles that dominated the land, sea, and air of the Mesozoic. The second group, the Synapsida, consists nowadays only of the mammals with their highenergy , active lifestyle and extraordinary ability to adapt successfully to so many terrestrial, subterranean, arboreal, aquatic, and aerial habitats in such a wide range of environmental conditions. The connection between the mammals and their remote Carboniferous relatives is one of the most remarkable parts of the fossil record. These animals, referred to as nonmammalian synapsids, or more affectionately if a trifle misleadingly as “mammal-like reptiles,” are the stem-group mammals , and they exhibit a range of different combinations of basal amniote and mammalian characters. Most of these nonmammalian synapsids are included in Therapsida, the monophyletic taxon that originated within a primitive, paraphyletic group called “pelycosaurs,” at least by the Middle Permian when the first undisputed therapsids are found. The therapsids are of fundamental importance in the history of terrestrial life on Earth for three reasons: first, because they dominated the terrestrial vertebrate scene from the Middle through the Late Permian and remained one of the most significant groups of the Triassic; second, because they established for the first time a terrestrial ecosystem based on very large numbers of fully terrestrial herbivorous tetrapods as the primary consumers, plus related carnivores as the major secondary consumers, a pattern to be repeated later by the dinosaurs of the Mesozoic and the mammals of the Tertiary; and third, because it was the therapsids that commenced the evolution of an elevated energy budget, and therefore a lifestyle in Introduction 1 Kemp 4 which relative independence of environmental fluctuations in temperature allowed continuous high levels of activity. Progressive development of this revolutionary biological strategy can be followed all the way to its expression in the fully endothermic mammals (Kemp 2006b, 2007b). The most recent overall review of the evolution of the synapsids is that of Kemp (2005). Reisz (1986) reviewed the pelycosaurs to genus level. Rubidge and Sidor (2001), and somewhat egregiously Ivakhnenko (2003), reviewed the major therapsid subtaxa. Synapsida is a monophyletic taxon recognized by a number of characters, including most distinctively the eponymous synapsid temporal fenestra in the skull roof, lying behind the orbit, and primitively bounded by the postparietal, parietal, squamosal, quadratojugal and jugal bones of the skull. The earliest record of synapsids consists of footprints of early Late Carboniferous age found in Germany, which predates the oldest actual fossil synapsids by 5–10 Ma (Voigt and Ganzelewski 2010). The latter consists of the humerus and other fragments described as Protoclepsydrops by Carroll (1964), which are dated as Westphalian B of the Late Carboniferous of Nova Scotia, and the slightly younger Westphalian D ophiacodontid pelycosaur Archaeothyris from the same region, which is represented by a skull lacking the mandible, and by the vertebral column and parts of the limbs (Reisz 1972). The Late Carboniferous and Early Permian witnessed a radiation of nontherapsid synapsids, usefully referred to as the paraphyletic group “Pelycosauria.” They retained many unmodified amniote features such as a more or less homodont dentition, short and heavily built sprawling limbs, and a long massive tail (Fig. 1.1). The great majority are known from North America, although there are also representatives in Europe and, at the very end of their temporal range, rare representatives of the family Varanopidae in South Africa (Modesto et al. 2001; Botha-Brink and Modesto 2009). In their classic review, Romer and Price (1940) divided all...

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