ABSTRACT
The Osteichthyes, including bony fi shes and tetrapods, is a highly speciose group of animals, comprising more than 42,000 living species. Two main osteichthyan groups are usually recognized: the Sarcopterygii (lobefi ns and tetrapods), with an estimate of more than 24,000 living species (e.g., Stiassny et al. 2004), and the Actinopterygii (rayfi ns), including more than 28,000 extant species (e.g., Nelson 2006). The Polypteridae (included in the Cladistia) are commonly considered the most basal extant actinopterygian taxon. The Acipenseridae and Polyodontidae (included in the Chondrostei) are usually considered the sister-group of a clade including the Lepisosteidae (included in the Ginglymodi) and the Amiidae (included in Halecomorphi) plus the Teleostei. Regarding the Teleostei, four main living clades are usually recognized in recent works: the Elopomorpha,
Osteoglossomorpha, Otocephala (Clupeomorpha + Ostariophysi) and Euteleostei. The Sarcopterygii includes two groups of extant fi shes, the coelacanths (Actinistia) and lungfi shes (Dipnoi), and the Tetrapoda. Within tetrapods, Amphibia is the sister-group of Amniota, which includes the Mammalia and the Reptilia (note: when we use the term ‘reptiles’ we refer to the group including lepidosaurs, birds, crocodylians and turtles, which, despite some controversy, continues to be considered a monophyletic taxon by most taxonomists: see, e.g., Gauthier et al. 1988; Kardong 2002; Dawkins 2004; Diogo 2007; Conrad 2008). The Amphibia include three main extant groups: caecilians (Gymnophiona or Caecilia), frogs (Anura), and salamanders (Caudata or Urodela), the two latter groups being possibly more closely related to each other than to the caecilians (see, e.g., the recent review of Carroll 2007). The Reptilia include four main extant groups: turtles (Testudines), lepidosaurs (Lepidosauria), crocodylians (Crocodylia) and birds (Aves). The Lepidosauria, Crocodylia and Aves are probably more closely related to each other than to Testudines, being thus usually included in the clade Diapsida; crocodylians are considered to be the closest living relatives of birds, these two groups being included in the clade Archosauria (see, e.g., Gauthier et al. 1988; Dilkes 2000; Kardong 2002; Meers 2003; Dawkins 2004; Conrad 2008). The Lepidosauria comprises the Rhynchocephalia, which includes a single extant genus, Sphenodon, and the Squamata, which according to the recent study of Conrad (2008) includes amphisbaenians, mosasaurs, snakes and ‘lizards’ (as explained by this author, ‘lizards’ do not form a monophyletic group, because some ‘lizards’ are more closely related to taxa such as snakes than to other ‘lizards’: see Conrad 2008 for more details on the interrelationships of squamates). The Mammalia includes the Monotremata and Theria, which comprises marsupials and placentals. Within the latter, the Primates (including modern humans), Dermoptera (including colugos or ‘fl ying lemurs’) and Scandentia (including tree-shrews) are included in the clade Euarchonta and are placed in an unresolved trichotomy in Fig. 1.1, because the relationships between these three groups remains mainly unresolved (some authors continue to group colugos with tree-shrews, others group tree-shrews with primates, and yet others group colugos with primates: see, e.g., Sargis 2002ab, 2004; Dawkins 2004; Marivaux et al. 2006; Janecka et al. 2007; Silcox et al. 2007; Diogo 2009).
