Introduction

Few areas of evolutionary investigation have benefited so much from the integration of molecular and morphological studies as that of the evolution of limbs (e.g. Shubin et al. 2009). Much effort has been concentrated on understanding the transition from ‘fins to limbs’ (Westoll 1943; Coates 1995; Shubin 1995; Shubin et al. 1997, 2004, 2006; Coates et al. 2002; Johanson et al. 2007; Larsson 2007) – for a recent summary of different aspects see Hall (2007) – but also major advances have been made concerning the origin and development of particularly specialized limbs such as those of bats (Sears et al. 2006; Sears 2008), whales (Thewissen et al. 2006), or their absence in snakes (Cohn and Tickle 1999). In fact, tetrapod limbs exhibit a stunning morphological diversity, reflecting their recruitment for a wide variety of functions ranging from walking to watch-making, often deployed as multipurpose structures serving in locomotion and support, personal hygiene, fighting, mating, manipulation and communication.

The reconstructed ground pattern of extant tetrapods may be viewed as a limb with a proximal stylopod (humeral/femoral region), a zeugopod (radial and ulnar/tibial and fibular region) and a distal, pentadactylous autopod (carpal/tarsal region), which contains the proximal mesopodium (wrist and ankle bones) and the terminal acropodium (the digits). With the notable exception of limbless tetrapods, most tetrapod limbs conservatively retain this pattern (Goodwin and Trainor 1983; Holder 1983). Based on these components, the diversity of skeletal morphology in tetrapod limbs is generated largely by differential growth and proportional changes of the common elements. Regarding the acropodia, hypodactyly, hyperphalangy and modifications of branching patterns involve various developmental mechanisms (Motani 1999; Hamrick 2001b, 2002; Richardson and Oelschläger 2002; Fedak and Hall 2004; Sears et al. 2007; Rolian 2008; Weisbecker and Nilsson 2008; Cooper and Dawson 2009).