Introduction

Turtles are characterized by the possession of shells. For acquisition of this structure, this animal group appears to have undergone various types of anatomical changes in their body plan, not only in their skeletal system, but also in the muscular, nervous and respiratory systems (Bojanus 1819; Thomson 1932). These features often lead to confusion in determining homology, especially of amniote ribs, as well as in establishing the phylogenetic position of this animal group (Goodrich 1930; Remane 1936). To understand the origins of the morphology of turtles, a number of embryologists and morphologists have studied their embryonic developmental patterns (e.g. Rathke 1848; Agassiz 1857; Mitsukuri and Ishikawa 1887; Mitsukuri 1894, 1896; Ogushi 1911, 1913; Ruckes 1929; Walker 1947; Burke 1989, 1991; Gilbert et al. 2001, 2008; Nagashima et al. 2005, 2007; 2009; Sánchez-Villagra et al. 2009; Werneburg et al. 2009; reviewed by Gilbert et al. 2008; Kuratani et al. 2011).

The phylogenetic position of turtles remains controversial, but considerable progress has been made. Although recent molecular phylogenetics and genomic analyses have placed this taxon close to or even within the archosaurians, including birds and crocodiles (Caspers et al. 1996; Zardoya and Meyer 1998, 2001; Hedges and Poling 1999; Kumazawa and Nishida 1999; Mannen and Li 1999; Mindell et al. 1999; Cao et al. 2000; Iwabe et al. 2005; Matsuda et al. 2005; Kuraku et al. 2006; Hugall et al. 2007; Chapus and Edwards 2009), morphological analyses do not always agree with this conclusion (reviewed by Kuratani et al. 2011). However, some early embryologists supported an affinity to archosaurians (Haeckel 1891; de Beer 1937; see also Figure 1.1 in Asher and Müller, this volume).