Chapter 6 Three Ways to Tackle the Turtle: Integrating Fossils, Comparative Embryology, and Microanatomy Torsten M. Scheyer, Ingmar Werneburg, Christian Mitgutsch, Massimo Delfino, and Marcelo R. Sa ´nchez-Villagra Abstract Herein we review a series of case studies covering the evolution and phylogenesis of turtles, and the ontogenetic development of one of the most peculiar body plans within the Craniota. Comparative analyses of skeletal development, ontogenetic timing, and bone micro- structure in both extant and extinct taxa are used to document patterns and make inferences about the origin of turtles, turtle ingroup relationships, and the evolution of turtle ontogenetic development. The need for a balanced sampling of both cryptodiran and pleurodiran turtle species for future comparative studies is highlighted. Keywords Bone histology Á Comparative anatomy Á Odontochelys Á Sequence heterochrony Á Testudinata Á Testudines Á Turtle origins Introduction Hypotheses on the position of turtles within the amniote tree of life are contentious (Carroll 2012; Lyson et al. 2010), despite many efforts over the past century to solve this major problem in vertebrate evolution (Rieppel 2008). This situation is largely due to the peculiar mor- phology of the turtle body plan, especially the unique shell (Nagashima et al. 2009, 2012; Kuratani et al. 2011; Rieppel 2012) and the anapsid condition of the skull (Gaffney and Meylan 1988; Lee 1997a), which renders comparisons to other vertebrates difficult at best. In recent years, turtles have been treated either as a group of parareptiles or as diapsids; for the latter, relationships with Lepidosauromorpha, Archosauromorpha, or Sauria have been hypothesized. The past two decades have brought refined techniques in molecular biology (Shaffer 2009) and evolutionary devel- opmental biology, as well as in comparative anatomy, all of which have helped clarify interrelationships among living turtles (e.g., see summary by Werneburg and Sánchez- Villagra 2009). For ontogenetic approaches, several methods have been developed to analyze developmental timing data within a phylogenetic framework. One of those methods, Parsimov, has been extensively used to assess information from organogenesis or ossification patterns, for which an expanding body of data is being assembled (e.g., Sánchez- Villagra et al. 2009 and references therein). However, atomizing heterochronic data as performed in event-pair based algorithms, as well as the robustness and the value of its information content, have recently been reassessed and alternative approaches have been suggested (Ziermann 2008; Germain and Laurin 2009; Werneburg 2010; Werneburg and Sánchez-Villagra 2011). In addition, new fossil discoveries continue to be impor- tant for documenting the evolution of turtles. Among the most important of these are fossils and taxa from Upper Triassic sediments on several continents, such as skeletons of T. M. Scheyer (&) Á I. Werneburg Á C. Mitgutsch Á M. R. Sánchez-Villagra Paläntologisches Institut und Museum, Universität Zürich, Karl Schmid-Strasse 4, 8006 Zürich, Switzerland e-mail: tscheyer@pim.uzh.ch; ingmar_werneburg@yahoo.de; christian.mitgutsch@gmail.com; m.sanchez@pim.uzh.ch M. Delfino Dipartimento di Scienze della Terra, Università di Torino, Via Valperga Caluso 35, 10125 Turin, Italy and Institut Català de Paleontologia Miquel Crusafont, Universitat Autònoma de Barcelona. Edifici ICP, Campus de la UAB s/n, 08193 Cerdanyola del Vallès, Barcelona, Spain e-mail: massimo.delfino@unito.it D. B. Brinkman et al. (eds.), Morphology and Evolution of Turtles, Vertebrate Paleobiology and Paleoanthropology, DOI: 10.1007/978-94-007-4309-0_6, Ó Springer Science+Business Media Dordrecht 2013 63