INTRODUCTION

In the past, muscles were mainly described and illustrated using monochrome line drawings often in a standard anatomical plane such as lateral view (e.g., Byerly 1925; Oelrich 1956; Haas 1973; Schumacher 1973; Gomes 1974; Wu 2003). The limitations are obvious when describing complex three-dimensional (3D) structures and may lead to ambiguous communication or misunderstandings. For example, Gorniak et al. (1982) provide a detailed illustration of the jaw muscles in Sphenodon (Günther 1867), but it is ambiguous as to the path of the muscle labelled as m. Adductor Mandibulae Externus Posterior and, indeed, it is uncertain whether this muscle is correctly identified (Wu 2003; Jones et al. 2009). From drawings made in lateral view alone it is also often difficult to appreciate the mediolateral orientation of muscles, for example the complexities of the pterygoideus muscle in the lizard Uromastyx (e.g., Haas 1973; Throckmorton 1978).

Hypotheses for the muscle arrangement in fossil taxa have long been carried out (e.g., Adams 1919; Anderson 1935; Fox 1964; Barghusen 1973; Rieppel 2002), with the depiction and communication of such hypotheses being subject to similar problems. More recent descriptions of muscle anatomy include photographic images on to which muscle groups are drawn and colour coded (e.g., Sniverly and Russell 2007; Tsuihiji 2007), and improve the clarity and understanding of muscle positioning and function.

Computer-based data collection and imaging has been used to some extent by palaeontologists and comparative anatomists for at least 25 years (e.g., Conroy and Vannier 1984; Conroy et al. 1990), but only with the recent advances in computational power have 3D imaging and functional analysis become more widespread (e.g., Hutchinson et al. 2005; Motani 2005; Grosse et al. 2007; Wickens 2007; Strait and Evans 2008; Sutton 2008). Examples include the description of the cranial anatomy of dinosaurs, which has been impressively presented in recent publications (Witmer and Ridgely 2008, 2009; Evans et al. 2009), and 3D bony anatomy derived from computed tomography (CT) data of dinosaur skulls that has been used as a frame on which to present muscle anatomy (e.g., Holliday 2009).

The modelling and analysis software known as multi-body dynamics analysis (MDA) is an engineering technique that has recently been used to calculate the kinetic and kinematic behaviour within skulls (e.g., Curtis et al. 2008, 2009; Moazen et al. 2008, 2009). Although not the primary purpose of this software, the way in which muscles are generated has the potential to make the muscle anatomy more clear and to help understand the function of individual muscle groups. Others apply this technology to understand function (e.g., Hutchinson et al. 2005), but the potential of this technique as a pure visualisation tool has not yet been explored or highlighted.

In order to demonstrate this potential, we present a thorough treatment of the head and neck muscles in Sphenodon. As the only extant rhynchocephalian (sensu Gauthier et al. 1988) this genus is an important reference taxon for work on the muscles of other amniotes (Schwenk 1986; Bryant and Russell 1992; Witmer 1995; Abdala and Moro 2003; Holliday and Witmer 2007), and it is therefore important that anatomical interpretations are both clear and detailed. The muscle arrangements of Sphenodon have been repeatedly examined and discussed (e.g., Nishi 1916; Byerly 1925; Anderson 1936; Poglayen-Neuwall 1953; Haas 1973; Gorniak et al. 1982; Wu 2003), although, as for many other taxa, they have mainly been described and illustrated in lateral view using line drawings. Apart from a few recent examples (e.g., Tsuihiji 2005, 2007; Holliday and Witmer 2007; Jones et al. 2009), authors were restricted to monochromatic images (e.g., Poglayen-Neuwall 1953; Haas 1973), and as descriptions have become more detailed, the limitations of these methods have become more apparent.

Here accurate 3D models of the skull, lower jaw, and neck of Sphenodon form a base on to which muscle groups are positioned. The muscles are represented as simple cylinders and thus allow the origins and insertions to be easily identified. Complex muscle groups are divided into clear, colour coded sections. A variation in bone transparency and orientation are used to achieve the best view of the muscle attachment, and the effective lines of action can be determined for each muscle group in both lateral and anterior views.

This paper visually presents all muscle groups in Sphenodon with only limited descriptive text; the muscle arrangements are based on a comprehensive descriptive review published by Jones et al. (2009).