Acta Zoologica (Stockholm) 90 (Suppl. 1): 297–305 (May 2009) doi: 10.1111/j.1463-6395.2008.00389.x © 2009 The Author Journal compilation © 2009 The Royal Swedish Academy of Sciences  Abstract Boisvert, C.A. 2009. The humerus of Panderichthys in three dimensions and its significance in the context of the fish–tetrapod transition. — Acta Zoologica (Stockholm) 90 (Suppl. 1): 297–305 The humerus of Panderichthys has been considered to represent a transitional form between that of tetrapodomorph fish such as Eusthenopteron and tetrapods such as Acanthostega. The previous description was based on flattened material and was analysed in the context of the few fossils known at the time. Since then, several new forms have been described such as Gogonasus, Tiktaalik and an isolated humerus from the Catskill Formation. The humeral morphology of Panderichthys rhombolepis and its interpretation in this new context are therefore reassessed with the help of a three-dimensional model produced with the  software based on a computed tomography scan of an unflattened specimen as well as comparisons with the originally described material. The humerus of Panderichthys displays a combination of primitive, derived, intermediate and unique characteristics. It is very similar to the morphology of Tiktaalik but when it differs from it, it is most often more derived despite the more basal phylogenetic position that Panderichthys occupies.What emerges from this study is a much more gradual transformation of the humerus morphology from fish to tetrapods and the ability to distinguish autapomorphies more easily. The picture is more complex than previously believed, with many morphological specializations probably reflecting the breadth of ecological specializations already present at the time. Catherine Boisvert, Evolutionary Organismal Biology, Institution for physiology and developmental biology, Evolutionary Biology Center, Uppsala University, Norbyvägen 18A, SE-75236 Uppsala, Sweden. E-mail: Catherine.Boisvert@ebc.uu.se Blackwell Publishing Ltd The humerus of Panderichthys in three dimensions and its significance in the context of the fish–tetrapod transition Catherine A. Boisvert Evolutionary Organismal Biology, Institution for Physiology and Developmental Biology, Evolutionary Biology Centre, Uppsala University, Uppsala, Sweden Keywords: fish-tetrapod transition, sarcopterygian, humeri, pectoral musculature Accepted for publication: 24 November 2008 Introduction The fish-tetrapod transition, which occurred in the Late Devonian, was accompanied by a breadth of morphological changes, of which the appendicular transformations are some of the most notable (Clack 2002). In conjunction with changes in the girdles, the limbs slowly become reoriented more laterally, towards a right angle to the body rather than directed posteriorly. This means that the muscles are now orientated perpendicular to the body rather than at an acute angle to it (Clack 2004). Muscle groups also become more elaborated and differentiated so that the flexor and extensor muscles become increasingly separated to lie on opposite faces of the bone. This, in connection with the anterior– posterior elongation of the humeral head, restricts shoulder rotation so that the appendage now moves in a more antero- posterior and dorsoventral manner (Shubin et al. 2004). Those changes are seen most obviously in the humerus in which the reorientation of the limbs is mirrored by a general change in morphology. Early tetrapod humeri are -shaped, dorsoventrally flattened and crests and ridges on their surfaces reflect the reorientation of muscle attachments (Shubin et al. 2004) from their arrangement in sarcopterygian fish humeri. On the dorsal (extensor) surface, proximalization