The humerus of Panderichthys in three dimensions and its significance in the context of the fish-tetrapod transition

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 mimics 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.