New pectoral fin material from the Late Devonian of Ellesmere Island, Nunavut, Canada and diversity of the radius in finned tetrapodomorphs

The paired fins of stem tetrapods (tetrapodomorphs) have been studied extensively to understand both the phylogenetic framework of early tetrapods and the functional context of the fin-to-limb transition. Attention has predominantly been focused on a few exemplar taxa with well-preserved pectoral fins, and macroevolutionary investigations on individual fin elements have, thus far, focused solely on the humerus. Here we present new isolated pectoral fin material (NUFV 1593) from the Late Devonian of Ellesmere Island, Nunavut, Canada that we identify as likely belonging to an elpistostegalian tetrapodomorph. NUFV 1593 is characterized by a slender, triangular radius and an intermedium with a prominent ventral groove, resembling features known from its contemporary Tiktaalik roseae. Geometric morphometric analyses of tetrapodomorph radii placed NUFV 1593 in a region of morphospace populated by elpistostegalians. However, the anatomy of NUFV 1593 shows a combination of distinctive features that we cannot attribute to any existing taxon. Differences include the relative proportions of the radius and intermedium, and the number of fin rays. Notably, the radius bears a bean-shaped proximal articular facet that is slender and dorsoventrally narrower than the rounded facets of described tetrapodomorphs, implying a movement pattern at the humeroradial joint not previously observed within the clade. The new material expands the functional diversity of tetrapodomorph fins and highlights the importance of incomplete material for understanding the evolution of the closest relatives of limbed vertebrates.