Data from: Fin ray patterns at the fin to limb transition

The fin-to-limb transition was marked by the origin of digits and the loss
of dermal fin rays. Paleontological research into this transformation has
focused on the evolution of the endoskeleton with little attention paid to
fin ray structure and function. To address this knowledge gap, we study
the dermal rays of the pectoral fins of three key tetrapodomorph
taxa—Sauripterus taylori (Rhizodontida), Eusthenopteron foordi
(Tristichopteridae), and Tiktaalik roseae (Elpistostegalia)—using computed
tomography. These data show several trends in the lineage leading to
digited forms, including the consolidation of fin rays (e.g., reduced
segmentation and branching), reduction of the fin web and, unexpectedly,
the evolution of asymmetry between dorsal and ventral hemitrichia. In
Eusthenopteron, dorsal rays cover the preaxial endoskeleton slightly more
than ventral rays. In Tiktaalik, dorsal rays fully cover the third and
fourth mesomeres, while ventral rays are restricted distal to these
elements, suggesting the presence of ventralized musculature at the fin
tip, analogous to a fleshy ‘palm.’ Asymmetry is also observed in cross
sectional area of dorsal and ventral rays. Eusthenopteron dorsal rays are
slightly larger than ventral rays; by contrast, Tiktaalik dorsal rays can
be several times larger than ventral rays, and degree of asymmetry appears
to be greater at larger sizes. Analysis of extant osteichthyans suggests
cross sectional asymmetry in the dermal rays of paired fins are
plesiomorphic to crown-group osteichthyans. The evolution of dermal rays
in crownward stem tetrapods reflects adaptation for a fin-supported
elevated posture and resistance to substrate-based loading prior to the
origin of digits.