Title: Fin ray patterns at the fin to limb transition
Authors: Thomas A. Stewart, Justin Lemberg, Natalia K. Taft, Ihna Yoo, Edward B. Daeschler, Neil H. Shubin
Corresponding Authors: tomstewart@uchicago.edu, nshubin@uchicago.edu
Abstract: 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, and little attention has been paid to fin ray structure and function. To address this knowledge gap, here we study the dermal rays of the pectoral fins of three key 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, and these differences scale isometrically with standard length. By contrast, Tiktaalik dorsal rays can be several times larger than ventral rays, and asymmetry scales allometrically, becoming more asymmetric at larger sizes. The evolution and development 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.
Project Summary: The files in this project folder were used to compare the dorsal and ventral hemitrichia in Tiktaalik, Eusthenopteron, Neoceratodus, Polypterus, and Acipenser.
The code is used to generate panels for Fig. 5, S7, S8, and Table S2. The figures were further tidied / organized using illustrator.
Additional supporting documents for the publication:
1 - Cross sectional area and second moment of area are deposited to Dryad Digital Repository: Stewart, T.A. et al. 2019. Data from: Fin ray patterns at the fin to limb transition, v4, Dryad, Dataset, https://doi.org/10.5061/dryad.2fqz612kd website: https://datadryad.org/stash/dataset/doi:10.5061/dryad.2fqz612kd
2 - Raw CT data are deposited to MorphoSource: Stewart, TA. et al. 2019. Project: Fin ray patterns at the fin to limb transition. MorphoSource. https://www.morphosource.org/Detail/ProjectDetail/Show/project_id/853. December 15th, 2019. website: https://www.morphosource.org/Detail/ProjectDetail/Show/project_id/853
3 - 3D models of select skeletal elements are free for download on SketchFab: https://sketchfab.com/tomstewartscience/collections/fin-ray-patterns-at-the-fin-to-limb-transition