A study uncovers fresh insights into the structural changes that enabled fins to give rise to limbs during vertebrate evolution. The transformation of fins into limbs is marked by the appearance of digits, an evolutionary milestone that paralleled the loss of dermal fin rays. Fin rays, which are skeletal rods found beneath the skin and above the endoskeleton, undergird fin webs. Previous efforts to explain the evolution of limbs from fins have largely focused on the underlying endoskeleton. Neil Shubin and colleagues used computed tomography to examine pectoral fin rays of adult and juvenile specimens of 3 tetrapodomorph fishes. The fishes--Sauripterus taylori, Eusthenopteron foordi, and Tiktaalik roseae--are thought to be evolutionary antecedents of tetrapods, or four-limbed vertebrates, with T. roseae being closest among them to tetrapods. Comparative analysis revealed a trend in the evolutionary progression toward digits: consolidated fin rays with reduced segmentation and branching, reduced fin webs, and asymmetric coverage of the endoskeleton by dorsal and ventral fin rays. The consolidated rays and foreshortened fins of Tiktaalik, which braved forays onto land, were likely adaptations for maneuvering at the water's murky bottom or on shallow mudflats. Additionally, the fin rays' dorsoventral asymmetry suggests that the fin's ventral surface was more muscular than its dorsal surface at the distal end, akin to a fleshy palm--a feature not previously found in tetrapodomorphs. Thus, the authors suggest, the asymmetry of dermal fin rays, together with previous findings on the endoskeleton and girdle, might help explain Tiktaalik's upright, fin-supported stance and represent an evolutionary precursor to limb architecture in tetrapods.
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Article #19-15983: "Fin ray patterns at the fin-to-limb transition," by Thomas Stewart et al.
MEDIA CONTACT: Neil Shubin, University of Chicago, IL; tel: 773-834-7472; e-mail: nshubin@uchicago.edu
Proceedings of the National Academy of Sciences