Fingers and toes were long thought to be novelties associated with the invasion of land by tetrapods. The recent identification of a variety of aquatic specializations in some early tetrapods has provoked a debate on whether digits arose in primarily terrestrial or aquatic animals1,2. We recently discovered a pectoral fin of a lobe-finned fish (Fig. 1a, b) that is remarkably similar to tetrapod limbs. This discovery reveals that major tetrapod novelties are also seen in the paddles of some closely related fish and therefore need not have arisen to meet the demands of a terrestrial existence.
We discovered the new fin along a roadside in north central Pennsylvania (Catskill Formation; Late Devonian; ∼370 Myr BP). Features of the shoulder girdle and fin rays clearly identify it as belonging to a rhizodontid sarcopterygian3. The specimen compares favourably with Sauripterus, known from a single specimen discovered in 1840, in having a broadly flattened radius. The quality of preservation of the new fin permits comparisons with limbs that were impossible until now.
The closest relatives of tetrapods, ‘osteo-lepiform’ and elpistostegalid sarcopterygians4, have fins designed on a simple bifurcate pattern and do not have structures that can be readily compared with digits (Fig. 1c). In contrast, the new fin contains an array of eight distally facing and jointed preaxial radials that are superficially similar in number and configuration to the digits of early tetrapods (Fig. 1d). In addition, six of these radials articulate with homologues of the carpus (intermedium and ulnare) at a common proximo-distal level. The radials of the new fin differ from tetrapod digits in that they are flattened and encased by stiff unjointed dermal fin rays (Fig. 1a, b).
Taken together, these characteristics suggest that the digit-like structures were not the primary load-bearing elements of the distal portion of the rhizodont appendage.
Phylogenetic comparisons with other sarcopterygians are complicated by missing data but current hypotheses place rhizodontids just outside a group containing ‘osteolepiformes’, elpistostegalids and tetrapods4,5. This suggests two possibilities: fingers are either primitive to stem tetrapods, or digit-like structures evolved independently in a closely related group of Devonian fish. Either phylogenetic interpretation forces us to question the use of digits as a key innovation associated with the origin of tetrapods.
This discovery reveals that some Devonian fish acquired a truly mosaic fin skeleton, possessing both an extensive and limb-like endoskeleton and elaborate dermal fin rays. The presence of digit-like structures in the paddle of an aquatic fish suggests that digits could have evolved for reasons other than bearing weight during terrestrial locomotion.
Edward B. Daeschler, Neil Shubin Vertebrate Biology, Academy of Natural Sciences of Philadelphia, 1900 Benjamin Franklin Parkway, Philadelphia, Pennsylvania 19103, USAand Department of Biology, University of Pennsylvania, Philadelphia, Pennsylvania 19104, USA
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Nature Ecology & Evolution (2017)