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Elpistostege and the origin of the vertebrate hand

Nature volume 579, pages 549–554 (2020)Cite this article

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Abstract

The evolution of fishes to tetrapods (four-limbed vertebrates) was one of the most important transformations in vertebrate evolution. Hypotheses of tetrapod origins rely heavily on the anatomy of a few tetrapod-like fish fossils from the Middle and Late Devonian period (393–359 million years ago)1. These taxa—known as elpistostegalians—include Panderichthys2, Elpistostege3,4 and Tiktaalik1,5, none of which has yet revealed the complete skeletal anatomy of the pectoral fin. Here we report a 1.57-metre-long articulated specimen of Elpistostege watsoni from the Upper Devonian period of Canada, which represents—to our knowledge—the most complete elpistostegalian yet found. High-energy computed tomography reveals that the skeleton of the pectoral fin has four proximodistal rows of radials (two of which include branched carpals) as well as two distal rows that are organized as digits and putative digits. Despite this skeletal pattern (which represents the most tetrapod-like arrangement of bones found in a pectoral fin to date), the fin retains lepidotrichia (fin rays) distal to the radials. We suggest that the vertebrate hand arose primarily from a skeletal pattern buried within the fairly typical aquatic pectoral fin of elpistostegalians. Elpistostege is potentially the sister taxon of all other tetrapods, and its appendages further blur the line between fish and land vertebrates.

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Fig. 1: Complete specimen of E. watsoni MHNM 06-2067.
Fig. 2: Pectoral girdle and fin of E. watsoni MHNM 06-2067.
Fig. 3: Pectoral fin of E. watsoni MHNM 06-2067.
Fig. 4: Comparative anatomy of pectoral limb endoskeleton and humerus of stem-tetrapod fish and early tetrapods.
Fig. 5: Phylogenetic analyses of elpistostegalian relationships, based on undated Bayesian and parsimony approaches.

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Data availability

Morphological data are deposited in MorphoBank at https://morphobank.org/index.php/Projects/ProjectOverview/project_id/3480. The complete character-by-taxon matrix, PAUP* and MrBayes scripts as well as full trees are presented in the Supplementary Information and available to download as source data files.

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Acknowledgements

We thank the Parc National de Miguasha—especially O. Matton, J. Kerr and F. Charest—for the loan of the specimen, and the special opportunity to work on this material; J. Willet for the skilful mechanical preparation of the specimen; A. E. Latimer and T. Senden for access to high-resolution micro-computed tomography; M. Coates and T. Grünbaum for comments and suggestions; S. Cumbaa for providing access to a Tiktaalik cast at the Canadian Museum of Nature; E. B. Daeschler and N. H. Shubin for discussions and providing us with casts of the skull, pectoral girdle and fin of Tiktaalik; and the Centre de Développement et de Recherche en Imagerie Numérique (CDRIN) for providing access to computer facilities The X-ray high-energy tomography was done by M. Colbert and J. A. Maisano. Some images were provided by J. Kerr and C. Rodrigue. This work was funded by a Research Laboratory in Paleontology and Evolutionary Biology at UQAR (Power Corporation), a Chantier 3 grant from Ministry of Education, Leasure, and Sports (Government of Québec) to R.C. and I.B., Natural Sciences and Engineering Research Council of Canada (NSERC) discovery grants to R.C., and Flinders University international exchange grant to R.C. J.A.L. and M.S.Y.L. acknowledge an Australian Research Council (ARC) DP grant; J.A.L. also acknowledges Flinders Strategic Professor Fund for travel to Canada in 2014/2016.

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Authors and Affiliations

  1. Université du Québec à Rimouski, Rimouski, Quebec, Canada

    Richard Cloutier, Roxanne Noël, Isabelle Béchard & Vincent Roy

  2. College of Science and Engineering, Flinders University, Adelaide, South Australia, Australia

    Richard Cloutier, Alice M. Clement, Michael S. Y. Lee & John A. Long

  3. Earth Sciences Section, South Australian Museum, Adelaide, South Australia, Australia

    Michael S. Y. Lee

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Contributions

R.C. conceived the project. A.M.C., I.B., R.N. and V.R. performed the image processing, including computerized tomography restorations. R.C. and J.A.L. defined the phylogenetic characters and coded the matrix. M.S.Y.L. and R.C. conducted the phylogenetic analyses. R.C. and J.A.L. discussed the results and prepared the manuscript, with input from M.S.Y.L. and A.M.C. J.A.L. prepared the illustrations from images provided by R.C., A.M.C., V.R. and I.B.

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Correspondence to Richard Cloutier.

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Extended data figures and tables

Extended Data Fig. 1 Complete specimen of E. watsoni MHNM 06-2067, with close-up views of skull, anocleithrum and supracleithrum, and pectoral fins.

a, Complete prepared specimen in dorsal view (high-resolution image). b, Close-up view of main skull block 8, showing spiracles and tabular horns. Photograph with ammonium chloride whitening. c, Close-up view of right anocleithrum and supracleithrum (scl). Photograph with ammonium chloride whitening. d, Ventral view of pectoral fins from blocks 8, 9 and 10.

Extended Data Fig. 2 Pectoral-fin endoskeleton of E. watsoni MHNM 06-2067, restored using Mimics v.18 from micro-computed tomography data.

a–f, Views of the left humerus: a, dorsal; b, preaxial (anteroventral); c, oblique preaxial; d, ventral; e, dorsal; f, ventral (distal) view. g, h, Close-up view of distal pectoral-fin elements in ventral (g) and dorsal (h) views. cap, caput humeri; d.rad, distal radials; ect.f, ectepicondyle foramen; h.r, humeral ridge; lat.d, latissimus dorsi process; m.pec, pectoralis muscle; rad, radius; rad. 1–6, preaxial–postaxial radial rows 1–6; rad.f, radial facet; sh.d, scapula–humeral depression; ul.f, ulnar facet.

Extended Data Fig. 3 E. watsoni lower jaw features restored from micro-computed tomography data.

a, b, MHNM 06-2067 in lateral view. a, Whole jaw. b, Close-up view of anterior region, individual bones and teeth segmented using Mimics v.18. c, d, MHNM 06-538, left lower jaw in dorsal view (c), and right lower jaw and symphyseal area of left jaw in dorsal view (d), volume rendered using Drishti v.2.6. ad.f, adsymphysial fang; adp, adsymphysial plate; cor.f, coronoid fang; den.f, dentary fang; pmx, premaxilla; pra, prearticular; sym, symphysis.

Extended Data Fig. 4 Phylogenetic relationships of Elpistostege and other tetrapodomorphs.

Core tetrapods shown in red, elpistostegalian-grade taxa in blue and outgroups in grey. a, Undated Bayesian analysis using MrBayes49. Majority-rule consensus of 32,000 post-burnin trees (mean log-likelihood of 1,694.88). Numbers at nodes refer to posterior probabilities. See Supplementary Information for full details of analysis, and the executable MrBayes script. b, Parsimony analysis using PAUP*48. Strict consensus of all 216 most-parsimonious trees (445 steps). Numbers at nodes denote bootstrap percentages, based on 200 replicates. See Supplementary Information for full details of analysis and the executable PAUP* script.

Supplementary information

Supplementary Information

This file contains phylogenetic data, methods used and character coding discussions.

Reporting Summary

Supplementary Data 1

MrBayes executable file (including formatted data), screen log, and consensus tree in Nexus/FigTree format. Multiple files (zipped).

Supplementary Data 2

PAUP executable files, all 216 MPTs, and consensus tree in Nexus/FigTree tree. Multiple files (zipped).

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Cloutier, R., Clement, A.M., Lee, M.S.Y. et al. Elpistostege and the origin of the vertebrate hand. Nature 579, 549–554 (2020). https://doi.org/10.1038/s41586-020-2100-8

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