Evolution: Fossil reveals early ‘fish fingers’

The discovery of a fossil fish with skeletal digit-like appendages in the pectoral fin provides insights into the origins of the vertebrate hand. The fossil, described in Nature, is the most complete specimen of Elpistostege watsoni, a fish thought to represent an evolutionary stage in the transition of fish to land vertebrates.

Skeletal remains suggest that the appearance of tetrapods (four-limbed vertebrates) occurred approximately 374 million years ago, but some older fossils contain impressions of limbs that could imply an even earlier origin. Until now, the evolutionary record of vertebrates as they moved onto land has relied heavily on the anatomy of a few tetrapod-like fish fossils called elpistostegalians, from the Middle and Late Devonian (393 - 359 million years ago). However, none of these fossils have revealed the complete skeletal anatomy of the pectoral fin (fore-fin).

Richard Cloutier, John Long and colleagues describe the most complete elpistostegalian fossil found to date; a 1.57 metre-long specimen of E. watsoni from the Escuminac Formation of Miguasha, Quebec, Canada. Using high-energy computed tomography (CT) scans, the authors were able to determine its pectoral fin anatomy. They found that the pectoral fin skeleton of E. watsoni had two identifiable digits and potentially three more putative digits, but the fin still retained fin rays. They conclude that this arrangement of bones is the most tetrapod-like example found in a pectoral fin and suggest that the vertebrate hand arose from a skeletal pattern within the pectoral fin of elpistostegalians.

doi:10.1038/s41586-020-2100-8

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