Elpistostege and the origin of the vertebrate hand (original) (raw)

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 Panderichthys 2 , Elpistostege 3,4 and Tiktaalik 1,5 , 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. The first tetrapods known from skeletal remains date back to the Late Devonian period (about 374 million years ago) 6,7 , while trackway fossils showing digitate impressions of limbs suggest an earlier origin for this clade 8. Over the past decade, fossils that provide information on the fish-to-tetrapod transition have been used to better understand anatomical transformations associated with locomotion 5,9-12 , breathing 13 , hearing 14 and feeding 11,15 , with regard to the change in habitat from water to land. Until now, the terrestrialization of vertebrates has primarily been a matter of comparing six relatively well-known Devonian taxa among stem-group tetrapods 16 : a true piscine sarcopterygian, Eusthe-nopteron foordi; a piscine elpistostegalian, Panderichthys rhombolepis; a near-tetrapod elpistostegalian, Tiktaalik roseae; and three true basal tetrapods, Acanthostega gunnari, Ventastega curonica and Ichthyostega sp. Here we adopt an apomorphy-based definition of tetrapods as 'all organisms derived from the first sarcopterygian to have possessed digits homologous with those in Homo sapiens' 17,18. However, these inferences regarding terrestrialization rely critically on the handful of specimens that have been referred to elpistostega-lians, none of which has been completely described. The postcranial anatomy of Panderichthys is primarily restricted to the morphology of the pectoral fins and girdle 2,19,20 , the vertebrae 2,21 , the scale patterning 22 and very little on the pelvic fin and girdle morphology 9. Although more than 60 specimens 1,10 of Tiktaalik have been found, most of the anatomy of this species has been described from a fairly complete individual for which the skull 1,15 , pectoral and pelvic fins and girdles 5,10,23 , scales 22 and the trunk region 1 anterior to the pelvic region are preserved.