A 380 million year old fossil fish which features in an article by R. Cloutier, A.M. Clement, and M.S.Y. Lee et al, in the current edition of Nature has provided us with a critical insight into the evolution of the vertebrate hand. Elpistostegi watsonii is the most complete epistostegalian (tetrapod-like fish) found to date. Discovered in Upper Devonian strata in Canada. Its importance lies in the preservation of the complete anatomy of the pectoral fin, which provides insigbht into the evolution of the tetrapod upper limb, and “further blurs[s] the line between fish and land vertebrates.” [1]
Prior to discovery of this largely-complete specimen, E. watsonia was known only from a posterior skull roof, an anterior skull half, and scattered scales and vertebral elements. The degree of preservation of this specimen is such that “it is the sole elpistostegalian for which we have complete knowledge of body shape and proportions.”
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. (Source)
The fish has a short head, an elongated and slender trunk, a relatively short caudal region, and a small anal fin. [3] The pectoral girdle resembles that of Tiktaalik, with a clavicle shape between that of Tiktaalik and the Devonian tetrapods Acanthostega and Ventastega. [4]
The pectoral fin has a humerus, ulna, and radius, while CT scans of the fossil have shown bones corresponding to wrist bones and digits:
"Nineteen radials are preserved and are organized into six preaxial–postaxial rows; the first of these rows articulates with the intermedium, and the last five articulate with the ulnare...We interpret the most-proximal two rows of radials in Elpistostege as carpals in addition to the row that includes the radiale, intermedium and ulnare, similar to numerous Carboniferous tetrapods that bear three rows of elements (the proximal, central and distal carpals)32; by contrast, in Tulerpeton only two carpal rows precede the digits (Fig. 4). The two most-distal rows of radials display a one-to- one relationship and are interpreted as digits (that is, phalanges). Thus, Elpistostege has two identifiable digits that are composed of two non-branching endoskeletal elements that articulate one-to-one proximodistally, and—potentially—three more digits that are each composed of a single preserved element." [5]
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. (Source)
The authors note that the pectoral fin of Elpistostege shows both digits and lepidotrichia, which suggests that the basic architecture of the vertebrate hand had appeared in the pectoral fin of lobe-finned fish while it was still functioning as an aquatic fin. [6] They conclude:
In summary, Elpistostege further blurs the line between fish and tetrapods in showing a greater number of tetrapod novelties than are present in any other ‘fish’. Our analyses place it crownward of Tiktaalik, at the node immediately below the unequivocally digitate tetrapods. If one adopts an apomorphy-based interpretation of Tetrapoda, and considers the parallel, unbranched distal radials in the Elpistostege fin to be true digits, then Elpistostege would represent the earliest and most primitive known tetrapod. The pectoral fin anatomy of Elpistostege provides a window into how fish developed digit-like structures within typical fins that retained lepidotrichia, and while these fish still occupied an aquatic lifestyle. The typical hand pattern—formed by serial rows of digits in tetrapods—began its assembly within advanced sarcopterygian fish fins, possibly driven by increasing evolutionary lability as these fishes began to foray into shallow water or briefly onto land. [7] (emphasis mine)
The evidence for tetrapod evolution, already well-described in the fossil record now gains even more clarity with a fossil possessing a pectoral fin with arm bones, wrist bones, and digits, as well as fin rays and scales.
References
1. Cloutier, R., Clement, A.M., Lee, M.S.Y. et al. Elpistostege and the origin of the vertebrate hand. Nature (2020). https://doi.org/10.1038/s41586-020-2100-8
2. ibid, p 1
3. ibid. p 1
4. ibid, p 2
5. ibid, p 3
6. ibid, p 4-5
7. ibid, p 5
