Evolution

Close to-complete transitional fossil sheds gentle on evolution of vertebrate hand


A 380 million yr outdated fossil fish which options in an article by R. Cloutier, A.M. Clement, and M.S.Y. Lee et al, within the present version of Nature has offered us with a crucial perception into the evolution of the vertebrate hand.  Elpistostegi watsonii is probably the most full epistostegalian (tetrapod-like fish) discovered thus far. Found in Higher Devonian strata in Canada. Its significance lies within the preservation of the whole anatomy of the pectoral fin, which offers insigbht into the evolution of the tetrapod higher limb, and “additional blurs[s] the road between fish and land vertebrates.” [1]

Previous to discovery of this largely-complete specimen, E. watsonia was recognized solely from a posterior cranium roof, an anterior cranium half, and scattered scales and vertebral parts. The diploma of preservation of this specimen is such that “it’s the sole elpistostegalian for which we’ve full data of physique form and proportions.”

a, Full ready specimen in dorsal view (high-resolution picture). b, Shut-up view of foremost cranium block 8, exhibiting spiracles and tabular horns. {Photograph} with ammonium chloride whitening. c, Shut-up view of proper anocleithrum and supracleithrum (scl). {Photograph} with ammonium chloride whitening. d, Ventral view of pectoral fins from blocks 8, 9 and 10. (Supply)

The fish has a brief head, an elongated and slender trunk, a comparatively brief caudal area, and a small anal fin. [3] The pectoral girdle resembles that of Tiktaalik, with a clavicle form between that of Tiktaalik and the Devonian tetrapods Acanthostega and Ventastega. [4]


The pectoral fin has a humerus, ulna, and radius, whereas CT scans of the fossil have proven bones akin to wrist bones and digits:

“Nineteen radials are preserved and are organized into six preaxial–postaxial rows; the primary of those rows articulates with the intermedium, and the final 5 articulate with the ulnare…We interpret the most-proximal two rows of radials in Elpistostege as carpals along with the row that features the radiale, intermedium and ulnare, much like quite a few Carboniferous tetrapods that bear three rows of parts (the proximal, central and distal carpals)32; in contrast, in Tulerpeton solely two carpal rows precede the digits (Fig. 4). The 2 most-distal rows of radials show a one-to- one relationship and are interpreted as digits (that’s, phalanges). Thus, Elpistostege has two identifiable digits which are composed of two non-branching endoskeletal parts that articulate one-to-one proximodistally, and—probably—three extra digits which are every composed of a single preserved factor.” [5]

s

af, Views of the left humerus: a, dorsal; b, preaxial (anteroventral); c, indirect preaxial; d, ventral; e, dorsal; f, ventral (distal) view. g, h, Shut-up view of distal pectoral-fin parts 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 course of; m.pec,
pectoralis muscle; rad, radius; rad. 1–6, preaxial–postaxial radial rows
1–6; rad.f, radial side; sh.d, scapula–humeral despair; ul.f, ulnar
side. (Supply)

The authors be aware that the pectoral fin of Elpistostege reveals each digits and lepidotrichia, which means that the essential structure of the vertebrate hand had appeared within the pectoral fin of lobe-finned fish whereas it was nonetheless functioning as an aquatic fin. [6] They conclude:

In abstract, Elpistostege additional blurs the road between fish and tetrapods in exhibiting a higher variety of tetrapod novelties than are current in every other ‘fish’. Our analyses place it crownward of Tiktaalik, on the node instantly under the unequivocally digitate tetrapods. If one adopts an apomorphy-based interpretation of Tetrapoda, and considers the parallel, unbranched distal radials within the Elpistostege fin to be true digits, then Elpistostege would symbolize the earliest and most primitive recognized tetrapod. The pectoral fin anatomy of Elpistostege offers a window into how fish developed digit-like constructions inside typical fins that retained lepidotrichia, and whereas these fish nonetheless occupied an aquatic way of life. The standard hand sample—fashioned by serial rows of digits in tetrapods—started its meeting inside superior sarcopterygian fish fins, presumably pushed by growing evolutionary lability as these fishes started to foray into shallow water or briefly onto land. [7] (emphasis mine)

The proof for tetrapod evolution, already well-described within the fossil report now beneficial properties much more readability with a fossil possessing a pectoral fin with arm bones, wrist bones, and digits, in addition to 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

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