Cloutier et al. 2020 bring us
a new complete specimen of Late Devonian Elpistostege (Fig. 1) along with CT scans of the manus-in-the-lobefin revealing finger buds inside (Fig. 2). Despite these buds, Elpistostege had no elbows (Fig. 2) and was too large (relative to its small posteriorly-oriented fins) to support itself above the substrate. That would come later. Meanwhile it could have wriggled ashore, insured from tipping over by its wide body and low center of gravity.
Cloutier et al. focused on the fingers,
providing an external view and CT scans, colored like a diagram (Fig. 2). I added pastel colors to the four fingers that extend beyond the more numerous nascent metacarpals. Even at this early stage, parallel interphalangeal lines (PILs) can be drawn where phalanges work in sets during extension and flexion.
The skull presents a bit of a tracing challenge
as the rostrum is largely missing here (Fig. 3).
The large reptile tree
(LRT, 1658+ taxa, subset Fig. 4) tests more taxa than the traditional cladogram published in Cloutier et al. 2020. They nest Panderichthys as the more basal taxon. In the LRT Panderichthys is more derived (also see Fig. 5). Missing from Cloutier et al. are Spathicephalus (a sister to Tiktaalik) and Koilops, a neotonous form basal to Elpistostege and the rest of the Tiktaalik clade. In the LRT that’s an offshoot clade from the main line of fin to fingers taxa. Trypanognathus has the phylogenetically first known fingers, but that LRT hypothesis of relationships has been overlooked in favor of the multi-digit taxa, Acanthostega and Ichthyostega.
Traditional transitional taxa,
like Tiktaalik, Acanthostega and Ichthyostega, are phylogenetically shifting off to the side at various nodes in the LRT. Other taxa that never get talked about (Fig. 4) are more in the main line of tetrapod evolution leading to reptiles. Cloutier et al. seem to be perpetuating the traditional myth that basal tetrapods had more than five fingers (corrected in Fig. 5). The evidence shows that four is the primitive number. We looked at basal tetrapod finger evolution earlier here. and here.
Elpistostege watsoni (Westoll 1938, Schultze and Arsenault 1985, Cloutier et al. 2020 is another flathead fish transitional to tetrapods, but off toward the Tiktaalik + Spathicephalus clade, derived from Koilops. Some tiny fingerbones are present in the lobefin. This fish is over a meter in length. It is the last taxon in this lineage with an anal fin. The radius continues to be as long as the ulna + ulnare with the intermedium separating them distally.
Cloutier R, Clement AM, Lee MSY, Noël R, Béchard I, Roy V and Long JA 2020. Elpistostege and the origin of the vertebrate hand. Nature https://doi.org/10.1038/s41586-020-2100-8
Schultze H-P and Arsenault M 1985. The panderichthyid fish Elpistostege: a close relative of tetrapods? Palaeontology 28, 293–309 (1985).
Swartz B 2012. A marine stem-tetrapod from the Devonian of Western North America. PLoS ONE. 7 (3): e33683. doi:10.1371/journal.pone.0033683
Westoll TS 1938. Ancestry of the tetrapods. Nature 141, 127–128 (1938).