Earlier we looked at the origin of fingers
in basal tetrapods (Fig. 1). The primitive number then, as now, was 4 fingers, 5 toes.
Figure 1. Graphing the presence of fingers and toes in basal tetrapods, updated today with the addition of 4 digits in Panderichthys. Sharp-eyed readers will note the switching of Panderichthys with the Tiktaalik clade here.
I overlooked a paper (Boisvert, et al., 2008) that found four proto-digits in the lobefin of Panderichthys (Fig. 2) and provided good data for the Tiktaalik manus that I did not have. With those corrections, a quick review is in order.
Figure 2. From Boisert et al. 2008, colors added. This is their ordering for the evolution of manual digits. Compare to figure 3 where Panderichthys and Tiktaalik switch places and several taxa are inserted transitional to Acanthostega.
For some reason, fingers are rarely preserved in basal tetrapods,
but most continue to have four. Proterogyrinus (Fig. 2) is an early exception with five. So is Acanthostega (Fig. 2) with eight. See chart above (Fig. 1) for all tested taxa in the LRT.
Tradition holds that eight is a primitive number,
later reduced to five or four. The large reptile tree (LRT, 1590 taxa, subset Fig. 1) flips that around. Eight is a derived number on a terminal taxon (Acanthostega) leaving no descendants. The primitive number is four (subset Fig. 1).
Boisvert, Mark-Kurik and Ahlberg 2008 used a CT-scanner
to find four proto-digits on the manus of Panderichthys (Fig. 2) and compared those to the traditional basal tetrapod taxa: Eusthenopteron, Tiktaalik and Acanthostega. Note the big phylogenetic leap they show between Tiktaalik and Acanthostega. And note the apparent reversal in Tiktaalik as the metacarpal seems to revert to a ray. These problems are corrected in figure 3.
Figure 3. Forelimb of several basal tetrapods rearranged to more closely fit the LRT. Compare to figure 2. As discovered here earlier, four fingers turns out to be the primitive number. Five is a recent mutation. Six was a short-lived experiment in Tulerpeton.
The large reptile tree
(LRT, 1590 taxa) recovers a different topology (subset Fig. 1). In the LRT Acanthostega is not a transitional taxon, but an aberrant one returning to a more aquatic lifestyle and leaving no descendants. On the other hand, the four digits in Panderichthys are retained by a wide variety of basal tetrapods. The number jumps to five with the addition of a lateral digit in only a few taxa (Fig. 1). Importantly, in Utegenia (Fig. 4) we see the most primitive appearance of five digits in our lineage despite its late appearance in the fossil record. More derived, but earlier, Late Devonian Tulerpeton had six fingers representing a failed experiment leaving no descendants in the lineage of reptiles, represented by Silvanerpeton in the Early Carboniferous, also pre-dating Utegenia.
So frogs did not lose a finger.
They retained the four that Panderichthys provided them. Four, not five or eight, is the primitive number of digits for basal tetrapods, as discovered earlier here in the LRT. Let me know if there was an earlier discovery for this hypothesis of interrelationships and I will promote that citation.
a salamander will have six fingers. We’ll look at that strange case soon.
Figure 4. Utegenia diagram showing five fingers on each hand. This is the most primitive taxon in our lineage to have all five.
In the course of this study
I learned that the Tiktaalik clade and Panderichthys needed to switch places on the LRT. This has been updated in most cases (Fig.1).
Figure 5. Tulerpeton manus and pes in situ, reconstructed by Lebdev and Coates 1995 and newly reconstructed here.
Evidently Boisvert et al. were using
an outdated tree topology and did not recognize the problem that arose between the digits presented by Tiktaalik and those presented by Panderichthys (Fig. 2). Of course that puts Tiktaalik cousins, Koilops and Spathicephalus in the lobefin grade (Fig. 1), lacking fingers. Both currently lack post-cranial data, but were originally thought to be tetrapods.
Boisvert CA, Mark-Kurik E and Ahlberg PE 2008. The pectoral fin of Panderichthys and the origin of digits. Nature 456:636–638.